GH-48679: [C++] Fix pivot_wider with non-monotonic group ids#50423
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GH-48679: [C++] Fix pivot_wider with non-monotonic group ids#50423pitrou wants to merge 1 commit into
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pitrou
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@zanmato1984 Do you think this is ok, or should FastGrouperImpl be fixed to always produce monotonic group ids? (by the way, I was looking for a way to force group ids to always be non-monotonic in debug mode, but that seems quite non-trivial) |
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Rationale for this change
The pivot_wider implementation was assuming that the
Grouperproduces monotonic group ids for the pivot key names.However, that turns out to not always be true when the
FastGrouperImplis involved (perhaps when hash collisions are involved?).What changes are included in this PR?
Compute a apply a mapping from pivot key group ids back to pivot key indices.
Are these changes tested?
Yes, by additional tests.
Are there any user-facing changes?
Only a bugfix.
hash_pivot_wider#48679