feat(android): pure idle-revert decision for UPnP output

android/app/src/main/java/com/fabledsword/minstrel/player/output/IdleRevertDecision.kt

@@ -0,0 +1,21 @@
+package com.fabledsword.minstrel.player.output
+
+/** Action the idle-revert collector should take on a player-state change. */
+internal enum class IdleRevertAction { ARM, CANCEL, IGNORE }
+
+/**
+ * Pure decision for the UPnP idle-revert timer. [armed] is whether the
+ * idle timer Job is currently running. We ARM only on the transition INTO
+ * "engaged + not playing" (so repeated paused emissions don't reset the
+ * countdown), CANCEL whenever playback resumes or UPnP disengages, and
+ * otherwise IGNORE.
+ */
+internal fun idleRevertAction(
+    upnpEngaged: Boolean,
+    isPlaying: Boolean,
+    armed: Boolean,
+): IdleRevertAction = when {
+    upnpEngaged && !isPlaying -> if (armed) IdleRevertAction.IGNORE else IdleRevertAction.ARM
+    armed -> IdleRevertAction.CANCEL
+    else -> IdleRevertAction.IGNORE
+}

android/app/src/test/java/com/fabledsword/minstrel/player/output/IdleRevertDecisionTest.kt

@@ -0,0 +1,55 @@
+package com.fabledsword.minstrel.player.output
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class IdleRevertDecisionTest {
+
+    @Test
+    fun `engaged and paused and not armed arms the timer`() {
+        assertEquals(
+            IdleRevertAction.ARM,
+            idleRevertAction(upnpEngaged = true, isPlaying = false, armed = false),
+        )
+    }
+
+    @Test
+    fun `engaged and paused and already armed is ignored`() {
+        assertEquals(
+            IdleRevertAction.IGNORE,
+            idleRevertAction(upnpEngaged = true, isPlaying = false, armed = true),
+        )
+    }
+
+    @Test
+    fun `resuming playback cancels an armed timer`() {
+        assertEquals(
+            IdleRevertAction.CANCEL,
+            idleRevertAction(upnpEngaged = true, isPlaying = true, armed = true),
+        )
+    }
+
+    @Test
+    fun `playing with no timer armed is ignored`() {
+        assertEquals(
+            IdleRevertAction.IGNORE,
+            idleRevertAction(upnpEngaged = true, isPlaying = true, armed = false),
+        )
+    }
+
+    @Test
+    fun `disengaging UPnP cancels an armed timer`() {
+        assertEquals(
+            IdleRevertAction.CANCEL,
+            idleRevertAction(upnpEngaged = false, isPlaying = false, armed = true),
+        )
+    }
+
+    @Test
+    fun `disengaged with no timer armed is ignored`() {
+        assertEquals(
+            IdleRevertAction.IGNORE,
+            idleRevertAction(upnpEngaged = false, isPlaying = true, armed = false),
+        )
+    }
+}