feat(format): name the filter in UnsupportedFilter errors; Blosc2/ZFP stay errors

Blosc2 (32026) is out of reach for now: hdf5plugin's Blosc2 filter stores
each HDF5 chunk as a Blosc2 super-chunk frame (msgpack header, a compressed
chunk-offset index, trailer metalayers) and, for 2-D and larger chunks, as
a B2ND array whose n-D blocks have to be reassembled - on top of the Blosc2
chunk format itself (extended header, filter pipeline, special-value
chunks). ZFP (32013) is out of scope. Both keep failing with
UnsupportedFilter, and the message now says what the ID is:
"unsupported filter: 32026 (Blosc2, not implemented by clawhdf5)", or,
for a filter this build left out, "... (Blosc; this build lacks the
`blosc` feature)". filter_registry::known_filter exposes the table.

tests/plugin_filters_interop.rs: hdf5plugin's Blosc2 and ZFP datasets
read as an error naming the filter, never as data.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/error.rs

@@ -406,9 +406,17 @@ impl fmt::Display for FormatError {
             FormatError::InvalidFilterPipelineVersion(v) => {
                 write!(f, "invalid filter pipeline version: {v}")
             }
-            FormatError::UnsupportedFilter(id) => {
-                write!(f, "unsupported filter: {id}")
-            }
+            FormatError::UnsupportedFilter(id) => match crate::filter_registry::known_filter(*id) {
+                Some((name, Some(feature))) => write!(
+                    f,
+                    "unsupported filter: {id} ({name}; this build lacks the `{feature}` feature)"
+                ),
+                Some((name, None)) => write!(
+                    f,
+                    "unsupported filter: {id} ({name}, not implemented by clawhdf5)"
+                ),
+                None => write!(f, "unsupported filter: {id}"),
+            },
             FormatError::FilterError(msg) => {
                 write!(f, "filter error: {msg}")
             }

crates/clawhdf5-format/src/filter_registry.rs

@@ -130,6 +130,30 @@ pub fn builtin_filter(id: u16) -> Option<&'static BuiltinFilter> {
     builtin_filters().iter().find(|f| f.id == id)
 }
 
+/// Why a filter ID may be missing from this build: the filter's name, and
+/// the cargo feature that provides it (`None`: clawhdf5 does not implement
+/// it — register a codec for it with [`register_filter`]). `None` for an ID
+/// clawhdf5 knows nothing about.
+pub fn known_filter(id: u16) -> Option<(&'static str, Option<&'static str>)> {
+    Some(match id {
+        1 => ("deflate", Some("deflate")),
+        4 => ("SZIP", Some("szip")),
+        307 => ("bzip2", Some("bzip2")),
+        480 => ("pcodec", Some("pcodec")),
+        32000 => ("LZF", Some("lzf")),
+        32001 => ("Blosc", Some("blosc")),
+        32004 => ("LZ4", Some("lz4")),
+        32008 => ("bitshuffle", Some("bitshuffle")),
+        32013 => ("ZFP", None),
+        32015 => ("Zstandard", Some("zstd")),
+        32019 => ("JPEG", None),
+        32022 => ("BitGroom", None),
+        32023 => ("Granular BitRound", None),
+        32026 => ("Blosc2", None),
+        _ => return None,
+    })
+}
+
 /// Whether a chunk filtered with `id` can be decoded: a built-in filter or a
 /// registered one.
 pub fn is_filter_available(id: u16) -> bool {
@@ -344,6 +368,23 @@ mod tests {
         assert!(builtin_filter(FILTER_SHUFFLE).is_some());
     }
 
+    #[test]
+    fn unsupported_filter_error_names_the_filter() {
+        let msg = FormatError::UnsupportedFilter(32026).to_string();
+        assert!(
+            msg.contains("Blosc2") && msg.contains("not implemented"),
+            "{msg}"
+        );
+        let msg = FormatError::UnsupportedFilter(32013).to_string();
+        assert!(msg.contains("ZFP"), "{msg}");
+        let msg = FormatError::UnsupportedFilter(32000).to_string();
+        assert!(msg.contains("LZF") && msg.contains("`lzf`"), "{msg}");
+        assert_eq!(
+            FormatError::UnsupportedFilter(399).to_string(),
+            "unsupported filter: 399"
+        );
+    }
+
     #[test]
     fn builtin_table_is_sorted_and_unique() {
         let ids: Vec<u16> = builtin_filters().iter().map(|f| f.id).collect();

crates/clawhdf5/tests/plugin_filters_interop.rs

@@ -380,3 +380,38 @@ fn lzf_written_by_clawhdf5_reads_in_h5py() {
         ds.with_lzf().without_shuffle();
     });
 }
+
+/// Blosc2 and ZFP are not implemented: reading them must be a clear error
+/// naming the filter, never data.
+#[test]
+fn unimplemented_filters_are_a_clear_error() {
+    if !have_python("h5py, hdf5plugin") {
+        return;
+    }
+    let dir = tempfile::tempdir().unwrap();
+    let path = dir.path().join("unimplemented.h5");
+    run_python(
+        r#"
+import sys
+import numpy as np, h5py, hdf5plugin
+with h5py.File(sys.argv[1], 'w') as f:
+    d = np.arange(4096, dtype='<f4').reshape(64, 64)
+    f.create_dataset('blosc2', data=d, chunks=(16, 16), **hdf5plugin.Blosc2())
+    f.create_dataset('zfp', data=d, chunks=(16, 16), **hdf5plugin.Zfp(reversible=True))
+"#,
+        &[path.to_str().unwrap()],
+    );
+    let file = File::open(&path).unwrap();
+    for (name, id, label) in [("blosc2", 32026u16, "Blosc2"), ("zfp", 32013, "ZFP")] {
+        let err = file
+            .dataset(name)
+            .unwrap()
+            .read_selection(&Selection::All)
+            .expect_err("an unimplemented filter must not read");
+        let msg = err.to_string();
+        assert!(
+            msg.contains(&id.to_string()) && msg.contains(label),
+            "{name}: {msg}"
+        );
+    }
+}