BUG: Fix buffered iterator stride after removing multi-index - #32116
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Backport of #31985.
PR summary
This PR fixes a critical bug where incorrect coordinates are returned after the iterator buffer boundary for large
(N, 1)index arrays. As seen in real-world applications (e.g., MRI reconstruction inmckib2/pygrappa#107), this can silently produce incorrect results without raising any exceptions.Root Cause:
The issue occurs because the transfer loop is fixed to a zero stride for a size-one inner axis, failing to reflect the updated runtime stride after
remove_multi_indexis applied.Fix:
To resolve this, I modified
numpy/_core/src/multiarray/nditer_constr.cto exclude mutable strides from the fixed-stride optimization.Testing & Verification:
nditerregression tests innumpy/_core/tests/test_nditer.py.unravel_indexregression tests innumpy/lib/tests/test_index_tricks.py.Closes #31980
First time committer introduction
Hi, I am snoopuppy582, a developer using NumPy. I investigated this issue because of its silent but critical impact on real-world calculations, and I hope this patch helps improve the stability of the library.
AI Disclosure
I used AI tools (OpenAI Codex / ChatGPT) strictly for review and verification purposes to ensure the quality of the fix.
I manually analyzed the root cause, wrote the C modifications and regression tests, and executed all local validations. I have fully reviewed and understood the entire diff. Also, this PR description, the commit messages, and all future communications are written entirely by me manually.