fidl_next_codec/
encoder.rs

1// Copyright 2024 The Fuchsia Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5//! The core [`Encoder`] trait.
6
7use core::marker::PhantomData;
8use core::mem::MaybeUninit;
9use core::slice::from_raw_parts;
10
11use crate::{Chunk, Encode, EncodeError, Wire, WireU64, CHUNK_SIZE};
12
13/// An encoder for FIDL handles (internal).
14pub trait InternalHandleEncoder {
15    /// Returns the number of handles written to the encoder.
16    ///
17    /// This method exposes details about Fuchsia resources that plain old FIDL shouldn't need to
18    /// know about. Do not use this method outside of this crate.
19    #[doc(hidden)]
20    fn __internal_handle_count(&self) -> usize;
21}
22
23/// An encoder for FIDL messages.
24pub trait Encoder: InternalHandleEncoder {
25    /// Returns the number of bytes written to the encoder.
26    fn bytes_written(&self) -> usize;
27
28    /// Writes zeroed bytes to the end of the encoder.
29    ///
30    /// Additional bytes are written to pad the written data to a multiple of [`CHUNK_SIZE`].
31    fn write_zeroes(&mut self, len: usize);
32
33    /// Copies bytes to the end of the encoder.
34    ///
35    /// Additional bytes are written to pad the written data to a multiple of [`CHUNK_SIZE`].
36    fn write(&mut self, bytes: &[u8]);
37
38    /// Rewrites bytes at a position in the encoder.
39    fn rewrite(&mut self, pos: usize, bytes: &[u8]);
40}
41
42impl InternalHandleEncoder for Vec<Chunk> {
43    #[inline]
44    fn __internal_handle_count(&self) -> usize {
45        0
46    }
47}
48
49impl Encoder for Vec<Chunk> {
50    #[inline]
51    fn bytes_written(&self) -> usize {
52        self.len() * CHUNK_SIZE
53    }
54
55    #[inline]
56    fn write_zeroes(&mut self, len: usize) {
57        let count = len.div_ceil(CHUNK_SIZE);
58        self.reserve(count);
59        let ptr = unsafe { self.as_mut_ptr().add(self.len()) };
60        unsafe {
61            ptr.write_bytes(0, count);
62        }
63        unsafe {
64            self.set_len(self.len() + count);
65        }
66    }
67
68    #[inline]
69    fn write(&mut self, bytes: &[u8]) {
70        if bytes.is_empty() {
71            return;
72        }
73
74        let count = bytes.len().div_ceil(CHUNK_SIZE);
75        self.reserve(count);
76
77        // Zero out the last chunk
78        unsafe {
79            self.as_mut_ptr().add(self.len() + count - 1).write(WireU64(0));
80        }
81        let ptr = unsafe { self.as_mut_ptr().add(self.len()).cast::<u8>() };
82
83        // Copy all the bytes
84        unsafe {
85            ptr.copy_from_nonoverlapping(bytes.as_ptr(), bytes.len());
86        }
87
88        // Set the new length
89        unsafe {
90            self.set_len(self.len() + count);
91        }
92    }
93
94    #[inline]
95    fn rewrite(&mut self, pos: usize, bytes: &[u8]) {
96        assert!(pos + bytes.len() <= self.bytes_written());
97
98        let ptr = unsafe { self.as_mut_ptr().cast::<u8>().add(pos) };
99        unsafe {
100            ptr.copy_from_nonoverlapping(bytes.as_ptr(), bytes.len());
101        }
102    }
103}
104
105/// Extension methods for [`Encoder`].
106pub trait EncoderExt {
107    /// Pre-allocates space for a slice of elements.
108    fn preallocate<T>(&mut self, len: usize) -> Preallocated<'_, Self, T>;
109
110    /// Encodes an iterator of elements.
111    ///
112    /// Returns `Err` if encoding failed.
113    fn encode_next_iter<T: Encode<Self>>(
114        &mut self,
115        values: impl ExactSizeIterator<Item = T>,
116    ) -> Result<(), EncodeError>;
117
118    /// Encodes a value.
119    ///
120    /// Returns `Err` if encoding failed.
121    fn encode_next<T: Encode<Self>>(&mut self, value: T) -> Result<(), EncodeError>;
122}
123
124impl<E: Encoder + ?Sized> EncoderExt for E {
125    fn preallocate<T>(&mut self, len: usize) -> Preallocated<'_, Self, T> {
126        let pos = self.bytes_written();
127
128        // Zero out the next `count` bytes
129        self.write_zeroes(len * size_of::<T>());
130
131        Preallocated {
132            encoder: self,
133            pos,
134            #[cfg(debug_assertions)]
135            remaining: len,
136            _phantom: PhantomData,
137        }
138    }
139
140    fn encode_next_iter<T: Encode<Self>>(
141        &mut self,
142        values: impl ExactSizeIterator<Item = T>,
143    ) -> Result<(), EncodeError> {
144        let mut outputs = self.preallocate::<T::Encoded>(values.len());
145
146        let mut out = MaybeUninit::<T::Encoded>::uninit();
147        <T::Encoded as Wire>::zero_padding(&mut out);
148        for value in values {
149            value.encode(outputs.encoder, &mut out)?;
150            unsafe {
151                outputs.write_next(out.assume_init_ref());
152            }
153        }
154
155        Ok(())
156    }
157
158    fn encode_next<T: Encode<Self>>(&mut self, value: T) -> Result<(), EncodeError> {
159        self.encode_next_iter(core::iter::once(value))
160    }
161}
162
163/// A pre-allocated slice of elements
164pub struct Preallocated<'a, E: ?Sized, T> {
165    /// The encoder.
166    pub encoder: &'a mut E,
167    pos: usize,
168    #[cfg(debug_assertions)]
169    remaining: usize,
170    _phantom: PhantomData<T>,
171}
172
173impl<E: Encoder + ?Sized, T> Preallocated<'_, E, T> {
174    /// Writes into the next pre-allocated slot in the encoder.
175    ///
176    /// # Safety
177    ///
178    /// All of the bytes of `value` must be initialized, including padding.
179    pub unsafe fn write_next(&mut self, value: &T) {
180        #[cfg(debug_assertions)]
181        {
182            assert!(self.remaining > 0, "attemped to write more slots than preallocated");
183            self.remaining -= 1;
184        }
185
186        let bytes_ptr = (value as *const T).cast::<u8>();
187        let bytes = unsafe { from_raw_parts(bytes_ptr, size_of::<T>()) };
188        self.encoder.rewrite(self.pos, bytes);
189        self.pos += size_of::<T>();
190    }
191}
fidl_next_codec/encoder.rs

fidl_next_codec/
encoder.rs
