Speed up sieve labeling and add convergence warning#1163
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Replace per-value scipy.ndimage.label calls with a single-pass numba union-find for connected-component labeling. Rework the adjacency builder to use 1-D np.unique on encoded int64 pairs instead of the slower np.unique(axis=0) on 2-D arrays. Add a UserWarning when the 50-iteration merge loop doesn't converge.
Move the convergence warning from _sieve_numpy into sieve() so stacklevel=2 always points at the caller regardless of backend. Return (result, converged) tuple from all backend functions. Remove unused import in test_sieve_convergence_warning. Add int32 limit note to _label_connected docstring.
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Closes #1162
Summary
scipy.ndimage.labelcalls with a single-pass numba union-find, cutting labeling cost from O(k*n) to O(n)np.uniqueon encoded int64 pairs instead of slower 2-Dnp.unique(axis=0)UserWarningwhen the 50-iteration merge loop doesn't converge (previously silent)Test plan
_MAX_ITERATIONS=0)