cadmpeg_core/decode/
arena.rs

1// SPDX-License-Identifier: Apache-2.0
2//! Append-only byte store with stable slice addresses.
3
4use std::cell::RefCell;
5
6/// Owns stable byte buffers allocated during a decode.
7#[derive(Debug, Default)]
8pub struct DecodeArena {
9    buffers: RefCell<Vec<Box<[u8]>>>,
10}
11
12impl DecodeArena {
13    /// Creates an empty arena.
14    pub fn new() -> Self {
15        Self::default()
16    }
17
18    /// Stores `bytes` and returns a borrow valid for the arena's lifetime.
19    ///
20    /// The returned slice is stable: later `alloc` calls never invalidate it.
21    pub fn alloc(&self, bytes: Box<[u8]>) -> &[u8] {
22        let mut buffers = self.buffers.borrow_mut();
23        buffers.push(bytes);
24        let slice: &[u8] = buffers.last().expect("a buffer was just pushed").as_ref();
25        // SAFETY: three legs keep the returned `&[u8]` valid:
26        // 1. Each buffer is a `Box<[u8]>`; its heap address is stable when
27        //    the outer `Vec` reallocates (only the box pointers move).
28        // 2. `DecodeArena` is `!Sync` (`RefCell`), so no concurrent mutation
29        //    can invalidate or alias the stored bytes across threads.
30        // 3. The returned slice is tied to `&self`; it cannot outlive the
31        //    arena that owns the boxes.
32        // The bytes are never mutated after being stored (the arena exposes
33        // no mutation), so aliasing `&[u8]` borrows do not conflict.
34        unsafe { std::slice::from_raw_parts(slice.as_ptr(), slice.len()) }
35    }
36}
37
38#[cfg(test)]
39mod tests {
40    use super::DecodeArena;
41
42    fn buffer(index: usize) -> Box<[u8]> {
43        let len = (index % 7) + 1;
44        vec![index as u8; len].into_boxed_slice()
45    }
46
47    fn check(index: usize, slice: &[u8]) {
48        assert_eq!(slice.len(), (index % 7) + 1, "length of buffer {index}");
49        assert!(
50            slice.iter().all(|&byte| byte == index as u8),
51            "contents of buffer {index}",
52        );
53    }
54
55    /// Alloc while earlier borrows stay live: each iteration allocates, then
56    /// re-reads every prior slice.
57    #[test]
58    fn interleaved_alloc_and_read_across_many_buffers() {
59        let arena = DecodeArena::new();
60        let mut borrows: Vec<&[u8]> = Vec::new();
61        for index in 0..256usize {
62            borrows.push(arena.alloc(buffer(index)));
63            for (i, slice) in borrows.iter().enumerate() {
64                check(i, slice);
65            }
66        }
67    }
68}
cadmpeg_core/decode/arena.rs

cadmpeg_core/decode/
arena.rs
