For #18836: add isHotStartForStartedActivity and tests.

app/src/main/java/org/mozilla/fenix/perf/StartupStateProvider.kt

@@ -89,4 +89,35 @@ class StartupStateProvider(
         )
         return isLastActivityCreatedStillStarted
     }
+
+    /**
+     * Returns true if the current startup state is HOT and the currently started activity is the
+     * first started activity for this start up (i.e. we can use it for performance measurements).
+     *
+     * This method must be called after the foreground activity is STARTED.
+     */
+    fun isHotStartForStartedActivity(activityClass: Class<out Activity>): Boolean {
+        // A hot start means:
+        // - the app was backgrounded and has since been started
+        // - the first started activity since the app was started is still active.
+        // - that activity was not created before being started
+        //
+        // For the activity log, we expect:
+        //   [... App-STOPPED, ... Activity-STARTED, App-STARTED]
+        // where:
+        // - App-STOPPED is the last STOPPED seen
+        // - App-CREATED is NOT called for this activity
+        // - we're assuming App-STARTED will only be last if one activity is started (as observed)
+        if (!startupLog.log.contains(LogEntry.AppStopped)) {
+            return false // if the app hasn't been stopped, it's not a hot start.
+        }
+        val afterLastStopped = startupLog.log.takeLastWhile { it != LogEntry.AppStopped }
+
+        val isLastActivityStartedStillStarted = afterLastStopped.takeLast(2) == listOf(
+            LogEntry.ActivityStarted(activityClass),
+            LogEntry.AppStarted
+        )
+        return !afterLastStopped.contains(LogEntry.ActivityCreated(activityClass)) &&
+            isLastActivityStartedStillStarted
+    }
 }

app/src/test/java/org/mozilla/fenix/perf/StartupStateProviderTest.kt

@@ -81,6 +81,27 @@ class StartupStateProviderTest {
         }
     }
 
+    @Test
+    fun `GIVEN the app started for an activity WHEN is cold start THEN hot start is false`() {
+        forEachColdStartEntries { index ->
+            assertFalse("$index", provider.isHotStartForStartedActivity(homeActivityClass))
+        }
+    }
+
+    @Test
+    fun `GIVEN the app started for an activity WHEN is warm start THEN hot start is false`() {
+        forEachWarmStartEntries { index ->
+            assertFalse("$index", provider.isHotStartForStartedActivity(homeActivityClass))
+        }
+    }
+
+    @Test
+    fun `GIVEN the app started for an activity WHEN is hot start THEN hot start is true` () {
+        forEachHotStartEntries { index ->
+            assertTrue("$index", provider.isHotStartForStartedActivity(homeActivityClass))
+        }
+    }
+
     @Test
     fun `GIVEN the app started for an activity WHEN we launched HA through a drawing IntentRA THEN start up is not cold`() {
         // These entries mimic observed behavior for local code changes.
@@ -111,6 +132,21 @@ class StartupStateProviderTest {
         assertFalse(provider.isWarmStartForStartedActivity(homeActivityClass))
     }
 
+    @Test
+    fun `GIVEN the app started for an activity WHEN we launched HA through a drawing IntentRA THEN start up is not hot`() {
+        // These entries mimic observed behavior for local code changes.
+        logEntries.addAll(listOf(
+            LogEntry.AppStopped,
+            LogEntry.ActivityStopped(homeActivityClass),
+            LogEntry.ActivityCreated(irActivityClass),
+            LogEntry.ActivityStarted(irActivityClass),
+            LogEntry.AppStarted,
+            LogEntry.ActivityStarted(homeActivityClass),
+            LogEntry.ActivityStopped(irActivityClass)
+        ))
+        assertFalse(provider.isHotStartForStartedActivity(homeActivityClass))
+    }
+
     @Test
     fun `GIVEN the app started for an activity WHEN two HomeActivities are created THEN start up is not cold`() {
         // We're making an assumption about how this would work based on previous observed patterns.
@@ -181,6 +217,26 @@ class StartupStateProviderTest {
         assertFalse(provider.isWarmStartForStartedActivity(homeActivityClass))
     }
 
+    @Test
+    fun `GIVEN the app has not been stopped WHEN an activity has not been created THEN it's not a hot start`() {
+        assertFalse(provider.isHotStartForStartedActivity(homeActivityClass))
+    }
+
+    @Test
+    fun `GIVEN the app has been stopped WHEN an activity has not been created THEN it's not a hot start`() {
+        logEntries.add(LogEntry.AppStopped)
+        assertFalse(provider.isHotStartForStartedActivity(homeActivityClass))
+    }
+
+    @Test
+    fun `GIVEN the app has been stopped WHEN an activity has not been started THEN it's not a hot start`() {
+        logEntries.addAll(listOf(
+            LogEntry.AppStopped,
+            LogEntry.ActivityCreated(homeActivityClass)
+        ))
+        assertFalse(provider.isHotStartForStartedActivity(homeActivityClass))
+    }
+
     private fun forEachColdStartEntries(block: (index: Int) -> Unit) {
         // These entries mimic observed behavior.
         //