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MINOR: [C++] Check size() instead of capacity() in TDigest::TDigestImpl::Validate() #52276

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MINOR: [C++] Check size() instead of capacity() in TDigest::TDigestImpl::Validate() #52276

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MINOR: [C++] Check size() instead of capacity() in TDigest::TDigestIm…
ezbr Oct 7, 2026
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Update comment in TDigestImpl::Validate()
ezbr Oct 9, 2026
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4 changes: 2 additions & 2 deletions cpp/src/arrow/util/tdigest.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -168,8 +168,8 @@ class TDigest::TDigestImpl {
if (total_weight != total_weight_) {
return Status::Invalid("tdigest total weight mismatch");
}
// check if buffer expanded
if (tdigests_[0].capacity() > delta_ || tdigests_[1].capacity() > delta_) {
// check if buffer size exceeded delta

@cyb70289 cyb70289 Oct 14, 2026 •
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The point is at line 104
td.push_back(centroid); // should never exceed capacity and trigger reallocation

A correctly implemented algorithm shall not write above pre-allocated capacity. I don't add check code there for performance, and write above capacity is not a disaster (the algorithm still works).

tdigests_[n] may be cleared during usage, size() doesn't guarantee the vector was not expanded.

I think we can record the actual buffer size at construction time, and check against that.

Note this is a loose check. As this PR points out, real capacity is often bigger than required.

@cyb70289 cyb70289 Oct 14, 2026

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Or we can just delete this check for simplicity. Looks to me I was overly cautious.

if (tdigests_[0].size() > delta_ || tdigests_[1].size() > delta_) {
return Status::Invalid("oversized tdigest buffer");
}
// check k-size
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