Modify the Rust implementation to initialize the ring buffer with the last two bytes set to 0

Oct 13, 2025 by wbrum.org 90 views

Introduction

Brotli is a compression algorithm developed by Google that is widely used in web applications to reduce the size of data. The algorithm uses a combination of LZ77 and Huffman coding to achieve high compression ratios. In this article, we will explore a case where the Rust implementation of Brotli decompressor fails to decode a specific input that works correctly with the C implementation.

The Issue

The issue arises when using the LZ77 shared dictionary in the Rust implementation of Brotli decompressor. The shared dictionary is a buffer that stores the last two bytes of the input data, which are used to look up the previous positions in the input buffer. However, in the Rust implementation, the end of the ring buffer is immediately overwritten with the shared dictionary, causing the p1 and p2 lookups to return the value of the last two bytes of the shared dictionary instead of 0.

The C Implementation

The C implementation of Brotli decompressor initializes the ring buffer with the last two bytes set to 0. This ensures that the p1 and p2 lookups return 0, which is the expected behavior.

// From https://github.com/google/brotli/blob/master/c/dec/decode.c#L1395
void init_ring_buffer(RingBuffer *rb) {
    rb->buf[0] = 0;
    rb->buf[1] = 0;
}

The Rust Implementation

The Rust implementation of Brotli decompressor initializes the ring buffer with the last two bytes set to 0, but then immediately overwrites the end of the ring buffer with the shared dictionary.

// From https://github.com/dropbox/rust-brotli-decompressor/blob/master/src/decode.rs#L1861
fn init_ring_buffer(&mut self) {
    self.ring_buffer.buf[0] = 0;
    self.ring_buffer.buf[1] = 0;
    self.ring_buffer.buf[2..].copy_from_slice(&self.shared_dict);
}

The Unit Test

The unit test provided in the Rust implementation of Brotli decompressor reproduces the issue.

#[test]
fn test_failing_patch() {
    // ...
}

Conclusion

In conclusion, the issue arises from the fact that the Rust implementation of Brotli decompressor overwrites the end of the ring buffer with the shared dictionary, causing the p1 and p2 lookups to return the value of the last two bytes of the shared dictionary instead of 0. To fix this issue, the Rust implementation should be modified to initialize the ring buffer with the last two bytes set to 0, and then use the shared dictionary to update the ring buffer.

Recommendations

Modify the Rust implementation to initialize the ring buffer with the last two bytes set to 0.
Use the shared dictionary to update the ring buffer.
Test the modified implementation to ensure that it works correctly.

By following these recommendations, the Rust implementation of Brotli decompressor should be able to decode the specific input that works correctly with the C implementation.

Code Changes

Here is the modified code for the Rust implementation of Brotli decompressor:

// From https://github.com/dropbox/rust-brotli-decompressor/blob/master/src/decode.rs#L1861
fn init_ring_buffer(&mut self) {
    self.ring_buffer.buf[0] = 0;
    self.ring_buffer.buf[1] = 0;
}

// From https://github.com/dropbox/rust-brotli-decompressor/blob/master/src/decode.rs#L1875
fn update_ring_buffer(&mut self) {
    self.ring_buffer.buf[2..].copy_from_slice(&self.shared_dict);
}

Commit Message

Here is a possible commit message for the modified code:

Fix issue with LZ77 shared dictionary in Rust implementation of Brotli decompressor

API Documentation

Here is a possible API documentation for the modified code:

/// Initializes the ring buffer with the last two bytes set to 0.
///
/// # Examples
///
/// ```
/// let mut ring_buffer = RingBuffer::new();
/// ring_buffer.init();
/// assert_eq!(ring_buffer.buf[0], 0);
/// assert_eq!(ring_buffer.buf[1], 0);
/// ```
pub fn init_ring_buffer(&mut self) {
    self.ring_buffer.buf[0] = 0;
    self.ring_buffer.buf[1] = 0;
}

/// Updates the ring buffer with the shared dictionary.
///
/// # Examples
///
/// ```
/// let mut ring_buffer = RingBuffer::new();
/// let shared_dict = [1, 2, 3, 4, 5];
/// ring_buffer.update(&shared_dict);
/// assert_eq!(ring_buffer.buf[2], 1);
/// assert_eq!(ring_buffer.buf[3], 2);
/// assert_eq!(ring_buffer.buf[4], 3);
/// assert_eq!(ring_buffer.buf[5], 4);
/// assert_eq!(ring_buffer.buf[6], 5);
/// ```
pub fn update_ring_buffer(&mut self) {
    self.ring_buffer.buf[2..].copy_from_slice(&self.shared_dict);
}
```<br/>
**Q&A: Decoding with LZ77 Shared Dictionary Fails on a Case that Works Correctly with the C Implementation**
====================================================================================

**Q: What is the issue with the Rust implementation of Brotli decompressor?**
--------------------------------------------------------------------------------

A: The issue arises from the fact that the Rust implementation of Brotli decompressor overwrites the end of the ring buffer with the shared dictionary, causing the p1 and p2 lookups to return the value of the last two bytes of the shared dictionary instead of 0.

**Q: Why does the C implementation work correctly?**
------------------------------------------------

A: The C implementation initializes the ring buffer with the last two bytes set to 0, which ensures that the p1 and p2 lookups return 0, which is the expected behavior.

**Q: What is the difference between the C and Rust implementations?**
----------------------------------------------------------------

A: The main difference between the C and Rust implementations is how they initialize and update the ring buffer. The C implementation initializes the ring buffer with the last two bytes set to 0 and then uses the shared dictionary to update the ring buffer. The Rust implementation, on the other hand, overwrites the end of the ring buffer with the shared dictionary, causing the issue.

**Q: How can the issue be fixed?**
--------------------------------

A: The issue can be fixed by modifying the Rust implementation to initialize the ring buffer with the last two bytes set to 0 and then use the shared dictionary to update the ring buffer.

**Q: What are the benefits of using the shared dictionary?**
------------------------------------------------------

A: The shared dictionary is used to store the last two bytes of the input data, which are used to look up the previous positions in the input buffer. This allows for more efficient compression and decompression.

**Q: What are the implications of this issue?**
------------------------------------------------

A: The issue has implications for the correctness and reliability of the Rust implementation of Brotli decompressor. If the issue is not fixed, it could lead to incorrect decompression and potentially cause data corruption or loss.

**Q: How can the issue be tested?**
--------------------------------

A: The issue can be tested by running the unit test provided in the Rust implementation of Brotli decompressor. The test should be modified to check for the correct behavior of the ring buffer and the shared dictionary.

**Q: What are the best practices for implementing the shared dictionary?**
---------------------------------------------------------------------

A: The best practices for implementing the shared dictionary include:

*   Initializing the ring buffer with the last two bytes set to 0
*   Using the shared dictionary to update the ring buffer
*   Testing the implementation to ensure that it works correctly

**Q: What are the next steps for fixing the issue?**
------------------------------------------------

A: The next steps for fixing the issue include:

*   Modifying the Rust implementation to initialize the ring buffer with the last two bytes set to 0
*   Using the shared dictionary to update the ring buffer
*   Testing the modified implementation to ensure that it works correctly

**Q: How can the issue be prevented in the future?**
------------------------------------------------

A: The issue can be prevented in the future by following best practices for implementing the shared dictionary, including initializing the ring buffer with the last two bytes set to 0 and using the shared dictionary to update the ring buffer.

**Q: What are the benefits of using a shared dictionary in Brotli decompressor?**
-------------------------------------------------------------------------

A: The benefits of using a shared dictionary in Brotli decompressor include:

*   More efficient compression and decompression
*   Improved compression ratios
*   Reduced memory usage

**Q: What are the implications of not using a shared dictionary in Brotli decompressor?**
--------------------------------------------------------------------------------

A: The implications of not using a shared dictionary in Brotli decompressor include:

*   Reduced compression ratios
*   Increased memory usage
*   Potential data corruption or loss

**Q: How can the shared dictionary be used in other compression algorithms?**
-------------------------------------------------------------------------

A: The shared dictionary can be used in other compression algorithms, such as LZ77 and LZ78, to improve compression ratios and reduce memory usage.

**Q: What are the best practices for implementing a shared dictionary in other compression algorithms?**
----------------------------------------------------------------------------------------------

A: The best practices for implementing a shared dictionary in other compression algorithms include:

*   Initializing the ring buffer with the last two bytes set to 0
*   Using the shared dictionary to update the ring buffer
*   Testing the implementation to ensure that it works correctly