[d-v-b]

Summary

performance win for pre-sorted coordinate indices.

🤖 AI text below 🤖

Writes whose indices land on a chunk in sorted order no longer sort them to drop repeated indices, so deduplication is linear in the number of points; writes without repeats also skip a gather copy. Unsorted indices behave as before.

_last_occurrences now checks in O(n) whether the flat positions are non-decreasing, the usual case since a chunk's points arrive in C order. If they are, repeats are adjacent and the last of each run is kept via np.flatnonzero(np.append(flat[1:] != flat[:-1], True)). If there are no repeats, or fewer than two positions, it returns slice(None), so _scatter_numpy and _scatter_planned index without copying. Unsorted input still goes through np.unique. Callers only use the result to pick matching rows of coordinates and values, so the order of the kept positions doesn't matter. The sorted-input check is borrowed from the last_occurrences helper that Bhaskar Gurram (bhaskargurram-ai) wrote in #4601; credit for the idea goes to them.

A new parametrized test, test_last_occurrences_matches_sequential_assignment, checks empty, single, sorted, sorted-with-repeats, unsorted-with-repeats and reversed input against sequential NumPy assignment.

Timing for _last_occurrences on 5M intp positions (NumPy 2.5.1, Python 3.13, macOS arm64, best of 5):

input	before	after	speedup
sorted, no repeats	12.9 ms	1.2 ms	11.0x
sorted, repeats	183.3 ms	3.1 ms	59.6x
unsorted, repeats	766.7 ms	765.2 ms	1.0x

The zarr-indexing suite passes on Python 3.13 (1745 passed), and just typecheck and just lint are clean. On Python 3.15, four tests fail on main too, because they match the __index__ error message that 3.15 changed.

🤖 Generated with Claude Code

Author attestation

• [ x ] I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)