From c93ab9e84edfffbae7e63894f6c960262a8c299e Mon Sep 17 00:00:00 2001
From: Chengzhong Wu <legendecas@gmail.com>
Date: Tue, 3 Jan 2023 23:53:25 +0800
Subject: [PATCH] fix(core): fix precision loss in numberToHrtime (#3480)

---
 CHANGELOG.md                                   |  1 +
 packages/opentelemetry-core/src/common/time.ts | 16 ++++------------
 .../test/common/time.test.ts                   | 18 +++++++++++++++++-
 3 files changed, 22 insertions(+), 13 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index cf293f3546..65b5e2e83e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -24,6 +24,7 @@ For experimental package changes, see the [experimental CHANGELOG](experimental/
     * `telemetry.sdk.version`
 * fix(selenium-tests): updated webpack version for selenium test issue [#3456](https://github.com/open-telemetry/opentelemetry-js/issues/3456) @SaumyaBhushan
 * fix(sdk-metrics): fix duplicated registration of metrics for collectors [#3488](https://github.com/open-telemetry/opentelemetry-js/pull/3488) @legendecas
+* fix(core): fix precision loss in numberToHrtime [#3480](https://github.com/open-telemetry/opentelemetry-js/pull/3480) @legendecas
 
 ### :books: (Refine Doc)
 
diff --git a/packages/opentelemetry-core/src/common/time.ts b/packages/opentelemetry-core/src/common/time.ts
index 64bc64d2d9..aa94aefec9 100644
--- a/packages/opentelemetry-core/src/common/time.ts
+++ b/packages/opentelemetry-core/src/common/time.ts
@@ -19,18 +19,12 @@ import { otperformance as performance } from '../platform';
 import { TimeOriginLegacy } from './types';
 
 const NANOSECOND_DIGITS = 9;
+const NANOSECOND_DIGITS_IN_MILLIS = 6;
+const MILLISECONDS_TO_NANOSECONDS = Math.pow(10, NANOSECOND_DIGITS_IN_MILLIS);
 const SECOND_TO_NANOSECONDS = Math.pow(10, NANOSECOND_DIGITS);
 
 /**
- * Converts a number to HrTime, HrTime = [number, number].
- * The first number is UNIX Epoch time in seconds since 00:00:00 UTC on 1 January 1970.
- * The second number represents the partial second elapsed since Unix Epoch time represented by first number in nanoseconds.
- * For example, 2021-01-01T12:30:10.150Z in UNIX Epoch time in milliseconds is represented as 1609504210150.
- * numberToHrtime calculates the first number by converting and truncating the Epoch time in milliseconds to seconds:
- * HrTime[0] = Math.trunc(1609504210150 / 1000) = 1609504210.
- * numberToHrtime calculates the second number by converting the digits after the decimal point of the subtraction, (1609504210150 / 1000) - HrTime[0], to nanoseconds:
- * HrTime[1] = Number((1609504210.150 - HrTime[0]).toFixed(9)) * SECOND_TO_NANOSECONDS = 150000000.
- * This is represented in HrTime format as [1609504210, 150000000].
+ * Converts a number of milliseconds from epoch to HrTime([seconds, remainder in nanoseconds]).
  * @param epochMillis
  */
 function numberToHrtime(epochMillis: number): api.HrTime {
@@ -38,9 +32,7 @@ function numberToHrtime(epochMillis: number): api.HrTime {
   // Decimals only.
   const seconds = Math.trunc(epochSeconds);
   // Round sub-nanosecond accuracy to nanosecond.
-  const nanos =
-    Number((epochSeconds - seconds).toFixed(NANOSECOND_DIGITS)) *
-    SECOND_TO_NANOSECONDS;
+  const nanos = Math.round((epochMillis % 1000) * MILLISECONDS_TO_NANOSECONDS);
   return [seconds, nanos];
 }
 
diff --git a/packages/opentelemetry-core/test/common/time.test.ts b/packages/opentelemetry-core/test/common/time.test.ts
index a8c284b9b0..d0b933e7a1 100644
--- a/packages/opentelemetry-core/test/common/time.test.ts
+++ b/packages/opentelemetry-core/test/common/time.test.ts
@@ -105,7 +105,7 @@ describe('time', () => {
     it('should convert Date hrTime', () => {
       const timeInput = new Date(1609297640313);
       const output = timeInputToHrTime(timeInput);
-      assert.deepStrictEqual(output, [1609297640, 312999964]);
+      assert.deepStrictEqual(output, [1609297640, 313000000]);
     });
 
     it('should convert epoch milliseconds hrTime', () => {
@@ -114,6 +114,22 @@ describe('time', () => {
       assert.deepStrictEqual(output[0], Math.trunc(timeInput / 1000));
     });
 
+    it('should convert arbitrary epoch milliseconds (with sub-millis precision) hrTime', () => {
+      sinon.stub(performance, 'timeOrigin').value(111.5);
+      const inputs = [
+        // [ input, expected ]
+        [1609297640313, [1609297640, 313000000]],
+        // inevitable precision loss without decimal arithmetics.
+        [1609297640313.333, [1609297640, 313333008]],
+        // eslint-disable-next-line @typescript-eslint/no-loss-of-precision
+        [1609297640313.333333333, [1609297640, 313333252]],
+      ] as const;
+      for (const [idx, input] of inputs.entries()) {
+        const output = timeInputToHrTime(input[0]);
+        assert.deepStrictEqual(output, input[1], `input[${idx}]: ${input}`);
+      }
+    });
+
     it('should convert performance.now() hrTime', () => {
       sinon.stub(performance, 'timeOrigin').value(111.5);