input_pipeline/gestures/

gesture_arena.rs

// Copyright 2022 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use super::{
    args, motion, one_finger_button, primary_tap, scroll, secondary_button, secondary_tap,
};
use crate::input_handler::{InputHandler, InputHandlerStatus};
use crate::utils::Size;
use crate::{input_device, mouse_binding, touch_binding};
use anyhow::{Context, Error, format_err};
use async_trait::async_trait;
use core::cell::RefCell;
use fidl_fuchsia_input_report as fidl_input_report;
use fuchsia_inspect::health::Reporter;
use fuchsia_inspect::{ArrayProperty, Node as InspectNode};
use fuchsia_inspect_contrib::nodes::BoundedListNode;
use std::any::Any;
use std::fmt::Debug;

struct GestureArenaInitialContenders {}

impl ContenderFactory for GestureArenaInitialContenders {
    fn make_contenders(&self) -> Vec<Box<dyn Contender>> {
        vec![
            Box::new(motion::InitialContender {
                min_movement_in_mm: args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
            }),
            Box::new(primary_tap::InitialContender {
                max_finger_displacement_in_mm: args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
                max_time_elapsed: args::TAP_TIMEOUT,
            }),
            Box::new(secondary_tap::InitialContender {
                max_finger_displacement_in_mm: args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
                max_time_elapsed: args::TAP_TIMEOUT,
            }),
            Box::new(scroll::InitialContender {
                motion_threshold_in_mm: args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
                min_movement_in_mm: args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
                max_movement_in_mm: args::MAX_SPURIOUS_TO_INTENTIONAL_SCROLL_THRESHOLD_MM,
                limit_tangent_for_direction: args::MAX_SCROLL_DIRECTION_SKEW_DEGREES
                    .to_radians()
                    .tan(),
            }),
            Box::new(one_finger_button::InitialContender {
                spurious_to_intentional_motion_threshold_mm:
                    args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
                spurious_to_intentional_motion_threshold_button_change_mm:
                    args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_BUTTON_CHANGE_MM,
                button_change_state_timeout: args::BUTTON_CHANGE_STATE_TIMEOUT,
            }),
            Box::new(secondary_button::InitialContender {
                spurious_to_intentional_motion_threshold_mm:
                    args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_MM,
                spurious_to_intentional_motion_threshold_button_change_mm:
                    args::SPURIOUS_TO_INTENTIONAL_MOTION_THRESHOLD_BUTTON_CHANGE_MM,
                button_change_state_timeout: args::BUTTON_CHANGE_STATE_TIMEOUT,
            }),
        ]
    }
}

pub fn make_input_handler(
    inspect_node: &InspectNode,
    input_handlers_node: &InspectNode,
) -> std::rc::Rc<dyn crate::input_handler::InputHandler> {
    // TODO(https://fxbug.dev/42056283): Remove log message.
    log::info!("touchpad: created input handler");
    std::rc::Rc::new(GestureArena::new_internal(
        Box::new(GestureArenaInitialContenders {}),
        inspect_node,
        MAX_TOUCHPAD_EVENT_LOG_ENTRIES,
        input_handlers_node,
    ))
}

pub(super) const PRIMARY_BUTTON: mouse_binding::MouseButton = 1;
pub(super) const SECONDARY_BUTTON: mouse_binding::MouseButton = 2;

// TODO(https://fxbug.dev/42053614): check that we've removed all leading `_` from types
// and variables in this file.
#[derive(Debug, Clone, PartialEq)]
pub(super) struct TouchpadEvent {
    pub(super) timestamp: zx::MonotonicInstant,
    // TODO(https://fxbug.dev/42053615): replace these fields with a field that embeds
    // `touch_data: super::touch_binding::TouchpadEvent`.
    pub(super) pressed_buttons: Vec<u8>,
    pub(super) contacts: Vec<touch_binding::TouchContact>,
    pub(super) filtered_palm_contacts: Vec<touch_binding::TouchContact>,
}

#[derive(Debug, PartialEq)]
pub(super) struct MouseEvent {
    pub(super) timestamp: zx::MonotonicInstant,
    pub(super) mouse_data: mouse_binding::MouseEvent,
}

#[derive(Debug)]
pub(super) struct DetailedReasonUint {
    pub(super) criterion: &'static str,
    pub(super) min: Option<u64>,
    pub(super) max: Option<u64>,
    pub(super) actual: usize,
}

#[derive(Debug)]
pub(super) struct DetailedReasonFloat {
    pub(super) criterion: &'static str,
    pub(super) min: Option<f32>,
    pub(super) max: Option<f32>,
    pub(super) actual: f32,
}

#[derive(Debug)]
pub(super) struct DetailedReasonInt {
    pub(super) criterion: &'static str,
    pub(super) min: Option<i64>,
    pub(super) max: Option<i64>,
    pub(super) actual: i64,
}

#[derive(Debug)]
pub(super) enum Reason {
    Basic(&'static str),
    DetailedUint(DetailedReasonUint),
    DetailedFloat(DetailedReasonFloat),
    DetailedInt(DetailedReasonInt),
}

#[derive(Debug)]
pub(super) enum ExamineEventResult {
    Contender(Box<dyn Contender>),
    MatchedContender(Box<dyn MatchedContender>),
    Mismatch(Reason),
}

pub(super) trait Contender: std::fmt::Debug + AsAny {
    /// Examines `event`, to determine whether or not the gesture
    /// is relevant to this `Recognizer`.
    ///
    /// Returns
    /// * `ExamineEventResult::MatchedContender` if this recognizer wants
    ///   to send (or start sending) events downstream, OR
    /// * `ExamineEventResult::Contender` if this recognizer is not yet
    ///   ready to send events downstream, but wants to continue
    ///   contending for the gesture, OR
    /// * `ExamineEventResult::Mismatch` if this recognizer no longer
    ///   wants to contend for this gesture
    fn examine_event(self: Box<Self>, event: &TouchpadEvent) -> ExamineEventResult;

    /// Returns a string that uniquely identifies the concrete type
    /// of this implementation of the `Contender` trait.
    fn get_type_name(&self) -> &'static str {
        std::any::type_name::<Self>()
    }

    /// If the gesture need to start from idle(no finger contacting the surface).
    fn start_from_idle(&self) -> bool {
        false
    }
}

pub trait AsAny {
    #[allow(dead_code)] // only used in test
    fn as_any(&self) -> &dyn Any;
}

impl<T: Any> AsAny for T {
    fn as_any(&self) -> &dyn Any {
        self
    }
}

#[derive(Debug)]
pub(super) enum VerifyEventResult {
    MatchedContender(Box<dyn MatchedContender>),
    Mismatch(Reason),
}

#[derive(Clone, Copy, Debug, PartialEq)]
pub(super) enum RecognizedGesture {
    /// Contains one variant for each recognizer, and the
    /// special value `Unrecognized` for when no recognizer
    /// claims the gesture.
    _Unrecognized,
    PrimaryTap,
    SecondaryTap,
    Motion,
    Scroll,
    OneButtonDown,
    SecondaryButtonDown,
}

#[derive(Debug)]
pub(super) struct ProcessBufferedEventsResult {
    pub(super) generated_events: Vec<MouseEvent>,
    pub(super) winner: Option<Box<dyn Winner>>,
    pub(super) recognized_gesture: RecognizedGesture, // for latency breakdown
}

pub(super) trait MatchedContender: std::fmt::Debug + AsAny {
    /// Verifies that `event` still matches the gesture that is relevant
    /// to this `Recognizer`.
    ///
    /// Returns
    /// * `VerifyEventResult::MatchedContender` if this recognizer wants
    ///   to send (or start sending) events downstream, OR
    /// * `VerifyEventResult::Mismatch` if this recognizer no longer
    ///   wants to contend for this gesture
    fn verify_event(self: Box<Self>, event: &TouchpadEvent) -> VerifyEventResult;

    /// Takes `events`, and generates corresponding `MouseEvent`s.
    ///
    /// Returns `ProcessBufferedEventsResult` with fields:
    /// * `generated_events`: the sequence of `MouseEvent`s needed
    ///   to effect the gesture downstream.
    /// * `winner`:
    ///   * `None` if the gesture is complete
    ///   * `Some` otherwise
    ///
    /// Note:
    /// * `generated_events` MAY be empty; for example, a palm
    ///   recognizer wants to discard unintended events
    /// * `events` is guaranteed to contains exactly the sequence of
    ///   `TouchpadEvent`s that this recognizer has already examined
    ///   and verified.
    /// * recognizers MAY choose to ignore `events`
    ///   e.g.:
    ///   * a one-finger-tap recognizer does not need to inspect
    ///     `events` to generate the `MouseEvent`s for the button click
    ///   * a motion recognizer, in contrast, needs the details in
    ///     `events` to generate the appropriate motion
    fn process_buffered_events(
        self: Box<Self>,
        events: Vec<TouchpadEvent>,
    ) -> ProcessBufferedEventsResult;

    /// Returns a string that uniquely identifies the concrete type
    /// of this implementation of the `MatchedContender` trait.
    fn get_type_name(&self) -> &'static str {
        std::any::type_name::<Self>()
    }
}

#[derive(Debug, PartialEq)]
pub(super) enum EndGestureEvent {
    // It still possible to use EndGestureEvent::GeneratedEvent when
    // we support scroll phase, keep it and related tests.
    #[allow(dead_code)]
    GeneratedEvent(MouseEvent),
    UnconsumedEvent(TouchpadEvent),
    NoEvent,
}

#[derive(Debug)]
pub(super) enum ProcessNewEventResult {
    ContinueGesture(Option<MouseEvent>, Box<dyn Winner>),
    EndGesture(EndGestureEvent, Reason),
}

pub(super) trait Winner: std::fmt::Debug {
    /// Takes `event`, and generates corresponding `MouseEvent`s.
    ///
    /// Returns:
    /// * `ContinueGesture(Some, …)` if the gesture is still
    ///   in progress, and a `MouseEvent` should be sent downstream;
    ///   might be used, e.g., by a motion recognizer
    /// * `ContinueGesutre(None, …)` if the gesture is still
    ///   in progress, and no `MouseEvent` should be sent downstream;
    ///   might be used, e.g., by a palm recognizer
    /// * `EndGesture(UnconsumedEvent, …)` if the gesture has ended because
    ///   `event` did not match; might be used, e.g., if the user
    ///    presses the touchpad down after a motion gesture
    /// * `EndGesture(GeneratedEvent, …)` if the gesture has ended because
    ///   `event` did end the gesture; might be used, e.g., if the user
    ///    release the touchpad down after a drag gesture
    /// * `EndGesture(NoEvent, …)` if `event` matches a normal end
    ///   of the gesture; might be used, e.g., if the user lifts
    ///   their finger off the touchpad after a motion gesture
    fn process_new_event(self: Box<Self>, event: TouchpadEvent) -> ProcessNewEventResult;

    /// Returns a string that uniquely defines the concrete type
    /// of this implementation of the `Winner` trait.
    fn get_type_name(&self) -> &'static str {
        std::any::type_name::<Self>()
    }
}

const MAX_TOUCHPAD_EVENT_LOG_ENTRIES: usize = 1250; // 125 Hz * 10 seconds

#[derive(Debug)]
enum MutableState {
    /// Looking for the start of any gesture.
    Idle,

    /// Looking for the start of a gesture that can be chained after
    /// a previous gesture. "Chaining" is using two or more gestures
    /// in sequence, while always having at least one finger in contact
    /// with the pad.
    ///
    /// Note that this does _not_ require any particular finger to
    /// be continually in contact with the pad. For example, the
    /// following is a valid chain:
    ///    1. Index finger
    ///    2. Index finger + middle finger
    ///    3. Middle finger
    Chain,

    /// Disambiguating the current gesture.
    Matching {
        contenders: Vec<Box<dyn Contender>>,
        matched_contenders: Vec<Box<dyn MatchedContender>>,
        first_event_timestamp: zx::MonotonicInstant,
        buffered_events: Vec<TouchpadEvent>,
    },

    /// The matching gesture has been identified, and is still in progress.
    Forwarding {
        winner: Box<dyn Winner>,
        current_gesture: RecognizedGesture,
        gesture_start_timestamp: zx::MonotonicInstant,
        num_events: usize,
    },

    /// A transient state during the processing of a single `InputEvent`.
    Invalid,
}

/// Names for the `MutableState`s. Exists to support the state machine debug log.
#[derive(Debug, PartialEq)]
enum StateName {
    Idle,
    Chain,
    Matching,
    Forwarding,
    Invalid,
}

trait ContenderFactory {
    fn make_contenders(&self) -> Vec<Box<dyn Contender>>;
}

pub(super) struct GestureArena {
    contender_factory: Box<dyn ContenderFactory>,
    mutable_state: RefCell<MutableState>,
    inspect_log: RefCell<BoundedListNode>,
    /// The inventory of this handler's Inspect status.
    inspect_status: InputHandlerStatus,
}

impl GestureArena {
    #[cfg(test)]
    fn new_for_test(
        contender_factory: Box<dyn ContenderFactory>,
        inspector: &fuchsia_inspect::Inspector,
        max_inspect_log_entries: usize,
    ) -> GestureArena {
        let test_node = inspector.root().create_child("test_node");
        Self::new_internal(contender_factory, inspector.root(), max_inspect_log_entries, &test_node)
    }

    fn new_internal(
        contender_factory: Box<dyn ContenderFactory>,
        inspect_node: &InspectNode,
        max_inspect_log_entries: usize,
        input_handlers_node: &InspectNode,
    ) -> GestureArena {
        let inspect_status = InputHandlerStatus::new(
            input_handlers_node,
            "gesture_arena",
            /* generates_events */ true,
        );
        GestureArena {
            contender_factory,
            mutable_state: RefCell::new(MutableState::Idle),
            inspect_log: RefCell::new(BoundedListNode::new(
                inspect_node.create_child("gestures_event_log"),
                max_inspect_log_entries,
            )),
            inspect_status,
        }
    }

    #[cfg(test)]
    pub(super) fn has_buffered_events(self: std::rc::Rc<Self>) -> bool {
        match &*self.mutable_state.borrow() {
            MutableState::Matching { buffered_events, .. } => buffered_events.len() > 0,
            _ => false,
        }
    }
}

impl TouchpadEvent {
    fn log_inspect(&self, log_entry_node: &InspectNode) {
        let touchpad_event_node = log_entry_node.create_child("touchpad_event");

        // Create an inspect array from the pressed buttons.
        let pressed_buttons_node =
            touchpad_event_node.create_uint_array("pressed_buttons", self.pressed_buttons.len());
        self.pressed_buttons.iter().enumerate().for_each(|(i, &button_id)| {
            pressed_buttons_node.set(i, button_id);
        });

        // Populate the touchpad event details
        log_common(&touchpad_event_node, self.timestamp);
        touchpad_event_node.record(pressed_buttons_node);
        touchpad_event_node.record_child("contacts", |contact_set_node| {
            self.contacts.iter().for_each(|contact| {
                contact_set_node.record_child(contact.id.to_string(), |contact_node| {
                    contact_node.record_double("pos_x_mm", f64::from(contact.position.x));
                    contact_node.record_double("pos_y_mm", f64::from(contact.position.y));
                    if let Some(contact_size) = contact.contact_size {
                        contact_node.record_double("width_mm", f64::from(contact_size.width));
                        contact_node.record_double("height_mm", f64::from(contact_size.height));
                    }
                })
            })
        });
        touchpad_event_node.record_child("filtered_palm_contacts", |contact_set_node| {
            self.filtered_palm_contacts.iter().for_each(|contact| {
                contact_set_node.record_child(contact.id.to_string(), |contact_node| {
                    contact_node.record_double("pos_x_mm", f64::from(contact.position.x));
                    contact_node.record_double("pos_y_mm", f64::from(contact.position.y));
                    if let Some(contact_size) = contact.contact_size {
                        contact_node.record_double("width_mm", f64::from(contact_size.width));
                        contact_node.record_double("height_mm", f64::from(contact_size.height));
                    }
                })
            })
        });

        // Pass ownership of the touchpad event node to the log.
        log_entry_node.record(touchpad_event_node);
    }
}

impl RecognizedGesture {
    fn to_str(&self) -> &'static str {
        match self {
            RecognizedGesture::_Unrecognized => "_unrecognized",
            RecognizedGesture::PrimaryTap => "primary_tap",
            RecognizedGesture::SecondaryTap => "secondary_tap",
            RecognizedGesture::Motion => "motion",
            RecognizedGesture::Scroll => "scroll",
            RecognizedGesture::OneButtonDown => "one_button_down",
            RecognizedGesture::SecondaryButtonDown => "secondary_button_down",
        }
    }
}

fn log_common(inspect_node: &InspectNode, driver_timestamp: zx::MonotonicInstant) {
    inspect_node.record_int("driver_monotonic_nanos", driver_timestamp.into_nanos());
    inspect_node.record_int(
        "entry_latency_micros",
        // Use lower precision for latency, to minimize space.
        (fuchsia_async::MonotonicInstant::now().into_zx() - driver_timestamp).into_micros(),
    );
}

impl MutableState {
    fn get_state_name(&self) -> StateName {
        match self {
            Self::Idle => StateName::Idle,
            Self::Chain => StateName::Chain,
            Self::Matching { .. } => StateName::Matching,
            Self::Forwarding { .. } => StateName::Forwarding,
            Self::Invalid => StateName::Invalid,
        }
    }
}

fn parse_touchpad_event(
    event_time: &zx::MonotonicInstant,
    touchpad_event: &touch_binding::TouchpadEvent,
    touchpad_descriptor: &touch_binding::TouchpadDeviceDescriptor,
) -> Result<TouchpadEvent, Error> {
    let position_divisor =
        get_position_divisor_to_mm(touchpad_descriptor).context("failed to compute divisor")?;
    Ok(TouchpadEvent {
        timestamp: *event_time,
        pressed_buttons: touchpad_event.pressed_buttons.iter().copied().collect::<Vec<_>>(),
        contacts: touchpad_event
            .injector_contacts
            .iter()
            .map(|contact| touch_binding::TouchContact {
                position: contact.position / position_divisor,
                contact_size: match contact.contact_size {
                    Some(size) => Some(Size {
                        width: size.width / position_divisor,
                        height: size.height / position_divisor,
                    }),
                    None => None,
                },
                ..*contact
            })
            .collect(),
        filtered_palm_contacts: vec![],
    })
}

fn filter_palm_contact(touchpad_event: TouchpadEvent) -> TouchpadEvent {
    // Button down will make the contact of finger larger. We are not able to
    // use the same threshold to distinguish finger or palm.
    if touchpad_event.pressed_buttons.len() > 0 {
        return touchpad_event;
    }
    let (contacts, filtered_palm_contacts) = touchpad_event.contacts.into_iter().fold(
        (
            Vec::<touch_binding::TouchContact>::default(),
            Vec::<touch_binding::TouchContact>::default(),
        ),
        |mut out, contact| {
            match contact.contact_size {
                Some(size) => {
                    if size.width < args::MIN_PALM_SIZE_MM && size.height < args::MIN_PALM_SIZE_MM {
                        out.0.push(contact);
                    } else {
                        out.1.push(contact);
                    }
                }
                None => {
                    out.0.push(contact);
                }
            }
            out
        },
    );

    TouchpadEvent { contacts, filtered_palm_contacts, ..touchpad_event }
}

const COUNTS_PER_MM: u32 = 12;

impl std::convert::From<MouseEvent> for input_device::InputEvent {
    fn from(mouse_event: MouseEvent) -> input_device::InputEvent {
        input_device::InputEvent {
            // TODO(https://fxbug.dev/42056058): Convert `mouse_event.mouse_data.wheel_delta_v`
            // and `mouse_event.mouse_data.wheel_delta_h` from micrometers to counts.
            device_event: input_device::InputDeviceEvent::Mouse(mouse_event.mouse_data),
            device_descriptor: input_device::InputDeviceDescriptor::Mouse(
                mouse_binding::MouseDeviceDescriptor {
                    // Use a large number for the `device_id` for the gesture arena's
                    // virtual mouse, to avoid conflicts with real devices (which start
                    // at ID 0).
                    //
                    // However, don't use u32::MAX, since the `InputDeviceRegistry`
                    // FIDL handling code uses that for the first registered device
                    // (and works downwards).
                    device_id: u32::MAX / 2,
                    absolute_x_range: Some(fidl_input_report::Range { min: -127, max: 127 }),
                    absolute_y_range: Some(fidl_input_report::Range { min: -127, max: 127 }),
                    wheel_v_range: Some(fidl_input_report::Axis {
                        // Set range based on the range of `position_y` values for the
                        // Atlas touchpad. The touchpad is 2.48 inches tall, and the
                        // position is reported in micrometers. That gives `position_y`
                        // a range of (0, 62992).
                        //
                        // Note that this range is likely larger than needed, since users
                        // probably won't swipe all the way up/down the pad in a single
                        // sampling interval (8msec).
                        //
                        // TODO(https://fxbug.dev/42056058): Adjust this range to reflect
                        // the range in counts instead of micrometers.
                        range: fidl_input_report::Range { min: -65535, max: 65535 },
                        unit: fidl_input_report::Unit {
                            type_: fidl_fuchsia_input_report::UnitType::Other,
                            exponent: 0,
                        },
                    }),
                    wheel_h_range: None,
                    buttons: Some(vec![PRIMARY_BUTTON, SECONDARY_BUTTON]),
                    // Downstream stages ignore `counts_per_mm`, so any value is fine.
                    // TODO(https://fxbug.dev/42072124): remove `counts_per_mm` from descriptor.
                    counts_per_mm: COUNTS_PER_MM,
                },
            ),
            event_time: mouse_event.timestamp,
            handled: input_device::Handled::No,
            trace_id: None,
        }
    }
}

impl GestureArena {
    /// Interprets `TouchpadEvent`s, and sends corresponding `MouseEvent`s downstream.
    fn handle_touchpad_event(
        self: std::rc::Rc<Self>,
        event_time: &zx::MonotonicInstant,
        touchpad_event: &touch_binding::TouchpadEvent,
        device_descriptor: &touch_binding::TouchpadDeviceDescriptor,
    ) -> Result<Vec<input_device::InputEvent>, Error> {
        let touchpad_event = parse_touchpad_event(event_time, touchpad_event, device_descriptor)
            .context("dropping touchpad event")?;
        let touchpad_event = filter_palm_contact(touchpad_event);
        self.inspect_log.borrow_mut().add_entry(|node| {
            touchpad_event.log_inspect(node);
        });

        let old_state_name = self.mutable_state.borrow().get_state_name();
        let (new_state, generated_events) = match self.mutable_state.replace(MutableState::Invalid)
        {
            MutableState::Idle => self.handle_event_while_idle(touchpad_event),
            MutableState::Chain => self.handle_event_while_chain(touchpad_event),
            MutableState::Matching {
                contenders,
                matched_contenders,
                first_event_timestamp,
                buffered_events,
            } => self.handle_event_while_matching(
                contenders,
                matched_contenders,
                first_event_timestamp,
                buffered_events,
                touchpad_event,
            ),
            MutableState::Forwarding {
                winner,
                current_gesture,
                gesture_start_timestamp,
                num_events,
            } => self.handle_event_while_forwarding(
                winner,
                current_gesture,
                gesture_start_timestamp,
                num_events + 1,
                touchpad_event,
            ),
            MutableState::Invalid => {
                unreachable!();
            }
        };
        log::debug!("gesture_arena: {:?} -> {:?}", old_state_name, new_state.get_state_name());
        self.mutable_state.replace(new_state);
        // self.log_mutable_state();  // uncomment to log contender set
        Ok(generated_events.into_iter().map(input_device::InputEvent::from).collect())
    }

    fn handle_event_while_idle(&self, new_event: TouchpadEvent) -> (MutableState, Vec<MouseEvent>) {
        let (contenders, matched_contenders) = self
            .contender_factory
            .make_contenders()
            .into_iter()
            .map(|contender| (contender.get_type_name(), contender.examine_event(&new_event)))
            .fold(
                (vec![], vec![]),
                |(mut contenders, mut matched_contenders), (type_name, examine_result)| {
                    match examine_result {
                        ExamineEventResult::Contender(contender) => {
                            contenders.push(contender);
                        }
                        ExamineEventResult::MatchedContender(matched_contender) => {
                            matched_contenders.push(matched_contender);
                        }
                        ExamineEventResult::Mismatch(mismatch_data) => {
                            self.inspect_log.borrow_mut().add_entry(|node| {
                                log_mismatch_reason(node, type_name, mismatch_data);
                            });
                        }
                    }
                    (contenders, matched_contenders)
                },
            );
        let event_timestamp = new_event.timestamp;
        self.transit_based_on_contenders_matched_contenders(
            vec![],
            new_event,
            event_timestamp,
            contenders,
            matched_contenders,
        )
    }

    fn handle_event_while_chain(
        &self,
        new_event: TouchpadEvent,
    ) -> (MutableState, Vec<MouseEvent>) {
        let (contenders, matched_contenders) = self
            .contender_factory
            .make_contenders()
            .into_iter()
            .filter(|contender| !contender.start_from_idle())
            .map(|contender| (contender.get_type_name(), contender.examine_event(&new_event)))
            .fold(
                (vec![], vec![]),
                |(mut contenders, mut matched_contenders), (type_name, examine_result)| {
                    match examine_result {
                        ExamineEventResult::Contender(contender) => {
                            contenders.push(contender);
                        }
                        ExamineEventResult::MatchedContender(matched_contender) => {
                            matched_contenders.push(matched_contender);
                        }
                        ExamineEventResult::Mismatch(mismatch_data) => {
                            self.inspect_log.borrow_mut().add_entry(|node| {
                                log_mismatch_reason(node, type_name, mismatch_data);
                            });
                        }
                    }
                    (contenders, matched_contenders)
                },
            );
        let event_timestamp = new_event.timestamp;
        self.transit_based_on_contenders_matched_contenders(
            vec![],
            new_event,
            event_timestamp,
            contenders,
            matched_contenders,
        )
    }

    fn handle_event_while_matching(
        &self,
        contenders: Vec<Box<dyn Contender>>,
        matched_contenders: Vec<Box<dyn MatchedContender>>,
        first_event_timestamp: zx::MonotonicInstant,
        buffered_events: Vec<TouchpadEvent>,
        new_event: TouchpadEvent,
    ) -> (MutableState, Vec<MouseEvent>) {
        // Note: This function processes `contenders` after `matched_contenders` to ensure
        // that a recognizer doesn't get invoked twice with the same event. Otherwise, it
        // would be possible for the following sequence of events to occur:
        //
        // 1. This function calls some_recognizer::Contender.examine_event(&new_event).
        // 2. The call returns some_recognizer::MatchedContender.
        // 3. This function calls some_recognizer::MatchedContender.verify_event(&new_event).
        //
        // See also: `does_not_repeat_event_to_matched_contender_returned_by_examine_event` test.
        let matched_contenders = matched_contenders
            .into_iter()
            .map(|matched_contender| {
                (matched_contender.get_type_name(), matched_contender.verify_event(&new_event))
            })
            .fold(vec![], |mut matched_contenders, (type_name, verify_result)| {
                match verify_result {
                    VerifyEventResult::MatchedContender(m) => {
                        matched_contenders.push(m);
                    }
                    VerifyEventResult::Mismatch(mismatch_data) => {
                        self.inspect_log.borrow_mut().add_entry(|node| {
                            log_mismatch_reason(node, type_name, mismatch_data);
                        });
                    }
                }
                matched_contenders
            });
        let (contenders, matched_contenders) = contenders
            .into_iter()
            .map(|contender| (contender.get_type_name(), contender.examine_event(&new_event)))
            .fold(
                (vec![], matched_contenders),
                |(mut contenders, mut matched_contenders), (type_name, examine_result)| {
                    match examine_result {
                        ExamineEventResult::Contender(contender) => {
                            contenders.push(contender);
                        }
                        ExamineEventResult::MatchedContender(matched_contender) => {
                            matched_contenders.push(matched_contender);
                        }
                        ExamineEventResult::Mismatch(mismatch_data) => {
                            self.inspect_log.borrow_mut().add_entry(|node| {
                                log_mismatch_reason(node, type_name, mismatch_data);
                            });
                        }
                    }
                    (contenders, matched_contenders)
                },
            );

        self.transit_based_on_contenders_matched_contenders(
            buffered_events,
            new_event,
            first_event_timestamp,
            contenders,
            matched_contenders,
        )
    }

    fn transit_based_on_contenders_matched_contenders(
        &self,
        buffered_events: Vec<TouchpadEvent>,
        new_event: TouchpadEvent,
        first_event_timestamp: zx::MonotonicInstant,
        contenders: Vec<Box<dyn Contender>>,
        matched_contenders: Vec<Box<dyn MatchedContender>>,
    ) -> (MutableState, Vec<MouseEvent>) {
        let has_finger_contact = new_event.contacts.len() > 0;
        let new_event_timestamp = new_event.timestamp;

        match (
            u8::try_from(contenders.len()).unwrap_or(u8::MAX),
            u8::try_from(matched_contenders.len()).unwrap_or(u8::MAX),
        ) {
            // No `Contender`s or `MatchedContender`s. The contest has ended without
            // identifying a gesture.
            (0, 0) => {
                if has_finger_contact {
                    // One more contacts transit to chain. This is required when user
                    // give an unknown gesture for example a 2 fingers swipe in no
                    // direction and movement larger than threshold. Then finger lift,
                    // Chain state will prevent it to be recognized as secondary tap.
                    (MutableState::Chain, vec![])
                } else {
                    (MutableState::Idle, vec![])
                }
            }
            // No `Contender`s, and exactly one `MatchedContender`. The contest has ended
            // with a recognized gesture.
            (0, 1) => {
                let num_previous_events = buffered_events.len();
                let mut buffered_events = buffered_events;
                buffered_events.push(new_event);

                let matched_contender = {
                    let mut matched_contenders = matched_contenders;
                    matched_contenders.remove(0)
                };
                let type_name = matched_contender.get_type_name();
                let ProcessBufferedEventsResult { generated_events, winner, recognized_gesture } =
                    matched_contender.process_buffered_events(buffered_events);
                self.inspect_log.borrow_mut().add_entry(|node| {
                    log_gesture_start(
                        node,
                        recognized_gesture,
                        num_previous_events,
                        new_event_timestamp - first_event_timestamp,
                    );
                });

                match winner {
                    Some(winner) => (
                        MutableState::Forwarding {
                            winner,
                            current_gesture: recognized_gesture,
                            gesture_start_timestamp: new_event_timestamp,
                            num_events: 0,
                        },
                        generated_events,
                    ),
                    None => {
                        self.inspect_log.borrow_mut().add_entry(|node| {
                            log_gesture_end(
                                node,
                                type_name,
                                recognized_gesture,
                                Reason::Basic("discrete-recognizer"),
                                0,
                                zx::MonotonicDuration::from_nanos(0),
                            );
                        });
                        if has_finger_contact {
                            (MutableState::Chain, generated_events)
                        } else {
                            (MutableState::Idle, generated_events)
                        }
                    }
                }
            }
            // At least 1 `Contender`, or 2 `MatchedContender`s; continue the contest.
            (1.., _) | (_, 2..) => {
                let mut buffered_events = buffered_events;
                buffered_events.push(new_event);
                (
                    MutableState::Matching {
                        contenders,
                        matched_contenders,
                        first_event_timestamp,
                        buffered_events,
                    },
                    vec![],
                )
            }
        }
    }

    fn handle_event_while_forwarding(
        &self,
        winner: Box<dyn Winner>,
        current_gesture: RecognizedGesture,
        gesture_start_timestamp: zx::MonotonicInstant,
        num_events: usize,
        new_event: TouchpadEvent,
    ) -> (MutableState, Vec<MouseEvent>) {
        let type_name = winner.get_type_name();
        let has_finger_contact = new_event.contacts.len() > 0;
        let new_event_timestamp = new_event.timestamp;

        match winner.process_new_event(new_event) {
            ProcessNewEventResult::ContinueGesture(generated_event, winner) => (
                MutableState::Forwarding {
                    winner,
                    current_gesture,
                    gesture_start_timestamp,
                    num_events,
                },
                generated_event.into_iter().collect(),
            ),
            ProcessNewEventResult::EndGesture(
                EndGestureEvent::GeneratedEvent(generated_event),
                reason,
            ) => {
                log::debug!("touchpad: {} ended: {:?}", type_name, reason);
                self.inspect_log.borrow_mut().add_entry(|node| {
                    log_gesture_end(
                        node,
                        type_name,
                        current_gesture,
                        reason,
                        num_events,
                        new_event_timestamp - gesture_start_timestamp,
                    );
                });
                if has_finger_contact {
                    (MutableState::Chain, vec![generated_event])
                } else {
                    (MutableState::Idle, vec![generated_event])
                }
            }
            ProcessNewEventResult::EndGesture(
                EndGestureEvent::UnconsumedEvent(unconsumed_event),
                reason,
            ) => {
                log::debug!("touchpad: {} ended: {:?}", type_name, reason);
                self.inspect_log.borrow_mut().add_entry(|node| {
                    log_gesture_end(
                        node,
                        type_name,
                        current_gesture,
                        reason,
                        num_events,
                        new_event_timestamp - gesture_start_timestamp,
                    );
                });
                if unconsumed_event.contacts.len() > 0 {
                    self.handle_event_while_chain(unconsumed_event)
                } else {
                    self.handle_event_while_idle(unconsumed_event)
                }
            }
            ProcessNewEventResult::EndGesture(EndGestureEvent::NoEvent, reason) => {
                log::debug!("touchpad: {} ended: {:?}", type_name, reason);
                self.inspect_log.borrow_mut().add_entry(|node| {
                    log_gesture_end(
                        node,
                        type_name,
                        current_gesture,
                        reason,
                        num_events,
                        new_event_timestamp - gesture_start_timestamp,
                    );
                });
                if has_finger_contact {
                    (MutableState::Chain, vec![])
                } else {
                    (MutableState::Idle, vec![])
                }
            }
        }
    }

    #[allow(dead_code)] // only called in developer debug builds and tests.
    pub(super) fn mutable_state_to_str(&self) -> String {
        match &*self.mutable_state.borrow() {
            MutableState::Idle => format!("touchpad: Idle"),
            MutableState::Chain => format!("touchpad: Chain"),
            MutableState::Matching {
                contenders,
                matched_contenders,
                first_event_timestamp,
                buffered_events,
            } => {
                format!(
                    "touchpad: Matching {{ \
                                contenders: [ {} ], \
                                matched_contenders: [ {} ], \
                                first_event_timestamp_nanos: {}, \
                                n_buffered_events: {} \
                              }}",
                    contenders.iter().fold(String::new(), |accum, item| {
                        accum + &format!("{}, ", item.get_type_name())
                    }),
                    matched_contenders.iter().fold(String::new(), |accum, item| {
                        accum + &format!("{}, ", item.get_type_name())
                    }),
                    first_event_timestamp.into_nanos(),
                    buffered_events.len()
                )
            }
            MutableState::Forwarding {
                winner,
                current_gesture,
                gesture_start_timestamp,
                num_events,
            } => {
                format!(
                    "touchpad: Forwarding {{ \
                        winner: {}, \
                        current_gesture: {:?}, \
                        gesture_start_timestamp_nanos: {}, \
                        num_events: {} \
                    }}",
                    winner.get_type_name(),
                    current_gesture,
                    gesture_start_timestamp.into_nanos(),
                    num_events
                )
            }
            MutableState::Invalid => format!("touchpad: Invalid"),
        }
    }

    #[allow(dead_code)] // only called in developer debug builds
    fn log_mutable_state(&self) {
        log::info!("{}", self.mutable_state_to_str());
    }
}

#[async_trait(?Send)]
impl InputHandler for GestureArena {
    /// Handle `input_event`:
    /// * For an unhandled touchpad event, try to interpret as a touchpad gesture.
    /// * For a keyboard event, log the timestamp, and pass the event onwards.
    /// * For any other event, pass onwards without logging.
    async fn handle_input_event(
        self: std::rc::Rc<Self>,
        input_event: input_device::InputEvent,
    ) -> Vec<input_device::InputEvent> {
        match input_event {
            input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(ref touchpad_event),
                ref event_time,
                device_descriptor:
                    input_device::InputDeviceDescriptor::Touchpad(ref touchpad_descriptor),
                handled: input_device::Handled::No,
                ..
            } => {
                self.inspect_status.count_received_event(&event_time);
                match self.handle_touchpad_event(event_time, touchpad_event, touchpad_descriptor) {
                    Ok(r) => r,
                    Err(e) => {
                        log::error!("{}", e);
                        vec![input_event]
                    }
                }
            }
            input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Keyboard(_),
                event_time,
                ..
            } => {
                self.inspect_log.borrow_mut().add_entry(|node| {
                    log_keyboard_event_timestamp(node, event_time);
                });
                vec![input_event]
            }
            _ => {
                vec![input_event]
            }
        }
    }

    fn set_handler_healthy(self: std::rc::Rc<Self>) {
        self.inspect_status.health_node.borrow_mut().set_ok();
    }

    fn set_handler_unhealthy(self: std::rc::Rc<Self>, msg: &str) {
        self.inspect_status.health_node.borrow_mut().set_unhealthy(msg);
    }
}

/// Returns the multiplier to translate position data for the device described by
/// `device_descriptor`, from the units in the corresponding `TouchpadEvent`, to
/// millimeters.
///
/// For example, if this function returns 1000, then the original data are in
/// micrometers, and dividing by 1000 will yield millimeters.
fn get_position_divisor_to_mm(
    touchpad_descriptor: &touch_binding::TouchpadDeviceDescriptor,
) -> Result<f32, Error> {
    const EXPONENT_MILLIS: i32 = -3;
    let divisors =
        touchpad_descriptor.contacts.iter().enumerate().map(|(i, contact_descriptor)| {
            match (contact_descriptor.x_unit, contact_descriptor.y_unit) {
                (
                    fidl_input_report::Unit {
                        type_: fidl_input_report::UnitType::Meters,
                        exponent: exponent_x,
                    },
                    fidl_input_report::Unit {
                        type_: fidl_input_report::UnitType::Meters,
                        exponent: exponent_y,
                    },
                ) => {
                    if exponent_x == exponent_y {
                        Ok(f32::powi(10.0, EXPONENT_MILLIS - exponent_x))
                    } else {
                        Err(format!(
                            "contact {}: mismatched exponents x={}, y={}",
                            i, exponent_x, exponent_y
                        ))
                    }
                }
                (
                    fidl_input_report::Unit { type_: x_unit_type, .. },
                    fidl_input_report::Unit { type_: fidl_input_report::UnitType::Meters, .. },
                ) => Err(format!(
                    "contact {}: expected x-unit-type of Meters but got {:?}",
                    i, x_unit_type
                )),
                (
                    fidl_input_report::Unit { type_: fidl_input_report::UnitType::Meters, .. },
                    fidl_input_report::Unit { type_: y_unit_type, .. },
                ) => Err(format!(
                    "contact {}: expected y-unit-type of Meters but got {:?}",
                    i, y_unit_type
                )),
                (
                    fidl_input_report::Unit { type_: x_unit_type, .. },
                    fidl_input_report::Unit { type_: y_unit_type, .. },
                ) => Err(format!(
                    "contact {}: expected x and y unit-types of Meters but got x={:?} and y={:?}",
                    i, x_unit_type, y_unit_type
                )),
            }
        });

    let (divisors, errors): (Vec<_>, Vec<_>) =
        divisors.fold((vec![], vec![]), |(mut divisors, mut errors), divisor| {
            match divisor {
                Ok(d) => divisors.push(d),
                Err(e) => errors.push(e),
            };
            (divisors, errors)
        });

    if !errors.is_empty() {
        return Err(format_err!(
            errors.into_iter().fold(String::new(), |prev_err_msgs, this_err_msg| prev_err_msgs
                + &this_err_msg
                + ", ")
        ));
    }

    let first_divisor = match divisors.first() {
        Some(&divisor) => divisor,
        None => return Err(format_err!("no contact descriptors!")),
    };

    if divisors.iter().any(|&divisor| divisor != first_divisor) {
        return Err(format_err!(
            divisors
                .iter()
                .enumerate()
                .filter(|(_i, &divisor)| divisor != first_divisor)
                .map(|(i, divisor)| format!(
                    "contact {} has a different divisor than the first contact ({:?} != {:?})",
                    i, divisor, first_divisor,
                ))
                .fold(String::new(), |prev_err_msgs, this_err_msg| prev_err_msgs
                    + &this_err_msg
                    + ", ")
        ));
    }

    Ok(first_divisor)
}

fn log_keyboard_event_timestamp(
    log_entry_node: &InspectNode,
    driver_timestamp: zx::MonotonicInstant,
) {
    log_entry_node.record_child("key_event", |key_event_node| {
        log_common(key_event_node, driver_timestamp);
    });
}

fn log_basic_reason(reason_node: &InspectNode, text: &'static str) {
    reason_node.record_string("reason", text);
}

fn log_detailed_reason_uint(reason_node: &InspectNode, reason_details: DetailedReasonUint) {
    reason_node.record_string("criterion", reason_details.criterion);
    reason_node.record_uint("actual", u64::try_from(reason_details.actual).unwrap_or(u64::MAX));
    reason_details.min.map(|min| reason_node.record_uint("min_allowed", min));
    reason_details.max.map(|max| reason_node.record_uint("max_allowed", max));
}

fn log_detailed_reason_float(reason_node: &InspectNode, reason_details: DetailedReasonFloat) {
    reason_node.record_string("criterion", reason_details.criterion);
    reason_node.record_double("actual", f64::from(reason_details.actual));
    reason_details.min.map(|min| reason_node.record_double("min_allowed", f64::from(min)));
    reason_details.max.map(|max| reason_node.record_double("max_allowed", f64::from(max)));
}

fn log_detailed_reason_int(reason_node: &InspectNode, reason_details: DetailedReasonInt) {
    reason_node.record_string("criterion", reason_details.criterion);
    reason_node.record_int("actual", reason_details.actual);
    reason_details.min.map(|min| reason_node.record_int("min_allowed", min));
    reason_details.max.map(|max| reason_node.record_int("max_allowed", max));
}

fn log_reason(reason_node: &InspectNode, contender_name: &'static str, reason: Reason) {
    // E.g. "input_pipeline_lib_test::gestures::gesture_arena::tests::utils::StubContender"
    // -> "utils::StubContender".
    let contender_name = match contender_name.rmatch_indices("::").nth(1) {
        Some((i, _matched_substr)) => &contender_name[i + 2..],
        None => contender_name,
    };
    reason_node.record_string("contender", contender_name);
    match reason {
        Reason::Basic(text) => log_basic_reason(reason_node, text),
        Reason::DetailedUint(mismatch_details) => {
            log_detailed_reason_uint(reason_node, mismatch_details)
        }
        Reason::DetailedFloat(mismatch_details) => {
            log_detailed_reason_float(reason_node, mismatch_details)
        }
        Reason::DetailedInt(mismatch_details) => {
            log_detailed_reason_int(reason_node, mismatch_details)
        }
    }
}

fn log_mismatch_reason(log_entry_node: &InspectNode, contender_name: &'static str, reason: Reason) {
    log::debug!("touchpad: {} mismatched: {:?}", contender_name, reason);
    log_entry_node.record_child("mismatch_event", |mismatch_event_node| {
        log_reason(mismatch_event_node, contender_name, reason);
    });
}

fn log_gesture_start(
    log_entry_node: &InspectNode,
    recognized_gesture: RecognizedGesture,
    num_previous_events: usize,
    elapsed_from_first_event: zx::MonotonicDuration,
) {
    log::debug!("touchpad: recognized start {:?}", recognized_gesture);
    log_entry_node.record_child("gesture_start", |gesture_start_node| {
        gesture_start_node.record_string("gesture_name", recognized_gesture.to_str());
        gesture_start_node.record_int(
            "latency_micros",
            // Reduce precision, to minimize space.
            elapsed_from_first_event.into_micros(),
        );
        gesture_start_node.record_uint(
            "latency_event_count",
            u64::try_from(num_previous_events).unwrap_or(u64::MAX),
        );
    });
}

fn log_gesture_end(
    log_entry_node: &InspectNode,
    contender_name: &'static str,
    current_gesture: RecognizedGesture,
    reason: Reason,
    num_events: usize,
    elapsed_from_gesture_start: zx::MonotonicDuration,
) {
    log::debug!("touchpad: recognized end {:?}", current_gesture);
    log_entry_node.record_child("gesture_end", |gesture_end_node| {
        gesture_end_node.record_string("gesture_name", current_gesture.to_str());
        gesture_end_node.record_int(
            "duration_micros",
            // Reduce precision, to minimize space.
            elapsed_from_gesture_start.into_micros(),
        );
        gesture_end_node.record_uint("event_count", u64::try_from(num_events).unwrap_or(u64::MAX));
        log_reason(gesture_end_node, contender_name, reason)
    });
}

#[cfg(test)]
mod tests {
    mod utils {
        use super::super::{
            COUNTS_PER_MM, Contender, ContenderFactory, ExamineEventResult, MatchedContender,
            PRIMARY_BUTTON, ProcessBufferedEventsResult, ProcessNewEventResult, TouchpadEvent,
            VerifyEventResult, Winner, args,
        };
        use crate::utils::Size;
        use crate::{Position, input_device, keyboard_binding, mouse_binding, touch_binding};
        use assert_matches::assert_matches;
        use fidl_fuchsia_input_report as fidl_input_report;
        use maplit::hashset;
        use std::cell::{Cell, RefCell};
        use std::rc::Rc;

        /// The gesture arena is mostly agnostic to the event details. Consequently, most
        /// tests can use the same lightly populated touchpad event.
        pub(super) fn make_unhandled_touchpad_event() -> input_device::InputEvent {
            input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            }
        }

        /// The gesture arena is mostly agnostic to the event details. Consequently, most
        /// tests can use the same lightly populated mouse event.
        pub(super) fn make_unhandled_mouse_event() -> input_device::InputEvent {
            input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Mouse(mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(
                        mouse_binding::RelativeLocation { millimeters: Position::zero() },
                    ),
                    wheel_delta_h: None,
                    wheel_delta_v: None,
                    phase: mouse_binding::MousePhase::Move,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                }),
                device_descriptor: make_mouse_descriptor(),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            }
        }

        /// The gesture arena is mostly agnostic to the event details. Consequently, most
        /// tests can use the same lightly populated keyboard event.
        pub(super) fn make_unhandled_keyboard_event() -> input_device::InputEvent {
            input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Keyboard(
                    keyboard_binding::KeyboardEvent::new(
                        fidl_fuchsia_input::Key::A,
                        fidl_fuchsia_ui_input3::KeyEventType::Pressed,
                    ),
                ),
                device_descriptor: make_keyboard_descriptor(),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            }
        }

        pub(super) fn make_touchpad_descriptor() -> input_device::InputDeviceDescriptor {
            input_device::InputDeviceDescriptor::Touchpad(touch_binding::TouchpadDeviceDescriptor {
                device_id: 1,
                contacts: vec![touch_binding::ContactDeviceDescriptor {
                    x_range: fidl_input_report::Range { min: 0, max: 10_000 },
                    y_range: fidl_input_report::Range { min: 0, max: 10_000 },
                    x_unit: fidl_input_report::Unit {
                        type_: fidl_input_report::UnitType::Meters,
                        exponent: -6,
                    },
                    y_unit: fidl_input_report::Unit {
                        type_: fidl_input_report::UnitType::Meters,
                        exponent: -6,
                    },
                    pressure_range: None,
                    width_range: Some(fidl_input_report::Range { min: 0, max: 10_000 }),
                    height_range: Some(fidl_input_report::Range { min: 0, max: 10_000 }),
                }],
            })
        }

        pub(super) fn make_mouse_descriptor() -> input_device::InputDeviceDescriptor {
            input_device::InputDeviceDescriptor::Mouse(mouse_binding::MouseDeviceDescriptor {
                device_id: 2,
                absolute_x_range: Some(fidl_input_report::Range { min: -127, max: 127 }),
                absolute_y_range: Some(fidl_input_report::Range { min: -127, max: 127 }),
                wheel_v_range: None,
                wheel_h_range: None,
                buttons: Some(vec![PRIMARY_BUTTON]),
                counts_per_mm: COUNTS_PER_MM,
            })
        }

        fn make_keyboard_descriptor() -> input_device::InputDeviceDescriptor {
            input_device::InputDeviceDescriptor::Keyboard(
                keyboard_binding::KeyboardDeviceDescriptor {
                    device_id: 3,
                    device_information: fidl_fuchsia_input_report::DeviceInformation {
                        vendor_id: Some(0),
                        product_id: Some(0),
                        version: Some(0),
                        polling_rate: Some(0),
                        ..Default::default()
                    },
                    keys: vec![fidl_fuchsia_input::Key::A],
                },
            )
        }

        #[derive(Clone, Debug)]
        /// Provides the ability to
        ///
        /// 1. Plumb a fake `Contender` into a `GestureArena`
        /// 2. Fake interactions between the two
        /// 3. Inspect interactions between the two
        ///
        /// To plumb the fake, pass a `Clone` of the `StubContender` to
        /// `ContenderFactoryOnceOrPanic`. To fake or inspect interactions, call the
        /// inherent methods on the struct.
        pub(super) struct StubContender {
            inner: Rc<RefCell<StubContenderInner>>,
            start_from_idle: bool,
        }

        impl StubContender {
            pub(super) fn new() -> Self {
                Self {
                    inner: Rc::new(RefCell::new(StubContenderInner {
                        next_result: None,
                        calls_received: 0,
                        last_touchpad_event: None,
                    })),
                    start_from_idle: false,
                }
            }

            pub(super) fn new_start_from_idle() -> Self {
                Self {
                    inner: Rc::new(RefCell::new(StubContenderInner {
                        next_result: None,
                        calls_received: 0,
                        last_touchpad_event: None,
                    })),
                    start_from_idle: true,
                }
            }

            /// Set the value to be returned on the next call to `examine_event()`.
            /// Aborts if a value is already set, since that suggests that a previously
            /// expected call was never made.
            pub(super) fn set_next_result(&self, next_result: ExamineEventResult) {
                self.assert_next_result_is_none();
                self.inner.borrow_mut().next_result = Some(next_result);
            }

            pub(super) fn assert_next_result_is_none(&self) {
                assert_matches!(self.inner.borrow().next_result, None);
            }

            pub(super) fn calls_received(&self) -> usize {
                self.inner.borrow().calls_received
            }

            pub(super) fn ref_count(&self) -> usize {
                Rc::strong_count(&self.inner)
            }

            pub(super) fn get_last_touchpad_event(&self) -> Option<TouchpadEvent> {
                self.inner.borrow_mut().last_touchpad_event.take()
            }
        }

        #[derive(Debug)]
        struct StubContenderInner {
            next_result: Option<ExamineEventResult>,
            calls_received: usize,
            last_touchpad_event: Option<TouchpadEvent>,
        }

        /// A factory that returns `Vec<Box<dyn Contender>>` from `contenders` on the
        /// first call, and `panic()`-s on the second call.
        ///
        /// Useful because
        /// a) `GestureArena` requires that the factory be invocable multiple times, BUT
        /// b) most of the gesture arena tests don't expect that to happen, SO
        /// c) the tests don't have logic to handle that case.
        ///
        /// To use: pass a lambda which invokes `make_contenders()` to
        /// `GestureArena::new_for_test()`.
        pub(super) struct ContenderFactoryOnceOrPanic {
            contenders: Cell<Option<Vec<Box<dyn Contender>>>>,
        }

        impl ContenderFactoryOnceOrPanic {
            pub(super) fn new(contenders: Vec<Box<dyn Contender>>) -> Self {
                Self { contenders: Cell::new(Some(contenders)) }
            }

            /// `ContenderFactoryOnceOrPanic::new_panic_when_call` will panic when calls
            /// `make_contenders`.
            pub(super) fn new_panic_when_call() -> Self {
                Self { contenders: Cell::new(None) }
            }
        }

        impl ContenderFactory for ContenderFactoryOnceOrPanic {
            fn make_contenders(&self) -> Vec<Box<dyn Contender>> {
                self.contenders
                    .take()
                    .expect("`contenders` has been consumed")
                    .into_iter()
                    .collect()
            }
        }

        impl Contender for StubContender {
            fn examine_event(self: Box<Self>, event: &TouchpadEvent) -> ExamineEventResult {
                let mut inner = self.inner.borrow_mut();
                inner.calls_received += 1;
                inner.last_touchpad_event = Some(event.clone());
                inner.next_result.take().unwrap_or_else(|| {
                    panic!("missing `next_result` on call {}", inner.calls_received)
                })
            }

            fn start_from_idle(&self) -> bool {
                self.start_from_idle
            }
        }

        #[derive(Clone)]
        pub(super) struct ContenderFactoryCalled {
            pub called: Rc<RefCell<bool>>,
        }

        impl ContenderFactoryCalled {
            pub(super) fn new() -> Self {
                Self { called: Rc::new(RefCell::new(false)) }
            }

            pub(super) fn was_called(&self) -> bool {
                *self.called.borrow()
            }
        }

        impl ContenderFactory for ContenderFactoryCalled {
            fn make_contenders(&self) -> Vec<Box<dyn Contender>> {
                self.called.replace(true);
                vec![]
            }
        }

        // A fake contender that always returns itself. Useful for keeping the
        // gesture arena from exiting the matching state. Keeping the arena in
        // the matching state eliminates the need for some tests to provide
        // return values for `process_buffered_events()`.
        #[derive(Debug)]
        pub(super) struct ContenderForever {}

        impl Contender for ContenderForever {
            fn examine_event(self: Box<Self>, _event: &TouchpadEvent) -> ExamineEventResult {
                ExamineEventResult::Contender(self)
            }
        }

        #[derive(Clone, Debug)]
        pub(super) struct StubMatchedContender {
            inner: Rc<RefCell<StubMatchedContenderInner>>,
        }

        impl StubMatchedContender {
            pub(super) fn new() -> Self {
                Self {
                    inner: Rc::new(RefCell::new(StubMatchedContenderInner {
                        next_verify_event_result: None,
                        next_process_buffered_events_result: None,
                        verify_event_calls_received: 0,
                        process_buffered_events_calls_received: 0,
                        last_process_buffered_events_args: None,
                    })),
                }
            }

            /// Set the value to be returned on the next call to `verify_event()`.
            /// Aborts if a value is already set, since that suggests that a previously
            /// expected call was never made.
            pub(super) fn set_next_verify_event_result(&self, next_result: VerifyEventResult) {
                self.assert_next_verify_event_result_is_none();
                self.inner.borrow_mut().next_verify_event_result = Some(next_result);
            }

            fn assert_next_verify_event_result_is_none(&self) {
                assert_matches!(self.inner.borrow().next_verify_event_result, None);
            }

            pub(super) fn verify_event_calls_received(&self) -> usize {
                self.inner.borrow().verify_event_calls_received
            }

            /// Set the value to be returned on the next call to `process_buffered_events()`.
            /// Aborts if a value is already set, since that suggests that a previously
            /// expected call was never made.
            pub(super) fn set_next_process_buffered_events_result(
                &self,
                next_result: ProcessBufferedEventsResult,
            ) {
                self.assert_next_process_buffered_events_result_is_none();
                self.inner.borrow_mut().next_process_buffered_events_result = Some(next_result);
            }

            pub(super) fn get_last_processed_buffered_events_args(
                &self,
            ) -> Option<Vec<TouchpadEvent>> {
                self.inner.borrow_mut().last_process_buffered_events_args.take()
            }

            fn assert_next_process_buffered_events_result_is_none(&self) {
                assert_matches!(self.inner.borrow().next_process_buffered_events_result, None);
            }

            pub(super) fn ref_count(&self) -> usize {
                Rc::strong_count(&self.inner)
            }
        }

        #[derive(Debug)]
        struct StubMatchedContenderInner {
            next_verify_event_result: Option<VerifyEventResult>,
            next_process_buffered_events_result: Option<ProcessBufferedEventsResult>,
            verify_event_calls_received: usize,
            process_buffered_events_calls_received: usize,
            last_process_buffered_events_args: Option<Vec<TouchpadEvent>>,
        }

        impl MatchedContender for StubMatchedContender {
            fn verify_event(self: Box<Self>, _event: &TouchpadEvent) -> VerifyEventResult {
                let mut inner = self.inner.borrow_mut();
                inner.verify_event_calls_received += 1;
                inner.next_verify_event_result.take().unwrap_or_else(|| {
                    panic!(
                        "missing `next_verify_event_result` on call {}",
                        inner.verify_event_calls_received
                    )
                })
            }

            fn process_buffered_events(
                self: Box<Self>,
                events: Vec<TouchpadEvent>,
            ) -> ProcessBufferedEventsResult {
                let mut inner = self.inner.borrow_mut();
                inner.last_process_buffered_events_args = Some(events);
                inner.process_buffered_events_calls_received += 1;
                inner.next_process_buffered_events_result.take().unwrap_or_else(|| {
                    panic!(
                        "missing `next_process_buffered_events_result` on call {}",
                        inner.process_buffered_events_calls_received
                    )
                })
            }
        }

        #[derive(Clone, Debug)]
        pub(super) struct StubWinner {
            inner: Rc<RefCell<StubWinnerInner>>,
        }

        impl StubWinner {
            pub(super) fn new() -> Self {
                Self {
                    inner: Rc::new(RefCell::new(StubWinnerInner {
                        next_result: None,
                        calls_received: 0,
                    })),
                }
            }

            /// Set the value to be returned on the next call to `examine_event()`.
            pub(super) fn set_next_result(&self, next_result: ProcessNewEventResult) {
                self.inner.borrow_mut().next_result = Some(next_result);
            }

            pub(super) fn calls_received(&self) -> usize {
                self.inner.borrow().calls_received
            }
        }

        #[derive(Debug)]
        struct StubWinnerInner {
            next_result: Option<ProcessNewEventResult>,
            calls_received: usize,
        }

        impl Winner for StubWinner {
            fn process_new_event(self: Box<Self>, _event: TouchpadEvent) -> ProcessNewEventResult {
                let mut inner = self.inner.borrow_mut();
                inner.calls_received += 1;
                inner.next_result.take().unwrap_or_else(|| {
                    panic!("missing `next_result` on call {}", inner.calls_received)
                })
            }
        }

        impl From<StubContender> for Box<dyn Contender> {
            fn from(stub_contender: StubContender) -> Box<dyn Contender> {
                Box::new(stub_contender)
            }
        }

        impl From<ContenderForever> for Box<dyn Contender> {
            fn from(contender_forever: ContenderForever) -> Box<dyn Contender> {
                Box::new(contender_forever)
            }
        }

        impl From<StubMatchedContender> for Box<dyn MatchedContender> {
            fn from(stub_matched_contender: StubMatchedContender) -> Box<dyn MatchedContender> {
                Box::new(stub_matched_contender)
            }
        }

        impl From<StubWinner> for Box<dyn Winner> {
            fn from(stub_winner: StubWinner) -> Box<dyn Winner> {
                Box::new(stub_winner)
            }
        }

        pub(super) const TOUCH_CONTACT_INDEX_FINGER: touch_binding::TouchContact =
            touch_binding::TouchContact {
                id: 0,
                position: Position { x: 0.0, y: 0.0 },
                pressure: None,
                contact_size: Some(Size {
                    width: args::MIN_PALM_SIZE_MM / 2.0,
                    height: args::MIN_PALM_SIZE_MM / 2.0,
                }),
            };
    }

    mod idle_chain_states {
        use super::super::{
            ExamineEventResult, GestureArena, InputHandler, MutableState,
            ProcessBufferedEventsResult, Reason, RecognizedGesture, TouchpadEvent, args,
        };
        use super::utils::{
            ContenderFactoryCalled, ContenderFactoryOnceOrPanic, StubContender,
            StubMatchedContender, StubWinner, TOUCH_CONTACT_INDEX_FINGER, make_touchpad_descriptor,
            make_unhandled_keyboard_event, make_unhandled_mouse_event,
            make_unhandled_touchpad_event,
        };
        use crate::input_handler::InputHandlerStatus;
        use crate::utils::Size;
        use crate::{Position, input_device, touch_binding};
        use assert_matches::assert_matches;

        use maplit::hashset;
        use std::cell::RefCell;
        use std::rc::Rc;
        use test_case::test_case;

        fn make_gesture_arena_with_state(
            contender_factory: ContenderFactoryOnceOrPanic,
            state: MutableState,
        ) -> Rc<GestureArena> {
            Rc::new(GestureArena {
                contender_factory: Box::new(contender_factory),
                mutable_state: RefCell::new(state),
                inspect_log: RefCell::new(fuchsia_inspect_contrib::nodes::BoundedListNode::new(
                    fuchsia_inspect::Inspector::default().root().create_child("some_key"),
                    1,
                )),
                inspect_status: InputHandlerStatus::default(),
            })
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_contender_factory_on_touchpad_event(state: MutableState) {
            let contender_factory = Box::new(ContenderFactoryCalled::new());
            let arena = Rc::new(GestureArena {
                contender_factory: contender_factory.clone(),
                mutable_state: RefCell::new(state),
                inspect_log: RefCell::new(fuchsia_inspect_contrib::nodes::BoundedListNode::new(
                    fuchsia_inspect::Inspector::default().root().create_child("some_key"),
                    1,
                )),
                inspect_status: InputHandlerStatus::default(),
            });
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            assert!(contender_factory.was_called());
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_invoke_contender_factory_on_mouse_event(state: MutableState) {
            let contender_factory = Box::new(ContenderFactoryCalled::new());
            let arena = Rc::new(GestureArena {
                contender_factory: contender_factory.clone(),
                mutable_state: RefCell::new(state),
                inspect_log: RefCell::new(fuchsia_inspect_contrib::nodes::BoundedListNode::new(
                    fuchsia_inspect::Inspector::default().root().create_child("some_key"),
                    1,
                )),
                inspect_status: InputHandlerStatus::default(),
            });
            arena.handle_input_event(make_unhandled_mouse_event()).await;
            assert!(!contender_factory.was_called());
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_invoke_contender_factory_on_keyboard_event(state: MutableState) {
            let contender_factory = Box::new(ContenderFactoryCalled::new());

            let arena = Rc::new(GestureArena {
                contender_factory: contender_factory.clone(),
                mutable_state: RefCell::new(state),
                inspect_log: RefCell::new(fuchsia_inspect_contrib::nodes::BoundedListNode::new(
                    fuchsia_inspect::Inspector::default().root().create_child("some_key"),
                    1,
                )),
                inspect_status: InputHandlerStatus::default(),
            });
            arena.handle_input_event(make_unhandled_keyboard_event()).await;
            assert!(!contender_factory.was_called());
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn calls_examine_event_on_contender(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            pretty_assertions::assert_eq!(contender.calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn calls_examine_event_on_idle_only_contender_while_idle() {
            let contender = Box::new(StubContender::new_start_from_idle());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, MutableState::Idle);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            pretty_assertions::assert_eq!(contender.calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_calls_examine_event_on_idle_only_contender_while_chain() {
            let contender = Box::new(StubContender::new_start_from_idle());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, MutableState::Chain);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            pretty_assertions::assert_eq!(contender.calls_received(), 0);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn calls_examine_event_on_all_contenders_even_if_first_matches(state: MutableState) {
            let first_contender = Box::new(StubContender::new());
            let second_contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![
                first_contender.clone(),
                second_contender.clone(),
            ]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            first_contender.set_next_result(ExamineEventResult::MatchedContender(
                StubMatchedContender::new().into(),
            ));
            // Second contender keeps gesture arena in Matching state.
            second_contender
                .set_next_result(ExamineEventResult::Contender(StubContender::new().into()));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            pretty_assertions::assert_eq!(first_contender.calls_received(), 1);
            pretty_assertions::assert_eq!(second_contender.calls_received(), 1);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn retains_reference_to_replacement_contender(state: MutableState) {
            // Create a gesture arena which will instantiate a `StubContender`.
            let initial_contender = Box::new(StubContender::new());
            let contender_factory =
                ContenderFactoryOnceOrPanic::new(vec![initial_contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);

            // Configure `initial_contender` to return a new `StubContender` when
            // `examine_event()` is called.
            let replacement_contender = StubContender::new();
            initial_contender.set_next_result(ExamineEventResult::Contender(
                replacement_contender.clone().into(),
            ));

            // Process a touchpad event. This should cause `arena` to consume the
            // `ExamineEventResult` set above.
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Verify that the `ExamineEventResult` was, in fact, consumed.
            initial_contender.assert_next_result_is_none();

            // Finally, verify that `replacement_contender` has two references.
            //
            // Note that:
            // * One of the references is from `replacement_contender`.
            // * The second one cannot be from the `ExamineEventResult` above,
            //   because the `ExamineEventResult` was consumed.
            //
            // Hence: the second reference (if it exists) must be in the gesture arena.
            pretty_assertions::assert_eq!(replacement_contender.ref_count(), 2);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn retains_reference_to_matched_contender(state: MutableState) {
            // Create a gesture arena which will instantiate a `StubContender`.
            let initial_contender = Box::new(StubContender::new());
            // Second contender keeps gesture arena in Matching state.
            let second_contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![
                initial_contender.clone(),
                second_contender.clone(),
            ]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            // Configure `initial_contender` to return a `StubMatchedContender` when
            // `examine_event()` is called.
            let replacement_contender = StubMatchedContender::new();
            initial_contender.set_next_result(ExamineEventResult::MatchedContender(
                replacement_contender.clone().into(),
            ));
            second_contender
                .set_next_result(ExamineEventResult::Contender(StubContender::new().into()));

            // Process a touchpad event. This should cause `arena` to consume the
            // `ExamineEventResult` set above.
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Verify that the `ExamineEventResult` was, in fact, consumed.
            initial_contender.assert_next_result_is_none();

            // Finally, verify that `replacement_contender` has two references.
            //
            // Note that:
            // * One of the references is from `replacement_contender`.
            // * The second one cannot be from the `ExamineEventResult` above,
            //   because the `ExamineEventResult` was consumed.
            //
            // Hence: the second reference (if it exists) must be in the gesture arena.
            pretty_assertions::assert_eq!(replacement_contender.ref_count(), 2);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn retains_touchpad_event_when_entering_matching(state: MutableState) {
            // Create a gesture arena which will instantiate a `StubContender`.
            let initial_contender = Box::new(StubContender::new());
            // Second contender keeps gesture arena in Matching state.
            let second_contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![
                initial_contender.clone(),
                second_contender.clone(),
            ]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            // Create the event which will be sent to the arena.
            let touchpad_event = input_device::InputEvent {
                event_time: zx::MonotonicInstant::from_nanos(123456),
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![TOUCH_CONTACT_INDEX_FINGER],
                        pressed_buttons: hashset! {0},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                trace_id: None,
                handled: input_device::Handled::No,
            };

            // Configure `initial_contender` to return a `StubMatchedContender` when
            // `examine_event()` is called.
            initial_contender.set_next_result(ExamineEventResult::MatchedContender(
                StubMatchedContender::new().into(),
            ));
            second_contender
                .set_next_result(ExamineEventResult::Contender(StubContender::new().into()));

            // Process `touchpad_event`. Because `initial_contender` returns
            // `ExamineEventResult::MatchedContender`, the gesture arena will enter
            // the `Matching` state.
            arena.clone().handle_input_event(touchpad_event).await;

            // Verify that `arena` retained the details of `touchpad_event`.
            assert_matches!(
                &*arena.mutable_state.borrow(),
                MutableState::Matching {
                    contenders: _,
                    matched_contenders: _,
                    buffered_events,
                    first_event_timestamp: _,
                } => pretty_assertions::assert_eq!(
                    buffered_events.as_slice(),
                    [TouchpadEvent {
                        timestamp: zx::MonotonicInstant::from_nanos(123456),
                        pressed_buttons: vec![0],
                        contacts: vec![
                            touch_binding::TouchContact {
                                contact_size: Some(Size{ width: args::MIN_PALM_SIZE_MM / 2000.0, height: args::MIN_PALM_SIZE_MM / 2000.0 }),
                                ..TOUCH_CONTACT_INDEX_FINGER
                            },
                        ],
                        filtered_palm_contacts: vec![],
                    }]
                )
            );
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_mismatch_entering_chain(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            let touchpad_event = input_device::InputEvent {
                event_time: zx::MonotonicInstant::from_nanos(123456),
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![touch_binding::TouchContact {
                            id: 1,
                            position: Position { x: 0.0, y: 0.0 },
                            ..TOUCH_CONTACT_INDEX_FINGER
                        }],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                trace_id: None,
                handled: input_device::Handled::No,
            };

            pretty_assertions::assert_eq!(
                arena.clone().handle_input_event(touchpad_event).await,
                vec![]
            );

            assert_matches!(*arena.mutable_state.borrow(), MutableState::Chain);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_mismatch_entering_idle(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            pretty_assertions::assert_eq!(
                arena.clone().handle_input_event(make_unhandled_touchpad_event()).await,
                vec![]
            );

            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_when_entering_matching(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            contender.set_next_result(ExamineEventResult::Contender(StubContender::new().into()));
            pretty_assertions::assert_eq!(
                arena.handle_input_event(make_unhandled_touchpad_event()).await,
                vec![]
            );
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn enters_idle_on_mismatch(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn enters_matching_on_contender_result(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);
            contender.set_next_result(ExamineEventResult::Contender(StubContender::new().into()));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Matching { .. });
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn enters_matching_on_matched_contender_result(state: MutableState) {
            let first_contender = Box::new(StubContender::new());
            // Second contender keeps gesture arena in Matching state.
            let second_contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![
                first_contender.clone(),
                second_contender.clone(),
            ]);
            let arena = make_gesture_arena_with_state(contender_factory, state);

            first_contender.set_next_result(ExamineEventResult::MatchedContender(
                StubMatchedContender::new().into(),
            ));
            second_contender
                .set_next_result(ExamineEventResult::Contender(StubContender::new().into()));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Matching { .. });
        }

        #[test_case(MutableState::Idle; "idle")]
        #[test_case(MutableState::Chain; "chain")]
        #[fuchsia::test(allow_stalls = false)]
        async fn enters_forward_on_only_one_matched_contender_result(state: MutableState) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = make_gesture_arena_with_state(contender_factory, state);

            let matched_contender = StubMatchedContender::new();
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: Some(Box::new(StubWinner::new())),
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            contender
                .set_next_result(ExamineEventResult::MatchedContender(matched_contender.into()));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Forwarding { .. });
        }
    }

    mod matching_state {
        use super::super::{
            Contender, ContenderFactory, ExamineEventResult, GestureArena, InputHandler,
            MouseEvent, MutableState, PRIMARY_BUTTON, ProcessBufferedEventsResult, Reason,
            RecognizedGesture, TouchpadEvent, VerifyEventResult,
        };
        use super::utils::{
            ContenderForever, StubContender, StubMatchedContender, StubWinner,
            TOUCH_CONTACT_INDEX_FINGER, make_touchpad_descriptor, make_unhandled_keyboard_event,
            make_unhandled_mouse_event, make_unhandled_touchpad_event,
        };
        use crate::input_handler::InputHandlerStatus;
        use crate::{Position, input_device, mouse_binding, touch_binding};
        use assert_matches::assert_matches;

        use maplit::hashset;
        use pretty_assertions::assert_eq;
        use std::cell::RefCell;
        use std::rc::Rc;
        use test_case::test_case;

        struct ContenderFactoryWarn {}

        impl ContenderFactory for ContenderFactoryWarn {
            fn make_contenders(&self) -> Vec<Box<dyn Contender>> {
                // Note: printing instead of panic()-ing here yields better
                // failure messages from the tests.
                eprintln!("factory invoked in matching state");
                vec![]
            }
        }

        fn make_matching_arena(
            contenders: Vec<StubContender>,
            matched_contenders: Vec<StubMatchedContender>,
            buffered_events: Vec<TouchpadEvent>,
            contender_forever: Option<ContenderForever>,
        ) -> Rc<GestureArena> {
            Rc::new(GestureArena {
                contender_factory: Box::new(ContenderFactoryWarn {}),
                mutable_state: RefCell::new(MutableState::Matching {
                    contenders: {
                        contenders
                            .into_iter()
                            .map(std::convert::From::<StubContender>::from)
                            .chain(
                                contender_forever
                                    .into_iter()
                                    .map(std::convert::From::<ContenderForever>::from),
                            )
                            .collect()
                    },
                    matched_contenders: {
                        matched_contenders
                            .into_iter()
                            .map(std::convert::From::<StubMatchedContender>::from)
                            .collect()
                    },
                    first_event_timestamp: zx::MonotonicInstant::ZERO,
                    buffered_events,
                }),
                inspect_log: RefCell::new(fuchsia_inspect_contrib::nodes::BoundedListNode::new(
                    fuchsia_inspect::Inspector::default().root().create_child("some_key"),
                    1,
                )),
                inspect_status: InputHandlerStatus::default(),
            })
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_examine_and_verify_event_on_touchpad_event() {
            let contender = StubContender::new();
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![contender.clone()],
                vec![matched_contender.clone()],
                vec![],
                None,
            );
            contender.set_next_result(ExamineEventResult::Contender(contender.clone().into()));
            matched_contender.set_next_verify_event_result(VerifyEventResult::Mismatch(
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            assert_eq!(contender.calls_received(), 1);
            assert_eq!(matched_contender.verify_event_calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_invoke_examine_or_verify_event_on_mouse_event() {
            let contender = StubContender::new();
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![contender.clone()],
                vec![matched_contender.clone()],
                vec![],
                None,
            );
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            matched_contender.set_next_verify_event_result(VerifyEventResult::Mismatch(
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_mouse_event()).await;
            assert_eq!(contender.calls_received(), 0);
            assert_eq!(matched_contender.verify_event_calls_received(), 0);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_invoke_examine_or_verify_event_on_keyboard_event() {
            let contender = StubContender::new();
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![contender.clone()],
                vec![matched_contender.clone()],
                vec![],
                None,
            );
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            matched_contender.set_next_verify_event_result(VerifyEventResult::Mismatch(
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_keyboard_event()).await;
            assert_eq!(contender.calls_received(), 0);
            assert_eq!(matched_contender.verify_event_calls_received(), 0);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_repeat_event_to_matched_contender_returned_by_examine_event() {
            let contender = StubContender::new();
            let arena = make_matching_arena(
                vec![contender.clone()],
                vec![],
                vec![],
                // Make sure that `arena` does not progress to the forwarding state,
                // even if the logic _does_ repeat the event.
                Some(ContenderForever {}),
            );

            // Configure `contender` to give a `MatchContender` to `arena`.
            let matched_contender = StubMatchedContender::new();
            contender.set_next_result(ExamineEventResult::MatchedContender(
                matched_contender.clone().into(),
            ));

            // Set the return value for `matched_contender`. If the implementation
            // is buggy, and `verify_event()` is called, having a return value for
            // that call makes this test fail at the final assertion, which is easier
            // to understand.
            matched_contender.set_next_verify_event_result(VerifyEventResult::Mismatch(
                Reason::Basic("some reason"),
            ));

            // Send the touchpad event, and validate that the arena did not call
            // `verify_event()` on the newly returned `MatchedContender`.
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            assert_eq!(matched_contender.verify_event_calls_received(), 0);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_examine_event_for_new_event_with_contender_replaced_by_contender() {
            // Set up an arena with a `StubContender` that will continue the
            // matching process with a new `StubContender`.
            let initial_contender = StubContender::new();
            let arena = make_matching_arena(vec![initial_contender.clone()], vec![], vec![], None);
            let replacement_contender = StubContender::new();
            initial_contender.set_next_result(ExamineEventResult::Contender(
                replacement_contender.clone().into(),
            ));

            // Process a touchpad event. This should cause `arena` to replace
            // `initial_contender` with `replacement_contender`.
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Process another touchpad event. This should cause `arena` to invoke
            // `examine_event()` on `replacement_contender`.
            replacement_contender
                .set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;

            // Verify that `replacement_contender` was called.
            assert_eq!(replacement_contender.calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_verify_event_for_new_event_with_contender_replaced_by_matched_contender() {
            // Set up an arena with
            // * a `StubContender` that will continue the matching process with
            //   a new `StubMatchedContender`
            // * a `ContenderForever`, which will keep the arena in the matching
            //   state
            let initial_contender = StubContender::new();
            let arena = make_matching_arena(
                vec![initial_contender.clone()],
                vec![],
                vec![],
                Some(ContenderForever {}),
            );
            let replacement_contender = StubMatchedContender::new();
            initial_contender.set_next_result(ExamineEventResult::MatchedContender(
                replacement_contender.clone().into(),
            ));

            // Process a touchpad event. This should cause `arena` to replace
            // `initial_contender` with `replacement_contender`.
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Process another touchpad event. This should cause `arena` to invoke
            // `examine_event()` on `replacement_contender`.
            replacement_contender.set_next_verify_event_result(VerifyEventResult::Mismatch(
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;

            // Verify that `replacement_contender` was called.
            assert_eq!(replacement_contender.verify_event_calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_verify_event_for_new_event_with_matched_contender_replaced_by_matched_contender()
         {
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![],
                vec![matched_contender.clone()],
                vec![],
                // Ensure that `arena` remains in the matching state. This simplifies
                // the test by eliminating the need to provide a response to a
                // potential `process_buffered_events()` call.
                Some(ContenderForever {}),
            );

            // Configure `matched_contender` to replace itself with
            // `replacement_matched_contender`.
            let replacement_matched_contender = StubMatchedContender::new();
            matched_contender.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                replacement_matched_contender.clone().into(),
            ));

            // Process a touchpad event. This should cause `arena` to retain
            // replace `contender` with `replacement_contender`.
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Set a return value for the expected call on `replacement_contender`.
            replacement_matched_contender.set_next_verify_event_result(
                VerifyEventResult::Mismatch(Reason::Basic("some reason")),
            );

            // Process another touchpad event, and verify that `replacement_contender`
            // is called.
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_eq!(replacement_matched_contender.verify_event_calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_mismatch_entering_idle() {
            let contender = StubContender::new();
            let arena = make_matching_arena(vec![contender.clone()], vec![], vec![], None);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            assert_eq!(
                arena.clone().handle_input_event(make_unhandled_touchpad_event()).await,
                vec![]
            );
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_mismatch_entering_chain() {
            let contender = StubContender::new();
            let arena = make_matching_arena(vec![contender.clone()], vec![], vec![], None);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            let touchpad_event = input_device::InputEvent {
                event_time: zx::MonotonicInstant::from_nanos(123456),
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![touch_binding::TouchContact {
                            id: 1,
                            position: Position { x: 0.0, y: 0.0 },
                            ..TOUCH_CONTACT_INDEX_FINGER
                        }],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                trace_id: None,
                handled: input_device::Handled::No,
            };

            pretty_assertions::assert_eq!(
                arena.clone().handle_input_event(touchpad_event).await,
                vec![]
            );

            assert_matches!(*arena.mutable_state.borrow(), MutableState::Chain);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_contender() {
            let contender = StubContender::new();
            let arena = make_matching_arena(vec![contender.clone()], vec![], vec![], None);
            contender.set_next_result(ExamineEventResult::Contender(contender.clone().into()));
            assert_eq!(arena.handle_input_event(make_unhandled_touchpad_event()).await, vec![]);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_multiple_matched_contenders() {
            let first_matched_contender = StubMatchedContender::new();
            let second_matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![],
                vec![first_matched_contender.clone(), second_matched_contender.clone()],
                vec![],
                None,
            );
            first_matched_contender.set_next_verify_event_result(
                VerifyEventResult::MatchedContender(first_matched_contender.clone().into()),
            );
            second_matched_contender.set_next_verify_event_result(
                VerifyEventResult::MatchedContender(second_matched_contender.clone().into()),
            );
            assert_eq!(arena.handle_input_event(make_unhandled_touchpad_event()).await, vec![]);
        }

        #[test_case(Some(StubWinner::new()); "with_winner")]
        #[test_case(None; "without_winner")]
        #[fuchsia::test(allow_stalls = false)]
        async fn generates_events_from_process_buffered_events_on_single_matched_contender(
            winner: Option<StubWinner>,
        ) {
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(vec![], vec![matched_contender.clone()], vec![], None);
            matched_contender.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender.clone().into(),
            ));
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![
                        MouseEvent {
                            timestamp: zx::MonotonicInstant::from_nanos(123),
                            mouse_data: mouse_binding::MouseEvent {
                                location: mouse_binding::MouseLocation::Relative(
                                    mouse_binding::RelativeLocation {
                                        millimeters: Position::zero(),
                                    },
                                ),
                                wheel_delta_v: None,
                                wheel_delta_h: None,
                                phase: mouse_binding::MousePhase::Down,
                                affected_buttons: hashset! { PRIMARY_BUTTON },
                                pressed_buttons: hashset! { PRIMARY_BUTTON },
                                is_precision_scroll: None,
                            },
                        },
                        MouseEvent {
                            timestamp: zx::MonotonicInstant::from_nanos(456),
                            mouse_data: mouse_binding::MouseEvent {
                                location: mouse_binding::MouseLocation::Relative(
                                    mouse_binding::RelativeLocation {
                                        millimeters: Position::zero(),
                                    },
                                ),
                                wheel_delta_v: None,
                                wheel_delta_h: None,
                                phase: mouse_binding::MousePhase::Up,
                                affected_buttons: hashset! { PRIMARY_BUTTON },
                                pressed_buttons: hashset! {},
                                is_precision_scroll: None,
                            },
                        },
                    ],
                    winner: winner.map(std::convert::From::<StubWinner>::from),
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            assert_matches!(
                arena
                    .handle_input_event(make_unhandled_touchpad_event())
                    .await
                    .as_slice(),
                [
                    input_device::InputEvent {
                        handled: input_device::Handled::No,
                        device_event: input_device::InputDeviceEvent::Mouse(
                            mouse_binding::MouseEvent {
                                pressed_buttons: first_pressed_buttons, ..
                            }
                        ),
                        ..
                    },
                    input_device::InputEvent {
                        handled: input_device::Handled::No,
                        device_event: input_device::InputDeviceEvent::Mouse(
                            mouse_binding::MouseEvent {
                                pressed_buttons: second_pressed_buttons, ..
                            }
                        ),
                        ..
                    },
                ] => {
                    pretty_assertions::assert_eq!(*first_pressed_buttons, hashset! { PRIMARY_BUTTON });
                    pretty_assertions::assert_eq!(*second_pressed_buttons, hashset! {});
                }
            );
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn passes_all_buffered_events_to_process_buffered_events() {
            // Create an arena, and seed it a buffered event (emulating what happens
            // as the arena transitions from Idle to Matching).
            let contender = StubContender::new();
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![contender.clone()],
                vec![matched_contender.clone()],
                vec![TouchpadEvent {
                    timestamp: zx::MonotonicInstant::from_nanos(123),
                    contacts: vec![],
                    pressed_buttons: vec![],
                    filtered_palm_contacts: vec![],
                }],
                None,
            );

            // Send another event to the arena, to exercise the case of an event
            // being buffered because the contest continued.
            contender.set_next_result(ExamineEventResult::Contender(contender.clone().into()));
            matched_contender.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender.clone().into(),
            ));
            arena
                .clone()
                .handle_input_event(input_device::InputEvent {
                    event_time: zx::MonotonicInstant::from_nanos(456),
                    device_event: input_device::InputDeviceEvent::Touchpad(
                        touch_binding::TouchpadEvent {
                            injector_contacts: vec![],
                            pressed_buttons: hashset! {},
                        },
                    ),
                    device_descriptor: make_touchpad_descriptor(),
                    trace_id: None,
                    handled: input_device::Handled::No,
                })
                .await;

            // Send the event that concludes the contest.
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            matched_contender.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender.clone().into(),
            ));
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: None,
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            arena
                .handle_input_event(input_device::InputEvent {
                    event_time: zx::MonotonicInstant::from_nanos(789),
                    device_event: input_device::InputDeviceEvent::Touchpad(
                        touch_binding::TouchpadEvent {
                            injector_contacts: vec![],
                            pressed_buttons: hashset! {},
                        },
                    ),
                    device_descriptor: make_touchpad_descriptor(),
                    trace_id: None,
                    handled: input_device::Handled::No,
                })
                .await;

            // Verify that the contender received all three events.
            assert_eq!(
                matched_contender
                    .get_last_processed_buffered_events_args()
                    .map(|vec| vec
                        .into_iter()
                        .map(|event| event.timestamp.into_nanos())
                        .collect::<Vec<_>>())
                    .as_deref(),
                Some([123, 456, 789].as_slice())
            );
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_idle_when_sole_contender_does_not_match() {
            let contender = StubContender::new();
            let arena = make_matching_arena(vec![contender.clone()], vec![], vec![], None);
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_idle_when_sole_matched_contender_does_not_match() {
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(vec![], vec![matched_contender.clone()], vec![], None);
            matched_contender.set_next_verify_event_result(VerifyEventResult::Mismatch(
                Reason::Basic("some reason"),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn remains_in_matching_when_a_contender_remains() {
            let contender = StubContender::new();
            let arena = make_matching_arena(vec![contender.clone()], vec![], vec![], None);
            contender.set_next_result(ExamineEventResult::Contender(contender.clone().into()));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Matching { .. });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn remains_in_matching_when_multiple_matched_contenders_remain() {
            let matched_contender_a = StubMatchedContender::new();
            let matched_contender_b = StubMatchedContender::new();
            let arena = make_matching_arena(
                vec![],
                vec![matched_contender_a.clone(), matched_contender_b.clone()],
                vec![],
                None,
            );
            matched_contender_a.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender_a.clone().into(),
            ));
            matched_contender_b.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender_b.clone().into(),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Matching { .. });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_idle_when_sole_matched_contender_returns_no_winner() {
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(vec![], vec![matched_contender.clone()], vec![], None);
            matched_contender.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender.clone().into(),
            ));
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: None,
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_forwarding_when_sole_matched_contender_returns_a_winner() {
            let matched_contender = StubMatchedContender::new();
            let arena = make_matching_arena(vec![], vec![matched_contender.clone()], vec![], None);
            matched_contender.set_next_verify_event_result(VerifyEventResult::MatchedContender(
                matched_contender.clone().into(),
            ));
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: Some(StubWinner::new().into()),
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Forwarding { .. });
        }
    }

    mod forwarding_state {
        use super::super::{
            Contender, EndGestureEvent, ExamineEventResult, GestureArena, InputHandler, MouseEvent,
            MutableState, ProcessNewEventResult, Reason, RecognizedGesture, TouchpadEvent,
        };
        use super::utils::{
            ContenderFactoryOnceOrPanic, ContenderForever, StubContender, StubWinner,
            TOUCH_CONTACT_INDEX_FINGER, make_touchpad_descriptor, make_unhandled_keyboard_event,
            make_unhandled_mouse_event, make_unhandled_touchpad_event,
        };
        use crate::input_handler::InputHandlerStatus;
        use crate::{Position, input_device, mouse_binding, touch_binding};
        use assert_matches::assert_matches;

        use maplit::hashset;
        use pretty_assertions::assert_eq;
        use std::cell::RefCell;
        use std::rc::Rc;
        use test_case::test_case;

        /// Creates an arena in the forwarding state, with
        /// a) the given `winner`, and
        /// b) an appropriate contender factory
        ///
        /// If `contender` is `None`, the contender factory will abort on the first
        /// call.
        ///
        /// If `contender` is `Some`, the contender factory will return the content
        /// of the `Option` on the first call, and abort on the second call.
        ///
        /// The former is the common case for the tests. The latter is useful for
        /// tests that exercise the arena's handling of an `EndGesture(Some)`.
        fn make_forwarding_arena(
            winner: StubWinner,
            contender: Option<Box<dyn Contender>>,
        ) -> Rc<GestureArena> {
            let contender_factory = match contender {
                Some(c) => Box::new(ContenderFactoryOnceOrPanic::new(vec![c])),
                None => Box::new(ContenderFactoryOnceOrPanic::new_panic_when_call()),
            };

            Rc::new(GestureArena {
                contender_factory,
                mutable_state: RefCell::new(MutableState::Forwarding {
                    winner: winner.into(),
                    current_gesture: RecognizedGesture::Motion,
                    // Tests that care about `gesture_start_timestamp` should take care
                    // to set that value themselves. Default to a value that should cause
                    // any test that relies on a good value to fail.
                    gesture_start_timestamp: zx::MonotonicInstant::INFINITE_PAST,
                    num_events: 0,
                }),
                inspect_log: RefCell::new(fuchsia_inspect_contrib::nodes::BoundedListNode::new(
                    fuchsia_inspect::Inspector::default().root().create_child("some_key"),
                    1,
                )),
                inspect_status: InputHandlerStatus::default(),
            })
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_process_new_event_on_touchpad_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::NoEvent,
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_touchpad_event()).await;
            assert_eq!(winner.calls_received(), 1);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_invoke_process_new_event_on_mouse_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::NoEvent,
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_mouse_event()).await;
            assert_eq!(winner.calls_received(), 0);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn does_not_invoke_process_new_event_on_keyboard_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::NoEvent,
                Reason::Basic("some reason"),
            ));
            arena.handle_input_event(make_unhandled_keyboard_event()).await;
            assert_eq!(winner.calls_received(), 0);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn invokes_process_new_event_for_multiple_new_events() {
            // Create `arena` with `winner` and a `ContenderForever`. The latter
            // makes the test fail in a more useful way if the `GestureArena`
            // is buggy.
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), Some(ContenderForever {}.into()));

            // Send two events to `arena`.
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(
                None,
                winner.clone().into(),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(
                None,
                winner.clone().into(),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Verify `winner` was called as expected.
            assert_eq!(winner.calls_received(), 2);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_event_on_continue_gesture_with_mouse_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(
                Some(MouseEvent {
                    timestamp: zx::MonotonicInstant::from_nanos(123),
                    mouse_data: mouse_binding::MouseEvent {
                        location: mouse_binding::MouseLocation::Relative(
                            mouse_binding::RelativeLocation { millimeters: Position::zero() },
                        ),
                        wheel_delta_v: None,
                        wheel_delta_h: None,
                        phase: mouse_binding::MousePhase::Move,
                        affected_buttons: hashset! {},
                        pressed_buttons: hashset! {},
                        is_precision_scroll: None,
                    },
                }),
                winner.clone().into(),
            ));
            assert_matches!(
                arena.handle_input_event(make_unhandled_touchpad_event()).await.as_slice(),
                [
                    input_device::InputEvent {
                        event_time,
                        handled: input_device::Handled::No,
                        device_event: input_device::InputDeviceEvent::Mouse(_),
                        ..
                    },
                ] => pretty_assertions::assert_eq!(*event_time, zx::MonotonicInstant::from_nanos(123))
            );
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_continue_gesture_without_mouse_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(
                None,
                winner.clone().into(),
            ));
            pretty_assertions::assert_eq!(
                arena.handle_input_event(make_unhandled_touchpad_event()).await.as_slice(),
                vec![]
            );
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_end_gesture_without_touchpad_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::NoEvent,
                Reason::Basic("some reason"),
            ));
            pretty_assertions::assert_eq!(
                arena.handle_input_event(make_unhandled_touchpad_event()).await.as_slice(),
                vec![]
            );
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_end_gesture_with_unconsumed_touchpad_event_entering_idle() {
            // Create `arena` with a `StubContender` for processing the unconsumed
            // `TouchpadEvent`.
            let winner = StubWinner::new();
            let contender = StubContender::new();
            let arena = make_forwarding_arena(winner.clone(), Some(contender.clone().into()));
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::UnconsumedEvent(TouchpadEvent {
                    contacts: vec![],
                    pressed_buttons: vec![],
                    timestamp: zx::MonotonicInstant::ZERO,
                    filtered_palm_contacts: vec![],
                }),
                Reason::Basic("some reason"),
            ));

            // Set a return value for the `examine_event()` call.
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            // Verify no events were generated.
            pretty_assertions::assert_eq!(
                arena.clone().handle_input_event(make_unhandled_touchpad_event()).await.as_slice(),
                vec![]
            );

            // Unconsumed event without contact will invoked in handle_event_while_idle,
            // but no contender want this event so stay in Idle state.
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle { .. });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_no_events_on_end_gesture_with_unconsumed_touchpad_event_entering_chain()
        {
            // Create `arena` with a `StubContender` for processing the unconsumed
            // `TouchpadEvent`.
            let winner = StubWinner::new();
            let contender = StubContender::new();
            let arena = make_forwarding_arena(winner.clone(), Some(contender.clone().into()));
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::UnconsumedEvent(TouchpadEvent {
                    contacts: vec![touch_binding::TouchContact {
                        id: 1,
                        position: Position { x: 0.0, y: 0.0 },
                        pressure: None,
                        contact_size: None,
                    }],
                    pressed_buttons: vec![],
                    timestamp: zx::MonotonicInstant::from_nanos(123456),
                    filtered_palm_contacts: vec![],
                }),
                Reason::Basic("some reason"),
            ));

            // Set a return value for the `examine_event()` call.
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            let touchpad_event = input_device::InputEvent {
                event_time: zx::MonotonicInstant::from_nanos(123456),
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![touch_binding::TouchContact {
                            id: 1,
                            position: Position { x: 0.0, y: 0.0 },
                            ..TOUCH_CONTACT_INDEX_FINGER
                        }],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                trace_id: None,
                handled: input_device::Handled::No,
            };

            // Verify no events were generated.
            pretty_assertions::assert_eq!(
                arena.clone().handle_input_event(touchpad_event).await.as_slice(),
                vec![]
            );

            // Unconsumed event with contact will invoked in handle_event_while_chain,
            // but no contender want this event so stay in Chain state.
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Chain { .. });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn generates_event_on_end_gesture_with_touchpad_event() {
            // Create `arena` with a `StubContender` for processing the unconsumed
            // `TouchpadEvent`.
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            let mouse_event = MouseEvent {
                timestamp: zx::MonotonicInstant::from_nanos(123),
                mouse_data: mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(
                        mouse_binding::RelativeLocation { millimeters: Position::zero() },
                    ),
                    wheel_delta_v: None,
                    wheel_delta_h: None,
                    phase: mouse_binding::MousePhase::Move,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                },
            };
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::GeneratedEvent(mouse_event),
                Reason::Basic("some reason"),
            ));

            // Verify events were generated.
            assert_matches!(
                arena.handle_input_event(make_unhandled_touchpad_event()).await.as_slice(),
                [
                    input_device::InputEvent {
                        event_time,
                        handled: input_device::Handled::No,
                        device_event: input_device::InputDeviceEvent::Mouse(_),
                        ..
                    },
                ] => pretty_assertions::assert_eq!(*event_time, zx::MonotonicInstant::from_nanos(123))
            );
        }

        #[test_case(Some(MouseEvent{
            timestamp: zx::MonotonicInstant::from_nanos(123),
            mouse_data: mouse_binding::MouseEvent {
                location: mouse_binding::MouseLocation::Relative(
                    mouse_binding::RelativeLocation {
                        millimeters: Position::zero(),
                    },
                ),
                wheel_delta_v: None,
                wheel_delta_h: None,
                phase: mouse_binding::MousePhase::Move,
                affected_buttons: hashset! {},
                pressed_buttons: hashset! {},
                is_precision_scroll: None,
            },
        }); "with_mouse_event")]
        #[test_case(None; "without_mouse_event")]
        #[fuchsia::test(allow_stalls = false)]
        async fn remains_in_forwarding_on_continue_gesture(mouse_event: Option<MouseEvent>) {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(
                mouse_event,
                winner.clone().into(),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Forwarding { .. });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_idle_on_end_gesture_with_touchpad_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            let mouse_event = MouseEvent {
                timestamp: zx::MonotonicInstant::from_nanos(123),
                mouse_data: mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(
                        mouse_binding::RelativeLocation { millimeters: Position::zero() },
                    ),
                    wheel_delta_v: None,
                    wheel_delta_h: None,
                    phase: mouse_binding::MousePhase::Move,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                },
            };
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::GeneratedEvent(mouse_event),
                Reason::Basic("some reason"),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle { .. });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_chain_on_end_gesture_with_touchpad_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            let mouse_event = MouseEvent {
                timestamp: zx::MonotonicInstant::from_nanos(123),
                mouse_data: mouse_binding::MouseEvent {
                    location: mouse_binding::MouseLocation::Relative(
                        mouse_binding::RelativeLocation { millimeters: Position::zero() },
                    ),
                    wheel_delta_v: None,
                    wheel_delta_h: None,
                    phase: mouse_binding::MousePhase::Move,
                    affected_buttons: hashset! {},
                    pressed_buttons: hashset! {},
                    is_precision_scroll: None,
                },
            };
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::GeneratedEvent(mouse_event),
                Reason::Basic("some reason"),
            ));
            let touchpad_event = input_device::InputEvent {
                event_time: zx::MonotonicInstant::from_nanos(123456),
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![touch_binding::TouchContact {
                            id: 1,
                            position: Position { x: 0.0, y: 0.0 },
                            ..TOUCH_CONTACT_INDEX_FINGER
                        }],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                trace_id: None,
                handled: input_device::Handled::No,
            };
            arena.clone().handle_input_event(touchpad_event).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Chain);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn transitions_to_idle_on_end_gesture_without_touchpad_event() {
            let winner = StubWinner::new();
            let arena = make_forwarding_arena(winner.clone(), None);
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::NoEvent,
                Reason::Basic("reason"),
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            assert_matches!(*arena.mutable_state.borrow(), MutableState::Idle);
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn starts_new_contest_on_end_gesture_with_touchpad_event() {
            // Set up an arena in the forwarding state with `winner`, and
            // with a factory that will return `contender`. The latter should
            // be called at the start of the new contest.
            let winner = StubWinner::new();
            let contender = StubContender::new();
            let arena = make_forwarding_arena(winner.clone(), Some(contender.clone().into()));

            // Set up `winner` to end the gesture and return an unconsumed event.
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                EndGestureEvent::UnconsumedEvent(TouchpadEvent {
                    timestamp: zx::MonotonicInstant::ZERO,
                    contacts: vec![],
                    pressed_buttons: vec![],
                    filtered_palm_contacts: vec![],
                }),
                Reason::Basic("reason"),
            ));

            // Set up `contender` to reply to the `examine_event()` call.
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            // Send an event into the arena.
            arena.handle_input_event(make_unhandled_touchpad_event()).await;

            // Verify that the arena started a new contest.
            assert_eq!(contender.calls_received(), 1);
        }
    }

    mod touchpad_event_payload {
        use super::super::{ExamineEventResult, GestureArena, InputHandler, Reason, args};
        use super::utils::{
            ContenderFactoryOnceOrPanic, StubContender, TOUCH_CONTACT_INDEX_FINGER,
        };
        use crate::utils::Size;
        use crate::{Position, input_device, touch_binding};
        use assert_matches::assert_matches;
        use fidl_fuchsia_input_report::{self as fidl_input_report, UnitType};

        use maplit::hashset;
        use std::rc::Rc;
        use test_case::test_case;
        use test_util::assert_near;

        fn make_touchpad_descriptor(
            units: Vec<(fidl_input_report::Unit, fidl_input_report::Unit)>,
        ) -> input_device::InputDeviceDescriptor {
            let contacts: Vec<_> = units
                .into_iter()
                .map(|(x_unit, y_unit)| touch_binding::ContactDeviceDescriptor {
                    x_range: fidl_input_report::Range { min: 0, max: 1_000_000 },
                    y_range: fidl_input_report::Range { min: 0, max: 1_000_000 },
                    x_unit,
                    y_unit,
                    pressure_range: None,
                    width_range: Some(fidl_input_report::Range { min: 0, max: 10_000 }),
                    height_range: Some(fidl_input_report::Range { min: 0, max: 10_000 }),
                })
                .collect();
            input_device::InputDeviceDescriptor::Touchpad(touch_binding::TouchpadDeviceDescriptor {
                device_id: 1,
                contacts,
            })
        }

        fn make_unhandled_touchpad_event_with_contacts(
            contact_position_units: Vec<(fidl_input_report::Unit, fidl_input_report::Unit)>,
            injector_contacts: Vec<touch_binding::TouchContact>,
        ) -> input_device::InputEvent {
            input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts,
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(contact_position_units),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            }
        }

        fn make_unhandled_touchpad_event_with_positions(
            contact_position_units: Vec<(fidl_input_report::Unit, fidl_input_report::Unit)>,
            positions: Vec<Position>,
        ) -> input_device::InputEvent {
            let injector_contacts: Vec<_> = positions
                .into_iter()
                .enumerate()
                .map(|(i, position)| touch_binding::TouchContact {
                    id: u32::try_from(i).unwrap(),
                    position,
                    contact_size: None,
                    pressure: None,
                })
                .collect();
            make_unhandled_touchpad_event_with_contacts(contact_position_units, injector_contacts)
        }

        #[test_case(
            vec![(
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
            )],
            vec![
                touch_binding::TouchContact{
                    id: 1,
                    position: Position { x: 200000.0, y: 100000.0 },
                    contact_size: Some(Size {
                        width: 2500.0,
                        height: 1500.0,
                    }),
                    pressure: None,
                }
            ]; "from_micrometers")]
        #[test_case(
            vec![(
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -2 },
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -2 },
            )],
            vec![
                touch_binding::TouchContact{
                    id: 1,
                    position: Position { x: 20.0, y: 10.0 },
                    contact_size: Some(Size {
                        width: 0.25,
                        height: 0.15,
                    }),
                    pressure: None,
                }
            ]; "from_centimeters")]
        #[fuchsia::test(allow_stalls = false)]
        async fn provides_recognizer_position_size_in_millimeters(
            contact_position_units: Vec<(fidl_input_report::Unit, fidl_input_report::Unit)>,
            contacts: Vec<touch_binding::TouchContact>,
        ) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(contender_factory),
                &fuchsia_inspect::Inspector::default(),
                1,
            ));
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena
                .handle_input_event(make_unhandled_touchpad_event_with_contacts(
                    contact_position_units,
                    contacts,
                ))
                .await;
            assert_matches!(
                contender.get_last_touchpad_event().unwrap().contacts.as_slice(),
                [touch_binding::TouchContact { position, contact_size: Some(size), .. }] => {
                    assert_near!(position.x, 200.0, 1.0);
                    assert_near!(position.y, 100.0, 1.0);
                    assert_near!(size.width, 2.5, 0.1);
                    assert_near!(size.height, 1.5, 0.1);
                }
            );
        }

        #[test_case(
            touch_binding::TouchpadEvent {
                injector_contacts: vec![
                    touch_binding::TouchContact{
                        id: 1,
                        position: Position { x: 0.0, y: 0.0 },
                        contact_size: Some(Size {
                            width: args::MIN_PALM_SIZE_MM,
                            height: 0.15,
                        }),
                        pressure: None,
                    }
                ],
                pressed_buttons: hashset! {},
            }; "only palm contact"
        )]
        #[fuchsia::test(allow_stalls = false)]
        async fn ignore_palm_contact(event: touch_binding::TouchpadEvent) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(contender_factory),
                &fuchsia_inspect::Inspector::default(),
                1,
            ));
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            let input_event = input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(event),
                device_descriptor: make_touchpad_descriptor(vec![(
                    fidl_input_report::Unit { type_: UnitType::Meters, exponent: -3 },
                    fidl_input_report::Unit { type_: UnitType::Meters, exponent: -3 },
                )]),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            };

            arena.handle_input_event(input_event).await;
            assert_matches!(contender.get_last_touchpad_event().unwrap().contacts.as_slice(), []);
        }

        #[test_case(
            touch_binding::TouchpadEvent {
                injector_contacts: vec![
                    TOUCH_CONTACT_INDEX_FINGER,
                    touch_binding::TouchContact{
                        id: 1,
                        position: Position { x: 0.0, y: 0.0 },
                        contact_size: Some(Size {
                            width: args::MIN_PALM_SIZE_MM,
                            height: 0.15,
                        }),
                        pressure: None,
                    }
                ],
                pressed_buttons: hashset! {},
            }, vec![]; "palm contact and finger"
        )]
        #[fuchsia::test(allow_stalls = false)]
        async fn ignore_palm_contact_keep_finger(
            event: touch_binding::TouchpadEvent,
            expect_buttons: Vec<u8>,
        ) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(contender_factory),
                &fuchsia_inspect::Inspector::default(),
                1,
            ));
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            let input_event = input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(event),
                device_descriptor: make_touchpad_descriptor(vec![(
                    fidl_input_report::Unit { type_: UnitType::Meters, exponent: -3 },
                    fidl_input_report::Unit { type_: UnitType::Meters, exponent: -3 },
                )]),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            };

            arena.handle_input_event(input_event).await;
            let got = contender.get_last_touchpad_event().unwrap();
            assert_eq!(got.contacts.as_slice(), [TOUCH_CONTACT_INDEX_FINGER]);
            assert_eq!(got.pressed_buttons, expect_buttons);
        }

        #[test_case(
            touch_binding::TouchpadEvent {
                injector_contacts: vec![
                    touch_binding::TouchContact{
                        id: 1,
                        position: Position { x: 0.0, y: 0.0 },
                        contact_size: Some(Size {
                            width: args::MIN_PALM_SIZE_MM,
                            height: 0.15,
                        }),
                        pressure: None,
                    }
                ],
                pressed_buttons: hashset! {1},
            }; "palm contact"
        )]
        #[test_case(
            touch_binding::TouchpadEvent {
                injector_contacts: vec![
                    touch_binding::TouchContact{
                        id: 1,
                        position: Position { x: 0.0, y: 0.0 },
                        contact_size: Some(Size {
                            width: args::MIN_PALM_SIZE_MM,
                            height: 0.15,
                        }),
                        pressure: None,
                    },
                    touch_binding::TouchContact{
                        id: 2,
                        position: Position { x: 5.0, y: 5.0 },
                        contact_size: Some(Size {
                            width: args::MIN_PALM_SIZE_MM / 2.0,
                            height: 0.15,
                        }),
                        pressure: None,
                    },
                ],
                pressed_buttons: hashset! {1},
            }; "palm and finger contact"
        )]
        #[fuchsia::test(allow_stalls = false)]
        async fn skip_palm_detection_when_button_down(event: touch_binding::TouchpadEvent) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(contender_factory),
                &fuchsia_inspect::Inspector::default(),
                1,
            ));
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));

            let count_of_contact = event.injector_contacts.len();
            let input_event = input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(event),
                device_descriptor: make_touchpad_descriptor(vec![(
                    fidl_input_report::Unit { type_: UnitType::Meters, exponent: -3 },
                    fidl_input_report::Unit { type_: UnitType::Meters, exponent: -3 },
                )]),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            };

            arena.handle_input_event(input_event).await;
            assert_eq!(
                contender.get_last_touchpad_event().unwrap().contacts.len(),
                count_of_contact
            );
        }

        #[test_case(
            vec![(
                fidl_input_report::Unit{ type_: UnitType::None, exponent: -6 },
                fidl_input_report::Unit{ type_: UnitType::None, exponent: -6 },
            )],
            vec![];
            "both units unspecified")]
        #[test_case(
            vec![(
                fidl_input_report::Unit{ type_: UnitType::None, exponent: -6 },
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
            )],
            vec![];
            "x unit unspecified")]
        #[test_case(
            vec![(
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
                fidl_input_report::Unit{ type_: UnitType::None, exponent: -6 },
            )],
            vec![];
            "y unit unspecified")]
        #[test_case(
            vec![(
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -3 },
                fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
            )],
            vec![];
            "mismatched exponents")]
        #[test_case(
            vec![
                (
                    fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -3 },
                    fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -3 },
                ),
                (
                    fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
                    fidl_input_report::Unit{ type_: UnitType::Meters, exponent: -6 },
                ),
            ],
            vec![];
            "unequal divisors")]
        #[fuchsia::test(allow_stalls = false)]
        async fn skips_contender_on_bad_descriptor(
            contact_position_units: Vec<(fidl_input_report::Unit, fidl_input_report::Unit)>,
            positions: Vec<Position>,
        ) {
            let contender = Box::new(StubContender::new());
            let contender_factory = ContenderFactoryOnceOrPanic::new(vec![contender.clone()]);
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(contender_factory),
                &fuchsia_inspect::Inspector::default(),
                1,
            ));
            contender.set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            arena
                .handle_input_event(make_unhandled_touchpad_event_with_positions(
                    contact_position_units,
                    positions,
                ))
                .await;
            assert_eq!(contender.calls_received(), 0);
        }
    }

    mod inspect {
        use super::super::{
            Contender, ContenderFactory, DetailedReasonFloat, DetailedReasonInt,
            DetailedReasonUint, EndGestureEvent, ExamineEventResult, GestureArena, InputHandler,
            MouseEvent, ProcessBufferedEventsResult, ProcessNewEventResult, Reason,
            RecognizedGesture, TouchpadEvent, args,
        };
        use super::utils::{
            ContenderFactoryOnceOrPanic, StubContender, StubMatchedContender, StubWinner,
            make_touchpad_descriptor, make_unhandled_keyboard_event, make_unhandled_mouse_event,
            make_unhandled_touchpad_event,
        };
        use crate::{Position, Size, input_device, keyboard_binding, mouse_binding, touch_binding};
        use assert_matches::assert_matches;
        use maplit::hashset;
        use std::rc::Rc;
        use test_case::test_case;
        use {fidl_fuchsia_input_report as fidl_input_report, fuchsia_async as fasync};

        struct EmptyContenderFactory {}

        impl ContenderFactory for EmptyContenderFactory {
            fn make_contenders(&self) -> Vec<Box<dyn crate::gestures::gesture_arena::Contender>> {
                vec![]
            }
        }

        #[fuchsia::test]
        async fn gesture_arena_initialized_with_inspect_node() {
            let inspector = fuchsia_inspect::Inspector::default();
            let fake_handlers_node = inspector.root().create_child("input_handlers_node");
            let _handler = GestureArena::new_internal(
                Box::new(EmptyContenderFactory {}),
                &inspector.root(),
                2,
                &fake_handlers_node,
            );
            diagnostics_assertions::assert_data_tree!(inspector, root: {
                gestures_event_log: {},
                input_handlers_node: {
                    gesture_arena: {
                        events_received_count: 0u64,
                        events_handled_count: 0u64,
                        last_received_timestamp_ns: 0u64,
                        "fuchsia.inspect.Health": {
                            status: "STARTING_UP",
                            // Timestamp value is unpredictable and not relevant in this context,
                            // so we only assert that the property is present.
                            start_timestamp_nanos: diagnostics_assertions::AnyProperty
                        },
                    }
                }
            });
        }

        #[fasync::run_singlethreaded(test)]
        async fn gesture_arena_inspect_counts_events() {
            let inspector = fuchsia_inspect::Inspector::default();
            let fake_handlers_node = inspector.root().create_child("input_handlers_node");
            let arena = Rc::new(GestureArena::new_internal(
                Box::new(EmptyContenderFactory {}),
                &inspector.root(),
                1,
                &fake_handlers_node,
            ));

            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;
            arena.clone().handle_input_event(make_unhandled_mouse_event()).await;
            arena.clone().handle_input_event(make_unhandled_keyboard_event()).await;
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            diagnostics_assertions::assert_data_tree!(inspector, root: contains {
                input_handlers_node: {
                    gesture_arena: {
                        events_received_count: 2u64,
                        events_handled_count: 0u64,
                        last_received_timestamp_ns: 0u64,
                        "fuchsia.inspect.Health": {
                            status: "STARTING_UP",
                            // Timestamp value is unpredictable and not relevant in this context,
                            // so we only assert that the property is present.
                            start_timestamp_nanos: diagnostics_assertions::AnyProperty
                        },
                    }
                }
            });
        }

        #[fuchsia::test]
        fn logs_to_inspect() {
            let mut executor = fasync::TestExecutor::new_with_fake_time();
            let basic_mismatch_contender = Box::new(StubContender::new());
            let detailed_uint_mismatch_contender = Box::new(StubContender::new());
            let detailed_float_mismatch_contender = Box::new(StubContender::new());
            let detailed_int_mismatch_contender = Box::new(StubContender::new());
            let gesture_matching_contender = Box::new(StubContender::new());
            basic_mismatch_contender
                .set_next_result(ExamineEventResult::Mismatch(Reason::Basic("some reason")));
            detailed_uint_mismatch_contender.set_next_result(ExamineEventResult::Mismatch(
                Reason::DetailedUint(DetailedReasonUint {
                    criterion: "num_goats_teleported",
                    min: Some(10),
                    max: Some(30),
                    actual: 42,
                }),
            ));
            detailed_float_mismatch_contender.set_next_result(ExamineEventResult::Mismatch(
                Reason::DetailedFloat(DetailedReasonFloat {
                    criterion: "teleportation_distance_kilometers",
                    min: Some(10.125),
                    max: Some(30.5),
                    actual: 42.0,
                }),
            ));
            detailed_int_mismatch_contender.set_next_result(ExamineEventResult::Mismatch(
                Reason::DetailedInt(DetailedReasonInt {
                    criterion: "budget_surplus_trillions",
                    min: Some(-10),
                    max: Some(1),
                    actual: -42,
                }),
            ));

            let inspector = fuchsia_inspect::Inspector::default();
            let contender_factory = Box::new(ContenderFactoryOnceOrPanic::new(vec![
                basic_mismatch_contender,
                detailed_uint_mismatch_contender,
                detailed_float_mismatch_contender,
                detailed_int_mismatch_contender,
                gesture_matching_contender.clone(),
            ]));
            let arena = Rc::new(GestureArena::new_for_test(contender_factory, &inspector, 100));
            let touchpad_descriptor = input_device::InputDeviceDescriptor::Touchpad(
                touch_binding::TouchpadDeviceDescriptor {
                    device_id: 1,
                    contacts: vec![touch_binding::ContactDeviceDescriptor {
                        x_range: fidl_input_report::Range { min: 0, max: 10_000 },
                        y_range: fidl_input_report::Range { min: 0, max: 10_000 },
                        x_unit: fidl_input_report::Unit {
                            // Use millimeters to avoid floating-point rounding.
                            type_: fidl_input_report::UnitType::Meters,
                            exponent: -3,
                        },
                        y_unit: fidl_input_report::Unit {
                            // Use millimeters to avoid floating-point rounding.
                            type_: fidl_input_report::UnitType::Meters,
                            exponent: -3,
                        },
                        pressure_range: None,
                        width_range: Some(fidl_input_report::Range { min: 0, max: 10_000 }),
                        height_range: Some(fidl_input_report::Range { min: 0, max: 10_000 }),
                    }],
                },
            );
            let keyboard_descriptor = input_device::InputDeviceDescriptor::Keyboard(
                keyboard_binding::KeyboardDeviceDescriptor {
                    device_id: 2,
                    device_information: fidl_fuchsia_input_report::DeviceInformation {
                        vendor_id: Some(0),
                        product_id: Some(0),
                        version: Some(0),
                        polling_rate: Some(0),
                        ..Default::default()
                    },
                    keys: vec![fidl_fuchsia_input::Key::A, fidl_fuchsia_input::Key::B],
                },
            );

            // Process a touchpad event without width/height.
            let mut handle_event_fut = arena.clone().handle_input_event(input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![
                            touch_binding::TouchContact {
                                id: 1u32,
                                position: Position { x: 2.0, y: 3.0 },
                                contact_size: None,
                                pressure: None,
                            },
                            touch_binding::TouchContact {
                                id: 2u32,
                                position: Position { x: 40.0, y: 50.0 },
                                contact_size: None,
                                pressure: None,
                            },
                        ],
                        pressed_buttons: hashset! {1},
                    },
                ),
                device_descriptor: touchpad_descriptor.clone(),
                event_time: zx::MonotonicInstant::from_nanos(12_300),
                trace_id: None,
                handled: input_device::Handled::No,
            });
            executor.set_fake_time(fasync::MonotonicInstant::from_nanos(10_000_000));
            gesture_matching_contender
                .set_next_result(ExamineEventResult::Contender(gesture_matching_contender.clone()));
            assert_matches!(
                executor.run_until_stalled(&mut handle_event_fut),
                std::task::Poll::Ready(_)
            );

            // Process a handled key event.
            let mut handle_event_fut = arena.clone().handle_input_event(input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Keyboard(
                    keyboard_binding::KeyboardEvent::new(
                        fidl_fuchsia_input::Key::A,
                        fidl_fuchsia_ui_input3::KeyEventType::Pressed,
                    ),
                ),
                device_descriptor: keyboard_descriptor.clone(),
                event_time: zx::MonotonicInstant::from_nanos(11_000_000),
                trace_id: None,
                handled: input_device::Handled::Yes,
            });
            executor.set_fake_time(fasync::MonotonicInstant::from_nanos(12_000_000));
            assert_matches!(
                executor.run_until_stalled(&mut handle_event_fut),
                std::task::Poll::Ready(_)
            );

            // Process an unhandled key event.
            let mut handle_event_fut = arena.clone().handle_input_event(input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Keyboard(
                    keyboard_binding::KeyboardEvent::new(
                        fidl_fuchsia_input::Key::B,
                        fidl_fuchsia_ui_input3::KeyEventType::Pressed,
                    ),
                ),
                device_descriptor: keyboard_descriptor,
                event_time: zx::MonotonicInstant::from_nanos(13_000_000),
                trace_id: None,
                handled: input_device::Handled::No,
            });
            executor.set_fake_time(fasync::MonotonicInstant::from_nanos(14_000_000));
            assert_matches!(
                executor.run_until_stalled(&mut handle_event_fut),
                std::task::Poll::Ready(_)
            );

            // Process a touchpad event with width/height, and end the contest with a match.
            let mut handle_event_fut = arena.clone().handle_input_event(input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![touch_binding::TouchContact {
                            id: 1u32,
                            position: Position { x: 2.0, y: 3.0 },
                            contact_size: Some(Size { width: 3.0, height: 4.0 }),
                            pressure: None,
                        }],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: touchpad_descriptor.clone(),
                event_time: zx::MonotonicInstant::from_nanos(18_000_000),
                trace_id: None,
                handled: input_device::Handled::No,
            });
            let matched_contender = Box::new(StubMatchedContender::new());
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: None,
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            gesture_matching_contender
                .set_next_result(ExamineEventResult::MatchedContender(matched_contender));
            executor.set_fake_time(fasync::MonotonicInstant::from_nanos(19_000_000));
            assert_matches!(
                executor.run_until_stalled(&mut handle_event_fut),
                std::task::Poll::Ready(_)
            );

            // Uncomment this block to generate a new example for the
            // documentation found at the bottom of this file.
            /*
            {
                use fuchsia_inspect::hierarchy::testing::JsonGetter;
                println!("{}", inspector.get_pretty_json());
            }
            */

            diagnostics_assertions::assert_data_tree!(@executor executor, inspector, root: {
                gestures_event_log: {
                    "0": {
                        touchpad_event: {
                            driver_monotonic_nanos: 12_300i64,
                            entry_latency_micros: 9987i64,  // 10_000_000 - 12_300 = 9_987_700 nsec
                            pressed_buttons: vec![ 1u64 ],
                            contacts: {
                                "1": {
                                    pos_x_mm: 2.0,
                                    pos_y_mm: 3.0,
                                },
                                "2": {
                                    pos_x_mm: 40.0,
                                    pos_y_mm: 50.0,
                                },
                            },
                            filtered_palm_contacts: {},
                        }
                    },
                    "1": {
                        mismatch_event: {
                            contender: "utils::StubContender",
                            reason: "some reason",
                        }
                    },
                    "2": {
                        mismatch_event: {
                            contender: "utils::StubContender",
                            criterion: "num_goats_teleported",
                            min_allowed: 10u64,
                            max_allowed: 30u64,
                            actual: 42u64,
                        }
                    },
                    "3": {
                        mismatch_event: {
                            contender: "utils::StubContender",
                            criterion: "teleportation_distance_kilometers",
                            min_allowed: 10.125,
                            max_allowed: 30.5,
                            actual: 42.0,
                        }
                    },
                    "4": {
                        mismatch_event: {
                            contender: "utils::StubContender",
                            criterion: "budget_surplus_trillions",
                            min_allowed: -10i64,
                            max_allowed: 1i64,
                            actual: -42i64,
                        }
                    },
                    "5": {
                        key_event: {
                            driver_monotonic_nanos: 11_000_000i64,
                            entry_latency_micros: 1_000i64,  // 12_000_000 - 11_000_000 = 1_000_00 nsec
                        }
                    },
                    "6": {
                        key_event: {
                            driver_monotonic_nanos: 13_000_000i64,
                            entry_latency_micros: 1_000i64,  // 14_000_000 - 13_000_000 = 1_000_00 nsec
                        }
                    },
                    "7": {
                        touchpad_event: {
                            driver_monotonic_nanos: 18_000_000i64,
                            entry_latency_micros: 1_000i64,  // 19_000_000 - 18_000_000 = 1_000_00 nsec
                            pressed_buttons: Vec::<u64>::new(),
                            contacts: {
                                "1": {
                                    pos_x_mm: 2.0,
                                    pos_y_mm: 3.0,
                                    width_mm: 3.0,
                                    height_mm: 4.0,
                                },
                            },
                            filtered_palm_contacts: {},
                        }
                    },
                    "8": {
                        gesture_start: {
                          gesture_name: "motion",
                          latency_event_count: 1u64,
                          latency_micros: 17_987i64,  // 18_000_000 - 12_300 = 17_987_700
                        }
                    },
                    "9": {
                        gesture_end: {
                          gesture_name: "motion",
                          contender: "utils::StubMatchedContender",
                          event_count: 0u64,
                          duration_micros: 0i64,
                          reason: "discrete-recognizer",
                        }
                    }
                }
            });
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn negative_matching_latency_is_logged_correctly() {
            let inspector = fuchsia_inspect::Inspector::default();
            let gesture_matching_contender = Box::new(StubContender::new());
            let contender_factory = Box::new(ContenderFactoryOnceOrPanic::new(vec![
                gesture_matching_contender.clone(),
            ]));
            let arena = Rc::new(GestureArena::new_for_test(contender_factory, &inspector, 100));

            gesture_matching_contender
                .set_next_result(ExamineEventResult::Contender(gesture_matching_contender.clone()));
            arena
                .clone()
                .handle_input_event(input_device::InputEvent {
                    event_time: zx::MonotonicInstant::from_nanos(15_000),
                    ..make_unhandled_touchpad_event()
                })
                .await;

            let matched_contender = Box::new(StubMatchedContender::new());
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: None,
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            gesture_matching_contender
                .set_next_result(ExamineEventResult::MatchedContender(matched_contender));
            arena
                .clone()
                .handle_input_event(input_device::InputEvent {
                    event_time: zx::MonotonicInstant::from_nanos(6_000),
                    ..make_unhandled_touchpad_event()
                })
                .await;

            diagnostics_assertions::assert_data_tree!(inspector, root: {
                gestures_event_log: {
                    "0": contains {},
                    "1": contains {},
                    "2": {
                        gesture_start: {
                          gesture_name: diagnostics_assertions::AnyProperty,
                          latency_event_count: 1u64,
                          latency_micros: -9i64,
                        }
                    },
                    "3": {
                        gesture_end: contains {}
                    },
                }
            })
        }

        struct ContenderFactoryOnceThenEmpty {
            contenders: std::cell::Cell<Vec<Box<dyn Contender>>>,
        }

        impl ContenderFactory for ContenderFactoryOnceThenEmpty {
            fn make_contenders(&self) -> Vec<Box<dyn Contender>> {
                self.contenders.take()
            }
        }

        #[test_case(EndGestureEvent::NoEvent; "end_gesture_no_event")]
        #[test_case(EndGestureEvent::UnconsumedEvent(TouchpadEvent {
            timestamp: zx::MonotonicInstant::ZERO,
            pressed_buttons: vec![],
            contacts: vec![],
            filtered_palm_contacts: vec![],
        }); "end_gesture_unconsumed_event")]
        #[test_case(EndGestureEvent::GeneratedEvent(MouseEvent {
            timestamp: zx::MonotonicInstant::ZERO,
            mouse_data: mouse_binding::MouseEvent {
                location: mouse_binding::MouseLocation::Relative(
                    mouse_binding::RelativeLocation {
                        millimeters: Position::zero(),
                    },
                ),
                wheel_delta_v: None,
                wheel_delta_h: None,
                phase: mouse_binding::MousePhase::Move,
                affected_buttons: hashset! {},
                pressed_buttons: hashset! {},
                is_precision_scroll: None,
            },
        }); "end_gesture_generated_event")]
        #[fuchsia::test(allow_stalls = false)]
        async fn multi_event_gesture_is_logged_correctly(end_gesture_event: EndGestureEvent) {
            // Set up the arena, and send the first touchpad event.
            let inspector = fuchsia_inspect::Inspector::default();
            let matching_contender = Box::new(StubContender::new());
            let arena = Rc::new(GestureArena::new_for_test(
                // In the `UnconsumedEvent` case, the gesture arena will try to start
                // a new contest. Hence, `ContenderFactoryOnceOrPanic` isn't appropriate
                // here. Nor is `ContenderFactoryOnceOrWarn`, since that would generate
                // a spurious warning, making this test harder to debug.
                Box::new(ContenderFactoryOnceThenEmpty {
                    contenders: std::cell::Cell::new(vec![matching_contender.clone()]),
                }),
                &inspector,
                100,
            ));
            matching_contender
                .set_next_result(ExamineEventResult::Contender(matching_contender.clone()));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Plumb things up for the next event to trigger the start of a multi-event gesture,
            // and send the next event.
            let matched_contender = Box::new(StubMatchedContender::new());
            let winner = Box::new(StubWinner::new());
            matching_contender
                .set_next_result(ExamineEventResult::MatchedContender(matched_contender.clone()));
            matched_contender.set_next_process_buffered_events_result(
                ProcessBufferedEventsResult {
                    generated_events: vec![],
                    winner: Some(winner.clone()),
                    recognized_gesture: RecognizedGesture::Motion,
                },
            );
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(None, winner.clone()));
            arena
                .clone()
                .handle_input_event(input_device::InputEvent {
                    device_event: input_device::InputDeviceEvent::Touchpad(
                        touch_binding::TouchpadEvent {
                            injector_contacts: vec![],
                            pressed_buttons: hashset! {},
                        },
                    ),
                    device_descriptor: make_touchpad_descriptor(),
                    event_time: zx::MonotonicInstant::from_nanos(123_000),
                    trace_id: None,
                    handled: input_device::Handled::No,
                })
                .await;

            // Send another event as part of the gesture.
            winner.set_next_result(ProcessNewEventResult::ContinueGesture(None, winner.clone()));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await;

            // Plumb things to end the gesture on the next event, and send that event.
            winner.set_next_result(ProcessNewEventResult::EndGesture(
                end_gesture_event,
                Reason::DetailedUint(DetailedReasonUint {
                    criterion: "num_goats_teleported",
                    min: Some(10),
                    max: Some(30),
                    actual: 42,
                }),
            ));
            arena
                .clone()
                .handle_input_event(input_device::InputEvent {
                    device_event: input_device::InputDeviceEvent::Touchpad(
                        touch_binding::TouchpadEvent {
                            injector_contacts: vec![],
                            pressed_buttons: hashset! {},
                        },
                    ),
                    device_descriptor: make_touchpad_descriptor(),
                    event_time: zx::MonotonicInstant::from_nanos(456_000),
                    trace_id: None,
                    handled: input_device::Handled::No,
                })
                .await;

            diagnostics_assertions::assert_data_tree!(inspector, root: {
                gestures_event_log: {
                    "0": { touchpad_event: contains {} },
                    "1": { touchpad_event: contains {} },
                    "2": { gesture_start: contains {} },
                    "3": { touchpad_event: contains {} },
                    "4": { touchpad_event: contains {} },
                    "5": {
                        gesture_end: contains {
                            gesture_name: "motion",
                            contender: "utils::StubWinner",
                            criterion: "num_goats_teleported",
                            min_allowed: 10u64,
                            max_allowed: 30u64,
                            actual: 42u64,
                            duration_micros: 333i64, // 456_000 - 123_000 = 333_000 nsec
                            event_count: 2u64,
                        },
                    },
                }
            })
        }

        #[fuchsia::test(allow_stalls = false)]
        async fn retains_latest_events_up_to_cap() {
            let inspector = fuchsia_inspect::Inspector::default();
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(EmptyContenderFactory {}),
                &inspector,
                2,
            ));
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await; // 0
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await; // 1
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await; // 2
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await; // 3
            arena.clone().handle_input_event(make_unhandled_touchpad_event()).await; // 4
            diagnostics_assertions::assert_data_tree!(inspector, root: {
                gestures_event_log: {
                    "3": contains {},
                    "4": contains {},
                }
            })
        }

        #[fuchsia::test]
        fn retains_palm_contacts() {
            let mut executor = fasync::TestExecutor::new_with_fake_time();
            let inspector = fuchsia_inspect::Inspector::default();
            let arena = Rc::new(GestureArena::new_for_test(
                Box::new(EmptyContenderFactory {}),
                &inspector,
                2,
            ));
            let mut handle_event_fut = arena.clone().handle_input_event(input_device::InputEvent {
                device_event: input_device::InputDeviceEvent::Touchpad(
                    touch_binding::TouchpadEvent {
                        injector_contacts: vec![touch_binding::TouchContact {
                            id: 1u32,
                            position: Position { x: 2_000.0, y: 1_000.0 },
                            contact_size: Some(Size {
                                width: (args::MIN_PALM_SIZE_MM + 0.1) * 1_000.0,
                                height: 4_000.0,
                            }),
                            pressure: None,
                        }],
                        pressed_buttons: hashset! {},
                    },
                ),
                device_descriptor: make_touchpad_descriptor(),
                event_time: zx::MonotonicInstant::ZERO,
                trace_id: None,
                handled: input_device::Handled::No,
            });
            executor.set_fake_time(fasync::MonotonicInstant::from_nanos(1_000_000));
            assert_matches!(
                executor.run_until_stalled(&mut handle_event_fut),
                std::task::Poll::Ready(_)
            );
            diagnostics_assertions::assert_data_tree!(@executor executor, inspector, root: {
                gestures_event_log: {
                    "0": {
                        touchpad_event: {
                            driver_monotonic_nanos: 0i64,
                            entry_latency_micros: 1_000i64,
                            pressed_buttons: Vec::<u64>::new(),
                            contacts: {},
                            filtered_palm_contacts: {
                                "1": {
                                    pos_x_mm: 2.0,
                                    pos_y_mm: 1.0,
                                    width_mm: (args::MIN_PALM_SIZE_MM + 0.1) as f64,
                                    height_mm: 4.0,
                                },
                            },
                        },
                    },
                },
            });
        }
    }
}

// Example JSON dump of inspect tree generated by `gesture_arena`, from a unit test:
//
// ```json
// {
//   "root": {
//     "gestures_event_log": {
//       "0": {
//         "touchpad_event": {
//           "driver_monotonic_nanos": 12300,
//           "entry_latency_micros": 9987,
//           "pressed_buttons": [
//             1
//           ],
//           "contacts": {
//             "1": {
//               "pos_x_mm": 2.0,
//               "pos_y_mm": 3.0
//             },
//             "2": {
//               "pos_x_mm": 40.0,
//               "pos_y_mm": 50.0
//             }
//           }
//         }
//       },
//       "1": {
//         "mismatch_event": {
//           "contender": "utils::StubContender",
//           "reason": "some reason"
//         }
//       },
//       "2": {
//         "mismatch_event": {
//           "actual": 42,
//           "contender": "utils::StubContender",
//           "criterion": "num_goats_teleported",
//           "max_allowed": 30,
//           "min_allowed": 10
//         }
//       },
//       "3": {
//         "mismatch_event": {
//           "actual": 42.0,
//           "contender": "utils::StubContender",
//           "criterion": "teleportation_distance_kilometers",
//           "max_allowed": 30.5,
//           "min_allowed": 10.125
//         }
//       },
//       "4": {
//         "mismatch_event": {
//           "actual": -42,
//           "contender": "utils::StubContender",
//           "criterion": "budget_surplus_trillions",
//           "max_allowed": 1,
//           "min_allowed": -10
//         }
//       },
//       "5": {
//         "key_event": {
//           "driver_monotonic_nanos": 11000000,
//           "entry_latency_micros": 1000
//         }
//       },
//       "6": {
//         "key_event": {
//           "driver_monotonic_nanos": 13000000,
//           "entry_latency_micros": 1000
//         }
//       },
//       "7": {
//         "touchpad_event": {
//           "driver_monotonic_nanos": 18000000,
//           "entry_latency_micros": 1000,
//           "pressed_buttons": [],
//           "contacts": {
//             "1": {
//               "height_mm": 4.0,
//               "pos_x_mm": 2.0,
//               "pos_y_mm": 3.0,
//               "width_mm": 3.0
//             }
//           }
//         }
//       },
//       "8": {
//         "gesture_start": {
//           "gesture_name": "click",
//           "latency_event_count": 1,
//           "latency_micros": 17987
//         }
//       },
//       "9": {
//         "gesture_end": {
//           "contender": "utils::StubMatchedContender",
//           "duration_micros": 0,
//           "event_count": 0,
//           "gesture_name": "click",
//           "reason": "discrete-recognizer"
//         }
//       }
//     }
//   }
// }
// ```

// Example `iquery` excerpt from a live device:
//
// ```json5
// core/ui/scene_manager:
//   metadata:
//     filename = fuchsia.inspect.Tree
//     component_url = fuchsia-pkg://fuchsia.com/scene_manager#meta/scene_manager.cm
//     timestamp = 375999103371
//   payload:
//     root:
//       fuchsia.inspect.Stats:
//         allocated_blocks = 9998
//         current_size = 163840
//         deallocated_blocks = 0
//         failed_allocations = 0
//         maximum_size = 307200
//         total_dynamic_children = 1
//       input_pipeline:
//         gestures_event_log:
//           0:
//             key_event:
//               driver_monotonic_nanos = 297873226402
//               entry_latency_micros = 32908
//           1:
//             key_event:
//               driver_monotonic_nanos = 297955554861
//               entry_latency_micros = 1403
//           /* ...many entries omitted... */
//           150:
//             touchpad_event:
//               driver_monotonic_nanos = 361816423302
//               entry_latency_micros = 14432
//               pressed_buttons = []
//               contacts:
//                 0:
//                   height_mm = 2.5840000
//                   pos_x_mm = 26.528000
//                   pos_y_mm = 23.712999
//                   width_mm = 2.9530000
//           /* mismatches on u64 properties */
//           151:
//             mismatch_event:
//               actual = 0
//               contender = one_finger_drag::InitialContender
//               criterion = num_pressed_buttons
//               max_allowed = 1
//               min_allowed = 1
//           152:
//             mismatch_event:
//               actual = 1
//               contender = scroll::InitialContender
//               criterion = num_contacts
//               max_allowed = 2
//               min_allowed = 2
//           /* ... many entries omitted ... */
//           159:
//             touchpad_event:
//               driver_monotonic_nanos = 361871136901
//               entry_latency_micros = 4745
//               pressed_buttons = []
//               contacts:
//                 0:
//                   height_mm = 2.5840000
//                   pos_x_mm = 27.162001
//                   pos_y_mm = 24.061001
//                   width_mm = 2.9530000
//           /* mismatches on float properties */
//           160:
//             mismatch_event:
//               actual = 0.723230
//               contender = click::UnpressedContender
//               criterion = displacement_mm
//               max_allowed = 0.500000
//           161:
//             mismatch_event:
//               actual = 0.723230
//               contender = primary_tap::FingerContactContender
//               criterion = displacement_mm
//               max_allowed = 0.500000
//           162:
//             mismatch_event:
//               actual = 0.723230
//               contender = secondary_tap::OneFingerContactContender
//               criterion = displacement_mm
//               max_allowed = 0.500000
//           /* gesture start */
//           163:
//             gesture_start:
//               gesture_name = motion
//               latency_event_count = 7
//               latency_micros = 54713
//           /* ... many entries omitted ... */
//           295:
//             touchpad_event:
//               driver_monotonic_nanos = 362903529428
//               entry_latency_micros = 3603
//               pressed_buttons = []
//               contacts:
//           /* gesture end */
//           296:
//             gesture_end:
//               actual = 0
//               contender = motion::Winner
//               criterion = num_contacts
//               duration_micros = 1032392
//               event_count = 132
//               gesture_name = motion
//               max_allowed = 1
//               min_allowed = 1
//           /* ... many entries omitted ... */
//           596:
//             touchpad_event:
//               driver_monotonic_nanos = 370902306135
//               entry_latency_micros = 4630
//               pressed_buttons = []
//               contacts:
//                 0:
//                   height_mm = 2.5840000
//                   pos_x_mm = 76.887001
//                   pos_y_mm = 25.962999
//                   width_mm = 2.9530000
//           /* ... many entries omitted ... */
//           752:
//             touchpad_event:
//               driver_monotonic_nanos = 372106779670
//               entry_latency_micros = 4607
//               pressed_buttons = []
//               contacts:
//                 0:
//                   height_mm = 3.2300000
//                   pos_x_mm = 76.949997
//                   pos_y_mm = 26.184999
//                   width_mm = 2.9530000
//           /* mismatches on i64 properties */
//           753:
//             mismatch_event:
//               actual = 1204473
//               contender = primary_tap::FingerContactContender
//               criterion = elapsed_time_micros
//               max_allowed = 1200000
//           754:
//             mismatch_event:
//               actual = 1204473
//               contender = secondary_tap::OneFingerContactContender
//               criterion = elapsed_time_micros
//               max_allowed = 1200000
// ```