support zero-sized chunks (#2434)

* support zero-sized chunks

* fix imports

* add min_side=0 to testing strategies

* fix property tests

---------

Co-authored-by: Deepak Cherian <deepak@cherian.net>

Author: brokkoli71

src/zarr/core/indexing.py

@@ -94,6 +94,8 @@ def __iter__(self) -> Iterator[ChunkProjection]: ...
 
 
 def ceildiv(a: float, b: float) -> int:
+    if a == 0:
+        return 0
     return math.ceil(a / b)
 
 
@@ -374,7 +376,7 @@ def __init__(self, dim_sel: slice, dim_len: int, dim_chunk_len: int) -> None:
 
     def __iter__(self) -> Iterator[ChunkDimProjection]:
         # figure out the range of chunks we need to visit
-        dim_chunk_ix_from = self.start // self.dim_chunk_len
+        dim_chunk_ix_from = 0 if self.start == 0 else self.start // self.dim_chunk_len
         dim_chunk_ix_to = ceildiv(self.stop, self.dim_chunk_len)
 
         # iterate over chunks in range

src/zarr/testing/strategies.py

@@ -65,7 +65,7 @@ def v2_dtypes() -> st.SearchStrategy[np.dtype]:
 stores = st.builds(MemoryStore, st.just({}), mode=st.just("w"))
 compressors = st.sampled_from([None, "default"])
 zarr_formats: st.SearchStrategy[Literal[2, 3]] = st.sampled_from([2, 3])
-array_shapes = npst.array_shapes(max_dims=4)
+array_shapes = npst.array_shapes(max_dims=4, min_side=0)
 
 
 @st.composite  # type: ignore[misc]
@@ -85,17 +85,24 @@ def numpy_arrays(
 @st.composite  # type: ignore[misc]
 def np_array_and_chunks(
     draw: st.DrawFn, *, arrays: st.SearchStrategy[np.ndarray] = numpy_arrays
-) -> tuple[np.ndarray, tuple[int]]:  # type: ignore[type-arg]
+) -> tuple[np.ndarray, tuple[int, ...]]:  # type: ignore[type-arg]
     """A hypothesis strategy to generate small sized random arrays.
 
     Returns: a tuple of the array and a suitable random chunking for it.
     """
     array = draw(arrays)
     # We want this strategy to shrink towards arrays with smaller number of chunks
     # 1. st.integers() shrinks towards smaller values. So we use that to generate number of chunks
-    numchunks = draw(st.tuples(*[st.integers(min_value=1, max_value=size) for size in array.shape]))
+    numchunks = draw(
+        st.tuples(
+            *[st.integers(min_value=0 if size == 0 else 1, max_value=size) for size in array.shape]
+        )
+    )
     # 2. and now generate the chunks tuple
-    chunks = tuple(size // nchunks for size, nchunks in zip(array.shape, numchunks, strict=True))
+    chunks = tuple(
+        size // nchunks if nchunks > 0 else 0
+        for size, nchunks in zip(array.shape, numchunks, strict=True)
+    )
     return (array, chunks)
 
 

tests/test_indexing.py

@@ -11,6 +11,7 @@
 from numpy.testing import assert_array_equal
 
 import zarr
+from zarr import Array
 from zarr.core.buffer import BufferPrototype, default_buffer_prototype
 from zarr.core.indexing import (
     BasicSelection,
@@ -31,7 +32,6 @@
 if TYPE_CHECKING:
     from collections.abc import AsyncGenerator
 
-    from zarr.core.array import Array
     from zarr.core.buffer.core import Buffer
     from zarr.core.common import ChunkCoords
 
@@ -1927,3 +1927,11 @@ def test_indexing_with_zarr_array(store: StorePath) -> None:
 
     assert_array_equal(a[ii], za[zii])
     assert_array_equal(a[ii], za.oindex[zii])
+
+
+@pytest.mark.parametrize("store", ["local", "memory"], indirect=["store"])
+@pytest.mark.parametrize("shape", [(0, 2, 3), (0), (3, 0)])
+def test_zero_sized_chunks(store: StorePath, shape: list[int]) -> None:
+    z = Array.create(store=store, shape=shape, chunk_shape=shape, zarr_format=3, dtype="f8")
+    z[...] = 42
+    assert_array_equal(z[...], np.zeros(shape, dtype="f8"))

tests/test_properties.py

@@ -6,7 +6,7 @@
 
 import hypothesis.extra.numpy as npst  # noqa: E402
 import hypothesis.strategies as st  # noqa: E402
-from hypothesis import given  # noqa: E402
+from hypothesis import assume, given  # noqa: E402
 
 from zarr.testing.strategies import arrays, basic_indices, numpy_arrays, zarr_formats  # noqa: E402
 
@@ -35,11 +35,13 @@ def test_basic_indexing(data: st.DataObject) -> None:
 @given(data=st.data())
 def test_vindex(data: st.DataObject) -> None:
     zarray = data.draw(arrays())
+    # integer_array_indices can't handle 0-size dimensions.
+    assume(all(s > 0 for s in zarray.shape))
     nparray = zarray[:]
 
     indexer = data.draw(
         npst.integer_array_indices(
-            shape=nparray.shape, result_shape=npst.array_shapes(max_dims=None)
+            shape=nparray.shape, result_shape=npst.array_shapes(min_side=1, max_dims=None)
         )
     )
     actual = zarray.vindex[indexer]