goodhistogram: add regression test for rightmost-bucket density fix - #13
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The rightmost-bucket density bias fixed in f4b1ed8 shipped without a regression test. Add TestQuantileTopBucketDensity, which places all mass in the last in-range bucket and asserts the median lands at or past the bucket midpoint. With the fix (right-edge density = last bucket's average) the density rises across the bucket and the median sits past the midpoint; under the bug (right-edge density = 0) it is pulled below. The test covers both the single-value (ValueAtQuantile) and batch (ValuesAtQuantiles) paths, which compute boundary densities independently. Verified the test fails when boundaryDensity[n] is reset to 0 and passes against the current code. Co-Authored-By: roachdev-claude <roachdev-claude-bot@cockroachlabs.com>
kyle-a-wong
approved these changes
Aug 26, 2026
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Summary
The rightmost-bucket density bias fixed in f4b1ed8 ("fix rightmost-bucket density bias in quantile interpolation") shipped without a regression test. This adds one.
TestQuantileTopBucketDensityplaces all mass in the last in-range bucket and asserts the median lands at or past the bucket midpoint:It covers both the single-value (
ValueAtQuantile) and batch (ValuesAtQuantiles) paths, since they compute boundary densities independently.Verification
boundaryDensity[n]is reset to0(median972810< midpoint980774).main.No production code changes.