fix: thread-safe player position logging

BUGS.md

@@ -1,5 +1,7 @@
 # Bugs
 
+- annotation playback: check what happens if some songs are not paired in the playlist. I believe the index is wrong and the player plays a different song from what is displayed as title and artist.
+
 - syncJukeboxIfToken() is always called at startup, even if we have a jukebox already
 
 - "Caveat: If a sync gets past the fetch phase, runs clearAllTables(), then dies while writing some playlists, you could still end up with a partial cache. Fixing that would mean a single Room transaction or a “swap” strategy; say if you want that next."

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/AndroidManifest.xml

@@ -2,6 +2,15 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
 
     <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.READ_MEDIA_AUDIO" />
+    <uses-permission
+        android:name="android.permission.READ_EXTERNAL_STORAGE"
+        android:maxSdkVersion="32" />
+    <uses-permission
+        android:name="android.permission.WRITE_EXTERNAL_STORAGE"
+        android:maxSdkVersion="28" />
+    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
+    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK" />

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

@@ -10,9 +10,14 @@ 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.MediaStoreMp3Scanner
 import at.lockstep.player.util.Mp3EmbeddedMetadata
 import at.lockstep.player.util.Mp3FolderCandidate
+import at.lockstep.player.util.RunDataStorage
+import at.lockstep.player.util.RunTrackDataSnapshot
 import at.lockstep.player.util.TrackFileMatching
 import at.lockstep.player.util.mp3DisplayNameFromUri
 import kotlinx.coroutines.Dispatchers
@@ -56,6 +61,32 @@ class LockstepViewModel(
             null,
         )
 
+    val annotationMode: StateFlow<Boolean> =
+        prefs.annotationMode.stateIn(
+            viewModelScope,
+            SharingStarted.Eagerly,
+            false,
+        )
+
+    val collectRunData: StateFlow<Boolean> =
+        prefs.collectRunData.stateIn(
+            viewModelScope,
+            SharingStarted.Eagerly,
+            false,
+        )
+
+    fun setAnnotationMode(enabled: Boolean) {
+        viewModelScope.launch {
+            prefs.setAnnotationMode(enabled)
+        }
+    }
+
+    fun setCollectRunData(enabled: Boolean) {
+        viewModelScope.launch {
+            prefs.setCollectRunData(enabled)
+        }
+    }
+
     private val context get() = getApplication<Application>()
 
     /**
@@ -120,6 +151,105 @@ 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,
+            )
+        }
+    }
+
+    /**
+     * Writes one JSON file under public Documents/Lockstep/{runSessionFolder}/ when a track finishes or is skipped.
+     * Skips when [samples] is empty or the track has no paired local URI.
+     */
+    fun persistRunData(
+        playlistId: String,
+        playlistDisplayName: String,
+        runSessionFolder: String,
+        event: TrackBoundaryEvent,
+        snapshot: RunTrackDataSnapshot,
+    ) {
+        if (snapshot.isEmpty()) {
+            return
+        }
+        viewModelScope.launch(Dispatchers.IO) {
+            val pairing = pairingDao.findForTrack(playlistId, event.trackId)
+            val meta = pairing?.localUri?.takeIf { it.isNotBlank() } ?: return@launch
+            RunDataStorage.writeRunDataFile(
+                context = getApplication(),
+                runSessionFolder = runSessionFolder,
+                playlistDisplayName = playlistDisplayName,
+                trackQueueIndex0Based = event.queueIndex,
+                metaContentUri = meta,
+                title = event.title,
+                artist = event.artist,
+                snapshot = snapshot,
+            )
+        }
+    }
+
+    /** Flush in-progress run data when leaving Now Playing before a track boundary fires. */
+    fun persistRunDataForCurrentTrack(
+        playlistId: String,
+        playlistDisplayName: String,
+        runSessionFolder: String,
+        trackId: String,
+        title: String,
+        artist: String,
+        queueIndex: Int,
+        snapshot: RunTrackDataSnapshot,
+    ) {
+        if (snapshot.isEmpty()) {
+            return
+        }
+        viewModelScope.launch(Dispatchers.IO) {
+            val pairing = pairingDao.findForTrack(playlistId, trackId)
+            val meta = pairing?.localUri?.takeIf { it.isNotBlank() } ?: return@launch
+            RunDataStorage.writeRunDataFile(
+                context = getApplication(),
+                runSessionFolder = runSessionFolder,
+                playlistDisplayName = playlistDisplayName,
+                trackQueueIndex0Based = queueIndex,
+                metaContentUri = meta,
+                title = title,
+                artist = artist,
+                snapshot = snapshot,
+            )
+        }
+    }
+
     suspend fun syncJukeboxIfToken(): String? {
         val token = spotifyAccessToken.value
         if (token.isNullOrBlank()) {
@@ -239,25 +369,7 @@ class LockstepViewModel(
             }
 
             val tracks = loadJukeboxTracksEnsuringDetail(playlistId)
-            val withId = tracks.count { !it.trackId.isNullOrBlank() }
+            logJukeboxTracks(tracks)
-            val withTitle = tracks.count { !it.trackName.isNullOrBlank() }
-            val withArtist = tracks.count { !it.artistName.isNullOrBlank() }
-            Log.d(
-                TAG,
-                "jukebox DB rows for playlist: total=${tracks.size} withSpotifyTrackId=$withId " +
-                    "withTitle=$withTitle withArtist=$withArtist",
-            )
-            tracks.take(12).forEach { row ->
-                Log.d(
-                    TAG,
-                    "  jukebox row pos=${row.position} id=${row.trackId} title='${row.trackName}' " +
-                        "artist='${row.artistName}' durationMs=${row.durationMs}",
-                )
-            }
-            if (tracks.size > 12) {
-                Log.d(TAG, "  … ${tracks.size - 12} more rows omitted from log")
-            }
-
             val (mp3Total, pool) =
                 withContext(Dispatchers.IO) {
                     val mp3s = FolderMp3Scanner.listMp3Uris(context, treeUri)
@@ -274,75 +386,117 @@ class LockstepViewModel(
                         }.toMutableList()
                     Pair(mp3s.size, p)
                 }
-
+            onFinished(matchPlaylistTracks(playlistId, tracks, pool, mp3Total))
-            if (pool.isEmpty()) {
-                Log.w(TAG, "No MP3 candidates after scan — cannot pair")
-                onFinished(
-                    FolderPairingResult(
-                        paired = 0,
-                        failed = 0,
-                        jukeboxRowCount = tracks.size,
-                        mp3Count = mp3Total,
-                        skippedNoSpotifyTrackId = 0,
-                    ),
-                )
-                return@launch
-            }
-
-            Log.d(
-                TAG,
-                "pool id3 coverage: ${pool.count { it.id3Title != null || it.id3Artist != null }}/${pool.size} " +
-                    "files have title or artist from tags",
-            )
-
-            var paired = 0
-            var failed = 0
-            var skippedNoSpotifyTrackId = 0
-            for (track in tracks.sortedBy { it.position }) {
-                val tid = track.trackId
-                if (tid.isNullOrBlank()) {
-                    skippedNoSpotifyTrackId++
-                    failed++
-                    Log.d(
-                        TAG,
-                        "skip row pos=${track.position}: no Spotify track id " +
-                            "(title='${track.trackName}' artist='${track.artistName}')",
-                    )
-                    continue
-                }
-                val title = track.trackName.orEmpty()
-                val artist = track.artistName.orEmpty()
-                val pick = TrackFileMatching.bestMatchForFolder(title, artist, pool)
-                if (pick == null) {
-                    failed++
-                    continue
-                }
-                val err =
-                    withContext(Dispatchers.IO) { AudioUriValidator.validateReadableAudio(context, pick) }
-                if (err != null) {
-                    Log.w(TAG, "match candidate failed validation spotifyTid=$tid local=$pick err=$err")
-                    upsertPairing(playlistId, tid, null, err)
-                    failed++
-                } else {
-                    upsertPairing(playlistId, tid, pick.toString(), null)
-                    paired++
-                    pool.removeAll { it.uri == pick }
-                }
-            }
-            Log.d(
-                TAG,
-                "pairPlaylistFromFolder done paired=$paired failed=$failed " +
-                    "skippedNoSpotifyTrackId=$skippedNoSpotifyTrackId jukeboxRows=${tracks.size} mp3=$mp3Total",
-            )
-            onFinished(
-                FolderPairingResult(
-                    paired = paired,
-                    failed = failed,
-                    jukeboxRowCount = tracks.size,
-                    mp3Count = mp3Total,
-                    skippedNoSpotifyTrackId = skippedNoSpotifyTrackId,
-                ),
-            )
         }
     }
+
+    /** Pair against all MP3s indexed on the device (Music, Downloads, etc.) — no folder picker needed. */
+    fun pairPlaylistFromDeviceAudio(
+        playlistId: String,
+        onFinished: (FolderPairingResult) -> Unit,
+    ) {
+        viewModelScope.launch {
+            Log.d(TAG, "pairPlaylistFromDeviceAudio playlistId=$playlistId")
+            val tracks = loadJukeboxTracksEnsuringDetail(playlistId)
+            logJukeboxTracks(tracks)
+            val (mp3Total, pool) =
+                withContext(Dispatchers.IO) {
+                    val candidates = MediaStoreMp3Scanner.listMp3Candidates(context)
+                    Pair(candidates.size, candidates.toMutableList())
+                }
+            onFinished(matchPlaylistTracks(playlistId, tracks, pool, mp3Total))
+        }
+    }
+
+    private fun logJukeboxTracks(tracks: List<TrackRow>) {
+        val withId = tracks.count { !it.trackId.isNullOrBlank() }
+        val withTitle = tracks.count { !it.trackName.isNullOrBlank() }
+        val withArtist = tracks.count { !it.artistName.isNullOrBlank() }
+        Log.d(
+            TAG,
+            "jukebox DB rows for playlist: total=${tracks.size} withSpotifyTrackId=$withId " +
+                "withTitle=$withTitle withArtist=$withArtist",
+        )
+        tracks.take(12).forEach { row ->
+            Log.d(
+                TAG,
+                "  jukebox row pos=${row.position} id=${row.trackId} title='${row.trackName}' " +
+                    "artist='${row.artistName}' durationMs=${row.durationMs}",
+            )
+        }
+        if (tracks.size > 12) {
+            Log.d(TAG, "  … ${tracks.size - 12} more rows omitted from log")
+        }
+    }
+
+    private suspend fun matchPlaylistTracks(
+        playlistId: String,
+        tracks: List<TrackRow>,
+        pool: MutableList<Mp3FolderCandidate>,
+        mp3Total: Int,
+    ): FolderPairingResult {
+        if (pool.isEmpty()) {
+            Log.w(TAG, "No MP3 candidates after scan — cannot pair")
+            return FolderPairingResult(
+                paired = 0,
+                failed = 0,
+                jukeboxRowCount = tracks.size,
+                mp3Count = mp3Total,
+                skippedNoSpotifyTrackId = 0,
+            )
+        }
+
+        Log.d(
+            TAG,
+            "pool id3 coverage: ${pool.count { it.id3Title != null || it.id3Artist != null }}/${pool.size} " +
+                "files have title or artist from tags",
+        )
+
+        var paired = 0
+        var failed = 0
+        var skippedNoSpotifyTrackId = 0
+        for (track in tracks.sortedBy { it.position }) {
+            val tid = track.trackId
+            if (tid.isNullOrBlank()) {
+                skippedNoSpotifyTrackId++
+                failed++
+                Log.d(
+                    TAG,
+                    "skip row pos=${track.position}: no Spotify track id " +
+                        "(title='${track.trackName}' artist='${track.artistName}')",
+                )
+                continue
+            }
+            val title = track.trackName.orEmpty()
+            val artist = track.artistName.orEmpty()
+            val pick = TrackFileMatching.bestMatchForFolder(title, artist, pool)
+            if (pick == null) {
+                failed++
+                continue
+            }
+            val err =
+                withContext(Dispatchers.IO) { AudioUriValidator.validateReadableAudio(context, pick) }
+            if (err != null) {
+                Log.w(TAG, "match candidate failed validation spotifyTid=$tid local=$pick err=$err")
+                upsertPairing(playlistId, tid, null, err)
+                failed++
+            } else {
+                upsertPairing(playlistId, tid, pick.toString(), null)
+                paired++
+                pool.removeAll { it.uri == pick }
+            }
+        }
+        Log.d(
+            TAG,
+            "playlist pairing done paired=$paired failed=$failed " +
+                "skippedNoSpotifyTrackId=$skippedNoSpotifyTrackId jukeboxRows=${tracks.size} mp3=$mp3Total",
+        )
+        return FolderPairingResult(
+            paired = paired,
+            failed = failed,
+            jukeboxRowCount = tracks.size,
+            mp3Count = mp3Total,
+            skippedNoSpotifyTrackId = skippedNoSpotifyTrackId,
+        )
+    }
 }

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

@@ -27,6 +27,18 @@ 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
+        }
+
+    /** When true, Now Playing records accelerometer samples per track into JSON under filesDir/run_data. */
+    val collectRunData: Flow<Boolean> =
+        dataStore.data.map { prefs ->
+            prefs[KEY_COLLECT_RUN_DATA] == true
+        }
+
     suspend fun setOnboardingComplete(done: Boolean) {
         dataStore.edit { prefs ->
             prefs[KEY_ONBOARDING_COMPLETE] = done
@@ -43,8 +55,22 @@ class UserPreferencesRepository(
         }
     }
 
+    suspend fun setAnnotationMode(enabled: Boolean) {
+        dataStore.edit { prefs ->
+            prefs[KEY_ANNOTATION_MODE] = enabled
+        }
+    }
+
+    suspend fun setCollectRunData(enabled: Boolean) {
+        dataStore.edit { prefs ->
+            prefs[KEY_COLLECT_RUN_DATA] = 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")
+        private val KEY_COLLECT_RUN_DATA = booleanPreferencesKey("collect_run_data")
     }
 }

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 +48,71 @@ class PlaybackService : Service() {
 
     private val binder = LocalBinder()
 
+    private var player: ExoPlayer? = null
+    private var positionPollJob: Job? = null
+    private var positionCachePollJob: 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
 
+    /** Updated on the main thread whenever progress is read from ExoPlayer — safe for sensor threads. */
+    @Volatile
+    private var cachedPlaybackPositionMs: Long = 0L
+
+    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 +140,12 @@ 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 +196,50 @@ 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()
+                    }
+                }
+            }
+        positionCachePollJob?.cancel()
+        positionCachePollJob =
+            scope.launch {
+                while (isActive) {
+                    delay(POSITION_CACHE_INTERVAL_MS)
+                    refreshCachedPlaybackPositionMs()
+                }
+            }
+    }
+
+    private fun releasePlayer() {
+        positionPollJob?.cancel()
+        positionPollJob = null
+        positionCachePollJob?.cancel()
+        positionCachePollJob = 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 +253,66 @@ 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 +326,58 @@ 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 refreshCachedPlaybackPositionMs() {
+        val p = player ?: return
+        if (queue.isEmpty()) {
+            cachedPlaybackPositionMs = 0L
+            return
+        }
+        val durationMs = currentDurationMs().coerceAtLeast(1L)
+        cachedPlaybackPositionMs = p.currentPosition.coerceIn(0L, durationMs)
+    }
+
+    private fun updateProgressFromPlayer() {
+        val p = player ?: return
+        refreshCachedPlaybackPositionMs()
+        if (queue.isEmpty()) {
+            return
+        }
+        val durationMs = currentDurationMs().coerceAtLeast(1L)
+        val positionMs = cachedPlaybackPositionMs
+        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 +385,15 @@ 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 +403,16 @@ 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()
     }
 
@@ -231,33 +430,19 @@ 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
-                        PlaybackStateCompat.ACTION_SKIP_TO_NEXT or
-                        PlaybackStateCompat.ACTION_SKIP_TO_PREVIOUS or
-                        PlaybackStateCompat.ACTION_SEEK_TO,
-                )
-                .setState(
-                    if (_uiState.value.isPlaying) {
-                        PlaybackStateCompat.STATE_PLAYING
-                    } else {
-                        PlaybackStateCompat.STATE_PAUSED
-                    },
-                    (_uiState.value.progress * _uiState.value.durationSeconds * 1000).toLong(),
-                    1f,
-                )
-                .build(),
-        )
-        refreshForegroundNotification()
     }
 
+    /**
+     * Milliseconds from the start of the current track — same timebase as [ExoPlayer.getCurrentPosition].
+     * Reads a main-thread cache so callers on background threads (e.g. run-data sensor collection) stay safe.
+     */
+    fun getPlaybackPositionMs(): Long = cachedPlaybackPositionMs
+
     private fun refreshForegroundNotification() {
         if (queue.isEmpty()) return
         val notification = buildNotification()
@@ -332,6 +517,9 @@ class PlaybackService : Service() {
     }
 
     override fun onDestroy() {
+        positionPollJob?.cancel()
+        positionCachePollJob?.cancel()
+        releasePlayer()
         mediaSession.run {
             isActive = false
             release()
@@ -345,6 +533,9 @@ 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 +545,9 @@ class PlaybackService : Service() {
                     progress = 0f,
                     durationSeconds = 180,
                     isPlaying = false,
+                    currentTrackId = null,
+                    currentQueueIndex = 0,
+                    queueSize = 0,
                 )
         }
     }
@@ -363,9 +557,15 @@ 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 POSITION_CACHE_INTERVAL_MS = 20L
+        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 = {
@@ -109,6 +115,22 @@ fun LockstepAppNavHost(
             NowPlayingRoute(
                 playlistId = playlistId,
                 playback = playback,
+                viewModel = viewModel,
+                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/NowPlayingScreen.kt

@@ -1,5 +1,9 @@
 package at.lockstep.player.ui
 
+import android.Manifest
+import android.content.pm.PackageManager
+import androidx.activity.compose.rememberLauncherForActivityResult
+import androidx.activity.result.contract.ActivityResultContracts
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
@@ -22,9 +26,11 @@ import androidx.compose.material3.Slider
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.DisposableEffect
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
+import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
@@ -34,8 +40,13 @@ import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.tooling.preview.Preview
 import androidx.compose.ui.unit.dp
+import androidx.core.content.ContextCompat
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import at.lockstep.player.LockstepViewModel
 import at.lockstep.player.playback.PlaybackService
 import at.lockstep.player.R
+import at.lockstep.player.util.RunDataCollector
+import at.lockstep.player.util.RunDataStorage
 
 data class NowPlayingUiState(
     val title: String,
@@ -153,10 +164,38 @@ fun NowPlayingScreen(
 fun NowPlayingRoute(
     playlistId: String,
     playback: PlaybackService?,
+    viewModel: LockstepViewModel,
     onBack: () -> Unit,
     modifier: Modifier = Modifier,
 ) {
     val context = LocalContext.current
+    val collectRunData by viewModel.collectRunData.collectAsStateWithLifecycle()
+    val collector = remember { RunDataCollector(context) }
+    val runSessionFolder = remember { RunDataStorage.newRunSessionFolderName() }
+
+    val locationPermissionLauncher =
+        rememberLauncherForActivityResult(
+            contract = ActivityResultContracts.RequestPermission(),
+            onResult = { granted ->
+                if (collectRunData) {
+                    collector.start(enableLocation = granted)
+                }
+            },
+        )
+
+    fun startRunDataCollection() {
+        val hasLocation =
+            ContextCompat.checkSelfPermission(context, Manifest.permission.ACCESS_FINE_LOCATION) ==
+                PackageManager.PERMISSION_GRANTED
+        if (!hasLocation) {
+            locationPermissionLauncher.launch(Manifest.permission.ACCESS_FINE_LOCATION)
+        }
+        collector.start(enableLocation = hasLocation)
+    }
+    var playlistDisplayName by remember { mutableStateOf("playlist") }
+    var currentTrackId by remember { mutableStateOf<String?>(null) }
+    var currentQueueIndex by remember { mutableIntStateOf(0) }
+
     var ui by remember {
         mutableStateOf(
             NowPlayingUiState(
@@ -170,9 +209,15 @@ fun NowPlayingRoute(
         )
     }
 
+    LaunchedEffect(playlistId) {
+        playlistDisplayName = viewModel.getPlaylistDisplayName(playlistId)
+    }
+
     LaunchedEffect(playback) {
         val service = playback ?: return@LaunchedEffect
         service.uiState.collect { p ->
+            currentTrackId = p.currentTrackId
+            currentQueueIndex = p.currentQueueIndex
             ui =
                 NowPlayingUiState(
                     title = p.title,
@@ -185,6 +230,64 @@ fun NowPlayingRoute(
         }
     }
 
+    LaunchedEffect(collectRunData, playback) {
+        if (!collectRunData) {
+            collector.setCollectingEnabled(false)
+            return@LaunchedEffect
+        }
+        val service = playback ?: run {
+            collector.setCollectingEnabled(false)
+            return@LaunchedEffect
+        }
+        collector.setPlaybackPositionMsProvider { service.getPlaybackPositionMs() }
+        collector.setCollectingEnabled(true)
+        var lastTrackId: String? = null
+        service.uiState.collect { state ->
+            val trackId = state.currentTrackId
+            if (trackId != null && trackId != lastTrackId) {
+                collector.markSongStart()
+                lastTrackId = trackId
+            }
+        }
+    }
+
+    LaunchedEffect(collectRunData, playback, playlistId, playlistDisplayName) {
+        if (!collectRunData) return@LaunchedEffect
+        val service = playback ?: return@LaunchedEffect
+        service.trackBoundaryEvents.collect { event ->
+            val snapshot = collector.snapshotAndClear()
+            viewModel.persistRunData(playlistId, playlistDisplayName, runSessionFolder, event, snapshot)
+        }
+    }
+
+    DisposableEffect(collectRunData) {
+        if (collectRunData) {
+            startRunDataCollection()
+        } else {
+            collector.stop()
+            collector.snapshotAndClear()
+        }
+        onDispose {
+            if (collectRunData) {
+                val snapshot = collector.snapshotAndClear()
+                val trackId = currentTrackId
+                if (!snapshot.isEmpty() && trackId != null) {
+                    viewModel.persistRunDataForCurrentTrack(
+                        playlistId = playlistId,
+                        playlistDisplayName = playlistDisplayName,
+                        runSessionFolder = runSessionFolder,
+                        trackId = trackId,
+                        title = ui.title,
+                        artist = ui.artist,
+                        queueIndex = currentQueueIndex,
+                        snapshot = snapshot,
+                    )
+                }
+            }
+            collector.release()
+        }
+    }
+
     Scaffold(
         modifier = modifier.fillMaxSize(),
         topBar = {

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/pairing/PairingScreen.kt

@@ -1,5 +1,6 @@
 package at.lockstep.player.ui.pairing
 
+import android.content.pm.PackageManager
 import android.widget.Toast
 import androidx.activity.compose.rememberLauncherForActivityResult
 import androidx.activity.result.contract.ActivityResultContracts
@@ -20,6 +21,7 @@ import androidx.compose.material3.Icon
 import androidx.compose.material3.IconButton
 import androidx.compose.material3.ListItem
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.OutlinedButton
 import androidx.compose.material3.Scaffold
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
@@ -33,10 +35,14 @@ import androidx.compose.runtime.setValue
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.unit.dp
+import androidx.core.content.ContextCompat
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import at.lockstep.jukebox.db.TrackRow
+import at.lockstep.player.FolderPairingResult
 import at.lockstep.player.LockstepViewModel
 import at.lockstep.player.R
+import at.lockstep.player.util.AudioReadPermission
+import at.lockstep.player.util.SafInitialUris
 
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -78,54 +84,44 @@ fun PairingScreen(
             onResult = { tree ->
                 if (tree != null) {
                     viewModel.pairPlaylistFromFolder(playlistId, tree) { r ->
-                        when {
+                        showPairingResultToast(context, r)
-                            r.mp3Count == 0 ->
-                                Toast.makeText(
-                                    context,
-                                    context.getString(R.string.pairing_no_mp3_in_folder),
-                                    Toast.LENGTH_SHORT,
-                                ).show()
-
-                            r.jukeboxRowCount == 0 ->
-                                Toast.makeText(
-                                    context,
-                                    context.getString(R.string.pairing_jukebox_empty),
-                                    Toast.LENGTH_LONG,
-                                ).show()
-
-                            r.skippedNoSpotifyTrackId == r.jukeboxRowCount && r.jukeboxRowCount > 0 ->
-                                Toast.makeText(
-                                    context,
-                                    context.getString(R.string.pairing_all_missing_spotify_id),
-                                    Toast.LENGTH_LONG,
-                                ).show()
-
-                            r.paired == 0 && r.failed == 0 ->
-                                Toast.makeText(
-                                    context,
-                                    context.getString(R.string.pairing_no_mp3_in_folder),
-                                    Toast.LENGTH_SHORT,
-                                ).show()
-
-                            r.failed > 0 ->
-                                Toast.makeText(
-                                    context,
-                                    context.getString(R.string.pairing_folder_mixed_result, r.paired, r.failed),
-                                    Toast.LENGTH_LONG,
-                                ).show()
-
-                            else ->
-                                Toast.makeText(
-                                    context,
-                                    context.getString(R.string.pairing_folder_ok, r.paired),
-                                    Toast.LENGTH_SHORT,
-                                ).show()
-                        }
                     }
                 }
             },
         )
 
+    val runDeviceScan = {
+        viewModel.pairPlaylistFromDeviceAudio(playlistId) { r ->
+            showPairingResultToast(context, r)
+        }
+    }
+
+    val audioPermissionLauncher =
+        rememberLauncherForActivityResult(
+            contract = ActivityResultContracts.RequestPermission(),
+            onResult = { granted ->
+                if (granted) {
+                    runDeviceScan()
+                } else {
+                    Toast
+                        .makeText(
+                            context,
+                            context.getString(R.string.pairing_audio_permission_denied),
+                            Toast.LENGTH_LONG,
+                        ).show()
+                }
+            },
+        )
+
+    fun scanDeviceAudio() {
+        val permission = AudioReadPermission.permissionName()
+        when {
+            ContextCompat.checkSelfPermission(context, permission) == PackageManager.PERMISSION_GRANTED ->
+                runDeviceScan()
+            else -> audioPermissionLauncher.launch(permission)
+        }
+    }
+
     Scaffold(
         modifier = modifier.fillMaxSize(),
         topBar = {
@@ -149,7 +145,22 @@ fun PairingScreen(
         ) {
             item {
                 Button(
-                    onClick = { openTree.launch(null) },
+                    onClick = { scanDeviceAudio() },
+                    modifier = Modifier.fillMaxWidth(),
+                ) {
+                    Text(context.getString(R.string.pairing_scan_device))
+                }
+            }
+            item {
+                Text(
+                    text = context.getString(R.string.pairing_scan_device_help),
+                    style = MaterialTheme.typography.bodySmall,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                )
+            }
+            item {
+                OutlinedButton(
+                    onClick = { openTree.launch(SafInitialUris.internalDocuments(context)) },
                     modifier = Modifier.fillMaxWidth(),
                 ) {
                     Text(context.getString(R.string.pairing_choose_folder))
@@ -215,3 +226,52 @@ fun PairingScreen(
         }
     }
 }
+
+private fun showPairingResultToast(
+    context: android.content.Context,
+    r: FolderPairingResult,
+) {
+    when {
+        r.mp3Count == 0 ->
+            Toast.makeText(
+                context,
+                context.getString(R.string.pairing_no_mp3_in_folder),
+                Toast.LENGTH_SHORT,
+            ).show()
+
+        r.jukeboxRowCount == 0 ->
+            Toast.makeText(
+                context,
+                context.getString(R.string.pairing_jukebox_empty),
+                Toast.LENGTH_LONG,
+            ).show()
+
+        r.skippedNoSpotifyTrackId == r.jukeboxRowCount && r.jukeboxRowCount > 0 ->
+            Toast.makeText(
+                context,
+                context.getString(R.string.pairing_all_missing_spotify_id),
+                Toast.LENGTH_LONG,
+            ).show()
+
+        r.paired == 0 && r.failed == 0 ->
+            Toast.makeText(
+                context,
+                context.getString(R.string.pairing_no_mp3_in_folder),
+                Toast.LENGTH_SHORT,
+            ).show()
+
+        r.failed > 0 ->
+            Toast.makeText(
+                context,
+                context.getString(R.string.pairing_folder_mixed_result, r.paired, r.failed),
+                Toast.LENGTH_LONG,
+            ).show()
+
+        else ->
+            Toast.makeText(
+                context,
+                context.getString(R.string.pairing_folder_ok, r.paired),
+                Toast.LENGTH_SHORT,
+            ).show()
+    }
+}

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 +36,8 @@ fun SettingsScreen(
     modifier: Modifier = Modifier,
 ) {
     val context = LocalContext.current
+    val annotationMode by viewModel.annotationMode.collectAsStateWithLifecycle()
+    val collectRunData by viewModel.collectRunData.collectAsStateWithLifecycle()
     Scaffold(
         modifier = modifier.fillMaxSize(),
         topBar = {
@@ -51,6 +59,54 @@ 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) },
+                )
+            }
+            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_collect_run_data),
+                        style = MaterialTheme.typography.titleMedium,
+                    )
+                    Text(
+                        text = context.getString(R.string.settings_collect_run_data_help),
+                        style = MaterialTheme.typography.bodySmall,
+                        color = MaterialTheme.colorScheme.onSurfaceVariant,
+                    )
+                }
+                Switch(
+                    checked = collectRunData,
+                    onCheckedChange = { viewModel.setCollectRunData(it) },
+                )
+            }
             Text(
                 text = context.getString(R.string.settings_stub_body),
                 style = MaterialTheme.typography.bodyLarge,

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

@@ -0,0 +1,13 @@
+package at.lockstep.player.util
+
+import android.Manifest
+import android.os.Build
+
+object AudioReadPermission {
+    fun permissionName(): String =
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+            Manifest.permission.READ_MEDIA_AUDIO
+        } else {
+            Manifest.permission.READ_EXTERNAL_STORAGE
+        }
+}

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/java/at/lockstep/player/util/MediaStoreMp3Scanner.kt

@@ -0,0 +1,60 @@
+package at.lockstep.player.util
+
+import android.content.ContentUris
+import android.content.Context
+import android.os.Build
+import android.provider.MediaStore
+import android.util.Log
+
+object MediaStoreMp3Scanner {
+    private const val TAG = "LockstepPairing"
+
+    fun listMp3Candidates(context: Context): List<Mp3FolderCandidate> {
+        val resolver = context.applicationContext.contentResolver
+        val collection =
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+                MediaStore.Audio.Media.getContentUri(MediaStore.VOLUME_EXTERNAL)
+            } else {
+                @Suppress("DEPRECATION")
+                MediaStore.Audio.Media.EXTERNAL_CONTENT_URI
+            }
+
+        val projection =
+            arrayOf(
+                MediaStore.Audio.Media._ID,
+                MediaStore.Audio.Media.DISPLAY_NAME,
+                MediaStore.Audio.Media.TITLE,
+                MediaStore.Audio.Media.ARTIST,
+            )
+
+        val selection =
+            "(${MediaStore.Audio.Media.MIME_TYPE} = ? OR " +
+                "${MediaStore.Audio.Media.DISPLAY_NAME} LIKE ?)"
+        val selectionArgs = arrayOf("audio/mpeg", "%.mp3")
+
+        val out = mutableListOf<Mp3FolderCandidate>()
+        resolver.query(collection, projection, selection, selectionArgs, null)?.use { cursor ->
+            val idCol = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media._ID)
+            val nameCol = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.DISPLAY_NAME)
+            val titleCol = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.TITLE)
+            val artistCol = cursor.getColumnIndexOrThrow(MediaStore.Audio.Media.ARTIST)
+            while (cursor.moveToNext()) {
+                val id = cursor.getLong(idCol)
+                val displayName = cursor.getString(nameCol).orEmpty()
+                val title = cursor.getString(titleCol)?.trim()?.takeIf { it.isNotEmpty() }
+                val artist = cursor.getString(artistCol)?.trim()?.takeIf { it.isNotEmpty() }
+                out.add(
+                    Mp3FolderCandidate(
+                        uri = ContentUris.withAppendedId(collection, id),
+                        fileBaseName = displayName.substringBeforeLast('.'),
+                        id3Title = title,
+                        id3Artist = artist,
+                    ),
+                )
+            }
+        } ?: Log.w(TAG, "MediaStore audio query returned null")
+
+        Log.d(TAG, "MediaStoreMp3Scanner found mp3Count=${out.size}")
+        return out.distinctBy { it.uri }.sortedBy { it.fileBaseName.lowercase() }
+    }
+}

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

@@ -0,0 +1,228 @@
+package at.lockstep.player.util
+
+import android.Manifest
+import android.content.Context
+import android.content.pm.PackageManager
+import android.hardware.Sensor
+import android.hardware.SensorEvent
+import android.hardware.SensorEventListener
+import android.hardware.SensorManager
+import android.location.Location
+import android.location.LocationListener
+import android.location.LocationManager
+import android.os.Build
+import android.os.Handler
+import android.os.HandlerThread
+import androidx.core.content.ContextCompat
+
+class RunDataCollector(
+    context: Context,
+) {
+    private val appContext = context.applicationContext
+    private val sensorManager = appContext.getSystemService(SensorManager::class.java)
+    private val locationManager = appContext.getSystemService(LocationManager::class.java)
+    private val accelerometer = sensorManager?.getDefaultSensor(Sensor.TYPE_ACCELEROMETER)
+    private val gyroscope = sensorManager?.getDefaultSensor(Sensor.TYPE_GYROSCOPE)
+    private val handlerThread = HandlerThread("RunDataCollect").apply { start() }
+    private val handler = Handler(handlerThread.looper)
+
+    private val accelBuffer = mutableListOf<RunAccelSample>()
+    private val gyroBuffer = mutableListOf<RunDataSample>()
+    private val gpsBuffer = mutableListOf<RunGpsSample>()
+
+    /** Baseline sensor/GPS time for the current song; set on the first sample after [markSongStart]. */
+    private var songStartElapsedRealtimeNanos: Long? = null
+
+    @Volatile
+    private var collectingEnabled = false
+
+    @Volatile
+    private var playbackPositionMsProvider: () -> Long = { 0L }
+
+    private var sensorsRegistered = false
+    private var locationRegistered = false
+
+    private val sensorListener =
+        object : SensorEventListener {
+            override fun onSensorChanged(event: SensorEvent) {
+                if (!collectingEnabled) return
+                val timestamp = relativeTimestampNanos(event.timestamp) ?: return
+                when (event.sensor.type) {
+                    Sensor.TYPE_ACCELEROMETER -> {
+                        val sample =
+                            RunAccelSample(
+                                timestampNanos = timestamp,
+                                positionMs = playbackPositionMsProvider(),
+                                values = floatArrayOf(event.values[0], event.values[1], event.values[2]),
+                            )
+                        synchronized(accelBuffer) {
+                            accelBuffer.add(sample)
+                        }
+                    }
+                    Sensor.TYPE_GYROSCOPE -> {
+                        val sample =
+                            RunDataSample(
+                                timestampNanos = timestamp,
+                                values = floatArrayOf(event.values[0], event.values[1], event.values[2]),
+                            )
+                        synchronized(gyroBuffer) {
+                            gyroBuffer.add(sample)
+                        }
+                    }
+                }
+            }
+
+            override fun onAccuracyChanged(
+                sensor: Sensor?,
+                accuracy: Int,
+            ) = Unit
+        }
+
+    private val locationListener =
+        LocationListener { location ->
+            if (!collectingEnabled) return@LocationListener
+            recordGpsLocation(location)
+        }
+
+    fun start(enableLocation: Boolean) {
+        startSensors()
+        if (enableLocation) {
+            startLocationUpdates()
+        }
+    }
+
+    private fun startSensors() {
+        if (sensorsRegistered || sensorManager == null) return
+        accelerometer?.let {
+            sensorManager.registerListener(
+                sensorListener,
+                it,
+                SensorManager.SENSOR_DELAY_GAME,
+                0,
+                handler,
+            )
+        }
+        gyroscope?.let {
+            sensorManager.registerListener(
+                sensorListener,
+                it,
+                SensorManager.SENSOR_DELAY_GAME,
+                0,
+                handler,
+            )
+        }
+        sensorsRegistered = accelerometer != null || gyroscope != null
+    }
+
+    private fun startLocationUpdates() {
+        if (locationRegistered || locationManager == null) return
+        if (!hasLocationPermission()) return
+        val providers =
+            listOf(LocationManager.GPS_PROVIDER, LocationManager.NETWORK_PROVIDER)
+                .filter { locationManager.isProviderEnabled(it) }
+        if (providers.isEmpty()) return
+        for (provider in providers) {
+            locationManager.requestLocationUpdates(
+                provider,
+                GPS_MIN_TIME_MS,
+                0f,
+                locationListener,
+                handler.looper,
+            )
+        }
+        locationRegistered = true
+    }
+
+    fun stop() {
+        stopLocationUpdates()
+        stopSensors()
+    }
+
+    private fun stopSensors() {
+        if (!sensorsRegistered || sensorManager == null) return
+        sensorManager.unregisterListener(sensorListener)
+        sensorsRegistered = false
+    }
+
+    private fun stopLocationUpdates() {
+        if (!locationRegistered || locationManager == null) return
+        locationManager.removeUpdates(locationListener)
+        locationRegistered = false
+    }
+
+    fun release() {
+        stop()
+        handlerThread.quitSafely()
+    }
+
+    fun markSongStart() {
+        songStartElapsedRealtimeNanos = null
+    }
+
+    fun setCollectingEnabled(enabled: Boolean) {
+        collectingEnabled = enabled
+    }
+
+    fun setPlaybackPositionMsProvider(provider: () -> Long) {
+        playbackPositionMsProvider = provider
+    }
+
+    fun snapshotAndClear(): RunTrackDataSnapshot =
+        RunTrackDataSnapshot(
+            accelerometer =
+                synchronized(accelBuffer) {
+                    accelBuffer.toList().also { accelBuffer.clear() }
+                },
+            gyroscope =
+                synchronized(gyroBuffer) {
+                    gyroBuffer.toList().also { gyroBuffer.clear() }
+                },
+            gps =
+                synchronized(gpsBuffer) {
+                    gpsBuffer.toList().also { gpsBuffer.clear() }
+                },
+        )
+
+    private fun relativeTimestampNanos(elapsedRealtimeNanos: Long): Long? {
+        val start =
+            songStartElapsedRealtimeNanos ?: run {
+                songStartElapsedRealtimeNanos = elapsedRealtimeNanos
+                elapsedRealtimeNanos
+            }
+        return elapsedRealtimeNanos - start
+    }
+
+    private fun recordGpsLocation(location: Location) {
+        val elapsedNs =
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
+                location.elapsedRealtimeNanos
+            } else {
+                @Suppress("DEPRECATION")
+                location.time * 1_000_000L
+            }
+        val timestamp = relativeTimestampNanos(elapsedNs) ?: return
+        synchronized(gpsBuffer) {
+            val last = gpsBuffer.lastOrNull()
+            if (last != null && timestamp - last.timestampNanos < GPS_MIN_TIME_NS) {
+                return
+            }
+            gpsBuffer.add(
+                RunGpsSample(
+                    timestampNanos = timestamp,
+                    latitude = location.latitude,
+                    longitude = location.longitude,
+                    altitude = location.altitude,
+                ),
+            )
+        }
+    }
+
+    private fun hasLocationPermission(): Boolean =
+        ContextCompat.checkSelfPermission(appContext, Manifest.permission.ACCESS_FINE_LOCATION) ==
+            PackageManager.PERMISSION_GRANTED
+
+    companion object {
+        private const val GPS_MIN_TIME_MS = 1_000L
+        private const val GPS_MIN_TIME_NS = 1_000_000_000L
+    }
+}

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

@@ -0,0 +1,19 @@
+package at.lockstep.player.util
+
+data class RunDataSample(
+    /** Nanoseconds since the current song started ([android.hardware.SensorEvent.timestamp] base). */
+    val timestampNanos: Long,
+    val values: FloatArray,
+) {
+    override fun equals(other: Any?): Boolean {
+        if (this === other) return true
+        if (other !is RunDataSample) return false
+        return timestampNanos == other.timestampNanos && values.contentEquals(other.values)
+    }
+
+    override fun hashCode(): Int {
+        var result = timestampNanos.hashCode()
+        result = 31 * result + values.contentHashCode()
+        return result
+    }
+}

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

@@ -0,0 +1,180 @@
+package at.lockstep.player.util
+
+import android.content.ContentValues
+import android.content.Context
+import android.net.Uri
+import android.os.Build
+import android.os.Environment
+import android.provider.MediaStore
+import at.lockstep.player.BuildConfig
+import org.json.JSONArray
+import org.json.JSONObject
+import java.io.File
+import java.text.SimpleDateFormat
+import java.util.Date
+import java.util.Locale
+
+object RunDataStorage {
+    private const val APP_DIR = "Lockstep"
+
+    /** e.g. `2026-05-24_18-36-42` — one folder per Now Playing run session. */
+    fun newRunSessionFolderName(): String =
+        SimpleDateFormat("yyyy-MM-dd_HH-mm-ss", Locale.US).format(Date())
+
+    /** Public path segment under Documents, for display: `Documents/Lockstep/{runSessionFolder}/`. */
+    fun documentsRelativePath(runSessionFolder: String): String =
+        "${Environment.DIRECTORY_DOCUMENTS}/$APP_DIR/$runSessionFolder"
+
+    fun writeRunDataFile(
+        context: Context,
+        runSessionFolder: String,
+        playlistDisplayName: String,
+        trackQueueIndex0Based: Int,
+        metaContentUri: String,
+        title: String,
+        artist: String,
+        snapshot: RunTrackDataSnapshot,
+    ): Uri? {
+        if (snapshot.isEmpty()) return null
+
+        val safeName =
+            playlistDisplayName
+                .replace(Regex("[\\\\/:*?\"<>|]"), "_")
+                .trim()
+                .ifBlank { "playlist" }
+                .take(120)
+        val suffix = String.format(Locale.US, "%03d", trackQueueIndex0Based + 1)
+        val fileName = "${safeName}_$suffix.json"
+
+        val jsonString =
+            JSONObject()
+                .apply {
+                    put("data", accelToJsonArray(snapshot.accelerometer))
+                    put("gyro", samplesToJsonArray(snapshot.gyroscope))
+                    put("gps", gpsToJsonArray(snapshot.gps))
+                    put("meta", metaContentUri)
+                    put("title", title)
+                    put("artist", artist)
+                    put("versionCode", BuildConfig.VERSION_CODE)
+                }.toString()
+
+        return if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+            writeViaMediaStore(context, runSessionFolder, fileName, jsonString)
+        } else {
+            writeViaPublicDocumentsDir(runSessionFolder, fileName, jsonString)
+        }
+    }
+
+    private fun accelToJsonArray(samples: List<RunAccelSample>): JSONArray {
+        val array = JSONArray()
+        for (sample in samples) {
+            array.put(
+                JSONObject().apply {
+                    put("timestamp", sample.timestampNanos)
+                    put("positionMs", sample.positionMs)
+                    put(
+                        "values",
+                        JSONArray().apply {
+                            for (v in sample.values) {
+                                put(v.toDouble())
+                            }
+                        },
+                    )
+                },
+            )
+        }
+        return array
+    }
+
+    private fun samplesToJsonArray(samples: List<RunDataSample>): JSONArray {
+        val array = JSONArray()
+        for (sample in samples) {
+            array.put(
+                JSONObject().apply {
+                    put("timestamp", sample.timestampNanos)
+                    put(
+                        "values",
+                        JSONArray().apply {
+                            for (v in sample.values) {
+                                put(v.toDouble())
+                            }
+                        },
+                    )
+                },
+            )
+        }
+        return array
+    }
+
+    private fun gpsToJsonArray(samples: List<RunGpsSample>): JSONArray {
+        val array = JSONArray()
+        for (sample in samples) {
+            array.put(
+                JSONObject().apply {
+                    put("timestamp", sample.timestampNanos)
+                    put(
+                        "values",
+                        JSONArray().apply {
+                            put(sample.latitude)
+                            put(sample.longitude)
+                            put(sample.altitude)
+                        },
+                    )
+                },
+            )
+        }
+        return array
+    }
+
+    private fun writeViaMediaStore(
+        context: Context,
+        runSessionFolder: String,
+        fileName: String,
+        jsonString: String,
+    ): Uri? {
+        val resolver = context.applicationContext.contentResolver
+        val relativePath = documentsRelativePath(runSessionFolder)
+        val pending =
+            ContentValues().apply {
+                put(MediaStore.MediaColumns.DISPLAY_NAME, fileName)
+                put(MediaStore.MediaColumns.MIME_TYPE, "application/json")
+                put(MediaStore.MediaColumns.RELATIVE_PATH, relativePath)
+                put(MediaStore.MediaColumns.IS_PENDING, 1)
+            }
+        val collection = MediaStore.Files.getContentUri(MediaStore.VOLUME_EXTERNAL_PRIMARY)
+        val uri = resolver.insert(collection, pending) ?: return null
+        try {
+            resolver.openOutputStream(uri)?.use { stream ->
+                stream.write(jsonString.toByteArray(Charsets.UTF_8))
+            } ?: return null
+            val published =
+                ContentValues().apply {
+                    put(MediaStore.MediaColumns.IS_PENDING, 0)
+                }
+            resolver.update(uri, published, null, null)
+            return uri
+        } catch (e: Exception) {
+            resolver.delete(uri, null, null)
+            throw e
+        }
+    }
+
+    @Suppress("DEPRECATION")
+    private fun writeViaPublicDocumentsDir(
+        runSessionFolder: String,
+        fileName: String,
+        jsonString: String,
+    ): Uri? {
+        val dir =
+            File(
+                Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS),
+                "$APP_DIR/$runSessionFolder",
+            )
+        if (!dir.exists() && !dir.mkdirs()) {
+            return null
+        }
+        val file = File(dir, fileName)
+        file.writeText(jsonString)
+        return Uri.fromFile(file)
+    }
+}

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

@@ -0,0 +1,17 @@
+package at.lockstep.player.util
+
+data class RunGpsSample(
+    val timestampNanos: Long,
+    val latitude: Double,
+    val longitude: Double,
+    val altitude: Double,
+)
+
+data class RunTrackDataSnapshot(
+    val accelerometer: List<RunAccelSample>,
+    val gyroscope: List<RunDataSample>,
+    val gps: List<RunGpsSample>,
+) {
+    fun isEmpty(): Boolean =
+        accelerometer.isEmpty() && gyroscope.isEmpty() && gps.isEmpty()
+}

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

@@ -0,0 +1,41 @@
+package at.lockstep.player.util
+
+import android.content.Context
+import android.net.Uri
+import android.os.Build
+import android.os.storage.StorageManager
+import android.provider.DocumentsContract
+
+object SafInitialUris {
+    private const val EXTERNAL_STORAGE_AUTHORITY = "com.android.externalstorage.documents"
+    private const val INITIAL_URI_EXTRA = "android.provider.extra.INITIAL_URI"
+
+    /**
+     * Internal-storage Documents. Uses [StorageManager] on Android 10+ so the system picker
+     * lands in a choosable folder instead of the blocked volume root on Pixel devices.
+     */
+    fun internalDocuments(context: Context): Uri {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+            val storageManager = context.getSystemService(StorageManager::class.java)
+            val intent = storageManager.primaryStorageVolume.createOpenDocumentTreeIntent()
+            val root =
+                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.TIRAMISU) {
+                    intent.getParcelableExtra(INITIAL_URI_EXTRA, Uri::class.java)
+                } else {
+                    @Suppress("DEPRECATION")
+                    intent.getParcelableExtra(INITIAL_URI_EXTRA)
+                }
+            if (root != null) {
+                val scheme =
+                    root
+                        .toString()
+                        .replace("/root/", "/document/") + "%3A" + "Documents"
+                return Uri.parse(scheme)
+            }
+        }
+        return DocumentsContract.buildDocumentUri(
+            EXTERNAL_STORAGE_AUTHORITY,
+            "primary:Documents",
+        )
+    }
+}

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

@@ -28,9 +28,21 @@
     <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="settings_collect_run_data">Collect run data</string>
+    <string name="settings_collect_run_data_help">When enabled, Now playing records accelerometer, gyroscope, and GPS (1 Hz) per song into Documents/Lockstep/ under a timestamped run folder.</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>
+    <string name="pairing_scan_device">Scan audio on this device</string>
+    <string name="pairing_scan_device_help">Finds MP3s in Music and other folders without using the folder picker.</string>
+    <string name="pairing_audio_permission_denied">Audio permission is required to scan MP3s on this device.</string>
     <string name="pairing_status_paired">Paired with local file</string>
     <string name="pairing_status_unpaired">Not paired — tap to pick an MP3</string>
     <string name="pairing_no_mp3_in_folder">No MP3 files found in that folder.</string>