#mame #decompression #chd


Rust implementation of the CHD File Format

17 releases

Uses new Rust 2021

new 0.2.3 Aug 13, 2022
0.2.2 Aug 13, 2022
0.1.9 Aug 9, 2022
0.1.0 Jun 4, 2022
0.0.1 Mar 29, 2021

#19 in Compression

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570 downloads per month
Used in chd-capi




Latest Version Docs License Minimum Supported Rust Version 1.59

Reimplementation of the CHD file format in pure Safe Rust, drop-in compatible with libchdr.

chd-rs aims to be a memory-safe, well documented, and clean from-scratch implementation of CHD, verifiable against chd.cpp while being easier to read and use as documentation to implement the format natively in other languages. It is standalone and can be built with just a Rust compiler, without the need for a full C/C++ toolchain.

Performance is competitive but a little slower than libchdr in benchmarks from using more immature (but fully correct) pure Rust implementations of compression codecs. Deflate (zlib) compression is backed by flate2, LZMA is backed by lzma-rs, and FLAC decompression is backed by claxon. While performance is not ignored, the focus is on readability and correctness.


Open a Chd with Chd::open, then iterate hunks from 0 to chd.header().hunk_count() to read hunks.

The size of the destination buffer must be exactly chd.header().hunk_size() to decompress with hunk.read_hunk_in, which takes the output slice and a buffer to hold compressed data.

fn main() -> Result<()> {
    let mut f = BufReader::new(File::open("image.chd")?;
    let mut chd = Chd::open(&mut f, None)?;
    let hunk_count = chd.header().hunk_count();
    let hunk_size = chd.header().hunk_size();
    // buffer to store decompressed hunks
    let mut out_buf = chd.get_hunksized_buffer();
    // buffer for temporary compressed
    let mut temp_buf = Vec::new();
    for hunk_num in 0..hunk_count {
        let mut hunk = chd.hunk(hunk_num)?;
        hunk.read_hunk_in(&mut temp_buf, &mut out_buf)?;

For more ergonomic but slower usage, chd::read provides buffered adapters that implement Read and Seek at the hunk level. A buffered adapter at the file level is also available.

Lending Iterators

With unstable_lending_iterators, hunks and metadata can be slightly more ergonomically iterated over albeit with a while let loop. This API is unstable until Generic Associated Types and the LendingIterator trait is stabilized.

chd = { version = "0.2", features = ["unstable_lending_iterators"] }

Then hunks can be iterated like so.

fn main() -> Result<()> {
    let mut f = BufReader::new(File::open("image.chd")?);
    let mut chd = Chd::open(&mut f, None)?;
    // buffer to store decompressed hunks
    let mut out_buf = chd.get_hunksized_buffer();
    // buffer for temporary compressed
    let mut temp_buf = Vec::new();
    let mut hunk_iter = chd.hunks();
    while let Some(mut hunk) = hunk_iter.next() {
        hunk.read_hunk_in(&mut temp_buf, &mut out_buf)?;

A similar API exists for metadata in Chd::metadata.

Verifying Hunk Checksums

By default, chd-rs does not verify the checksums of decompressed hunks for performance. The feature verify_block_crc should be enabled to verify hunk checksums.

chd = { version = "0.2", features = ["verify_block_crc"] }

Supported Codecs

chd-rs supports the following compression codecs, with wider coverage than libchdr. For implementation details, see the chd::compression module.

V1-4 Codecs

⚠️V1-4 support has not been as rigorously tested as V5 support. ⚠️


V5 Codecs

  • Deflate (CHD_CODEC_ZLIB)
  • Huffman (CHD_CODEC_HUFF)
  • CD Deflate (CHD_CODEC_CD_ZLIB)

Codecs and Huffman API

By default, the codecs and static Huffman implementations are not exposed as part of the public API, but can be enabled with the codec_api and huffman_api features respectively. These APIs are subject to change but should be considered mostly stable.

In particular the type signature for HuffmanDecoder is subject to change once generic_const_exprs is stabilized.

rchdman command line tool

As a proof of concept, chd-rs implements an extremely basic reimplementation of chdman for read-only purposes. The following functions are available with rchdman.

  • info Displays information about a CHD.
  • verify Verify the integrity of a CHD. Metadata integrity is not verified.
  • extractraw Extract the raw file from a CHD input file.
  • dumpmeta Dump metadata from the CHD to stdout or to a file.

The results from rchdman should be identical from chdman. rchdman is intended to be basic and does not implement multithreading or other functions, so in general it is slower than chdman. There are no plans to implement write-operations into rchdman.


By default, chd-rs uses pure Rust codecs but if maximum performance is needed, max_perf can be enabled. This enables the zlib-ng backend of flate2 as well as using experimental APIs in a custom lzma-rs fork for some improvements in performance at the expense of higher memory usage. Combined with codegen-units=1 and Profile Guided Optimization, chd-rs is within 1% of libchdr performance.

Without max_perf, chd-rs is already within 15% of libchdr without needing to link with C libraries like zlib-ng.

libchdr API

⚠️The C API has not been heavily tested. Use at your own risk. ⚠️

chd-rs provides a C API compatible with chd.h. ABI compatibility is detailed below but is untested when compiling as a dynamic library. See /chd-rs-capi for more details.


~36K SLoC