feat: beat annotator

app/build.gradle.kts

@@ -84,6 +84,7 @@ dependencies {
     implementation("androidx.datastore:datastore-preferences:1.1.1")
 
     implementation("androidx.media:media:1.7.0")
+    implementation("androidx.media3:media3-exoplayer:1.5.1")
     implementation("androidx.browser:browser:1.8.0")
     implementation("androidx.documentfile:documentfile:1.0.1")
     implementation("com.squareup.okhttp3:okhttp:4.12.0")

app/src/main/java/at/lockstep/player/LockstepViewModel.kt

@@ -10,6 +10,8 @@ import at.lockstep.jukebox.db.TrackRow
 import at.lockstep.player.data.UserPreferencesRepository
 import at.lockstep.player.data.db.TrackPairingEntity
 import at.lockstep.player.util.AudioUriValidator
+import at.lockstep.player.playback.TrackBoundaryEvent
+import at.lockstep.player.util.BeatAnnotationStorage
 import at.lockstep.player.util.FolderMp3Scanner
 import at.lockstep.player.util.Mp3EmbeddedMetadata
 import at.lockstep.player.util.Mp3FolderCandidate
@@ -56,6 +58,19 @@ class LockstepViewModel(
             null,
         )
 
+    val annotationMode: StateFlow<Boolean> =
+        prefs.annotationMode.stateIn(
+            viewModelScope,
+            SharingStarted.Eagerly,
+            false,
+        )
+
+    fun setAnnotationMode(enabled: Boolean) {
+        viewModelScope.launch {
+            prefs.setAnnotationMode(enabled)
+        }
+    }
+
     private val context get() = getApplication<Application>()
 
     /**
@@ -120,6 +135,45 @@ class LockstepViewModel(
 
     suspend fun hasPairedTracks(playlistId: String): Boolean = pairingDao.countPaired(playlistId) > 0
 
+    suspend fun getPlaylistDisplayName(playlistId: String): String =
+        withContext(Dispatchers.IO) {
+            app.playlistRepository.getPlaylists()
+                .find { it.id == playlistId }
+                ?.name
+                ?.trim()
+                .orEmpty()
+                .ifBlank { "playlist" }
+        }
+
+    /**
+     * Writes one JSON file under app files dir ([BeatAnnotationStorage]) for the track described
+     * by [event]. Skips when [beatTimesMs] is empty.
+     */
+    fun persistBeatAnnotation(
+        playlistId: String,
+        playlistDisplayName: String,
+        event: TrackBoundaryEvent,
+        beatTimesMs: List<Long>,
+    ) {
+        if (beatTimesMs.isEmpty()) {
+            return
+        }
+        viewModelScope.launch(Dispatchers.IO) {
+            val pairing = pairingDao.findForTrack(playlistId, event.trackId)
+            val docId = BeatAnnotationStorage.mp3DocumentContentId(pairing?.localUri)
+            val contentId = docId.ifBlank { event.trackId }
+            BeatAnnotationStorage.writeAnnotationsFile(
+                context = getApplication(),
+                playlistDisplayName = playlistDisplayName,
+                trackQueueIndex0Based = event.queueIndex,
+                contentId = contentId,
+                title = event.title,
+                artist = event.artist,
+                beatTimesMs = beatTimesMs,
+            )
+        }
+    }
+
     suspend fun syncJukeboxIfToken(): String? {
         val token = spotifyAccessToken.value
         if (token.isNullOrBlank()) {

app/src/main/java/at/lockstep/player/data/UserPreferencesRepository.kt

@@ -27,6 +27,12 @@ class UserPreferencesRepository(
             prefs[KEY_SPOTIFY_ACCESS_TOKEN]
         }
 
+    /** When true, opening a paired playlist navigates to beat annotation instead of Now Playing. */
+    val annotationMode: Flow<Boolean> =
+        dataStore.data.map { prefs ->
+            prefs[KEY_ANNOTATION_MODE] == true
+        }
+
     suspend fun setOnboardingComplete(done: Boolean) {
         dataStore.edit { prefs ->
             prefs[KEY_ONBOARDING_COMPLETE] = done
@@ -43,8 +49,15 @@ class UserPreferencesRepository(
         }
     }
 
+    suspend fun setAnnotationMode(enabled: Boolean) {
+        dataStore.edit { prefs ->
+            prefs[KEY_ANNOTATION_MODE] = enabled
+        }
+    }
+
     companion object {
         private val KEY_ONBOARDING_COMPLETE = booleanPreferencesKey("onboarding_complete")
         private val KEY_SPOTIFY_ACCESS_TOKEN = stringPreferencesKey("spotify_access_token")
+        private val KEY_ANNOTATION_MODE = booleanPreferencesKey("annotation_mode")
     }
 }

app/src/main/java/at/lockstep/player/data/db/PairingDao.kt

@@ -24,6 +24,14 @@ interface PairingDao {
     @Query("SELECT * FROM track_pairings WHERE playlistId = :playlistId")
     suspend fun listForPlaylist(playlistId: String): List<TrackPairingEntity>
 
+    @Query(
+        "SELECT * FROM track_pairings WHERE playlistId = :playlistId AND trackId = :trackId LIMIT 1",
+    )
+    suspend fun findForTrack(
+        playlistId: String,
+        trackId: String,
+    ): TrackPairingEntity?
+
     @Insert(onConflict = OnConflictStrategy.REPLACE)
     suspend fun upsert(row: TrackPairingEntity)
 }

app/src/main/java/at/lockstep/player/playback/PlaybackService.kt

@@ -13,18 +13,34 @@ import android.support.v4.media.session.MediaSessionCompat
 import android.support.v4.media.session.PlaybackStateCompat
 import androidx.core.app.NotificationCompat
 import androidx.media.app.NotificationCompat.MediaStyle
+import androidx.media3.common.MediaItem
+import androidx.media3.common.Player
+import androidx.media3.exoplayer.ExoPlayer
 import at.lockstep.player.LockstepApplication
 import at.lockstep.player.MainActivity
 import at.lockstep.player.R
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.MutableSharedFlow
 import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.SharedFlow
 import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asSharedFlow
 import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.channels.BufferOverflow
+import kotlinx.coroutines.isActive
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 
+/**
+ * Local MP3 playback via Media3 [ExoPlayer]. UI progress and beat taps use
+ * [ExoPlayer.getCurrentPosition] (milliseconds from the start of the current media item), which is
+ * the same timeline used for seeking and end-of-track reporting — see Media3
+ * [Player] / [ExoPlayer] documentation.
+ */
 class PlaybackService : Service() {
     private val scope = CoroutineScope(SupervisorJob() + Dispatchers.Main.immediate)
     private lateinit var mediaSession: MediaSessionCompat
@@ -32,12 +48,66 @@ class PlaybackService : Service() {
 
     private val binder = LocalBinder()
 
+    private var player: ExoPlayer? = null
+    private var positionPollJob: Job? = null
+
     private val _uiState = MutableStateFlow(PlaybackUiState.initial())
     val uiState: StateFlow<PlaybackUiState> = _uiState.asStateFlow()
 
+    private val _trackBoundary =
+        MutableSharedFlow<TrackBoundaryEvent>(
+            extraBufferCapacity = 32,
+            onBufferOverflow = BufferOverflow.DROP_OLDEST,
+        )
+    val trackBoundaryEvents: SharedFlow<TrackBoundaryEvent> = _trackBoundary.asSharedFlow()
+
     private var queue: List<TrackQueueItem> = emptyList()
     private var index: Int = 0
 
+    private val playerListener =
+        object : Player.Listener {
+            override fun onPlaybackStateChanged(playbackState: Int) {
+                if (playbackState == Player.STATE_ENDED) {
+                    if (queue.isEmpty()) {
+                        return
+                    }
+                    if (index < queue.lastIndex) {
+                        emitTrackBoundaryForQueueIndex(
+                            index,
+                            TrackBoundaryReason.ADVANCED_TO_OTHER_TRACK,
+                        )
+                        index++
+                        publishCurrentTrack()
+                    } else {
+                        emitTrackBoundaryForQueueIndex(
+                            index,
+                            TrackBoundaryReason.LAST_TRACK_FINISHED,
+                        )
+                        setPlaying(false)
+                    }
+                }
+            }
+        }
+
+    private fun emitTrackBoundaryForQueueIndex(
+            i: Int,
+            reason: TrackBoundaryReason,
+    ) {
+        val item = queue.getOrNull(i) ?: return
+        scope.launch {
+            _trackBoundary.emit(
+                    TrackBoundaryEvent(
+                            trackId = item.id,
+                            title = item.title,
+                            artist = item.artist,
+                            queueIndex = i,
+                            queueSize = queue.size,
+                            reason = reason,
+                    ),
+            )
+        }
+    }
+
     inner class LocalBinder : Binder() {
         fun getService(): PlaybackService = this@PlaybackService
     }
@@ -65,6 +135,12 @@ class PlaybackService : Service() {
                 override fun onSkipToPrevious() {
                     skipDelta(-1)
                 }
+
+                override fun onSeekTo(pos: Long) {
+                    val p = player ?: return
+                    p.seekTo(pos)
+                    updateProgressFromPlayer()
+                }
             },
         )
         mediaSession.isActive = true
@@ -115,6 +191,40 @@ class PlaybackService : Service() {
         }
     }
 
+    private fun ensurePlayer(): ExoPlayer {
+        player?.let {
+            return it
+        }
+        return ExoPlayer.Builder(this)
+            .build()
+            .also {
+                it.addListener(playerListener)
+                player = it
+                startPositionPolling()
+            }
+    }
+
+    private fun startPositionPolling() {
+        positionPollJob?.cancel()
+        positionPollJob =
+            scope.launch {
+                while (isActive) {
+                    delay(UPDATE_INTERVAL_MS)
+                    if (player != null && queue.isNotEmpty()) {
+                        updateProgressFromPlayer()
+                    }
+                }
+            }
+    }
+
+    private fun releasePlayer() {
+        positionPollJob?.cancel()
+        positionPollJob = null
+        player?.removeListener(playerListener)
+        player?.release()
+        player = null
+    }
+
     private fun startPlaylist(pid: String) {
         scope.launch(Dispatchers.IO) {
             val rows = app.playlistRepository.getTracks(pid)
@@ -128,45 +238,66 @@ class PlaybackService : Service() {
                     if (uriStr.isNullOrBlank() || pairing.pairingError != null) {
                         return@mapNotNull null
                     }
+                    val hint =
+                        row.durationMs?.takeIf { it > 0 }
+                            ?: DEFAULT_DURATION_HINT_MS
                     TrackQueueItem(
                         id = tid,
                         title = row.trackName ?: "—",
                         artist = row.artistName ?: "—",
                         localUri = Uri.parse(uriStr),
+                        durationMsHint = hint,
                     )
                 }
             index = 0
             if (queue.isEmpty()) {
-                stopForeground(STOP_FOREGROUND_REMOVE)
+                withContext(Dispatchers.Main) {
-                stopSelf()
+                    releasePlayer()
+                    stopForeground(STOP_FOREGROUND_REMOVE)
+                    stopSelf()
+                }
                 return@launch
             }
             withContext(Dispatchers.Main) {
+                ensurePlayer()
                 setPlaying(true)
                 publishCurrentTrack()
+                refreshForegroundNotification()
             }
         }
     }
 
     private fun publishCurrentTrack() {
         val item = queue.getOrNull(index) ?: return
-        val durationSec = 180
+        applyCurrentMediaItem(item)
+        val durationSec =
+            (player?.duration?.takeIf { it > 0 }?.div(1000)?.toInt())
+                ?: (item.durationMsHint / 1000).coerceAtLeast(1)
         _uiState.value =
             PlaybackUiState(
                 title = item.title,
                 artist = item.artist,
-                progress = _uiState.value.progress,
+                progress = 0f,
                 durationSeconds = durationSec,
                 isPlaying = _uiState.value.isPlaying,
+                currentTrackId = item.id,
+                currentQueueIndex = index,
+                queueSize = queue.size,
             )
-        updateSessionMetadata(
+        updateProgressFromPlayer()
-            item,
+        updateSessionMetadata(item, durationSec)
-            durationSec,
+        updatePlaybackStateFromPlayer()
-        )
-        updatePlaybackState()
         refreshForegroundNotification()
     }
 
+    private fun applyCurrentMediaItem(item: TrackQueueItem) {
+        val uri = item.localUri ?: return
+        val p = ensurePlayer()
+        p.setMediaItem(MediaItem.fromUri(uri))
+        p.prepare()
+        p.playWhenReady = _uiState.value.isPlaying
+    }
+
     private fun updateSessionMetadata(
         item: TrackQueueItem,
         durationSec: Int,
@@ -180,14 +311,47 @@ class PlaybackService : Service() {
         )
     }
 
-    private fun updatePlaybackState() {
+    private fun currentDurationMs(): Long {
+        val p = player
+        val fromPlayer = p?.duration?.takeIf { it > 0 }
+        if (fromPlayer != null) {
+            return fromPlayer
+        }
+        val hint = queue.getOrNull(index)?.durationMsHint?.toLong() ?: DEFAULT_DURATION_HINT_MS.toLong()
+        return hint
+    }
+
+    private fun updateProgressFromPlayer() {
+        val p = player ?: return
+        val durationMs = currentDurationMs().coerceAtLeast(1L)
+        val positionMs = p.currentPosition.coerceIn(0L, durationMs)
+        if (queue.isEmpty()) {
+            return
+        }
+        val progress = (positionMs.toFloat() / durationMs.toFloat()).coerceIn(0f, 1f)
+        val durationSec = (durationMs / 1000L).toInt().coerceAtLeast(1)
+        _uiState.value =
+            _uiState.value.copy(
+                progress = progress,
+                durationSeconds = durationSec,
+                currentTrackId = queue.getOrNull(index)?.id ?: _uiState.value.currentTrackId,
+                currentQueueIndex = index,
+                queueSize = queue.size,
+            )
+        updatePlaybackStateFromPlayer()
+    }
+
+    private fun updatePlaybackStateFromPlayer() {
+        val p = player
+        val positionMs = p?.currentPosition ?: 0L
         val actions =
             PlaybackStateCompat.ACTION_PLAY or
                 PlaybackStateCompat.ACTION_PAUSE or
                 PlaybackStateCompat.ACTION_SKIP_TO_NEXT or
-                PlaybackStateCompat.ACTION_SKIP_TO_PREVIOUS
+                PlaybackStateCompat.ACTION_SKIP_TO_PREVIOUS or
+                PlaybackStateCompat.ACTION_SEEK_TO
         val state =
-            if (_uiState.value.isPlaying) {
+            if (_uiState.value.isPlaying && p?.isPlaying == true) {
                 PlaybackStateCompat.STATE_PLAYING
             } else {
                 PlaybackStateCompat.STATE_PAUSED
@@ -195,14 +359,15 @@ class PlaybackService : Service() {
         mediaSession.setPlaybackState(
             PlaybackStateCompat.Builder()
                 .setActions(actions)
-                .setState(state, PlaybackStateCompat.PLAYBACK_POSITION_UNKNOWN, 1f)
+                .setState(state, positionMs, 1f)
                 .build(),
         )
     }
 
     private fun setPlaying(playing: Boolean) {
         _uiState.value = _uiState.value.copy(isPlaying = playing)
-        updatePlaybackState()
+        player?.playWhenReady = playing
+        updatePlaybackStateFromPlayer()
         refreshForegroundNotification()
     }
 
@@ -212,8 +377,16 @@ class PlaybackService : Service() {
     }
 
     private fun skipDelta(delta: Int) {
-        if (queue.isEmpty()) return
+        if (queue.isEmpty()) {
-        index = (index + delta).coerceIn(0, queue.lastIndex)
+            return
+        }
+        val oldIndex = index
+        val newIndex = (oldIndex + delta).coerceIn(0, queue.lastIndex)
+        if (newIndex == oldIndex) {
+            return
+        }
+        emitTrackBoundaryForQueueIndex(oldIndex, TrackBoundaryReason.ADVANCED_TO_OTHER_TRACK)
+        index = newIndex
         publishCurrentTrack()
     }
 
@@ -231,31 +404,19 @@ class PlaybackService : Service() {
 
     fun requestSeek(fraction: Float) {
         if (queue.isEmpty()) return
-        _uiState.value =
+        val p = player ?: return
-            _uiState.value.copy(
+        val durationMs = currentDurationMs()
-                progress = fraction.coerceIn(0f, 1f),
+        val positionMs = (fraction.coerceIn(0f, 1f) * durationMs).toLong()
-            )
+        p.seekTo(positionMs)
-        mediaSession.setPlaybackState(
+        updateProgressFromPlayer()
-            PlaybackStateCompat.Builder()
+    }
-                .setActions(
+
-                    PlaybackStateCompat.ACTION_PLAY or
+    /**
-                        PlaybackStateCompat.ACTION_PAUSE or
+     * Milliseconds from the start of the current track — same timebase as [ExoPlayer.getCurrentPosition].
-                        PlaybackStateCompat.ACTION_SKIP_TO_NEXT or
+     * May be called from any thread per Media3 [Player] contract.
-                        PlaybackStateCompat.ACTION_SKIP_TO_PREVIOUS or
+     */
-                        PlaybackStateCompat.ACTION_SEEK_TO,
+    fun getPlaybackPositionMs(): Long {
-                )
+        return player?.currentPosition ?: 0L
-                .setState(
-                    if (_uiState.value.isPlaying) {
-                        PlaybackStateCompat.STATE_PLAYING
-                    } else {
-                        PlaybackStateCompat.STATE_PAUSED
-                    },
-                    (_uiState.value.progress * _uiState.value.durationSeconds * 1000).toLong(),
-                    1f,
-                )
-                .build(),
-        )
-        refreshForegroundNotification()
     }
 
     private fun refreshForegroundNotification() {
@@ -332,6 +493,8 @@ class PlaybackService : Service() {
     }
 
     override fun onDestroy() {
+        positionPollJob?.cancel()
+        releasePlayer()
         mediaSession.run {
             isActive = false
             release()
@@ -345,6 +508,9 @@ class PlaybackService : Service() {
         val progress: Float,
         val durationSeconds: Int,
         val isPlaying: Boolean,
+        val currentTrackId: String?,
+        val currentQueueIndex: Int,
+        val queueSize: Int,
     ) {
         companion object {
             fun initial() =
@@ -354,6 +520,9 @@ class PlaybackService : Service() {
                     progress = 0f,
                     durationSeconds = 180,
                     isPlaying = false,
+                    currentTrackId = null,
+                    currentQueueIndex = 0,
+                    queueSize = 0,
                 )
         }
     }
@@ -363,9 +532,14 @@ class PlaybackService : Service() {
         val title: String,
         val artist: String,
         val localUri: Uri?,
+        /** Fallback when [ExoPlayer] has not reported duration yet (from jukebox or default). */
+        val durationMsHint: Int,
     )
 
     companion object {
+        private const val UPDATE_INTERVAL_MS = 250L
+        private const val DEFAULT_DURATION_HINT_MS = 180_000
+
         const val ACTION_START_PLAYLIST = "at.lockstep.player.action.START_PLAYLIST"
         const val ACTION_TOGGLE_PAUSE = "at.lockstep.player.action.TOGGLE_PAUSE"
         const val ACTION_SKIP_NEXT = "at.lockstep.player.action.SKIP_NEXT"

app/src/main/java/at/lockstep/player/playback/TrackBoundaryEvent.kt

@@ -0,0 +1,23 @@
+package at.lockstep.player.playback
+
+/**
+ * Fired when annotation should flush beats for the track being left:
+ * user skipped, auto-advance after a track ended, or the last track finished.
+ */
+data class TrackBoundaryEvent(
+    val trackId: String,
+    val title: String,
+    val artist: String,
+    /** 0-based index in the current play queue when this track was current. */
+    val queueIndex: Int,
+    val queueSize: Int,
+    val reason: TrackBoundaryReason,
+)
+
+enum class TrackBoundaryReason {
+    /** Left this track for another (skip, previous, or middle track ended). */
+    ADVANCED_TO_OTHER_TRACK,
+
+    /** Playback of the last track in the playlist completed. */
+    LAST_TRACK_FINISHED,
+}

app/src/main/java/at/lockstep/player/ui/AnnotationScreen.kt

@@ -0,0 +1,200 @@
+package at.lockstep.player.ui
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.gestures.awaitEachGesture
+import androidx.compose.foundation.gestures.awaitFirstDown
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.heightIn
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.foundation.lazy.itemsIndexed
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.material3.Icon
+import androidx.compose.material3.IconButton
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Text
+import androidx.compose.material3.TopAppBar
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateListOf
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.input.pointer.pointerInput
+import androidx.compose.ui.platform.LocalContext
+import androidx.compose.ui.unit.dp
+import at.lockstep.player.LockstepViewModel
+import at.lockstep.player.R
+import at.lockstep.player.playback.PlaybackService
+
+/**
+ * Beat taps record [PlaybackService.getPlaybackPositionMs] (Media3 [androidx.media3.common.Player]
+ * timeline). JSON is written only when the track changes (skip/prev/next or natural advance) or when
+ * the last track in the playlist finishes — see [PlaybackService.trackBoundaryEvents].
+ */
+@OptIn(ExperimentalMaterial3Api::class)
+@Composable
+fun AnnotationRoute(
+    playlistId: String,
+    playback: PlaybackService?,
+    viewModel: LockstepViewModel,
+    onBack: () -> Unit,
+    modifier: Modifier = Modifier,
+) {
+    val context = LocalContext.current
+    var ui by remember {
+        mutableStateOf(
+            NowPlayingUiState(
+                title = context.getString(R.string.now_playing_idle_title),
+                artist = context.getString(R.string.now_playing_idle_artist),
+                stepFrequencyDisplay = context.getString(R.string.step_frequency_placeholder),
+                progress = 0f,
+                durationSeconds = 180,
+                isPlaying = false,
+            ),
+        )
+    }
+
+    LaunchedEffect(playback) {
+        val service = playback ?: return@LaunchedEffect
+        service.uiState.collect { p ->
+            ui =
+                NowPlayingUiState(
+                    title = p.title,
+                    artist = p.artist,
+                    stepFrequencyDisplay = context.getString(R.string.step_frequency_placeholder),
+                    progress = p.progress,
+                    durationSeconds = p.durationSeconds,
+                    isPlaying = p.isPlaying,
+                )
+        }
+    }
+
+    val beatTimesMs = remember { mutableStateListOf<Long>() }
+
+    var playlistDisplayName by remember { mutableStateOf("playlist") }
+
+    LaunchedEffect(playlistId) {
+        playlistDisplayName = viewModel.getPlaylistDisplayName(playlistId)
+    }
+
+    LaunchedEffect(playback, playlistId, playlistDisplayName) {
+        val service = playback ?: return@LaunchedEffect
+        service.trackBoundaryEvents.collect { event ->
+            val snapshot = beatTimesMs.toList()
+            beatTimesMs.clear()
+            viewModel.persistBeatAnnotation(playlistId, playlistDisplayName, event, snapshot)
+        }
+    }
+
+    Scaffold(
+        modifier = modifier.fillMaxSize(),
+        topBar = {
+            TopAppBar(
+                title = { Text(text = context.getString(R.string.annotation_title)) },
+                navigationIcon = {
+                    IconButton(onClick = onBack) {
+                        Icon(Icons.AutoMirrored.Filled.ArrowBack, contentDescription = null)
+                    }
+                },
+            )
+        },
+    ) { padding ->
+        Column(
+            modifier =
+                Modifier
+                    .fillMaxSize()
+                    .padding(padding),
+        ) {
+            Text(
+                text = context.getString(R.string.annotation_subtitle),
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.onSurfaceVariant,
+                modifier = Modifier.padding(horizontal = 24.dp, vertical = 8.dp),
+            )
+            Column(
+                modifier =
+                    Modifier
+                        .fillMaxWidth()
+                        .heightIn(max = 420.dp)
+                        .padding(horizontal = 24.dp),
+                verticalArrangement = Arrangement.spacedBy(16.dp),
+            ) {
+                NowPlayingScreen(
+                    state = ui,
+                    onProgressChange = { fraction ->
+                        playback?.requestSeek(fraction)
+                        ui = ui.copy(progress = fraction)
+                    },
+                    onPrevious = { playback?.requestSkipPrevious() },
+                    onTogglePlayPause = { playback?.requestTogglePause() },
+                    onNext = { playback?.requestSkipNext() },
+                    modifier = Modifier.fillMaxWidth(),
+                )
+            }
+            Text(
+                text = context.getString(R.string.annotation_beat_count, beatTimesMs.size),
+                style = MaterialTheme.typography.titleMedium,
+                modifier = Modifier.padding(horizontal = 24.dp, vertical = 4.dp),
+            )
+            Box(
+                modifier =
+                    Modifier
+                        .weight(1f)
+                        .fillMaxWidth()
+                        .padding(horizontal = 16.dp, vertical = 8.dp)
+                        .background(MaterialTheme.colorScheme.primaryContainer)
+                        .pointerInput(playback) {
+                            awaitEachGesture {
+                                awaitFirstDown(requireUnconsumed = false)
+                                val t = playback?.getPlaybackPositionMs() ?: 0L
+                                beatTimesMs.add(t)
+                            }
+                        },
+                contentAlignment = Alignment.Center,
+            ) {
+                Text(
+                    text = context.getString(R.string.annotation_tap_area_label),
+                    style = MaterialTheme.typography.headlineSmall,
+                    color = MaterialTheme.colorScheme.onPrimaryContainer,
+                )
+            }
+            LazyColumn(
+                modifier =
+                    Modifier
+                        .fillMaxWidth()
+                        .heightIn(max = 160.dp)
+                        .padding(horizontal = 24.dp, vertical = 8.dp),
+                verticalArrangement = Arrangement.spacedBy(4.dp),
+            ) {
+                itemsIndexed(beatTimesMs) { i, ms ->
+                    Row(
+                        modifier = Modifier.fillMaxWidth(),
+                        horizontalArrangement = Arrangement.SpaceBetween,
+                    ) {
+                        Text(
+                            text = "${i + 1}.",
+                            style = MaterialTheme.typography.labelLarge,
+                            color = MaterialTheme.colorScheme.onSurfaceVariant,
+                        )
+                        Text(
+                            text = context.getString(R.string.annotation_time_ms, ms.toInt()),
+                            style = MaterialTheme.typography.bodyLarge,
+                        )
+                    }
+                }
+            }
+        }
+    }
+}

app/src/main/java/at/lockstep/player/ui/LockstepAppNavHost.kt

@@ -12,6 +12,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.core.content.ContextCompat
 import androidx.navigation.NavHostController
 import androidx.navigation.NavType
@@ -32,6 +33,7 @@ fun LockstepAppNavHost(
     navController: NavHostController,
     viewModel: LockstepViewModel,
 ) {
+    val annotationMode by viewModel.annotationMode.collectAsStateWithLifecycle()
     var playback by remember { mutableStateOf<PlaybackService?>(null) }
 
     DisposableEffect(Unit) {
@@ -70,7 +72,11 @@ fun LockstepAppNavHost(
                         navController.navigate(Routes.pairing(playlist.id))
                     } else {
                         startPlaylistPlayback(activity, playlist.id)
-                        navController.navigate(Routes.nowPlaying(playlist.id))
+                        if (annotationMode) {
+                            navController.navigate(Routes.annotation(playlist.id))
+                        } else {
+                            navController.navigate(Routes.nowPlaying(playlist.id))
+                        }
                     }
                 },
                 onOpenSettings = {
@@ -112,6 +118,21 @@ fun LockstepAppNavHost(
                 onBack = { navController.popBackStack() },
             )
         }
+        composable(
+            route = Routes.Annotation,
+            arguments =
+                listOf(
+                    navArgument("playlistId") { type = NavType.StringType },
+                ),
+        ) { entry ->
+            val playlistId = entry.arguments?.getString("playlistId").orEmpty()
+            AnnotationRoute(
+                playlistId = playlistId,
+                playback = playback,
+                viewModel = viewModel,
+                onBack = { navController.popBackStack() },
+            )
+        }
     }
 }
 

app/src/main/java/at/lockstep/player/ui/navigation/Routes.kt

@@ -6,8 +6,11 @@ object Routes {
     const val Settings = "settings"
     const val Pairing = "pairing/{playlistId}"
     const val NowPlaying = "nowPlaying/{playlistId}"
+    const val Annotation = "annotation/{playlistId}"
 
     fun pairing(playlistId: String) = "pairing/$playlistId"
 
     fun nowPlaying(playlistId: String) = "nowPlaying/$playlistId"
+
+    fun annotation(playlistId: String) = "annotation/$playlistId"
 }

app/src/main/java/at/lockstep/player/ui/settings/SettingsScreen.kt

@@ -2,7 +2,9 @@ package at.lockstep.player.ui.settings
 
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.filled.ArrowBack
@@ -13,12 +15,16 @@ import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Switch
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.unit.dp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import at.lockstep.player.LockstepViewModel
 import at.lockstep.player.R
 
@@ -30,6 +36,7 @@ fun SettingsScreen(
     modifier: Modifier = Modifier,
 ) {
     val context = LocalContext.current
+    val annotationMode by viewModel.annotationMode.collectAsStateWithLifecycle()
     Scaffold(
         modifier = modifier.fillMaxSize(),
         topBar = {
@@ -51,6 +58,30 @@ fun SettingsScreen(
                     .padding(24.dp),
             verticalArrangement = Arrangement.spacedBy(12.dp),
         ) {
+            Row(
+                modifier =
+                    Modifier
+                        .fillMaxWidth()
+                        .padding(vertical = 8.dp),
+                verticalAlignment = Alignment.CenterVertically,
+                horizontalArrangement = Arrangement.SpaceBetween,
+            ) {
+                Column(modifier = Modifier.weight(1f).padding(end = 16.dp)) {
+                    Text(
+                        text = context.getString(R.string.settings_annotation_mode),
+                        style = MaterialTheme.typography.titleMedium,
+                    )
+                    Text(
+                        text = context.getString(R.string.settings_annotation_mode_help),
+                        style = MaterialTheme.typography.bodySmall,
+                        color = MaterialTheme.colorScheme.onSurfaceVariant,
+                    )
+                }
+                Switch(
+                    checked = annotationMode,
+                    onCheckedChange = { viewModel.setAnnotationMode(it) },
+                )
+            }
             Text(
                 text = context.getString(R.string.settings_stub_body),
                 style = MaterialTheme.typography.bodyLarge,

app/src/main/java/at/lockstep/player/util/BeatAnnotationStorage.kt

@@ -0,0 +1,63 @@
+package at.lockstep.player.util
+
+import android.content.Context
+import android.net.Uri
+import android.provider.DocumentsContract
+import org.json.JSONArray
+import org.json.JSONObject
+import java.io.File
+import java.util.Locale
+
+object BeatAnnotationStorage {
+
+    private const val DIR_NAME = "beat_annotations"
+
+    fun annotationsDir(context: Context): File =
+        File(context.filesDir, DIR_NAME).apply { mkdirs() }
+
+    /**
+     * [playlistDisplayName] + "_" + 1-based track index (000) + ".json".
+     */
+    fun writeAnnotationsFile(
+        context: Context,
+        playlistDisplayName: String,
+        trackQueueIndex0Based: Int,
+        contentId: String,
+        title: String,
+        artist: String,
+        beatTimesMs: List<Long>,
+    ): File {
+        val safeName =
+            playlistDisplayName
+                .replace(Regex("[\\\\/:*?\"<>|]"), "_")
+                .trim()
+                .ifBlank { "playlist" }
+                .take(120)
+        val suffix =
+            String.format(Locale.US, "%03d", trackQueueIndex0Based + 1)
+        val file = File(annotationsDir(context), "${safeName}_$suffix.json")
+        val sec = beatTimesMs.map { it / 1000.0 }
+        val json =
+            JSONObject().apply {
+                put("contentId", contentId)
+                put("title", title)
+                put("artist", artist)
+                put("beatTimesSec", JSONArray(sec))
+            }
+        file.writeText(json.toString(2))
+        return file
+    }
+
+    /** Document id for a content [Uri] when available; otherwise last path segment. */
+    fun mp3DocumentContentId(localUri: String?): String {
+        if (localUri.isNullOrBlank()) {
+            return ""
+        }
+        val u = Uri.parse(localUri)
+        return try {
+            DocumentsContract.getDocumentId(u)
+        } catch (_: Exception) {
+            u.lastPathSegment ?: ""
+        }
+    }
+}

app/src/main/res/values/strings.xml

@@ -28,6 +28,13 @@
     <string name="settings_stub_body">More controls will land here in a later milestone.</string>
     <string name="settings_logout_spotify">Sign out of Spotify</string>
     <string name="settings_logout_spotify_help">Clears your stored access token and returns to the welcome steps so you can log in again. Use this if the app gets HTTP 401 from the server.</string>
+    <string name="settings_annotation_mode">Annotation mode</string>
+    <string name="settings_annotation_mode_help">When enabled, choosing a playlist opens beat annotation (tap in time) instead of Now playing.</string>
+    <string name="annotation_title">Beat annotation</string>
+    <string name="annotation_subtitle">Tap on each beat; times use the same clock as playback (ExoPlayer position).</string>
+    <string name="annotation_tap_area_label">Tap here on the beat</string>
+    <string name="annotation_beat_count">Beats recorded: %1$d</string>
+    <string name="annotation_time_ms">%1$d ms</string>
 
     <string name="pairing_title">Pair local MP3s</string>
     <string name="pairing_choose_folder">Choose folder of MP3s</string>