From 7048e444d143eff4e40bfec4a3632f91d202111e Mon Sep 17 00:00:00 2001
From: Mark Tozzi <mark.tozzi@gmail.com>
Date: Thu, 5 Dec 2024 16:25:13 -0500
Subject: [PATCH] additional tests and comments

---
 .../java/org/elasticsearch/common/time/DateUtils.java     | 2 ++
 .../org/elasticsearch/common/time/DateUtilsTests.java     | 8 ++++++++
 2 files changed, 10 insertions(+)

diff --git a/server/src/main/java/org/elasticsearch/common/time/DateUtils.java b/server/src/main/java/org/elasticsearch/common/time/DateUtils.java
index 1372cade41a30..a6f58e71d82c2 100644
--- a/server/src/main/java/org/elasticsearch/common/time/DateUtils.java
+++ b/server/src/main/java/org/elasticsearch/common/time/DateUtils.java
@@ -317,6 +317,8 @@ public static int compareNanosToMillis(long nanos, long millis) {
         if (millis > MAX_NANOSECOND_IN_MILLIS) {
             return -1;
         }
+        // This can't overflow, because we know millis is between 0 and  MAX_NANOSECOND_IN_MILLIS,
+        // and MAX_NANOSECOND_IN_MILLIS * 1_000_000 doesn't overflow.
         long diff = nanos - (millis * 1_000_000);
         return diff == 0 ? 0 : diff < 0 ? -1 : 1;
     }
diff --git a/server/src/test/java/org/elasticsearch/common/time/DateUtilsTests.java b/server/src/test/java/org/elasticsearch/common/time/DateUtilsTests.java
index beeb013dff9ea..e15bbbf75a529 100644
--- a/server/src/test/java/org/elasticsearch/common/time/DateUtilsTests.java
+++ b/server/src/test/java/org/elasticsearch/common/time/DateUtilsTests.java
@@ -22,6 +22,7 @@
 
 import static org.elasticsearch.common.time.DateUtils.MAX_MILLIS_BEFORE_MINUS_9999;
 import static org.elasticsearch.common.time.DateUtils.MAX_NANOSECOND_INSTANT;
+import static org.elasticsearch.common.time.DateUtils.MAX_NANOSECOND_IN_MILLIS;
 import static org.elasticsearch.common.time.DateUtils.clampToNanosRange;
 import static org.elasticsearch.common.time.DateUtils.compareNanosToMillis;
 import static org.elasticsearch.common.time.DateUtils.toInstant;
@@ -31,12 +32,19 @@
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.lessThan;
 
 public class DateUtilsTests extends ESTestCase {
 
     public void testCompareNanosToMillis() {
+        assertThat(MAX_NANOSECOND_IN_MILLIS * 1_000_000, lessThan(Long.MAX_VALUE));
+
         assertThat(compareNanosToMillis(toLong(Instant.EPOCH), Instant.EPOCH.toEpochMilli()), is(0));
 
+        // This should be 1, because the millisecond version should truncate a bit
+        assertThat(compareNanosToMillis(toLong(MAX_NANOSECOND_INSTANT), MAX_NANOSECOND_INSTANT.toEpochMilli()), is(1));
+
+        assertThat(compareNanosToMillis(toLong(MAX_NANOSECOND_INSTANT), -1000), is(1));
         // millis before epoch
         assertCompareInstants(
             randomInstantBetween(Instant.EPOCH, MAX_NANOSECOND_INSTANT),