Replace O(n⁴) regions() with scipy union-find, add dask/cupy backends#898
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brendancol merged 1 commit intomasterfrom Feb 25, 2026
Merged
Replace O(n⁴) regions() with scipy union-find, add dask/cupy backends#898brendancol merged 1 commit intomasterfrom
brendancol merged 1 commit intomasterfrom
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The old _area_connectivity algorithm did full-array scans inside the main pixel loop for conflict resolution, giving O(n⁴) worst case. Replace it with scipy.ndimage.label (union-find, ~O(n)) per unique value, and add backend dispatch for dask, cupy, and dask+cupy arrays.
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Summary
_area_connectivitywithscipy.ndimage.label(union-find, ~O(n) per unique value). The old algorithm did full-array scans inside the main pixel loop for conflict resolution.cupyx.scipy.ndimage.label), and dask+cupy arrays.install_requires— it was already a transitive dependency of datashader and in test deps.['numpy', 'dask+numpy']backends and add edge-case tests (single pixel, all-same, all-NaN, numpy/dask match).Test plan
pytest -k regions -v)