FIX Restore path compression in UnionFind.fast_find - #34872

Reference Issues/PRs

Fixes #34626.

What does this implement/fix? Explain your changes.

UnionFind.fast_find finds the correct root, but its existing path-compression loop does not repoint the queried node. The chained assignment advances the traversal variable before using it as the parent-array index. A lookup that starts at a root can also write through parent[-1] because the method enables Cython wraparound. The returned root generally remains correct, so the defect is visible primarily as quadratic linkage-labeling time on adversarial trees rather than incorrect clustering output.

This changes fast_find to use two explicit passes: first locate the root, then walk the original path while saving each next parent and repointing the current node to that root. It also removes @cython.wraparound(True) because negative indexing is no longer needed.

Performance

Direct local timings of the workload used by SingleLinkageLabelBenchmark produced:

The measured revisions were 8a6f766c6d (baseline) and 0bec25b5d9 (fix).

These are direct local timings rather than an asv continuous result. The loop counts
per repeat were 100 and 2 for the 2,000- and 16,000-sample baseline cases, and 5,000
and 500 for the corresponding fixed cases.

Introduce yourself

• what prompted you to work on UnionFind.fast_find's path compression never touches the queried node — quadratic linkage labelling #34626
  • I enjoy working on performance and efficiency related code.
• is this your first contribution
  • I have submitted one other pull request that was approved but ultimately not merged

AI usage disclosure

I used AI assistance for:

• Some code generation
  • some AI generated code for the initial version of the code but tweaked and completed by human
• Test and benchmark generation
• Research and understanding