From bc7926ae8ed38f2832076b5e0c3e5ec057271562 Mon Sep 17 00:00:00 2001
From: Osamaali313 <s.osamaali72@gmail.com>
Date: Wed, 1 Jul 2026 20:22:13 +0300
Subject: [PATCH] Fix histogram bin redistribution dropping counts at the range
 boundary

buildNormalizedHistograms redistributes input bins into binCount
equal-width result bins. The last result bin's right edge was computed
as resultLeft + dx == left + (binCount-1)*dx + dx, which accumulates
floating-point error and can land just below range.right (e.g. for
range [0, 1] with binCount 6 it is 0.9999999999999999). An input bin
whose right edge sits exactly on range.right then satisfies
'bin.x + bin.dx > resultRight' in the inner loop and the loop breaks
without consuming it, so its y counts are silently dropped from the
normalized histogram.

Clamp the last result bin's right edge to range.right (its true upper
bound by construction). Add regression tests asserting count
conservation when a bin's right edge sits on range.right.
---
 .../widgets/histogram/histogram_util.ts       |  8 ++++-
 .../widgets/histogram/histogram_util_test.ts  | 32 +++++++++++++++++++
 2 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/tensorboard/webapp/widgets/histogram/histogram_util.ts b/tensorboard/webapp/widgets/histogram/histogram_util.ts
index fafaf61f9a7..6f5cf79d09e 100644
--- a/tensorboard/webapp/widgets/histogram/histogram_util.ts
+++ b/tensorboard/webapp/widgets/histogram/histogram_util.ts
@@ -119,8 +119,14 @@ function rebuildBins(bins: Bin[], range: Range, binCount: number): Bin[] {
   let nextBinContribution = 0;
   for (let i = 0; i < binCount; i++) {
     const resultLeft = left + i * dx;
-    const resultRight = resultLeft + dx;
     const isLastResultBin = i === binCount - 1;
+    // The last result bin must extend exactly to `range.right`. Computing it as
+    // `resultLeft + dx` accumulates floating-point error in the repeated `i * dx`
+    // sum and can land just below `range.right`; an input bin whose right edge
+    // sits exactly on `range.right` then satisfies `bin.x + bin.dx > resultRight`
+    // and is skipped without being consumed, dropping its counts. Clamp the last
+    // result bin's right edge to `range.right` so those counts are preserved.
+    const resultRight = isLastResultBin ? right : resultLeft + dx;
 
     let resultY = nextBinContribution;
     nextBinContribution = 0;
diff --git a/tensorboard/webapp/widgets/histogram/histogram_util_test.ts b/tensorboard/webapp/widgets/histogram/histogram_util_test.ts
index 8aee2f66dfb..7a934ed718f 100644
--- a/tensorboard/webapp/widgets/histogram/histogram_util_test.ts
+++ b/tensorboard/webapp/widgets/histogram/histogram_util_test.ts
@@ -54,6 +54,38 @@ describe('histogram util', () => {
       });
     });
 
+    describe('count conservation at the range boundary', () => {
+      function totalCount(bins: Bin[]): number {
+        return bins.reduce((sum, bin) => sum + bin.y, 0);
+      }
+
+      it('preserves a bin whose right edge sits on range.right', () => {
+        // A zero-width bin at x=1 sits exactly on the range's right edge (the
+        // widest histogram spans [0, 1]). It must not be dropped when the bins
+        // are redistributed into `binCount` result bins.
+        const input: Bin[] = [
+          {x: 0, dx: 1, y: 1},
+          {x: 1, dx: 0, y: 5},
+        ];
+        const [result] = histogramsToBins(
+          buildNormalizedHistograms([binsToHistogram(input)], 6)
+        );
+        expect(totalCount(result)).toBeCloseTo(totalCount(input), 6);
+      });
+
+      it('preserves counts when the final normal bin ends on range.right', () => {
+        const input: Bin[] = [
+          {x: 0, dx: 1, y: 2},
+          {x: 1, dx: 1, y: 3},
+          {x: 2, dx: 1, y: 4},
+        ];
+        const [result] = histogramsToBins(
+          buildNormalizedHistograms([binsToHistogram(input)], 7)
+        );
+        expect(totalCount(result)).toBeCloseTo(totalCount(input), 6);
+      });
+    });
+
     describe('single histogram', () => {
       it('converts a 0 width bin into a default bin', () => {
         expect(