feat(android): per-playlist fullyCached via cache-index join

CachedPlaylistDao.observeCachedCounts LEFT-JOINs cached_playlist_tracks ×
audio_cache_index per playlist; PlaylistsRepository.observeAll combines it in
and stamps PlaylistRef.fullyCached (trackCount>0 && cached>=trackCount). Merge
extracted to a pure mergePlaylistsWithCache for Android-free unit tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/cache/db/dao/CachedPlaylistDao.kt

@@ -31,6 +31,20 @@ interface CachedPlaylistDao {
     @Query("SELECT * FROM cached_playlists WHERE id = :id")
     suspend fun getById(id: String): CachedPlaylistEntity?
 
+    /**
+     * Per-playlist count of member tracks resident in the audio cache index.
+     * LEFT JOINs so playlists with zero cached tracks still appear
+     * (cachedCount = 0). Drives the offline "fully cached" greying.
+     */
+    @Query(
+        "SELECT p.id AS playlistId, COUNT(a.trackId) AS cachedCount " +
+            "FROM cached_playlists p " +
+            "LEFT JOIN cached_playlist_tracks t ON t.playlistId = p.id " +
+            "LEFT JOIN audio_cache_index a ON a.trackId = t.trackId " +
+            "GROUP BY p.id",
+    )
+    fun observeCachedCounts(): Flow<List<PlaylistCachedCount>>
+
     @Insert(onConflict = OnConflictStrategy.REPLACE)
     suspend fun upsertAll(rows: List<CachedPlaylistEntity>)
 

android/app/src/main/java/com/fabledsword/minstrel/cache/db/dao/PlaylistCachedCount.kt

@@ -0,0 +1,11 @@
+package com.fabledsword.minstrel.cache.db.dao
+
+/**
+ * Projection: how many of a playlist's member tracks are resident in the audio
+ * cache index. Backs the offline "fully cached" greying — a playlist is fully
+ * available offline when [cachedCount] reaches its `trackCount`.
+ */
+data class PlaylistCachedCount(
+    val playlistId: String,
+    val cachedCount: Int,
+)

android/app/src/main/java/com/fabledsword/minstrel/models/Playlist.kt

@@ -20,6 +20,8 @@ data class PlaylistRef(
     val trackCount: Int = 0,
     val coverUrl: String = "",
     val ownerUsername: String = "",
+    /** All member tracks resident in the audio cache — playable fully offline. */
+    val fullyCached: Boolean = false,
 ) {
     val isSystem: Boolean get() = systemVariant != null
 

android/app/src/main/java/com/fabledsword/minstrel/playlists/data/PlaylistsRepository.kt

@@ -7,6 +7,7 @@ import retrofit2.HttpException
 import java.net.HttpURLConnection
 import com.fabledsword.minstrel.cache.db.dao.CachedPlaylistDao
 import com.fabledsword.minstrel.cache.db.dao.CachedPlaylistTrackDao
+import com.fabledsword.minstrel.cache.db.dao.PlaylistCachedCount
 import com.fabledsword.minstrel.cache.db.entities.CachedPlaylistEntity
 import com.fabledsword.minstrel.cache.db.entities.CachedPlaylistTrackEntity
 import com.fabledsword.minstrel.cache.mutations.MutationQueue
@@ -18,6 +19,7 @@ import com.fabledsword.minstrel.models.wire.PlaylistTrackWire
 import com.fabledsword.minstrel.models.wire.PlaylistWire
 import com.fabledsword.minstrel.shared.resolveServerUrl
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.map
 import retrofit2.Retrofit
 import retrofit2.create
@@ -51,9 +53,16 @@ class PlaylistsRepository @Inject constructor(
 
     // ── Reads (Flow, cache-first; the cache is the source of truth) ──
 
-    /** Owned + public; UI splits by isSystem / userId comparison. */
+    /**
+     * Owned + public; UI splits by isSystem / userId comparison. Joined with
+     * per-playlist cache counts so [PlaylistRef.fullyCached] drives the offline
+     * greying without each consumer re-querying the cache index.
+     */
     fun observeAll(): Flow<List<PlaylistRef>> =
-        playlistDao.observeAll().map { rows -> rows.map { it.toDomain() } }
+        combine(
+            playlistDao.observeAll(),
+            playlistDao.observeCachedCounts(),
+        ) { rows, counts -> mergePlaylistsWithCache(rows, counts) }
 
     /** User-owned playlists only (systemVariant IS NULL). */
     fun observeUserPlaylists(): Flow<List<PlaylistRef>> =
@@ -233,6 +242,23 @@ fun List<PlaylistTrackRef>.toPlayableTrackRefs(): List<TrackRef> =
 
 // ── Mappers (internal — keep Room + wire types out of the UI layer) ──
 
+/**
+ * Maps cached playlist rows to domain refs, stamping [PlaylistRef.fullyCached]
+ * from the cache-index counts. A playlist is fully cached when it has tracks
+ * and every member track is resident (`cachedCount >= trackCount`). Pulled out
+ * of the Flow so the predicate is unit-testable without a Room database.
+ */
+internal fun mergePlaylistsWithCache(
+    rows: List<CachedPlaylistEntity>,
+    counts: List<PlaylistCachedCount>,
+): List<PlaylistRef> {
+    val cachedById = counts.associate { it.playlistId to it.cachedCount }
+    return rows.map { row ->
+        val cached = cachedById[row.id] ?: 0
+        row.toDomain().copy(fullyCached = row.trackCount > 0 && cached >= row.trackCount)
+    }
+}
+
 private fun CachedPlaylistEntity.toDomain(): PlaylistRef =
     PlaylistRef(
         id = id,

android/app/src/test/java/com/fabledsword/minstrel/playlists/data/PlaylistsRepositoryCacheMergeTest.kt

@@ -0,0 +1,64 @@
+package com.fabledsword.minstrel.playlists.data
+
+import com.fabledsword.minstrel.cache.db.dao.PlaylistCachedCount
+import com.fabledsword.minstrel.cache.db.entities.CachedPlaylistEntity
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFalse
+import kotlin.test.assertTrue
+
+class PlaylistsRepositoryCacheMergeTest {
+
+    private fun playlist(id: String, trackCount: Int) =
+        CachedPlaylistEntity(id = id, userId = "u", name = id, trackCount = trackCount)
+
+    @Test
+    fun `fully cached when every member track is resident`() {
+        val out = mergePlaylistsWithCache(
+            rows = listOf(playlist("p", trackCount = 3)),
+            counts = listOf(PlaylistCachedCount("p", cachedCount = 3)),
+        )
+        assertTrue(out.single().fullyCached)
+    }
+
+    @Test
+    fun `not fully cached when only some tracks are resident`() {
+        val out = mergePlaylistsWithCache(
+            rows = listOf(playlist("p", trackCount = 3)),
+            counts = listOf(PlaylistCachedCount("p", cachedCount = 2)),
+        )
+        assertFalse(out.single().fullyCached)
+    }
+
+    @Test
+    fun `not fully cached when no tracks are resident (missing count row)`() {
+        val out = mergePlaylistsWithCache(
+            rows = listOf(playlist("p", trackCount = 3)),
+            counts = emptyList(),
+        )
+        assertFalse(out.single().fullyCached)
+    }
+
+    @Test
+    fun `empty playlist is never fully cached`() {
+        val out = mergePlaylistsWithCache(
+            rows = listOf(playlist("p", trackCount = 0)),
+            counts = listOf(PlaylistCachedCount("p", cachedCount = 0)),
+        )
+        assertFalse(out.single().fullyCached)
+    }
+
+    @Test
+    fun `each playlist gets its own cache verdict`() {
+        val out = mergePlaylistsWithCache(
+            rows = listOf(playlist("full", 2), playlist("partial", 2)),
+            counts = listOf(
+                PlaylistCachedCount("full", cachedCount = 2),
+                PlaylistCachedCount("partial", cachedCount = 1),
+            ),
+        )
+        val byId = out.associateBy { it.id }
+        assertEquals(true, byId.getValue("full").fullyCached)
+        assertEquals(false, byId.getValue("partial").fullyCached)
+    }
+}