Fix NaN handling in ttftBucketIndex

common/src/util/__tests__/ttft-histogram.test.ts

@@ -70,6 +70,16 @@ describe('ttftBucketIndex', () => {
     expect(ttftBucketIndex(60 * 60 * 1000)).toBeLessThan(last)
   })
 
+  it('routes NaN to bucket 0, keeps Infinity behavior correct', () => {
+    // Only NaN should default to 0. Negative Infinity should also go to 0
+    // because Math.max(-Infinity, 1) = 1, log(1) = 0, then bucket 0
+    expect(ttftBucketIndex(NaN)).toBe(0)
+    expect(ttftBucketIndex(-Infinity)).toBe(0)
+    // Positive Infinity flows through Math.max/Math.log and clamps to top
+    const last = TTFT_HISTOGRAM_BUCKET_COUNT - 1
+    expect(ttftBucketIndex(Infinity)).toBe(last)
+  })
+
   it('keeps every reported value within half a bucket, plus ms rounding', () => {
     // The geometric half-width is the real guarantee; the extra 0.5ms is
     // ttftBucketMs rounding to whole milliseconds, which only matters at

common/src/util/ttft-histogram.ts

@@ -37,7 +37,12 @@ const LN_BASE = Math.log(TTFT_HISTOGRAM_BASE)
  * Sub-millisecond and zero samples land in bucket 0 rather than at -Infinity.
  */
 export function ttftBucketIndex(ttftMs: number): number {
-  const index = Math.floor(Math.log(Math.max(ttftMs, 1)) / LN_BASE)
+  // Only special-case NaN. Infinity naturally flows through Math.max/Math.log
+  // and gets clamped to the top bucket by the Math.min below, which is the
+  // correct behavior. Treating Infinity as 0 would route it to the wrong end
+  // of the histogram.
+  const safeTtftMs = Number.isNaN(ttftMs) ? 0 : ttftMs
+  const index = Math.floor(Math.log(Math.max(safeTtftMs, 1)) / LN_BASE)
   return Math.min(TTFT_HISTOGRAM_BUCKET_COUNT - 1, Math.max(0, index))
 }