Fix UnionFind.fast_find path compression bug (#34626) - #34881
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The path compression in UnionFind.fast_find was broken due to a Cython tuple assignment issue. The original code: evaluates the right-hand side as a tuple first, then assigns left to right. This means is updated before is assigned, so the path compression doesn't work correctly. The fix walks to the root first, then compresses every node on the path in a second pass: This restores O(α(n)) amortized time complexity for repeated fast_find calls, where α is the inverse Ackermann function. Without this fix, single-linkage clustering was O(n²) instead of O(n·α(n)). Fixes scikit-learn#34626
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This PR fixes issue #34626 - a path compression bug in UnionFind.fast_find that caused O(n²) performance in single-linkage clustering.
Problem
The path compression in UnionFind.fast_find was broken due to a Cython tuple assignment issue:
This evaluates the right-hand side as a tuple first, then assigns left to right. This means is updated before is assigned, so the path compression doesn't work correctly.
Solution
The fix walks to the root first, then compresses every node on the path in a second pass:
Impact
Testing
This pull request includes code written with the assistance of AI.
The code has not yet been reviewed by a human.