fix(chunk-grids): make array creation O(1) in per-dimension chunk count

changes/4174.bugfix.md

@@ -0,0 +1,13 @@
+Array creation is now O(1) in the number of chunks per dimension. Chunk
+normalization returns a `ChunkGrid` whose uniform dimensions are stored as a
+size + extent pair (`FixedDimension`) instead of being expanded to one entry
+per chunk, so creating arrays like
+`zarr.create_array(store, shape=(2**62,), chunks=(1,), dtype='int32')` succeeds
+instantly instead of raising `ValueError` or allocating gigabytes of memory.
+The intermediate `ChunksTuple` representation was removed in the process, and
+`ChunksLike` now admits per-dimension specs that mix a bare int (uniform chunk
+size) with explicit edge-length sequences, matching what the normalizer and
+the rectilinear grid spec already accepted.
+This fixes the array-creation half of #4174; the coordinate-selection
+allocation reported there is still tracked in that issue (#4172 fixed the
+related case of sorted 1-D coordinate selections).

changes/4272.bugfix.md

@@ -0,0 +1,27 @@
+Explicit per-chunk size lists now always produce a rectilinear chunk grid,
+even when the sizes happen to describe a regular grid (all equal, or all equal
+with a smaller trailing chunk). Previously such input was silently collapsed to
+a regular grid, which changed resize semantics: a regular grid grows by
+extending the uniform pattern, while a rectilinear grid appends a new edge
+chunk — the behavior an append-oriented layout like `(168,) * 13 + (24,)`
+relies on. The grid kind now follows the input syntax, matching 3.2.x:
+scalar chunk sizes (including numpy integers and the `-1` sentinel) produce a
+regular grid, nested sequences produce a rectilinear grid. Rectilinear grids
+remain gated behind `zarr.config.set({"array.rectilinear_chunks": True})`.
+See #4174 for the accompanying O(1) chunk normalization change.
+
+`zarr.from_array` with the default `chunks="keep"` / `shards="keep"` now
+reproduces the source's stored grid exactly: a rectilinear grid is passed
+through in O(number of dimensions), with uniform dimensions keeping their
+bare-int shorthand; sharding under a rectilinear shard grid is preserved
+instead of being silently dropped; and the default `write_data=True` copy
+works for every grid kind. `Array.chunks` is now defined for any sharded array
+(the inner chunks of a shard are always regular), and for sharded arrays with
+a rectilinear shard grid `Array.info` no longer raises — it reports the shard
+shape as `<variable>` — while `Array.nchunks_initialized` counts the chunks of
+each initialized shard individually instead of raising.
+
+Everything described here concerns rectilinear chunk grids, which remain an
+experimental feature gated behind
+`zarr.config.set({"array.rectilinear_chunks": True})`; arrays with regular
+chunk grids are unaffected.

docs/user-guide/arrays.md

@@ -674,8 +674,12 @@ z_regular = zarr.create_array(
 print(z_regular.write_chunk_sizes)
 ```
 
-Note that the `.chunks` property is only available for regular chunk grids. For
-rectilinear arrays, use `.write_chunk_sizes` (or `.read_chunk_sizes`) instead.
+Note that the `.chunks` property is not available for non-sharded rectilinear
+arrays, since there is no single uniform chunk shape — use `.write_chunk_sizes`
+(or `.read_chunk_sizes`) instead. Sharded arrays always have `.chunks`: it
+returns the inner chunk shape, which is always regular — the sharding codec
+requires a single uniform inner chunk shape, so only the shard boundaries can
+be rectilinear (see [Rectilinear shard boundaries](#rectilinear-shard-boundaries)).
 
 ### Resizing and appending
 
@@ -746,6 +750,17 @@ print(z[50:70, 40:60])
 Note that rectilinear inner chunks with sharding are not supported — only the
 shard boundaries can be rectilinear.
 
+For such arrays, `.chunks` returns the (regular) inner chunk shape, while
+`.shards` raises `NotImplementedError` since there is no single uniform shard
+shape — use `.write_chunk_sizes` for the per-dimension shard sizes. `.info`
+reports the shard shape as `<variable>`:
+
+```python exec="true" session="arrays" source="above" result="ansi"
+print(f"chunks={z.chunks}")
+print(f"shard sizes={z.write_chunk_sizes}")
+print(z.info)
+```
+
 ### Metadata format
 
 Rectilinear chunk grid metadata uses run-length encoding (RLE) for compact

src/zarr/core/_info.py

@@ -82,7 +82,9 @@ class ArrayInfo:
     _data_type: ZDType[TBaseDType, TBaseScalar]
     _fill_value: object
     _shape: tuple[int, ...]
-    _shard_shape: tuple[int, ...] | None = None
+    # "<variable>" marks a sharded array whose shard grid is rectilinear, so
+    # there is no uniform shard shape; None means the array is not sharded.
+    _shard_shape: tuple[int, ...] | Literal["<variable>"] | None = None
     _chunk_shape: tuple[int, ...] | None = None
     _order: Literal["C", "F"]
     _read_only: bool