support zero-sized chunks

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

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,
@@ -1927,3 +1928,11 @@ 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"))