playback: preserve detached fallback queue state

app/src/main/kotlin/com/subtracks/playback/PlaybackController.kt

@@ -161,6 +161,7 @@ class PlaybackController(
     private var windowFiltered = false
     private var detachedWindow = false
     private var detachedPosition: Long? = null
+    private var detachedFallbackPosition: Long? = null
 
     init {
         queueRepository.setResetListener { reset ->
@@ -1505,9 +1506,11 @@ class PlaybackController(
     private fun markDetached(
         window: List<QueueWindowItem>,
         position: Long?,
+        fallbackPosition: Long?,
     ) {
         detachedWindow = true
         detachedPosition = position
+        detachedFallbackPosition = fallbackPosition
         windowFiltered = offline
         windowPositions = LongArray(window.size + 1) { if (it == 0) DETACHED_POSITION else window[it - 1].position }
         windowStart = window.firstOrNull()?.position ?: 0L
@@ -1517,6 +1520,7 @@ class PlaybackController(
     private fun clearDetached() {
         detachedWindow = false
         detachedPosition = null
+        detachedFallbackPosition = null
         windowFiltered = offline
     }
 
@@ -1536,7 +1540,7 @@ class PlaybackController(
         windowEnd = windowPositions.lastOrNull() ?: -1L
     }
 
-    private fun currentQueuePosition(): Long? = currentPosition() ?: detachedPosition
+    private fun currentQueuePosition(): Long? = currentPosition() ?: detachedPosition ?: detachedFallbackPosition
 
     private suspend fun rebuildWindow(
         window: List<QueueWindowItem>,
@@ -1586,7 +1590,7 @@ class PlaybackController(
         if (!detachedWindow || currentPosition() != null) return false
         queueRepository.setCursor(position)
         rebuildDetachedSurvivors(snapshot, position)
-        refresh(position)
+        refresh(lastPosition)
         return true
     }
 
@@ -1608,7 +1612,7 @@ class PlaybackController(
             updating = false
         }
         val next = window.firstOrNull()?.position
-        markDetached(window, next)
+        markDetached(window, next, nearest.takeIf { snapshot.size > 0L })
         lastPosition = next
     }
 

app/src/test/kotlin/com/subtracks/playback/PlaybackControllerTest.kt

@@ -4021,6 +4021,59 @@ class PlaybackControllerTest {
         assertEquals(0L, controller.state.value.position)
     }
 
+    @Test
+    fun offlineDetachedPlaybackWithoutAForwardSurvivorCanPlayPrevious() {
+        detachOfflineTrackWithoutAForwardSurvivor()
+
+        controller.previousTrack()
+
+        await { handle.currentItem?.id == "s1" && controller.state.value.position == 0L }
+    }
+
+    @Test
+    fun offlineDetachedPlaybackWithoutAForwardSurvivorCanRemoveAnEarlierTrack() {
+        detachOfflineTrackWithoutAForwardSurvivor()
+
+        runBlocking { controller.removeAt(0) }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(1L, snapshot.size)
+        assertEquals("s3", runBlocking { queues.itemAt(snapshot, 0) }?.song?.id)
+        assertEquals(listOf("s2"), handle.items.map { it.id })
+        assertNull(controller.state.value.position)
+    }
+
+    @Test
+    fun offlineDetachedPlaybackWithoutAForwardSurvivorReloadsWhenOnline() {
+        detachOfflineTrackWithoutAForwardSurvivor()
+
+        setOnline()
+
+        await { handle.items.map { it.id } == listOf("s2", "s3") }
+        controller.next()
+        await { handle.currentItem?.id == "s3" && controller.state.value.position == 1L }
+    }
+
+    @Test
+    fun clearingTheFinalDetachedSurvivorPublishesNoQueuePosition() {
+        seedAlbum(1, sourceId = 1)
+        seedSong("x1", "al2")
+        controller.playAlbum(1, "al1", 0)
+        await { handle.currentItem?.id == "s1" && controller.state.value.isPlaying }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { queues.snapshot().upNextSize } == 1L }
+        val point = runBlocking { queues.captureResetPoint() }
+        runBlocking { db.libraryDao().deleteSongs(1, listOf("s1")) }
+        runBlocking { queues.resetChangedOrigins(setOf(QueueOrigin(QueueKind.Album, 1, "al1")), point) }
+        await { handle.items.map { it.id } == listOf("s1", "x1") && controller.state.value.position == 0L }
+
+        controller.clearUpNext()
+
+        await { handle.items.map { it.id } == listOf("s1") }
+        assertEquals(0L, runBlocking { queues.snapshot().size })
+        assertNull(controller.state.value.position)
+    }
+
     @Test
     fun aResetRemapsAContextTrackThatMovedToUpNext() {
         seedPlaylist(5)