subtracks/docs/implementation-plan.md

Implementation plan

The rewrite is delivered in vertical slices. Each phase leaves the app in a working, testable state.

Phase 0 - Foundation (done)

Nix devshell (JDK, Gradle, Android SDK, nushell, navidrome, gonic), Gradle Kotlin DSL at the repo root, a Compose skeleton, Roborazzi/Robolectric screenshot testing, and the carried-over launcher and notification assets. The app ships as com.subtracks.next.

Phase 1 - Data foundation (done)

Everything needed to mirror a server library locally:

• MusicSource abstraction with SubsonicClient, SubsonicXml and SubsonicSource.
• Room 3 schema on bundled SQLite, so the SQLite version is identical on every device.
• SyncService: stream each entity in batches, upserting per batch and pruning the IDs the server no longer lists; the sync is not atomic.
• Unit tests (client, XML, sync), native integration tests against navidrome and gonic, and CI (unit, lint, integration) with warm caches.

Phase 2 - UI vertical slice (done)

Goal: a usable app for browsing a server, end to end.

• Koin DI and DataStore-backed preferences (album sort); the unused app_settings Room table was removed in favour of DataStore.
• App shell: a root gate that shows the add-server flow until a server exists, then the tabbed library screen matching the Flutter app (icon-only tabs for albums, artists and playlists, with sync and settings actions), plus album and playlist detail screens and a monochrome theme.
• Source setup: name/address/username/password with token-auth toggle, a connection test, save-and-sync, plus server switching and removal in settings.
• Library browsing: albums (covers-only grid), artists and playlists paged from Room (PagingSource) and rendered with LazyPagingItems, with Coil cover art served from stable media URLs.
• Artist detail: tapping an artist opens their albums, newest first, and an album can be opened from there.
• Roborazzi screenshots for the new screens, rendered on demand for review (not committed).

Not done yet in this slice (deliberately): cover-art tonal colour extraction, long-press menus, editing an existing server, and per-section sort/filter controls (only album sort is wired).

Phase 3 - Playback (in progress)

• (done) Media3/ExoPlayer playback service, queue and now-playing UI: a MediaSessionService plus a mini player and a Now Playing screen.
• (done) Media notifications and headset/Bluetooth controls, from the media session.
• (done) A SQL-backed queue of references (whole playlist / album / song, each with an optional ordinal range) resolved lazily, with only a bounded window in memory and in the player.
• (done) Streaming bitrate and preferred format from settings (maxBitRate/format, so the server transcodes as configured).
• (done) A queue view that lists the resolved queue, jumps to a track on tap, removes tracks and reorders them by dragging, with a one-step undo.
• (done) Adding to the queue from the library (play next / add to queue), backed by a separate, editable up-next list that plays between the current track and the rest of the context.
• Gapless format preferences.
• Scrobbling and "now playing" back to the server.
• Optional download/caching.

Phase 4 - Search, playlists, offline

• (done) Search: a substring filter on the library queries, scoped to the active tab, with a docked search field and per-tab sort/filter controls.
• Playlist browsing and editing (create/add/remove), syncing changes back to the server.
• Offline mode and downloads.

Phase 5 - Release

• Change applicationId from com.subtracks.next to com.subtracks.
• Signing, versioning and Fastlane metadata.
• F-Droid submission. Their buildserver must have Gradle >= 9.6 and SDK 37, which this project targets; do not rely on the Gradle wrapper there (fdroidserver deletes gradlew and uses its own Gradle).

Scale hardening backlog

Follow-up work for very large libraries, playlists and long sessions, grouped so each block can be one PR.

• Library read path. Index the sort keys used by album/artist/playlist browsing (a denormalized albumArtist on albums plus NOCASE indices) and replace LIMIT 1 OFFSET with keyset/cursor seeks in QueueRepository/QueueDao. Shared dependency for the read-side fixes; can split into "indices" then "keyset" if too large for one review.
• Shuffle. Landed: the permutation is derived from (seed, size) (a seekable Feistel in Shuffle.kt) and positions resolve through the same positional lookup, so shuffle_order and the retained whole-order/whole-id arrays are gone. The context is read-only while shuffled; manual ordering goes through the up-next block.
• Sync pipeline. Prune with a memory-bounded id set, remove fetchRanks and the write-only rank columns, fetch playlists/albums with bounded concurrency and skip unchanged ones, and raise/make configurable the page cap with a clear "library too large" failure. Independent of the read path.
• Queue view and edits. Bound the queue view's loaded rows (or page over the queue ordinal) and scope its ViewModel to the overlay; compact adjacent same-ref ranges after an edit and cache resolved entry lengths. The view half is independent; the edit half shares QueueRepository with the read-path PR.
• Artwork. Give artwork_seeds a sourceId with ON DELETE CASCADE and an LRU/TTL (or drop the table and rely on Coil's disk cache); prefetch the thumbnail, not the original, on now-playing transitions. Independent.
• FTS5 search. Deferred; the revival requirements are recorded in architecture.md (per-entity trigram tables indexing every scanned field, incremental maintenance, three-character floor).
• UI/flow overhead. Subscribe only the active tab's paging flow; move the position ticker off the shared PlaybackState. Independent.

A syncGen column was considered for the sync prune and rejected: it is part of the upsert's update set, so its always-changing value defeats upsertChanged's changed-only WHERE predicate and rewrites every row on every full sync. The prune should instead keep the two-phase diff with a memory-bounded id set (primitive long hashes) or diff a staging seen table in SQL.

Deferred decisions

• Whether to drop the plaintext-auth fallback or keep it behind an "insecure server" toggle.
• Robolectric runs at SDK 35 while targetSdk is 37; aligning them changes the on-demand Roborazzi renders (nothing committed to re-record).
• Whether to extract :core:data and friends once the UI has grown.