feat: write complex numbers, incl. HDF5 2.0 native complex (class 11)

- Datatype::Complex serializes class 11 version 5 byte-identically to
  libhdf5 2.2.0; containers holding it are written as version 5.
- DatasetBuilder::with_complex_f32/f64_data (h5py's {r, i} compound,
  default) and with_native_complex_f32/f64_data (class 11, opt-in);
  make_(native_)complex_f32/f64_type for attributes.
- Dataset::read_complex_f64/f32 read either form.
- Python create_dataset accepts complex64/complex128 (compound form).
- Parsing unchanged: class 11 still surfaces as {r, i}.
- Tests vs h5py 3.16 / libhdf5 2.0.0 and h5dump 2.2.0; docs.

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

crates/clawhdf5-py/tests/test_write_read.py

@@ -247,6 +247,51 @@ def test_roundtrip_uint8(tmp_h5):
         assert result.dtype == np.uint8
 
 
+@pytest.mark.parametrize("dtype", [np.complex64, np.complex128])
+def test_roundtrip_complex(tmp_h5, dtype):
+    """Complex arrays are written as h5py writes them (a compound {r, i});
+    h5py and clawhdf5 both read them back as the same numpy complex dtype."""
+    import h5py
+
+    original = (np.arange(12).reshape(3, 4) * (1.5 - 0.25j)).astype(dtype)
+    with clawhdf5.File(tmp_h5, "w") as f:
+        f.create_dataset("z", data=original)
+        f.create_dataset("zc", data=original, chunks=(2, 2), compression="gzip")
+    with clawhdf5.File(tmp_h5, "r") as f:
+        for name in ["z", "zc"]:
+            result = f[name][:]
+            assert result.dtype == dtype
+            np.testing.assert_array_equal(result, original)
+    with h5py.File(tmp_h5, "r") as f:
+        for name in ["z", "zc"]:
+            assert f[name].dtype == dtype
+            assert f[name].id.get_type().get_class() == h5py.h5t.COMPOUND
+            np.testing.assert_array_equal(f[name][:], original)
+
+
+def test_read_native_complex_from_h5py(tmp_h5):
+    """HDF5 2.0's native complex type (class 11), written through h5py's
+    low-level API, reads as numpy complex."""
+    import h5py
+    from h5py import h5s, h5t
+
+    if not getattr(h5py.get_config(), "has_native_complex", False):
+        pytest.skip("h5py's libhdf5 predates 2.0")
+    original = np.array([1 + 2j, -3.5 + 0j, 0 - 1e-3j])
+    with h5py.File(tmp_h5, "w") as f:
+        for name, t, dt in [
+            (b"n64", h5t.COMPLEX_IEEE_F32LE, np.complex64),
+            (b"n128", h5t.COMPLEX_IEEE_F64LE, np.complex128),
+        ]:
+            d = h5py.h5d.create(f.id, name, t, h5s.create_simple((3,)))
+            d.write(h5s.ALL, h5s.ALL, original.astype(dt), mtype=t)
+    with clawhdf5.File(tmp_h5, "r") as f:
+        for name, dt in [("n64", np.complex64), ("n128", np.complex128)]:
+            result = f[name][:]
+            assert result.dtype == dt
+            np.testing.assert_array_equal(result, original.astype(dt))
+
+
 # ---------------------------------------------------------------------------
 # Test: chunked + compressed datasets
 # ---------------------------------------------------------------------------