From 7505a3dcb1f03dc73e88bbbc288b4375a3170de8 Mon Sep 17 00:00:00 2001
From: Sebastian Reddig <sgcr@gmx.net>
Date: Fri, 30 Aug 2024 19:46:36 +0200
Subject: [PATCH] Fix: Correct contract of equals and hashCode of HODateTime

---
 src/main/java/core/util/HODateTime.java     | 20 +++++-
 src/test/java/core/util/HODateTimeTest.java | 75 ++++++++++++++++++++-
 2 files changed, 93 insertions(+), 2 deletions(-)

diff --git a/src/main/java/core/util/HODateTime.java b/src/main/java/core/util/HODateTime.java
index 8364e51ab..a4950ce8e 100644
--- a/src/main/java/core/util/HODateTime.java
+++ b/src/main/java/core/util/HODateTime.java
@@ -218,7 +218,25 @@ public int compareTo(@NotNull HODateTime o) {
         return instant.compareTo(o.instant);
     }
 
-    public boolean equals(HODateTime t){ return this.instant.equals(t.instant);}
+    @Override
+    public boolean equals(Object o) {
+        if (o == this) {
+            return true;
+        }
+        if (o instanceof HODateTime that) {
+            return that.canEqual(this) && instant.equals(that.instant);
+        }
+        return false;
+    }
+
+    @Override
+    public int hashCode() {
+        return instant.hashCode();
+    }
+
+    protected boolean canEqual(Object o) {
+        return o instanceof HODateTime;
+    }
 
     public HODateTime minus(int i, ChronoUnit unit) {
         return new HODateTime(instant.minus(i, unit));
diff --git a/src/test/java/core/util/HODateTimeTest.java b/src/test/java/core/util/HODateTimeTest.java
index 575cb4a0f..07bb15622 100644
--- a/src/test/java/core/util/HODateTimeTest.java
+++ b/src/test/java/core/util/HODateTimeTest.java
@@ -13,15 +13,22 @@
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
 import java.time.temporal.ChronoUnit;
+import java.util.Objects;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
 import java.util.stream.Stream;
 
 import static java.math.BigDecimal.ONE;
 import static java.math.BigDecimal.ZERO;
-import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+import static org.assertj.core.data.Offset.offset;
 import static org.junit.jupiter.params.provider.Arguments.of;
 
 class HODateTimeTest {
 
+    private static final HODateTime HO_DATE_TIME = HODateTime.fromHT("2024-01-01 00:00:00");
+
         @Test
         void test() {
             var nextTraining = HODateTime.fromHT("2022-03-31 08:30:00");
@@ -74,6 +81,72 @@ void test() {
 
         }
 
+    static Stream<Arguments> equals() {
+        return Stream.of(
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2025-10-30 20:00:00"), false),
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2024-11-30 20:00:00"), false),
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2024-10-31 20:00:00"), false),
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2024-10-30 21:00:00"), false),
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2024-10-30 20:01:00"), false),
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2024-10-30 20:00:01"), false),
+                Arguments.of(HO_DATE_TIME, HO_DATE_TIME, true),
+                Arguments.of(HODateTime.fromHT("2024-10-30 20:00:00"), HODateTime.fromHT("2024-10-30 20:00:00"), true),
+                Arguments.of(HO_DATE_TIME, null, false)
+        );
+    }
+
+    @ParameterizedTest
+    @MethodSource
+    void equals(HODateTime lhs, HODateTime rhs, boolean result) {
+        assertThat(lhs.equals(rhs)).isEqualTo(result);
+    }
+
+
+    @Test
+    void testHashCode_consistencyOnTwoCalls() {
+        // given
+        final var now = HODateTime.now();
+
+        // when
+        final var hashCode1 = now.hashCode();
+        final var hashCode2 = now.hashCode();
+
+        // then
+        assertThat(hashCode1).isEqualTo(hashCode2);
+    }
+
+    @Test
+    void testHashCode_twoEqualObjectsResultsEqualHashCodes() {
+        // given
+        final var htTimeString = "2024-12-31 23:59:59";
+        final var hoDateTime1 = HODateTime.fromHT(htTimeString);
+        final var hoDateTime2 = HODateTime.fromHT(htTimeString);
+
+        // when
+        final var hashCode1 = hoDateTime1.hashCode();
+        final var hashCode2 = hoDateTime2.hashCode();
+
+        // then
+        assertThat(hashCode1).isEqualTo(hashCode2);
+    }
+
+    @Test
+    void testHashCode_givenMultipleObjects_whenTestingHashCodeDistribution_thenEvenDistributionOfHashCodes() {
+        // given
+        final var localDateTime = LocalDateTime.now();
+        final var objects = IntStream.range(0, 1000).mapToObj(i -> generate(i, localDateTime)).toList();
+
+        // when
+        final Set<Integer> hashCodes = objects.stream().map(Objects::hashCode).collect(Collectors.toSet());
+
+        // then
+        assertThat(hashCodes.size()).isCloseTo( objects.size(), offset(10));
+    }
+
+    private static HODateTime generate(int i, LocalDateTime localDateTime) {
+        return fromLocalDateTime(localDateTime.plusDays(i));
+    }
+
     @Test
     void daysFromNow_future() {
         // given