address review: anchor relocation and empty-context positions

- clamp anchorPlay to the context size, so an empty context with a queued
  block puts the block at combined 0 (previously position 0 was
  unresolvable and the queue view emptied).
- move/removeAt now derive the playing track's new position from its id
  after an edit, instead of assuming a fixed list delta; moving or
  removing the anchor relocates the whole block, which movedCursor
  ignored.
- removing the currently playing track reloads at the model's next
  position (the anchor may have relocated) rather than removing it from
  the player in place.
- the queue view's remove reconciles its rows from the repository instead
  of decrementing positions by one, so a block relocation stays in sync.

app/src/main/kotlin/com/subtracks/data/repo/QueueRepository.kt

@@ -58,10 +58,12 @@ data class QueueSnapshot(
     val shuffled: Boolean get() = shuffleOrder != null
 
     val anchorPlay: Long =
+        (
             when (val order = shuffleOrder) {
                 null -> upNextAnchor
                 else -> order.indexOf(upNextAnchor).takeIf { it >= 0 }?.toLong() ?: 0L
             }
+        ).coerceAtMost(contextSize - 1L)
 
     fun isUpNext(position: Long): Boolean = upNextSize > 0L && position > anchorPlay && position <= anchorPlay + upNextSize
 

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

@@ -349,15 +349,8 @@ class PlaybackController(
                 if (position != current) {
                     rebuildWindow(queueRepository.window(updated, target, QUEUE_WINDOW_RADIUS), target)
                     refresh(target)
-                } else if (target - windowStart in 0 until player.itemCount) {
-                    updating = true
-                    player.removeAt((target - windowStart).toInt())
-                    updating = false
-                    windowEnd--
-                    shiftWindowLocked(target)
-                    refresh(target)
                 } else {
-                    loadWindow(target, autoplay = playing)
+                    loadWindow(target, autoplay = playing, startPositionMs = 0)
                 }
                 return@withLock
             }
@@ -400,10 +393,17 @@ class PlaybackController(
         val cursor = queueRepository.cursor()
         val entries = snapshot.entries.map { it.entry }
         val upNext = snapshot.upNext.map { it.entry }
+        val wasUpNext = snapshot.isUpNext(current)
         if (!queueRepository.move(snapshot, from, to)) return@withLock
         val updated = readSnapshot()
         this.snapshot = updated
-        val target = movedCursor(current, from, to)
+        val currentId = player.currentItem?.id
+        val target =
+            (
+                currentId?.let {
+                    if (wasUpNext) queueRepository.combinedIndexOf(updated, it) else queueRepository.combinedContextIndexOf(updated, it)
+                } ?: movedCursor(current, from, to)
+            ).coerceIn(0, updated.size - 1)
         lastPosition = target
         windowJob?.cancel()
         val fromInWindow = from in windowStart..windowEnd

app/src/main/kotlin/com/subtracks/ui/playback/QueueScreen.kt

@@ -134,13 +134,15 @@ class QueueViewModel(
     }
 
     fun reconcile() {
-        viewModelScope.launch {
+        viewModelScope.launch { mutex.withLock { reconcileLocked() } }
-            mutex.withLock {
+    }
-                if (!ready) return@withLock
+
+    private suspend fun reconcileLocked() {
+        if (!ready) return
         val snapshot = queueRepository.snapshot()
         if (snapshot.size == 0L) {
             rows.clear()
-                    return@withLock
+            return
         }
         val start = (rows.firstOrNull()?.position ?: 0L).coerceIn(0, snapshot.size - 1)
         val end = (start + rows.size.coerceAtLeast(QUEUE_CHUNK) - 1).coerceAtMost(snapshot.size - 1)
@@ -154,8 +156,6 @@ class QueueViewModel(
             rows.addAll(updated)
         }
     }
-        }
-    }
 
     fun loadOlder() {
         val top = rows.firstOrNull()?.position ?: return
@@ -228,24 +228,9 @@ class QueueViewModel(
         viewModelScope.launch {
             mutex.withLock {
                 val index = rows.indexOfFirst { it.position == position }
-                if (index >= 0) {
+                if (index >= 0) rows.removeAt(index)
-                    rows.removeAt(index)
-                    for (i in index until rows.size) {
-                        val row = rows[i]
-                        rows[i] = row.copy(position = row.position - 1)
-                    }
-                }
                 playbackController.removeAt(position)
-                val snapshot = queueRepository.snapshot()
+                reconcileLocked()
-                if (rows.isEmpty()) {
-                    reload()
-                    return@withLock
-                }
-                val next = rows.last().position + 1
-                if (next <= snapshot.size - 1) {
-                    queueRepository.range(snapshot, next, next).firstOrNull()?.let { rows.add(newRow(it)) }
-                }
-                while (rows.size > QUEUE_WINDOW_ROWS) rows.removeAt(0)
             }
         }
     }

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

@@ -926,6 +926,81 @@ class PlaybackControllerTest {
         assertFalse(handle.operations.any { it.startsWith("setWindow") })
     }
 
+    @Test
+    fun removingTheLastContextTrackWithAQueuedTrackKeepsTheQueue() {
+        seedAlbum(1, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 0)
+        await {
+            controller.state.value.item
+                ?.id == "s1"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        runBlocking { controller.removeAt(0) }
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(1L, snapshot.size)
+        assertTrue(runBlocking { queues.range(snapshot, 0, 0) }.isNotEmpty())
+    }
+
+    @Test
+    fun movingTheAnchorKeepsThePlayingTrackPositionInSync() {
+        seedAlbum(4, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 1)
+        await {
+            controller.state.value.item
+                ?.id == "s2"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+        controller.next()
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+
+        runBlocking { controller.move(1, 0) }
+        await {
+            controller.state.value.item
+                ?.id == "x1"
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(runBlocking { queues.combinedIndexOf(snapshot, "x1") }, controller.state.value.position)
+    }
+
+    @Test
+    fun removingThePlayingAnchorTrackPlaysTheModelNextTrack() {
+        seedAlbum(4, sourceId = 1)
+        seedSong("x1", "al2")
+
+        controller.playAlbum(1, "al1", 1)
+        await {
+            controller.state.value.item
+                ?.id == "s2"
+        }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        runBlocking { controller.removeAt(1) }
+        await {
+            controller.state.value.item
+                ?.id == "s3"
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        assertEquals(runBlocking { queues.combinedIndexOf(snapshot, "s3") }, controller.state.value.position)
+    }
+
     @Test
     fun nextFollowsTheReorderedQueue() {
         seedAlbum(3, sourceId = 1)

app/src/test/kotlin/com/subtracks/ui/playback/QueueViewModelTest.kt

@@ -6,6 +6,7 @@ import androidx.sqlite.driver.bundled.BundledSQLiteDriver
 import androidx.test.core.app.ApplicationProvider
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.subtracks.data.db.SubtracksDatabase
+import com.subtracks.data.model.QueueKind
 import com.subtracks.data.model.Song
 import com.subtracks.data.model.SongListItem
 import com.subtracks.data.model.Source
@@ -225,6 +226,37 @@ class QueueViewModelTest {
         assertEquals((220L..399L).toList(), viewModel.rows.map { it.position })
     }
 
+    @Test
+    fun removingATrackUnderTheBlockReloadsRows() {
+        runBlocking { seedSongs(10) }
+        runBlocking {
+            db.libraryDao().upsertSongs(
+                listOf(Song(1, "x1", "al2", "ar1", "X", "Other Album", "Artist", 100, 1, 1, null, null)),
+            )
+        }
+        controller.playAlbum(1, "al1", 2)
+        await { controller.state.value.position == 2L }
+        controller.addToQueue(1, QueueKind.Song, "x1")
+        await { runBlocking { controller.upcomingItem()?.id } == "x1" }
+
+        viewModel.open()
+        await { viewModel.ready }
+
+        viewModel.remove(2)
+        await {
+            val positions = viewModel.rows.map { it.position }
+            positions.isNotEmpty() && positions == (positions.first()..positions.last()).toList()
+        }
+
+        val snapshot = runBlocking { queues.snapshot() }
+        val expected =
+            runBlocking {
+                (viewModel.rows.first().position..viewModel.rows.last().position)
+                    .mapNotNull { queues.itemAt(snapshot, it)?.song?.id }
+            }
+        assertEquals(expected, viewModel.rows.map { it.song.song.id })
+    }
+
     @Test
     fun removeAndUndoKeepTheBoundedWindowConsistent() {
         runBlocking { seedSongs(400) }