fix: thread-safe player position logging

app/src/main/AndroidManifest.xml

@@ -6,6 +6,11 @@
     <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

@@ -16,6 +16,8 @@ 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
@@ -66,12 +68,25 @@ class LockstepViewModel(
             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>()
 
     /**
@@ -175,6 +190,66 @@ class LockstepViewModel(
         }
     }
 
+    /**
+     * 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()) {

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

@@ -33,6 +33,12 @@ class UserPreferencesRepository(
             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
@@ -55,9 +61,16 @@ class UserPreferencesRepository(
         }
     }
 
+    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/playback/PlaybackService.kt

@@ -50,6 +50,7 @@ class PlaybackService : Service() {
 
     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()
@@ -64,6 +65,10 @@ class PlaybackService : Service() {
     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) {
@@ -215,11 +220,21 @@ class PlaybackService : Service() {
                     }
                 }
             }
+        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
@@ -321,13 +336,24 @@ class PlaybackService : Service() {
         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
-        val durationMs = currentDurationMs().coerceAtLeast(1L)
+        refreshCachedPlaybackPositionMs()
-        val positionMs = p.currentPosition.coerceIn(0L, durationMs)
         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 =
@@ -413,11 +439,9 @@ class PlaybackService : Service() {
 
     /**
      * Milliseconds from the start of the current track — same timebase as [ExoPlayer.getCurrentPosition].
-     * May be called from any thread per Media3 [Player] contract.
+     * Reads a main-thread cache so callers on background threads (e.g. run-data sensor collection) stay safe.
      */
-    fun getPlaybackPositionMs(): Long {
+    fun getPlaybackPositionMs(): Long = cachedPlaybackPositionMs
-        return player?.currentPosition ?: 0L
-    }
 
     private fun refreshForegroundNotification() {
         if (queue.isEmpty()) return
@@ -494,6 +518,7 @@ class PlaybackService : Service() {
 
     override fun onDestroy() {
         positionPollJob?.cancel()
+        positionCachePollJob?.cancel()
         releasePlayer()
         mediaSession.run {
             isActive = false
@@ -538,6 +563,7 @@ class PlaybackService : Service() {
 
     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"

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

@@ -115,6 +115,7 @@ fun LockstepAppNavHost(
             NowPlayingRoute(
                 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/settings/SettingsScreen.kt

@@ -37,6 +37,7 @@ fun SettingsScreen(
 ) {
     val context = LocalContext.current
     val annotationMode by viewModel.annotationMode.collectAsStateWithLifecycle()
+    val collectRunData by viewModel.collectRunData.collectAsStateWithLifecycle()
     Scaffold(
         modifier = modifier.fillMaxSize(),
         topBar = {
@@ -82,6 +83,30 @@ fun SettingsScreen(
                     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/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/res/values/strings.xml

@@ -30,6 +30,8 @@
     <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>