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. Queries of three or more characters run through per-entity FTS5 trigram tables (external content, trigger-maintained), with the instr scan as the below-three-character fallback.
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. Landed: per-entity external-content trigram tables (album_search, artist_search, playlist_search) with Room-generated content-sync triggers, a three-character floor and an instr fallback below it.
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.

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