feat: add canonical beat pattern TYPE_BEATS, not synced to server

DESIGN.md

@@ -11,6 +11,64 @@ Product goals, DSP scope, and repo layout are summarized in [SPECS.md](SPECS.md)
   - Collection mode: app records sensor data during a run
 - Settings: allows to enable modes above, logout button
 
+## Data files
+
+Filesystem layout of JSON files on primary storage (e.g. Pixel 5: `/storage/emulated/0/Documents/Lockstep/`):
+
+```
+/storage/emulated/0/Documents/Lockstep/
+├── 2026-05-31_10-15-30/          ← annotation session (TYPE_ANNOTATION)
+│   ├── Running Mix_001.json
+│   └── Running Mix_002.json
+├── 2026-05-31_14-22-05/          ← now-playing collection session (TYPE_COLLECTION)
+│   ├── Running Mix_001.json
+│   └── Running Mix_002.json
+└── Beats/                        ← canonical beats per playlist (TYPE_BEATS)
+    └── Running Mix/
+        ├── 001.json
+        └── 002.json
+```
+
+Each type is recorded in Room `file_metadata` (`FileMetadataEntity`) with `fileUri`, `trackId`, `type`, `version`, `synced`, and `lastSyncedAt` (epoch millis).
+
+| Type | Constant | Written by | Path pattern | Server sync |
+|------|----------|------------|--------------|-------------|
+| **Annotation session** | `TYPE_ANNOTATION` | Annotation screen (`BeatAnnotationStorage`) | `Documents/Lockstep/{sessionFolder}/{playlist}_{NNN}.json` — one timestamped folder per annotation session | Yes — uploaded via `POST /metadata` when signed in |
+| **Run collection** | `TYPE_COLLECTION` | Now Playing with collect-run-data enabled (`RunDataStorage`) | `Documents/Lockstep/{sessionFolder}/{playlist}_{NNN}.json` — one timestamped folder per run session | Yes |
+| **Canonical beats** | `TYPE_BEATS` | Annotation screen (`BeatAnnotationStorage`) — written alongside each session annotation | `Documents/Lockstep/Beats/{playlist_name}/{NNN}.json` — one folder per playlist; **overwrites** on re-annotation | **No** — tracked locally only; `synced = 1` and `lastSyncedAt` set at write time |
+
+During an **annotation session**, leaving a track writes **both** `TYPE_ANNOTATION` (append-only session archive) and `TYPE_BEATS` (latest beats for that playlist slot).
+
+### Beat JSON schema (`TYPE_ANNOTATION` and `TYPE_BEATS`)
+
+Same schema for session annotations and canonical beats:
+
+```json
+{
+  "contentId": "primary:Documents/Music/track.mp3",
+  "title": "Track Title",
+  "artist": "Artist Name",
+  "beatTimesSec": [1.23, 2.45, 3.67]
+}
+```
+
+- **`contentId`** — SAF document id of the paired local MP3 when available; otherwise Spotify track id.
+- **`beatTimesSec`** — user-tapped beat times in seconds (playback position).
+
+### Collection JSON schema (`TYPE_COLLECTION`)
+
+```json
+{
+  "data": [ { "timestamp": 0, "positionMs": 0, "values": [0.0, 0.0, 9.8] } ],
+  "gyro": [ { "timestamp": 0, "values": [0.0, 0.0, 0.0] } ],
+  "gps": [ { "timestamp": 0, "values": [48.2, 16.3, 200.0] } ],
+  "meta": "content://…",
+  "title": "Track Title",
+  "artist": "Artist Name",
+  "versionCode": 1
+}
+```
+
 ## Already captured from SPECS.md
 
 - **Product**: Pace-aware playlist ordering + real-time playback adaptation via accelerometer, **libpasada** (C++/JNI) + **Oboe**, user-supplied **MP3** via file descriptors, feedback loop for sensor-driven playback.

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

@@ -167,8 +167,9 @@ class LockstepViewModel(
         }
 
     /**
-     * Writes one JSON file under public Documents/Lockstep/{sessionFolder}/ for the track described
-     * by [event], records file metadata, and uploads when signed in. Skips when [beatTimesMs] is empty.
+     * Writes session annotation JSON under Documents/Lockstep/{sessionFolder}/ (synced when signed in)
+     * and canonical beat JSON under Documents/Lockstep/Beats/{playlist_name}/ (local file_metadata only).
+     * Skips when [beatTimesMs] is empty.
      */
     fun persistBeatAnnotation(
         playlistId: String,
@@ -184,9 +185,10 @@ class LockstepViewModel(
             val pairing = pairingDao.findForTrack(playlistId, event.trackId)
             val docId = BeatAnnotationStorage.mp3DocumentContentId(pairing?.localUri)
             val contentId = docId.ifBlank { event.trackId }
-            val uri =
+            val appContext = getApplication<Application>()
+            val annotationUri =
                 BeatAnnotationStorage.writeAnnotationsFile(
-                    context = getApplication(),
+                    context = appContext,
                     sessionFolder = sessionFolder,
                     playlistDisplayName = playlistDisplayName,
                     trackQueueIndex0Based = event.queueIndex,
@@ -195,18 +197,66 @@ class LockstepViewModel(
                     artist = event.artist,
                     beatTimesMs = beatTimesMs,
                 ) ?: return@launch
-            val entry =
-                FileMetadataEntity(
-                    fileUri = uri.toString(),
+            recordMetadataEntry(
+                fileUri = annotationUri.toString(),
                 trackId = event.trackId,
                 type = FileMetadataEntity.TYPE_ANNOTATION,
+            )
+            val beatsUri =
+                BeatAnnotationStorage.writeBeatsFile(
+                    context = appContext,
+                    playlistDisplayName = playlistDisplayName,
+                    trackQueueIndex0Based = event.queueIndex,
+                    contentId = contentId,
+                    title = event.title,
+                    artist = event.artist,
+                    beatTimesMs = beatTimesMs,
+                ) ?: return@launch
+            recordOrUpdateMetadataEntry(
+                fileUri = beatsUri.toString(),
+                trackId = event.trackId,
+                type = FileMetadataEntity.TYPE_BEATS,
+            )
+        }
+    }
+
+    private suspend fun recordMetadataEntry(
+        fileUri: String,
+        trackId: String,
+        type: String,
+    ) {
+        val now = System.currentTimeMillis()
+        val beats = type == FileMetadataEntity.TYPE_BEATS
+        val entry =
+            FileMetadataEntity(
+                fileUri = fileUri,
+                trackId = trackId,
+                type = type,
                 version = BuildConfig.VERSION_CODE,
+                synced = beats,
+                lastSyncedAt = if (beats) now else null,
             )
         val id = fileMetadataDao.insert(entry)
+        if (!beats) {
             syncMetadataEntry(entry.copy(id = id))
         }
     }
 
+    private suspend fun recordOrUpdateMetadataEntry(
+        fileUri: String,
+        trackId: String,
+        type: String,
+    ) {
+        val version = BuildConfig.VERSION_CODE
+        val now = System.currentTimeMillis()
+        val existing = fileMetadataDao.findByTrackIdAndType(trackId, type)
+        if (existing != null) {
+            fileMetadataDao.updateFile(existing.id, fileUri, version, now)
+        } else {
+            recordMetadataEntry(fileUri, trackId, type)
+        }
+    }
+
     /**
      * 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.
@@ -333,7 +383,7 @@ class LockstepViewModel(
                     version = entry.version,
                     collection = collection,
                 )
-                fileMetadataDao.markSynced(entry.id)
+                fileMetadataDao.markSynced(entry.id, System.currentTimeMillis())
                 true
             } catch (e: IOException) {
                 Log.w(METADATA_TAG, "metadata sync failed id=${entry.id}", e)

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

@@ -33,7 +33,7 @@ class UserPreferencesRepository(
             prefs[KEY_ANNOTATION_MODE] == true
         }
 
-    /** When true, Now Playing records accelerometer samples per track into JSON under filesDir/run_data. */
+    /** When true, Now Playing records sensor samples per track into JSON under Documents/Lockstep/{sessionFolder}/. */
     val collectRunData: Flow<Boolean> =
         dataStore.data.map { prefs ->
             prefs[KEY_COLLECT_RUN_DATA] == true

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

@@ -10,7 +10,7 @@ import androidx.room.RoomDatabase
         TrackPairingEntity::class,
         FileMetadataEntity::class,
     ],
-    version = 4,
+    version = 5,
     exportSchema = false,
 )
 abstract class AppDatabase :

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

@@ -9,9 +9,17 @@ interface FileMetadataDao {
     @Insert
     suspend fun insert(row: FileMetadataEntity): Long
 
-    @Query("SELECT * FROM file_metadata WHERE synced = 0 ORDER BY id ASC")
+    @Query("SELECT * FROM file_metadata WHERE synced = 0 AND type != 'beats' ORDER BY id ASC")
     suspend fun listUnsynced(): List<FileMetadataEntity>
 
-    @Query("UPDATE file_metadata SET synced = 1 WHERE id = :id")
-    suspend fun markSynced(id: Long)
+    @Query("UPDATE file_metadata SET synced = 1, lastSyncedAt = :syncedAt WHERE id = :id")
+    suspend fun markSynced(id: Long, syncedAt: Long)
+
+    @Query("SELECT * FROM file_metadata WHERE trackId = :trackId AND type = :type LIMIT 1")
+    suspend fun findByTrackIdAndType(trackId: String, type: String): FileMetadataEntity?
+
+    @Query(
+        "UPDATE file_metadata SET fileUri = :fileUri, version = :version, synced = 1, lastSyncedAt = :syncedAt WHERE id = :id",
+    )
+    suspend fun updateFile(id: Long, fileUri: String, version: Int, syncedAt: Long)
 }

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

@@ -11,14 +11,18 @@ data class FileMetadataEntity(
     val fileUri: String,
     /** Spotify track id for the collected run. */
     val trackId: String,
-    /** e.g. [TYPE_COLLECTION] or [TYPE_ANNOTATION]. */
+    /** e.g. [TYPE_COLLECTION], [TYPE_ANNOTATION], or [TYPE_BEATS]. */
     val type: String,
     /** [BuildConfig.VERSION_CODE] at write time. */
     val version: Int,
     val synced: Boolean = false,
+    /** Epoch millis when [synced] was set; set on local write for [TYPE_BEATS], on server sync otherwise. */
+    val lastSyncedAt: Long? = null,
 ) {
     companion object {
         const val TYPE_COLLECTION = "collection"
         const val TYPE_ANNOTATION = "annotation"
+        /** Canonical beat files under Documents/Lockstep/Beats/; tracked locally, not synced to server. */
+        const val TYPE_BEATS = "beats"
     }
 }

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

@@ -23,24 +23,53 @@ object BeatAnnotationStorage {
         artist: String,
         beatTimesMs: List<Long>,
     ): Uri? {
-        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 fileName = trackFileName(playlistDisplayName, trackQueueIndex0Based)
+        val jsonString = buildAnnotationJson(contentId, title, artist, beatTimesMs).toString(2)
+        return RunDataStorage.writePublicJsonFile(context, sessionFolder, fileName, jsonString)
+    }
+
+    /**
+     * 1-based track index (000) + ".json" under Documents/Lockstep/Beats/{playlist_name}/.
+     * Overwrites an existing file for the same track index.
+     */
+    fun writeBeatsFile(
+        context: Context,
+        playlistDisplayName: String,
+        trackQueueIndex0Based: Int,
+        contentId: String,
+        title: String,
+        artist: String,
+        beatTimesMs: List<Long>,
+    ): Uri? {
+        val suffix = String.format(Locale.US, "%03d", trackQueueIndex0Based + 1)
+        val fileName = "$suffix.json"
+        val jsonString = buildAnnotationJson(contentId, title, artist, beatTimesMs).toString(2)
+        val relativePath = RunDataStorage.beatsPlaylistRelativePath(playlistDisplayName)
+        return RunDataStorage.writeOrReplacePublicJsonFile(context, relativePath, fileName, jsonString)
+    }
+
+    fun buildAnnotationJson(
+        contentId: String,
+        title: String,
+        artist: String,
+        beatTimesMs: List<Long>,
+    ): JSONObject {
         val sec = beatTimesMs.map { it / 1000.0 }
-        val jsonString =
-            JSONObject().apply {
+        return JSONObject().apply {
             put("contentId", contentId)
             put("title", title)
             put("artist", artist)
             put("beatTimesSec", JSONArray(sec))
-            }.toString(2)
-        return RunDataStorage.writePublicJsonFile(context, sessionFolder, fileName, jsonString)
+        }
+    }
+
+    private fun trackFileName(
+        playlistDisplayName: String,
+        trackQueueIndex0Based: Int,
+    ): String {
+        val safeName = RunDataStorage.sanitizeFileLabel(playlistDisplayName)
+        val suffix = String.format(Locale.US, "%03d", trackQueueIndex0Based + 1)
+        return "${safeName}_$suffix.json"
     }
 
     /** Document id for a content [Uri] when available; otherwise last path segment. */

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

@@ -1,5 +1,6 @@
 package at.lockstep.player.util
 
+import android.content.ContentUris
 import android.content.ContentValues
 import android.content.Context
 import android.net.Uri
@@ -16,15 +17,27 @@ import java.util.Locale
 
 object RunDataStorage {
     private const val APP_DIR = "Lockstep"
+    private const val BEATS_DIR = "Beats"
 
     /** 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())
 
+    fun sanitizeFileLabel(name: String): String =
+        name
+            .replace(Regex("[\\\\/:*?\"<>|]"), "_")
+            .trim()
+            .ifBlank { "playlist" }
+            .take(120)
+
     /** Public path segment under Documents, for display: `Documents/Lockstep/{runSessionFolder}/`. */
     fun documentsRelativePath(runSessionFolder: String): String =
         "${Environment.DIRECTORY_DOCUMENTS}/$APP_DIR/$runSessionFolder"
 
+    /** Public path segment: `Documents/Lockstep/Beats/{playlist_name}/`. */
+    fun beatsPlaylistRelativePath(playlistDisplayName: String): String =
+        "${Environment.DIRECTORY_DOCUMENTS}/$APP_DIR/$BEATS_DIR/${sanitizeFileLabel(playlistDisplayName)}"
+
     fun writeRunDataFile(
         context: Context,
         runSessionFolder: String,
@@ -37,12 +50,7 @@ object RunDataStorage {
     ): Uri? {
         if (snapshot.isEmpty()) return null
 
-        val safeName =
-            playlistDisplayName
-                .replace(Regex("[\\\\/:*?\"<>|]"), "_")
-                .trim()
-                .ifBlank { "playlist" }
-                .take(120)
+        val safeName = sanitizeFileLabel(playlistDisplayName)
         val suffix = String.format(Locale.US, "%03d", trackQueueIndex0Based + 1)
         val fileName = "${safeName}_$suffix.json"
 
@@ -67,11 +75,25 @@ object RunDataStorage {
         sessionFolder: String,
         fileName: String,
         jsonString: String,
+    ): Uri? =
+        writeOrReplacePublicJsonFile(
+            context,
+            documentsRelativePath(sessionFolder),
+            fileName,
+            jsonString,
+        )
+
+    /** Writes or overwrites [jsonString] at public Documents/{relativePath}/{fileName}. */
+    fun writeOrReplacePublicJsonFile(
+        context: Context,
+        relativePath: String,
+        fileName: String,
+        jsonString: String,
     ): Uri? =
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
-            writeViaMediaStore(context, sessionFolder, fileName, jsonString)
+            writeOrReplaceViaMediaStore(context, relativePath, fileName, jsonString)
         } else {
-            writeViaPublicDocumentsDir(sessionFolder, fileName, jsonString)
+            writeOrReplaceViaPublicDocumentsDir(relativePath, fileName, jsonString)
         }
 
     private fun accelToJsonArray(samples: List<RunAccelSample>): JSONArray {
@@ -135,22 +157,57 @@ object RunDataStorage {
         return array
     }
 
-    private fun writeViaMediaStore(
+    private fun writeOrReplaceViaMediaStore(
         context: Context,
-        runSessionFolder: String,
+        relativePath: String,
         fileName: String,
         jsonString: String,
     ): Uri? {
         val resolver = context.applicationContext.contentResolver
-        val relativePath = documentsRelativePath(runSessionFolder)
+        val collection = MediaStore.Files.getContentUri(MediaStore.VOLUME_EXTERNAL_PRIMARY)
+        val existingUri = findMediaStoreFileUri(resolver, collection, relativePath, fileName)
+        if (existingUri != null) {
+            resolver.openOutputStream(existingUri, "wt")?.use { stream ->
+                stream.write(jsonString.toByteArray(Charsets.UTF_8))
+            } ?: return null
+            return existingUri
+        }
+        return insertViaMediaStore(resolver, collection, relativePath, fileName, jsonString)
+    }
+
+    private fun findMediaStoreFileUri(
+        resolver: android.content.ContentResolver,
+        collection: Uri,
+        relativePath: String,
+        fileName: String,
+    ): Uri? {
+        val projection = arrayOf(MediaStore.MediaColumns._ID)
+        val selection =
+            "${MediaStore.MediaColumns.RELATIVE_PATH} = ? AND ${MediaStore.MediaColumns.DISPLAY_NAME} = ?"
+        val selectionArgs = arrayOf(relativePathWithTrailingSlash(relativePath), fileName)
+        resolver.query(collection, projection, selection, selectionArgs, null)?.use { cursor ->
+            if (cursor.moveToFirst()) {
+                val id = cursor.getLong(cursor.getColumnIndexOrThrow(MediaStore.MediaColumns._ID))
+                return ContentUris.withAppendedId(collection, id)
+            }
+        }
+        return null
+    }
+
+    private fun insertViaMediaStore(
+        resolver: android.content.ContentResolver,
+        collection: Uri,
+        relativePath: String,
+        fileName: String,
+        jsonString: String,
+    ): Uri? {
         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.RELATIVE_PATH, relativePathWithTrailingSlash(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 ->
@@ -168,17 +225,23 @@ object RunDataStorage {
         }
     }
 
+    /** MediaStore stores directory relative paths with a trailing separator. */
+    private fun relativePathWithTrailingSlash(relativePath: String): String =
+        if (relativePath.endsWith("/")) relativePath else "$relativePath/"
+
     @Suppress("DEPRECATION")
-    private fun writeViaPublicDocumentsDir(
-        runSessionFolder: String,
+    private fun writeOrReplaceViaPublicDocumentsDir(
+        relativePath: String,
         fileName: String,
         jsonString: String,
     ): Uri? {
-        val dir =
-            File(
-                Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS),
-                "$APP_DIR/$runSessionFolder",
-            )
+        val documentsRoot = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOCUMENTS)
+        val lockstepPrefix = "$APP_DIR/"
+        val subPath =
+            relativePath.removePrefix("${Environment.DIRECTORY_DOCUMENTS}/")
+                .removePrefix("${Environment.DIRECTORY_DOCUMENTS}${File.separator}")
+                .removePrefix(lockstepPrefix)
+        val dir = File(documentsRoot, "$APP_DIR/$subPath")
         if (!dir.exists() && !dir.mkdirs()) {
             return null
         }