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Fix exponential histogram midpoint computation #133719

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Merged
merged 4 commits into from
Aug 29, 2025
Merged

Fix exponential histogram midpoint computation #133719

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91067e3
Fix exponential histogram midpoint computation
JonasKunz Aug 28, 2025
cc9ff63
Fix comment
JonasKunz Aug 28, 2025
486a6b2
Revert basic computation to original
JonasKunz Aug 28, 2025
5b51c4a
Merge branch 'main' into fix-exp-histo-nan
JonasKunz Aug 29, 2025
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14 changes: 12 additions & 2 deletions ...histogram/src/main/java/org/elasticsearch/exponentialhistogram/ExponentialScaleUtils.java
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Original file line number Diff line number Diff line change
Expand Up @@ -234,9 +234,19 @@ static double exponentiallyScaledToDoubleValue(long index, int scale) {
*/
public static double getPointOfLeastRelativeError(long bucketIndex, int scale) {
checkIndexAndScaleBounds(bucketIndex, scale);
double upperBound = getUpperBucketBoundary(bucketIndex, scale);
double histogramBase = Math.pow(2, Math.scalb(1, -scale));
return 2 / (histogramBase + 1) * upperBound;
if (Double.isFinite(histogramBase)) {
double upperBound = getUpperBucketBoundary(bucketIndex, scale);
return 2 / (histogramBase + 1) * upperBound;
} else {
if (bucketIndex >= 0) {
// the bucket is (1, +inf), approximate point of least error as inf
return Double.POSITIVE_INFINITY;
} else {
// the bucket is (1/(Inf), 1), approximate point of least error as 0
return 0;
}
}
}

/**
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6 changes: 6 additions & 0 deletions ...gram/src/test/java/org/elasticsearch/exponentialhistogram/ExponentialScaleUtilsTests.java
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Expand Up @@ -79,6 +79,12 @@ public void testExtremeValueIndexing() {
}
}

public void testPointOfLeastErrorAtInfinity() {
assertThat(getPointOfLeastRelativeError(0, MIN_SCALE), equalTo(Double.POSITIVE_INFINITY));
assertThat(getPointOfLeastRelativeError(-1, MIN_SCALE), equalTo(0.0));
assertThat(getPointOfLeastRelativeError(10, MIN_SCALE + 2), equalTo(Double.POSITIVE_INFINITY));
}

public void testRandomValueIndexing() {
for (int i = 0; i < 100_000; i++) {
// generate values in the range 10^-100 to 10^100
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