Fix backup jitter and add unit tests.

app/src/test/java/org/thoughtcrime/securesms/service/BackListenerTest.kt

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2024 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.service
+
+import org.junit.Assert
+import org.junit.Test
+import org.thoughtcrime.securesms.BaseUnitTest
+import java.time.LocalDateTime
+import java.util.concurrent.TimeUnit
+
+class BackListenerTest : BaseUnitTest() {
+
+  @Test
+  fun testBackupJitterExactlyWithinJitterWindow() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 2, 55)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(8, next.dayOfMonth)
+  }
+
+  @Test
+  fun testBackupJitterWithinJitterWindow() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 2, 58)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(8, next.dayOfMonth)
+  }
+
+  @Test
+  fun testBackupJitterJustOutsideOfWindow() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 2, 54, 59)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(7, next.dayOfMonth)
+  }
+
+  @Test
+  fun testBackupJitter() {
+    val jitterWindowSeconds = Math.toIntExact(TimeUnit.MINUTES.toSeconds(10))
+    val now = LocalDateTime.of(2024, 6, 7, 3, 15, 0)
+    val next = MessageBackupListener.getNextDailyBackupTimeFromNowWithJitter(now, 3, 0, jitterWindowSeconds)
+    Assert.assertEquals(8, next.dayOfMonth)
+  }
+}