crates/clawhdf5-format/src/chunked_write.rs

@@ -444,6 +444,27 @@ fn serialize_v4_fixed_array(
     element_size: u32,
     max_bits: u8,
 ) -> Vec<u8> {
+    let mut buf = layout_v4_chunked_prefix(chunk_dims, element_size);
+
+    // chunk index type = 3 (Fixed Array)
+    buf.push(3);
+
+    // max_dblk_page_nelmts_bits — must match FAHD max_nelmts_bits
+    buf.push(max_bits);
+
+    // Fixed Array header address
+    match offset_size {
+        4 => buf.extend_from_slice(&(fixed_array_address as u32).to_le_bytes()),
+        8 => buf.extend_from_slice(&fixed_array_address.to_le_bytes()),
+        _ => {}
+    }
+
+    buf
+}
+
+/// The part of a v4 chunked layout message before the chunk index type:
+/// version, class, flags and the chunk dimensions (plus the element size).
+fn layout_v4_chunked_prefix(chunk_dims: &[u32], element_size: u32) -> Vec<u8> {
     let mut buf = Vec::new();
     buf.push(4); // version
     buf.push(2); // class = chunked
@@ -483,20 +504,6 @@ fn serialize_v4_fixed_array(
         4 => buf.extend_from_slice(&element_size.to_le_bytes()),
         _ => {}
     }
-
-    // chunk index type = 3 (Fixed Array)
-    buf.push(3);
-
-    // max_dblk_page_nelmts_bits — must match FAHD max_nelmts_bits
-    buf.push(max_bits);
-
-    // Fixed Array header address
-    match offset_size {
-        4 => buf.extend_from_slice(&(fixed_array_address as u32).to_le_bytes()),
-        8 => buf.extend_from_slice(&fixed_array_address.to_le_bytes()),
-        _ => {}
-    }
-
     buf
 }
 
@@ -733,7 +740,8 @@ pub fn build_chunked_data_from_precompressed(
         data_buf.resize(aligned_idx, 0u8);
     }
 
-    let layout_message = if let ChunkIndexPlan::ExtensibleArray(grid) = &index {
+    let layout_message = match &index {
+        ChunkIndexPlan::ExtensibleArray(grid) => {
             let ea_address = base_address + data_buf.len() as u64;
             let slots = index_slots(grid, &pre.shape, &pre.chunk_dims, &written_chunks, None)?;
             let ea_bytes = ea_writer::build_extensible_array_at(
@@ -750,7 +758,8 @@ pub fn build_chunked_data_from_precompressed(
                 offset_size,
                 element_size as u32,
             )
-    } else if matches!(index, ChunkIndexPlan::SingleChunk) {
+        }
+        ChunkIndexPlan::SingleChunk => {
             let chunk_addr = written_chunks[0].address;
             let filtered_size = if pre.has_filters {
                 Some(written_chunks[0].compressed_size)
@@ -766,7 +775,8 @@ pub fn build_chunked_data_from_precompressed(
                 offset_size,
                 element_size as u32,
             )
-    } else if let ChunkIndexPlan::FixedArray(grid, nslots) = &index {
+        }
+        ChunkIndexPlan::FixedArray(grid, nslots) => {
             let fa_address = base_address + data_buf.len() as u64;
             let slots = index_slots(
                 grid,
@@ -790,8 +800,31 @@ pub fn build_chunked_data_from_precompressed(
                 element_size as u32,
                 FA_PAGE_BITS,
             )
-    } else {
+        }
-        unreachable!("every chunk index plan is handled above")
+        ChunkIndexPlan::BTreeV2 => {
+            let bt_address = base_address + data_buf.len() as u64;
+            let records: Vec<(Vec<u64>, &WrittenChunk)> = written_chunks
+                .iter()
+                .enumerate()
+                .map(|(i, c)| (scaled_coords(&pre.shape, &pre.chunk_dims, i), c))
+                .collect();
+            let (bt_bytes, node_size) = build_btree_v2_chunk_index_at(
+                pre.shape.len(),
+                &records,
+                offset_size,
+                length_size,
+                pre.has_filters,
+                bt_address,
+            )?;
+            data_buf.extend_from_slice(&bt_bytes);
+            serialize_v4_btree_v2(
+                &chunk_dims_u32,
+                bt_address,
+                offset_size,
+                element_size as u32,
+                node_size,
+            )
+        }
     };
 
     Ok(ChunkedDataResult {
@@ -807,14 +840,15 @@ pub fn build_chunked_data_from_precompressed(
 const MAX_FIXED_ARRAY_SLOTS: u64 = 1 << 26;
 
 /// Which chunk index a dataset gets, following the library's choice in
-/// `H5D__layout_set_latest_indexing`: Extensible Array for exactly one
+/// `H5D__layout_set_latest_indexing`: version-2 B-tree for more than one
-/// unlimited dimension, Fixed Array for a finite maxshape, Single Chunk when
+/// unlimited dimension, Extensible Array for exactly one, Fixed Array for a
-/// the whole maximum extent is one chunk.
+/// finite maxshape, Single Chunk when the whole maximum extent is one chunk.
 enum ChunkIndexPlan {
     SingleChunk,
     /// The grid and the number of array elements (chunks of the max extent).
     FixedArray(ChunkGrid, usize),
     ExtensibleArray(ChunkGrid),
+    BTreeV2,
 }
 
 impl ChunkIndexPlan {
@@ -860,10 +894,7 @@ impl ChunkIndexPlan {
                 Some(max),
                 chunk_dims,
             )?)),
-            _ => Err(bad(
+            _ => Ok(Self::BTreeV2),
-                "more than one unlimited dimension needs a B-tree v2 chunk index, \
-                 which the writer does not support",
-            )),
         }
     }
 }
@@ -879,20 +910,9 @@ fn index_slots(
     chunks: &[WrittenChunk],
     len: Option<usize>,
 ) -> Result<Vec<Option<WrittenChunk>>, FormatError> {
-    let rank = shape.len();
-    let cur: Vec<u64> = shape
-        .iter()
-        .zip(chunk_dims)
-        .map(|(&s, &c)| s.div_ceil(c))
-        .collect();
     let mut placed: Vec<(usize, &WrittenChunk)> = Vec::with_capacity(chunks.len());
-    let mut scaled = vec![0u64; rank];
     for (i, chunk) in chunks.iter().enumerate() {
-        let mut rem = i as u64;
+        let scaled = scaled_coords(shape, chunk_dims, i);
-        for d in (0..rank).rev() {
-            scaled[d] = rem % cur[d];
-            rem /= cur[d];
-        }
         let idx = usize::try_from(grid.linear_index(&scaled))
             .map_err(|_| FormatError::Overflow("chunk index slot".into()))?;
         placed.push((idx, chunk));
@@ -907,6 +927,136 @@ fn index_slots(
     Ok(slots)
 }
 
+/// Scaled coordinates (`offset / chunk_dim`) of the `i`-th chunk in the
+/// row-major order `split_into_chunks` produces over the current extent.
+fn scaled_coords(shape: &[u64], chunk_dims: &[u64], i: usize) -> Vec<u64> {
+    let rank = shape.len();
+    let mut scaled = vec![0u64; rank];
+    let mut rem = i as u64;
+    for d in (0..rank).rev() {
+        let n = shape[d].div_ceil(chunk_dims[d]);
+        scaled[d] = rem % n;
+        rem /= n;
+    }
+    scaled
+}
+
+/// Node size the library gives a chunk index B-tree (`H5D_BT2_NODE_SIZE`),
+/// with its split and merge percentages.
+const BT2_NODE_SIZE: u32 = 2048;
+const BT2_SPLIT_PERCENT: u8 = 100;
+const BT2_MERGE_PERCENT: u8 = 40;
+/// B-tree v2 record types for chunk indexes (`H5B2_CDSET_ID`,
+/// `H5B2_CDSET_FILT_ID`).
+const BT2_CHUNK_UNFILTERED: u8 = 10;
+const BT2_CHUNK_FILTERED: u8 = 11;
+
+/// Build a version-2 B-tree chunk index (the library's index for datasets
+/// with more than one unlimited dimension) at a known absolute address.
+///
+/// `records` are `(scaled coordinates, chunk)` in lexicographic order of the
+/// coordinates, which is the order the library's comparator
+/// (`H5VM_vector_cmp_u`) keeps them in. The tree is a single leaf: the
+/// library's 2048-byte node when the records fit, otherwise a leaf node
+/// sized to hold them all (the root's record count is 16-bit, so at most
+/// 65535 chunks). Returns the bytes and the node size the layout message
+/// must record.
+fn build_btree_v2_chunk_index_at(
+    rank: usize,
+    records: &[(Vec<u64>, &WrittenChunk)],
+    offset_size: u8,
+    length_size: u8,
+    has_filters: bool,
+    base_address: u64,
+) -> Result<(Vec<u8>, u32), FormatError> {
+    let os = offset_size as usize;
+    let nrec = u16::try_from(records.len()).map_err(|_| {
+        FormatError::ChunkedReadError(
+            "more than 65535 chunks with more than one unlimited dimension: \
+             use larger chunks"
+                .into(),
+        )
+    })?;
+    let chunk_size_bytes = has_filters.then(|| {
+        let slots: Vec<Option<WrittenChunk>> =
+            records.iter().map(|(_, c)| Some((*c).clone())).collect();
+        filtered_chunk_size_len(&slots)
+    });
+    let record_size = os + chunk_size_bytes.map_or(0, |n| n + 4) + 8 * rank;
+    // Leaf: signature, version, type, records, checksum.
+    let leaf_len = 4 + 1 + 1 + records.len() * record_size + 4;
+    let node_size = u32::try_from(leaf_len)
+        .map_err(|_| FormatError::Overflow("B-tree v2 leaf size".into()))?
+        .max(BT2_NODE_SIZE);
+    let tree_type = if has_filters {
+        BT2_CHUNK_FILTERED
+    } else {
+        BT2_CHUNK_UNFILTERED
+    };
+
+    let hdr_len = 4 + 1 + 1 + 4 + 2 + 2 + 1 + 1 + os + 2 + length_size as usize + 4;
+    let leaf_address = base_address + hdr_len as u64;
+
+    let mut out = Vec::with_capacity(hdr_len + node_size as usize);
+    out.extend_from_slice(b"BTHD");
+    out.push(0); // version
+    out.push(tree_type);
+    out.extend_from_slice(&node_size.to_le_bytes());
+    out.extend_from_slice(&(record_size as u16).to_le_bytes());
+    out.extend_from_slice(&0u16.to_le_bytes()); // depth
+    out.push(BT2_SPLIT_PERCENT);
+    out.push(BT2_MERGE_PERCENT);
+    if records.is_empty() {
+        out.extend(core::iter::repeat_n(0xFF, os));
+    } else {
+        push_addr(&mut out, leaf_address, offset_size);
+    }
+    out.extend_from_slice(&nrec.to_le_bytes());
+    match length_size {
+        4 => out.extend_from_slice(&(records.len() as u32).to_le_bytes()),
+        _ => out.extend_from_slice(&(records.len() as u64).to_le_bytes()),
+    }
+    let sum = jenkins_lookup3(&out);
+    out.extend_from_slice(&sum.to_le_bytes());
+    debug_assert_eq!(out.len(), hdr_len);
+    if records.is_empty() {
+        return Ok((out, node_size));
+    }
+
+    let leaf_start = out.len();
+    out.extend_from_slice(b"BTLF");
+    out.push(0); // version
+    out.push(tree_type);
+    for (scaled, chunk) in records {
+        push_index_element(&mut out, Some(chunk), offset_size, chunk_size_bytes);
+        for &c in scaled {
+            out.extend_from_slice(&c.to_le_bytes());
+        }
+    }
+    let sum = jenkins_lookup3(&out[leaf_start..]);
+    out.extend_from_slice(&sum.to_le_bytes());
+    // The library reads whole nodes; pad the leaf out to the node size.
+    out.resize(leaf_start + node_size as usize, 0);
+    Ok((out, node_size))
+}
+
+/// Serialize a v4 layout message for a version-2 B-tree chunk index.
+fn serialize_v4_btree_v2(
+    chunk_dims: &[u32],
+    btree_address: u64,
+    offset_size: u8,
+    element_size: u32,
+    node_size: u32,
+) -> Vec<u8> {
+    let mut buf = layout_v4_chunked_prefix(chunk_dims, element_size);
+    buf.push(5); // chunk index type = 5 (version-2 B-tree)
+    buf.extend_from_slice(&node_size.to_le_bytes());
+    buf.push(BT2_SPLIT_PERCENT);
+    buf.push(BT2_MERGE_PERCENT);
+    push_addr(&mut buf, btree_address, offset_size);
+    buf
+}
+
 /// Build chunked data with absolute addresses.
 /// If `maxshape` has unlimited dims, uses Extensible Array index.
 pub fn build_chunked_data_at(

crates/clawhdf5/tests/chunk_index_interop.rs

@@ -515,16 +515,35 @@ fn we_write_maxshape_larger_than_shape() {
     check_we_write(&cases);
 }
 
-/// More than one unlimited dimension needs a B-tree v2 chunk index; the
+/// More than one unlimited dimension needs a version-2 B-tree chunk index,
-/// writer must not produce a file libhdf5 cannot open.
+/// as the library uses; an Extensible Array for `(None, None)` made libhdf5
+/// refuse the whole file ("already found unlimited dimension").
 #[test]
-fn two_unlimited_dims_are_refused() {
+fn we_write_btree_v2_for_several_unlimited_dims() {
+    const U: u64 = u64::MAX;
+    let mut cases = vec![
+        wcase("unl_unl", &[20, 30], &[5, 5], Some(&[U, U])),
+        wcase("unl_fin_unl", &[6, 7, 8], &[4, 3, 5], Some(&[U, 9, U])),
+        // More records than the library's 2048-byte node holds (84 here).
+        wcase("unl_unl_2400", &[40, 60], &[1, 1], Some(&[U, U])),
+        wcase("unl_unl_empty", &[0, 0], &[4, 4], Some(&[U, U])),
+    ];
+    let mut filtered = wcase("unl_unl_deflate", &[6, 7, 8], &[4, 3, 5], Some(&[U, U, U]));
+    filtered.deflate = true;
+    cases.push(filtered);
+    check_we_write(&cases);
+}
+
+/// A single-leaf B-tree has a 16-bit record count; beyond it the writer
+/// refuses rather than writing a tree libhdf5 would misread.
+#[test]
+fn btree_v2_index_past_one_leaf_is_refused() {
     let mut b = FileBuilder::new();
     b.create_dataset("d")
-        .with_i32_data(&(0..600).collect::<Vec<i32>>())
+        .with_i32_data(&vec![0i32; 70_000])
-        .with_shape(&[20, 30])
+        .with_shape(&[70_000, 1])
-        .with_chunks(&[5, 5])
+        .with_chunks(&[1, 1])
         .with_maxshape(&[u64::MAX, u64::MAX]);
     let dir = tempfile::tempdir().unwrap();
-    assert!(b.write(dir.path().join("unl_unl.h5")).is_err());
+    assert!(b.write(dir.path().join("too_many.h5")).is_err());
 }