crates/clawhdf5-format/Cargo.toml

@@ -28,6 +28,7 @@ ruzstd = { version = "0.9", optional = true }
 # bzip2 with its default backend, libbz2-rs-sys: a pure-Rust port of
 # libbzip2 (no C is compiled, despite the -sys name).
 bzip2 = { version = "0.6", optional = true }
+snap = { version = "1", optional = true }
 
 [dev-dependencies]
 half = { workspace = true }
@@ -69,6 +70,10 @@ lzf = []
 bitshuffle = ["lz4_flex", "ruzstd"]
 # bzip2 (307).
 bzip2 = ["dep:bzip2", "std"]
+# Blosc 1 (32001) with its BloscLZ, LZ4, Snappy, Zlib and Zstandard codecs.
+blosc = ["lz4_flex", "ruzstd", "snap", "deflate", "std"]
+# Every plugin filter above.
+plugin-filters = ["lzf", "bitshuffle", "bzip2", "blosc"]
 
 [[bench]]
 name = "parallel_decompress_bench"

crates/clawhdf5-format/src/chunked_write.rs

@@ -12,8 +12,8 @@ use crate::chunk_grid::ChunkGrid;
 use crate::ea_writer;
 use crate::error::FormatError;
 use crate::filter_pipeline::{
-    FILTER_BITSHUFFLE, FILTER_BZIP2, FILTER_DEFLATE, FILTER_FLETCHER32, FILTER_LZ4, FILTER_LZF,
+    FILTER_BITSHUFFLE, FILTER_BLOSC, FILTER_BZIP2, FILTER_DEFLATE, FILTER_FLETCHER32, FILTER_LZ4,
-    FILTER_PCODEC, FILTER_PCODEC_NAME, FILTER_SHUFFLE, FILTER_ZSTD, FilterDescription,
+    FILTER_LZF, FILTER_PCODEC, FILTER_PCODEC_NAME, FILTER_SHUFFLE, FILTER_ZSTD, FilterDescription,
     FilterPipeline,
 };
 use crate::filters::compress_chunk;
@@ -77,6 +77,41 @@ pub enum PluginFilter {
         /// Block size 1-9 (9 = hdf5plugin's default).
         level: u32,
     },
+    /// Blosc 1 (filter 32001): `codec` at `level` (0-9; 0 stores), after
+    /// `shuffle`. Needs the `blosc` feature.
+    Blosc {
+        /// The codec inside the Blosc frame.
+        codec: BloscCodec,
+        /// Compression level 0-9 (0 stores the data uncompressed).
+        level: u32,
+        /// The shuffle Blosc applies first.
+        shuffle: BloscShuffle,
+    },
+}
+
+/// The codec inside a Blosc frame that clawhdf5 can write. (It reads
+/// BloscLZ too, but cannot write it.)
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BloscCodec {
+    /// LZ4.
+    Lz4,
+    /// Snappy.
+    Snappy,
+    /// Zlib, at the Blosc level.
+    Zlib,
+    /// Zstandard (clawhdf5's pure-Rust encoder has one level, about zstd 1).
+    Zstd,
+}
+
+/// The shuffle Blosc applies before compressing.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum BloscShuffle {
+    /// None.
+    None,
+    /// Byte shuffle (Blosc's default).
+    Byte,
+    /// Bit shuffle.
+    Bit,
 }
 
 /// What bitshuffle compresses its blocks with.
@@ -102,6 +137,7 @@ impl PluginFilter {
             PluginFilter::Lzf => false,
             PluginFilter::Bitshuffle { .. } => true,
             PluginFilter::Bzip2 { .. } => false,
+            PluginFilter::Blosc { .. } => true,
         }
     }
 
@@ -120,6 +156,35 @@ impl PluginFilter {
             },
             // bshuf_h5_set_local: version 0.4, element size, block size,
             // compression (0 none, 2 LZ4, 3 Zstandard), Zstandard level.
+            // hdf5-blosc's blosc_set_local: filter revision 2, Blosc format
+            // 2, type size, chunk size, then level, shuffle, compressor.
+            PluginFilter::Blosc {
+                codec,
+                level,
+                shuffle,
+            } => FilterDescription {
+                filter_id: FILTER_BLOSC,
+                name: Some("blosc".into()),
+                flags: 1,
+                client_data: vec![
+                    2,
+                    2,
+                    element_size,
+                    chunk_bytes,
+                    (*level).min(9),
+                    match shuffle {
+                        BloscShuffle::None => 0,
+                        BloscShuffle::Byte => 1,
+                        BloscShuffle::Bit => 2,
+                    },
+                    match codec {
+                        BloscCodec::Lz4 => 1,
+                        BloscCodec::Snappy => 3,
+                        BloscCodec::Zlib => 4,
+                        BloscCodec::Zstd => 5,
+                    },
+                ],
+            },
             PluginFilter::Bzip2 { level } => FilterDescription {
                 filter_id: FILTER_BZIP2,
                 name: Some("bzip2".into()),

crates/clawhdf5-format/src/filters.rs

@@ -206,6 +206,13 @@ pub(crate) static BUILTIN_FILTERS: &[BuiltinFilter] = &[
         decode: crate::filters_lzf::lzf_decode,
         encode: Some(crate::filters_lzf::lzf_encode),
     },
+    #[cfg(feature = "blosc")]
+    BuiltinFilter {
+        id: crate::filter_pipeline::FILTER_BLOSC,
+        name: "blosc",
+        decode: crate::filters_blosc::blosc_decode,
+        encode: Some(crate::filters_blosc::blosc_encode),
+    },
     #[cfg(feature = "lz4")]
     BuiltinFilter {
         id: FILTER_LZ4,

crates/clawhdf5-format/src/filters_bitshuffle.rs

@@ -218,7 +218,7 @@ pub(crate) fn bitshuffle_decode(
 }
 
 /// Decode Zstandard frames into exactly `dst`, failing if they hold more.
-#[cfg(feature = "bitshuffle")]
+#[cfg(any(feature = "bitshuffle", feature = "blosc"))]
 pub(crate) fn zstd_decode_into(
     decoder: &mut ruzstd::decoding::FrameDecoder,
     frames: &[u8],
@@ -231,7 +231,7 @@ pub(crate) fn zstd_decode_into(
 
 /// Compress with ruzstd. It implements one level (roughly zstd's level 1),
 /// so the requested level only matters to other encoders.
-#[cfg(feature = "bitshuffle")]
+#[cfg(any(feature = "bitshuffle", feature = "blosc"))]
 pub(crate) fn zstd_encode(data: &[u8]) -> Vec<u8> {
     ruzstd::encoding::compress_to_vec(data, ruzstd::encoding::CompressionLevel::Fastest)
 }

crates/clawhdf5-format/src/filters_blosc.rs

@@ -0,0 +1,604 @@
+//! Blosc 1 (HDF5 filter 32001, `hdf5-blosc`, hdf5plugin's `Blosc`), in pure
+//! Rust: the Blosc 1 frame, its byte shuffle and bit shuffle, and the
+//! BloscLZ, LZ4/LZ4HC, Snappy, Zlib and Zstandard codecs inside it.
+//!
+//! **Frame** (c-blosc 1.x, format version 2). A 16-byte header — version
+//! (2), codec format version (1), flags, type size, then little-endian `u32`
+//! decoded size, block size and frame size. Flags: bit 0 byte shuffle, bit
+//! 1 stored raw ("memcpyed": the data follows the header), bit 2 bit
+//! shuffle, bit 4 "do not split", bits 5-7 the codec (0 BloscLZ, 1 LZ4 and
+//! LZ4HC, 2 Snappy, 3 Zlib, 4 Zstandard). Unless stored raw, a table of
+//! `u32` block offsets follows, one per block of `block size` bytes (the
+//! last one may be shorter). A block is one stream, or — when the "do not
+//! split" flag is clear, the type size is at most 16, the block holds at
+//! least 128 elements, and it is not the short last block — `type size`
+//! streams, one per byte plane. Each stream is a `u32` length and the
+//! codec's output; a length equal to the stream's decoded size means the
+//! bytes are stored raw. The decoded block is then unshuffled (byte shuffle
+//! for type size > 1; bit shuffle when the block holds a multiple of 8
+//! elements, the trailing partial element copied as is).
+//!
+//! **Filter** (`blosc_filter.c`) `cd_values`: `[0]` filter revision, `[1]`
+//! Blosc format version, `[2]` type size, `[3]` chunk size in bytes, `[4]`
+//! compression level, `[5]` shuffle (0 none, 1 byte, 2 bit), `[6]`
+//! compressor (0 blosclz, 1 lz4, 2 lz4hc, 3 snappy, 4 zlib, 5 zstd). The
+//! decoder needs only the frame.
+
+use crate::error::FormatError;
+use crate::filter_registry::FilterContext;
+use crate::filters_bitshuffle::{bitshuffle_block, bitunshuffle_block};
+
+const HEADER: usize = 16;
+const FLAG_SHUFFLE: u8 = 0x01;
+const FLAG_MEMCPYED: u8 = 0x02;
+const FLAG_BITSHUFFLE: u8 = 0x04;
+const FLAG_FUTURE: u8 = 0x08;
+const FLAG_DONT_SPLIT: u8 = 0x10;
+const MAX_SPLITS: usize = 16;
+const MIN_BUFFERSIZE: usize = 128;
+
+fn err(msg: &str) -> FormatError {
+    FormatError::DecompressionError(format!("blosc: {msg}"))
+}
+
+fn le32(b: &[u8], at: usize) -> Result<usize, FormatError> {
+    b.get(at..at + 4)
+        .map(|s| u32::from_le_bytes(s.try_into().unwrap()) as usize)
+        .ok_or_else(|| err("truncated frame"))
+}
+
+/// The codec inside a Blosc frame (flags bits 5-7).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum Codec {
+    BloscLz,
+    Lz4,
+    Snappy,
+    Zlib,
+    Zstd,
+}
+
+impl Codec {
+    fn from_flags(flags: u8) -> Result<Codec, FormatError> {
+        match flags >> 5 {
+            0 => Ok(Codec::BloscLz),
+            1 => Ok(Codec::Lz4),
+            2 => Ok(Codec::Snappy),
+            3 => Ok(Codec::Zlib),
+            4 => Ok(Codec::Zstd),
+            other => Err(err(&format!("unknown codec {other}"))),
+        }
+    }
+}
+
+/// Decode one codec stream into exactly `dst`.
+fn decode_stream(
+    codec: Codec,
+    src: &[u8],
+    dst: &mut [u8],
+    zstd: &mut Option<ruzstd::decoding::FrameDecoder>,
+) -> Result<(), FormatError> {
+    let n = match codec {
+        Codec::BloscLz => blosclz_decompress(src, dst),
+        Codec::Lz4 => {
+            lz4_flex::block::decompress_into(src, dst).map_err(|e| err(&format!("lz4: {e}")))?
+        }
+        Codec::Snappy => {
+            let len = snap::raw::decompress_len(src).map_err(|e| err(&format!("snappy: {e}")))?;
+            if len != dst.len() {
+                return Err(err("snappy stream has the wrong size"));
+            }
+            snap::raw::Decoder::new()
+                .decompress(src, dst)
+                .map_err(|e| err(&format!("snappy: {e}")))?
+        }
+        Codec::Zlib => {
+            let out = crate::filters::inflate_bounded(src, dst.len(), dst.len())
+                .map_err(|e| err(&format!("zlib: {e}")))?;
+            let n = out.len();
+            if n == dst.len() {
+                dst.copy_from_slice(&out);
+            }
+            n
+        }
+        Codec::Zstd => crate::filters_bitshuffle::zstd_decode_into(
+            zstd.get_or_insert_with(ruzstd::decoding::FrameDecoder::new),
+            src,
+            dst,
+        )?,
+    };
+    if n != dst.len() {
+        return Err(err("stream decoded to the wrong size"));
+    }
+    Ok(())
+}
+
+/// Decode a Blosc-filtered chunk: one Blosc 1 frame.
+pub(crate) fn blosc_decode(input: &[u8], ctx: &FilterContext<'_>) -> Result<Vec<u8>, FormatError> {
+    blosc_decompress(input, ctx.output_limit())
+}
+
+/// Decompress a Blosc 1 frame, refusing more than `limit` bytes of output.
+pub fn blosc_decompress(input: &[u8], limit: usize) -> Result<Vec<u8>, FormatError> {
+    if input.len() < HEADER {
+        return Err(err("truncated header"));
+    }
+    let version = input[0];
+    let codec_version = input[1];
+    let flags = input[2];
+    let typesize = input[3] as usize;
+    let nbytes = le32(input, 4)?;
+    let blocksize = le32(input, 8)?;
+    let cbytes = le32(input, 12)?;
+    if version != 1 && version != 2 {
+        return Err(err(&format!(
+            "frame format version {version} is not Blosc 1 (a Blosc 2 chunk?)"
+        )));
+    }
+    if flags & FLAG_FUTURE != 0 {
+        return Err(err("unknown header flags"));
+    }
+    if nbytes > limit {
+        return Err(err("decoded size exceeds the chunk size"));
+    }
+    if cbytes > input.len() {
+        return Err(err("frame is longer than the chunk"));
+    }
+    let src = &input[..cbytes];
+    if nbytes == 0 {
+        return Ok(Vec::new());
+    }
+    if blocksize == 0 || typesize == 0 {
+        return Err(err("bad block or type size"));
+    }
+    let mut out = vec![0u8; nbytes];
+    if flags & FLAG_MEMCPYED != 0 {
+        if cbytes != nbytes + HEADER {
+            return Err(err("stored frame has the wrong size"));
+        }
+        out.copy_from_slice(&src[HEADER..]);
+        return Ok(out);
+    }
+    let codec = Codec::from_flags(flags)?;
+    if codec_version != 1 {
+        return Err(err(&format!(
+            "unsupported {codec:?} format version {codec_version}"
+        )));
+    }
+    let nblocks = nbytes.div_ceil(blocksize);
+    let leftover = nbytes % blocksize;
+    if nblocks > (cbytes - HEADER) / 4 {
+        return Err(err("block table is truncated"));
+    }
+    let block_len = blocksize.min(nbytes);
+    let mut tmp = vec![0u8; block_len];
+    let mut zstd = None;
+    let dont_split = flags & FLAG_DONT_SPLIT != 0;
+    for j in 0..nblocks {
+        let is_leftover = j == nblocks - 1 && leftover > 0;
+        let bsize = if is_leftover { leftover } else { blocksize };
+        let nsplits = if !dont_split
+            && typesize <= MAX_SPLITS
+            && bsize / typesize >= MIN_BUFFERSIZE
+            && !is_leftover
+        {
+            typesize
+        } else {
+            1
+        };
+        let neblock = bsize / nsplits;
+        let mut pos = le32(src, HEADER + 4 * j)?;
+        let tmp = &mut tmp[..bsize];
+        for s in 0..nsplits {
+            if pos + 4 > src.len() {
+                return Err(err("block offset out of range"));
+            }
+            let clen = le32(src, pos)?;
+            pos += 4;
+            let stream = src
+                .get(pos..pos.saturating_add(clen))
+                .ok_or_else(|| err("stream runs past the frame"))?;
+            let dst = &mut tmp[s * neblock..(s + 1) * neblock];
+            if clen == neblock {
+                dst.copy_from_slice(stream);
+            } else {
+                decode_stream(codec, stream, dst, &mut zstd)?;
+            }
+            pos += clen;
+        }
+        // `bsize` is a whole number of splits by construction (`nsplits` > 1
+        // only for full blocks, and c-blosc sizes those in whole elements).
+        if nsplits * neblock != bsize {
+            return Err(err("block is not a whole number of streams"));
+        }
+        let dest = &mut out[j * blocksize..j * blocksize + bsize];
+        unshuffle_block(flags, typesize, tmp, dest);
+    }
+    Ok(out)
+}
+
+/// Undo the frame's shuffle on one decoded block.
+fn unshuffle_block(flags: u8, typesize: usize, src: &[u8], dest: &mut [u8]) {
+    let bsize = src.len();
+    if flags & FLAG_SHUFFLE != 0 && typesize > 1 {
+        let n = bsize / typesize;
+        for i in 0..n {
+            for b in 0..typesize {
+                dest[i * typesize + b] = src[b * n + i];
+            }
+        }
+        dest[n * typesize..].copy_from_slice(&src[n * typesize..]);
+    } else if flags & FLAG_BITSHUFFLE != 0 && bsize >= typesize {
+        let n = bsize / typesize;
+        if n.is_multiple_of(8) {
+            let body = n * typesize;
+            bitunshuffle_block(&src[..body], &mut dest[..body], n, typesize);
+            dest[body..].copy_from_slice(&src[body..]);
+        } else {
+            dest.copy_from_slice(src);
+        }
+    } else {
+        dest.copy_from_slice(src);
+    }
+}
+
+/// BloscLZ decompression (c-blosc 1.21 `blosclz_decompress`): returns the
+/// number of bytes written, or 0 on malformed input — exactly as the C
+/// decoder, including stopping before a match that ends the stream, so a
+/// stream libblosc rejects is rejected here too.
+///
+/// Instructions: a control byte `ctrl`. Below 32, a literal run of
+/// `ctrl + 1` bytes. Otherwise a match: length `(ctrl >> 5) + 2`, extended
+/// by following bytes while they are 255 when the top three bits are all
+/// set; distance `((ctrl & 31) << 8) + next byte + 1`, or — when that byte
+/// is 255 and the high bits are 31 — a 16-bit big-endian distance plus 8192.
+/// The first instruction is always a literal.
+pub(crate) fn blosclz_decompress(input: &[u8], out: &mut [u8]) -> usize {
+    const MAX_DISTANCE: usize = 8191;
+    let limit = input.len();
+    if limit == 0 {
+        return 0;
+    }
+    let mut ip = 1usize;
+    let mut op = 0usize;
+    let mut ctrl = (input[0] & 31) as usize;
+    loop {
+        if ctrl >= 32 {
+            let mut len = (ctrl >> 5) - 1;
+            let ofs = (ctrl & 31) << 8;
+            if len == 6 {
+                loop {
+                    if ip + 1 >= limit {
+                        return 0;
+                    }
+                    let code = input[ip] as usize;
+                    ip += 1;
+                    len += code;
+                    if code != 255 {
+                        break;
+                    }
+                }
+            } else if ip + 1 >= limit {
+                return 0;
+            }
+            let code = input[ip] as usize;
+            ip += 1;
+            len += 3;
+            // The copy source is `distance` bytes back.
+            let mut distance = ofs + code + 1;
+            if code == 255 && ofs == 31 << 8 {
+                if ip + 1 >= limit {
+                    return 0;
+                }
+                let far = ((input[ip] as usize) << 8) + input[ip + 1] as usize;
+                ip += 2;
+                distance = far + MAX_DISTANCE + 1;
+            }
+            if op + len > out.len() {
+                return 0;
+            }
+            if distance > op {
+                return 0;
+            }
+            if ip >= limit {
+                break;
+            }
+            ctrl = input[ip] as usize;
+            ip += 1;
+            let start = op - distance;
+            if distance >= len {
+                out.copy_within(start..start + len, op);
+            } else {
+                for k in 0..len {
+                    out[op + k] = out[start + k];
+                }
+            }
+            op += len;
+        } else {
+            let run = ctrl + 1;
+            if op + run > out.len() || ip + run > limit {
+                return 0;
+            }
+            out[op..op + run].copy_from_slice(&input[ip..ip + run]);
+            op += run;
+            ip += run;
+            if ip >= limit {
+                break;
+            }
+            ctrl = input[ip] as usize;
+            ip += 1;
+        }
+    }
+    op
+}
+
+/// The codec our encoder puts inside the frame.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum EncodeCodec {
+    Lz4,
+    Snappy,
+    Zlib,
+    Zstd,
+}
+
+impl EncodeCodec {
+    /// From the filter's `cd_values[6]` compressor code.
+    fn from_cd(code: u32) -> Result<EncodeCodec, FormatError> {
+        match code {
+            1 | 2 => Ok(EncodeCodec::Lz4),
+            3 => Ok(EncodeCodec::Snappy),
+            4 => Ok(EncodeCodec::Zlib),
+            5 => Ok(EncodeCodec::Zstd),
+            0 => Err(FormatError::CompressionError(
+                "blosc: clawhdf5 cannot write BloscLZ; choose lz4, snappy, zlib or zstd".into(),
+            )),
+            other => Err(FormatError::CompressionError(format!(
+                "blosc: unknown compressor {other}"
+            ))),
+        }
+    }
+
+    fn flags(self) -> u8 {
+        (match self {
+            EncodeCodec::Lz4 => 1,
+            EncodeCodec::Snappy => 2,
+            EncodeCodec::Zlib => 3,
+            EncodeCodec::Zstd => 4,
+        }) << 5
+    }
+
+    fn encode(self, data: &[u8], level: u32) -> Result<Vec<u8>, FormatError> {
+        match self {
+            EncodeCodec::Lz4 => Ok(lz4_flex::block::compress(data)),
+            EncodeCodec::Snappy => snap::raw::Encoder::new()
+                .compress_vec(data)
+                .map_err(|e| FormatError::CompressionError(format!("blosc: snappy: {e}"))),
+            EncodeCodec::Zlib => crate::filters::deflate_bounded(data, level.min(9))
+                .map_err(|e| FormatError::CompressionError(format!("blosc: zlib: {e}"))),
+            EncodeCodec::Zstd => Ok(crate::filters_bitshuffle::zstd_encode(data)),
+        }
+    }
+}
+
+/// Block size our encoder uses: at most 256 KiB, a whole number of
+/// elements (and, for bit shuffle, of 8-element groups).
+fn encode_block_size(nbytes: usize, typesize: usize, bitshuffle: bool) -> usize {
+    let unit = if bitshuffle { 8 * typesize } else { typesize };
+    let target = (256 * 1024).min(nbytes);
+    if target < unit {
+        return nbytes.max(1);
+    }
+    target / unit * unit
+}
+
+/// Encode a chunk as one Blosc 1 frame. `cd_values` as hdf5-blosc:
+/// `[2]` type size, `[4]` level (0 = store), `[5]` shuffle, `[6]` codec.
+pub(crate) fn blosc_encode(input: &[u8], ctx: &FilterContext<'_>) -> Result<Vec<u8>, FormatError> {
+    let cd = ctx.client_data();
+    let cerr = |m: &str| FormatError::CompressionError(format!("blosc: {m}"));
+    let typesize = match cd.get(2) {
+        Some(&t) if t != 0 => t as usize,
+        _ => ctx.element_size.max(1),
+    };
+    // Blosc records the type size in one byte; c-blosc treats larger types
+    // as bytes.
+    let typesize = if typesize > 255 { 1 } else { typesize };
+    let level = cd.get(4).copied().unwrap_or(5);
+    let shuffle = cd.get(5).copied().unwrap_or(1);
+    let codec = EncodeCodec::from_cd(cd.get(6).copied().unwrap_or(1))?;
+    let nbytes = input.len();
+    if nbytes > i32::MAX as usize - HEADER {
+        return Err(cerr("chunk too large for a Blosc frame"));
+    }
+    let mut flags = codec.flags();
+    match shuffle {
+        0 => {}
+        1 => flags |= FLAG_SHUFFLE,
+        2 => flags |= FLAG_BITSHUFFLE,
+        other => return Err(cerr(&format!("unknown shuffle mode {other}"))),
+    }
+    let blocksize = encode_block_size(nbytes, typesize, shuffle == 2);
+    let header = |flags: u8, blocksize: usize, cbytes: usize| {
+        let mut h = Vec::with_capacity(HEADER);
+        h.extend_from_slice(&[2, 1, flags, typesize as u8]);
+        h.extend_from_slice(&(nbytes as u32).to_le_bytes());
+        h.extend_from_slice(&(blocksize as u32).to_le_bytes());
+        h.extend_from_slice(&(cbytes as u32).to_le_bytes());
+        h
+    };
+    let stored = || {
+        let mut out = header(
+            FLAG_MEMCPYED | (flags & !(FLAG_SHUFFLE | FLAG_BITSHUFFLE)),
+            blocksize,
+            nbytes + HEADER,
+        );
+        out.extend_from_slice(input);
+        out
+    };
+    if level == 0 || nbytes == 0 {
+        return Ok(stored());
+    }
+
+    let nblocks = nbytes.div_ceil(blocksize);
+    let leftover = nbytes % blocksize;
+    let mut body = Vec::with_capacity(nbytes / 2);
+    let mut starts = Vec::with_capacity(nblocks);
+    let table_end = HEADER + 4 * nblocks;
+    let mut shuffled = vec![0u8; blocksize];
+    for j in 0..nblocks {
+        let is_leftover = j == nblocks - 1 && leftover > 0;
+        let bsize = if is_leftover { leftover } else { blocksize };
+        let block = &input[j * blocksize..j * blocksize + bsize];
+        let sh = &mut shuffled[..bsize];
+        shuffle_block(flags, typesize, block, sh);
+        starts.push(table_end + body.len());
+        let nsplits = if typesize <= MAX_SPLITS
+            && bsize / typesize >= MIN_BUFFERSIZE
+            && !is_leftover
+            && bsize.is_multiple_of(typesize)
+        {
+            typesize
+        } else {
+            1
+        };
+        let neblock = bsize / nsplits;
+        for s in 0..nsplits {
+            let part = &sh[s * neblock..(s + 1) * neblock];
+            let comp = codec.encode(part, level)?;
+            if comp.len() < neblock {
+                body.extend_from_slice(&(comp.len() as u32).to_le_bytes());
+                body.extend_from_slice(&comp);
+            } else {
+                body.extend_from_slice(&(neblock as u32).to_le_bytes());
+                body.extend_from_slice(part);
+            }
+        }
+        if table_end + body.len() >= nbytes + HEADER {
+            // Incompressible: store instead, as c-blosc does.
+            return Ok(stored());
+        }
+    }
+    // A split block must decode as split: the decoder infers splitting from
+    // the same rule, which requires a whole number of elements per block.
+    let cbytes = table_end + body.len();
+    let mut out = header(flags, blocksize, cbytes);
+    for s in starts {
+        out.extend_from_slice(&(s as u32).to_le_bytes());
+    }
+    out.extend_from_slice(&body);
+    Ok(out)
+}
+
+/// Apply the frame's shuffle to one block (the inverse of
+/// [`unshuffle_block`]).
+fn shuffle_block(flags: u8, typesize: usize, src: &[u8], dest: &mut [u8]) {
+    let bsize = src.len();
+    if flags & FLAG_SHUFFLE != 0 && typesize > 1 {
+        let n = bsize / typesize;
+        for i in 0..n {
+            for b in 0..typesize {
+                dest[b * n + i] = src[i * typesize + b];
+            }
+        }
+        dest[n * typesize..].copy_from_slice(&src[n * typesize..]);
+    } else if flags & FLAG_BITSHUFFLE != 0 && bsize >= typesize {
+        let n = bsize / typesize;
+        if n.is_multiple_of(8) {
+            let body = n * typesize;
+            bitshuffle_block(&src[..body], &mut dest[..body], n, typesize);
+            dest[body..].copy_from_slice(&src[body..]);
+        } else {
+            dest.copy_from_slice(src);
+        }
+    } else {
+        dest.copy_from_slice(src);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::filter_pipeline::{FILTER_BLOSC, FilterDescription};
+
+    /// A blosclz stream: literal "abc", then a 9-byte match 3 back (a run
+    /// of "abc"), then literal "Z".
+    #[test]
+    fn blosclz_decodes_literals_and_overlapping_matches() {
+        // Match: length (ctrl >> 5) + 2 = 8, distance ofs + code + 1 = 3.
+        let stream = [2, b'a', b'b', b'c', (6 << 5), 2, 0, b'Z'];
+        let mut out = [0u8; 12];
+        assert_eq!(blosclz_decompress(&stream, &mut out), 12);
+        assert_eq!(&out, b"abcabcabcabZ");
+        // A stream cut inside a match is malformed.
+        let mut out = [0u8; 11];
+        assert_eq!(blosclz_decompress(&stream[..6], &mut out), 0);
+        // A match before the start of the output is malformed.
+        assert_eq!(blosclz_decompress(&[0, b'a', 32, 5, 0, b'x'], &mut out), 0);
+    }
+
+    fn desc(cd: Vec<u32>) -> FilterDescription {
+        FilterDescription {
+            filter_id: FILTER_BLOSC,
+            name: None,
+            flags: 0,
+            client_data: cd,
+        }
+    }
+
+    #[test]
+    fn frame_round_trips_every_codec_and_shuffle() {
+        for ts in [1usize, 2, 4, 8, 3, 32] {
+            for n in [0usize, 5, 100, 1000, 70_000, 300_001] {
+                if n * ts > 1 << 20 && ts > 1 {
+                    continue;
+                }
+                let data: Vec<u8> = (0..n * ts)
+                    .map(|i| ((i / ts) % 200) as u8 ^ (i % ts) as u8)
+                    .collect();
+                for codec in [1u32, 3, 4, 5] {
+                    for shuffle in [0u32, 1, 2] {
+                        for level in [0u32, 5] {
+                            let f = desc(vec![2, 2, ts as u32, 0, level, shuffle, codec]);
+                            let ctx = FilterContext {
+                                filter: &f,
+                                element_size: ts,
+                                max_output: data.len(),
+                            };
+                            let enc = blosc_encode(&data, &ctx).unwrap();
+                            let dec = blosc_decode(&enc, &ctx).unwrap_or_else(|e| {
+                                panic!("ts={ts} n={n} codec={codec} shuffle={shuffle}: {e}")
+                            });
+                            assert!(
+                                dec == data,
+                                "ts={ts} n={n} codec={codec} shuffle={shuffle} level={level}"
+                            );
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn rejects_bad_frames() {
+        let data = vec![9u8; 50_000];
+        let f = desc(vec![2, 2, 4, 0, 5, 1, 1]);
+        let ctx = FilterContext {
+            filter: &f,
+            element_size: 4,
+            max_output: data.len(),
+        };
+        let enc = blosc_encode(&data, &ctx).unwrap();
+        assert!(blosc_decode(&enc[..enc.len() - 3], &ctx).is_err());
+        let small = FilterContext {
+            max_output: 49_999,
+            ..ctx
+        };
+        assert!(blosc_decode(&enc, &small).is_err());
+        let mut v3 = enc.clone();
+        v3[0] = 3;
+        assert!(blosc_decode(&v3, &ctx).is_err());
+        let f0 = desc(vec![2, 2, 4, 0, 5, 1, 0]);
+        let ctx0 = FilterContext { filter: &f0, ..ctx };
+        assert!(blosc_encode(&data, &ctx0).is_err());
+    }
+}

crates/clawhdf5-format/src/lib.rs

@@ -73,8 +73,10 @@ pub mod fill_value;
 pub mod filter_pipeline;
 pub mod filter_registry;
 pub mod filters;
-#[cfg(feature = "bitshuffle")]
+#[cfg(any(feature = "bitshuffle", feature = "blosc"))]
 mod filters_bitshuffle;
+#[cfg(feature = "blosc")]
+pub mod filters_blosc;
 #[cfg(feature = "bzip2")]
 mod filters_bzip2;
 #[cfg(feature = "lzf")]

crates/clawhdf5-format/src/type_builders.rs

@@ -770,6 +770,22 @@ impl DatasetBuilder {
         self.with_plugin_filter(crate::chunked_write::PluginFilter::Bzip2 { level })
     }
 
+    /// Enable Blosc (filter 32001) with `codec` at `level` (0-9) after
+    /// `shuffle`. Implies chunked storage; no extra HDF5 shuffle is added.
+    /// Requires the `blosc` cargo feature.
+    pub fn with_blosc(
+        &mut self,
+        codec: crate::chunked_write::BloscCodec,
+        level: u32,
+        shuffle: crate::chunked_write::BloscShuffle,
+    ) -> &mut Self {
+        self.with_plugin_filter(crate::chunked_write::PluginFilter::Blosc {
+            codec,
+            level,
+            shuffle,
+        })
+    }
+
     /// Enable Pcodec lossless numerical compression (private clawhdf5 filter
     /// ID 480).
     ///

crates/clawhdf5/Cargo.toml

@@ -46,6 +46,9 @@ pcodec = ["clawhdf5-format/pcodec"]
 lzf = ["clawhdf5-format/lzf"]
 bitshuffle = ["clawhdf5-format/bitshuffle"]
 bzip2 = ["clawhdf5-format/bzip2"]
+blosc = ["clawhdf5-format/blosc"]
+# Every plugin filter.
+plugin-filters = ["lzf", "bitshuffle", "bzip2", "blosc"]
 # Dataset::verify_provenance() — recompute a dataset's SHA-256 and compare
 # against its stored _provenance_sha256 attribute. On by default, matching
 # clawhdf5-format's own default-on `provenance` feature.

crates/clawhdf5/tests/plugin_filters_interop.rs

@@ -76,7 +76,7 @@ try:
 except ImportError:
     hdf5plugin = None
 path = sys.argv[1]
-FILTERS = eval(sys.argv[2])
+FILTERS = eval('(' + sys.argv[2] + ')')
 cases = [
     ('<u1', (1000,), (128,), 'ramp'),
     ('<i2', (37, 53), (10, 16), 'ramp'),
@@ -303,6 +303,56 @@ fn bzip2_written_by_clawhdf5_reads_in_hdf5plugin() {
     });
 }
 
+#[cfg(feature = "blosc")]
+#[test]
+fn blosc_written_by_hdf5plugin_reads_exactly() {
+    if !have_python("h5py, hdf5plugin") {
+        return;
+    }
+    // Every codec hdf5plugin's Blosc offers, each shuffle mode, and levels
+    // from "store" to maximum.
+    check_h5py_written(
+        "blosc",
+        r#"[(f'{c} {s} {l}', hdf5plugin.Blosc(cname=c, clevel=l, shuffle=s))
+            for c in ['blosclz', 'lz4', 'lz4hc', 'snappy', 'zlib', 'zstd']
+            for s, l in [(hdf5plugin.Blosc.NOSHUFFLE, 5),
+                         (hdf5plugin.Blosc.SHUFFLE, 9),
+                         (hdf5plugin.Blosc.BITSHUFFLE, 1)]]
+           + [('blosclz level 0', hdf5plugin.Blosc(cname='blosclz', clevel=0))]"#,
+    );
+}
+
+#[cfg(feature = "blosc")]
+#[test]
+fn blosc_written_by_clawhdf5_reads_in_hdf5plugin() {
+    use clawhdf5_format::chunked_write::{BloscCodec, BloscShuffle};
+    if !have_python("h5py, hdf5plugin") {
+        return;
+    }
+    for codec in [
+        BloscCodec::Lz4,
+        BloscCodec::Snappy,
+        BloscCodec::Zlib,
+        BloscCodec::Zstd,
+    ] {
+        for (shuffle, level) in [
+            (BloscShuffle::None, 5),
+            (BloscShuffle::Byte, 9),
+            (BloscShuffle::Bit, 1),
+            (BloscShuffle::Byte, 0),
+        ] {
+            check_ours_read_by_h5py(
+                &format!("blosc_{codec:?}_{shuffle:?}_{level}"),
+                32001,
+                "blosc",
+                |ds| {
+                    ds.with_blosc(codec, level, shuffle);
+                },
+            );
+        }
+    }
+}
+
 #[cfg(feature = "lzf")]
 #[test]
 fn lzf_written_by_h5py_reads_exactly() {