.cue Sheet Support

Two independent code paths that consume .cue sheets. The Stream Ripper writes cues (one TRACK per chapter — see the stream-ripper skill); this feature reads them.

• Part A — Direct play (virtual split): open a .cue, or an audio file with a sibling .cue, and the single backing file is played virtually split into its cue tracks → N rows in the now-playing playlist. Nothing is written to disk; nothing is added to the Local Library. Satisfies issue #273.
• Part B — Library split-on-import (off by default): when the toggle is on, a .cue encountered by the Local Library scan causes the backing file to be physically split into per-track FLACs via ffmpeg; those are added to the library and the original is excluded. When off, the scan ignores .cue files entirely and imports the backing file as one normal track.

The two paths share only the parser. Part A is unaffected by the Part B toggle.

Key files

Area	File
Shared parser	Data/Models/CueSheet.swift
Track cue fields	Data/Models/Track.swift (cueStartOffset, cueEndOffset, cueSourceURL, isCueTrack)
Playback + boundary detector + entry helper	Audio/AudioEngine.swift
Part B discovery	Utilities/LocalFileDiscovery.swift (cueExtensions / cueFiles bucket)
Part B splitter	Utilities/CueAlbumSplitter.swift
Part B scan pre-pass + exclusion + pref threading	Data/Models/MediaLibrary.swift
Part B toggle UI	App/ContextMenuBuilder.swift (cueSplitOnImportEnabled)
Entry points (Part A)	App/AppDelegate.swift, Windows/MainWindow/MainWindowView.swift, Windows/ModernMainWindow/ModernMainWindowView.swift, Windows/Playlist/PlaylistView.swift, Windows/ModernPlaylist/ModernPlaylistView.swift, Windows/ModernLibraryBrowser/ModernLibraryBrowserView.swift, Windows/PlexBrowser/PlexBrowserView.swift
Tests	Tests/NullPlayerAppTests/CueSheetTests.swift, AudioEngineCueBoundaryDetectorTests.swift, CueAlbumSplitterTests.swift

Shared parser — CueSheet.swift

CueSheet holds top-level performer (→ artist fallback), title (→ album), fileName (first FILE only), and [Entry] (number, title, performer?, startTime).

• parse(from:) throws — line scan; top-level vs track-level PERFORMER/TITLE are disambiguated by whether FILE has been seen yet. INDEX 01 preferred, INDEX 00 as fallback. Multiple FILE entries → warn, use first only. Throws if no FILE, or if entries.count > CueSheet.maxEntries (10 000 — DoS guard against a pathological cue spawning unbounded jobs/rows).
• parseCueTimestamp(_:) — inverse of the writer's cueTimestamp: MM:SS:FF @ 75 fps → seconds. Must round-trip with StreamRipper.cueTimestamp.
• resolveBackingFile(for:fileName:) — absolute paths honored; relative resolved against the cue's directory. Untrusted-input guard: a relative path that escapes the cue's own directory (../../…) is treated as missing (returns a non-existent sentinel) rather than reading an arbitrary file. A .cue travels with downloaded media — treat it as untrusted.
• siblingCue(for:) — returns <basename>.cue next to an audio file if present.
• expandToTracks(cue:cueFileURL:) — virtual [Track] for Part A. Empty cue → []. For entry i: cueStartOffset = entries[i].startTime, cueEndOffset = entries[i+1].startTime (guarded i+1 < count; nil for the last entry → play to EOF). A missing backing file is just a cue track whose url doesn't exist → the engine's load-failure skip shows it as an unplayable row.

Part A — playback (AudioEngine.swift)

• Entry helper tracksForCueOrSibling(url:) -> [Track]? — every entry point calls this before the normal Track-from-URL path: returns expanded tracks for a .cue, or for an audio file with a sibling cue, else nil.

• Load: a cue track schedules scheduleSegment from cueStart*sr to EOF (not to cueEnd), .dataPlayedBack, generation-guarded — the continuous schedule is what makes gapless possible. _currentTime/lastReportedTime reset to 0 so the clock is 0-relative; the boundary base index resets.

• duration getter: cueEnd - cueStart, or fileDuration - cueStart when cueEnd == nil.

• Seek: clamps to [0, duration], then offsets by cueStart and reschedules to EOF.

• Gapless boundary detector advanceCueTrackIfBoundaryCrossed() (called from the 0.1 s time-update timer) wraps the pure, unit-tested decision shouldAdvanceCueTrackAtBoundary(...). When currentTime >= cueEnd and the next playlist entry is a same-cueSourceURL cue track, it advances currentIndex/currentTrack, resets _currentTime/lastReportedTime, and resets playbackStartDate = Date() (without this the 0-relative clock keeps climbing). Audio is untouched → no gap; currentTrack didSet posts the change so title/seek-bar/Now Playing update.

  Gapless is provided only with shuffle off and repeat-single off. The detector returns early under shuffle or repeat-single (it assumes the next track is playlist[currentIndex+1], which is false under shuffle) and guards currentIndex+1 < playlist.count. In those modes the schedule runs to real EOF and the normal trackDidFinish path advances (a gap is acceptable — documented limitation).

Part B — library split (CueAlbumSplitter.swift, off by default)

Gated by UserDefaults bool cueSplitOnImportEnabled (default false, mirrors the includeLegacyWMA scan-flag pattern). When off, LocalFileDiscovery does not collect .cue files and MediaLibrary excludes nothing.

LocalFileDiscovery exposes a separate cueExtensions = ["cue"] / cueFiles bucket — do not reuse playlistExtensions (that would make cues into LocalPlaylist browser nodes). The MediaLibrary scan runs a pre-pass before the cleanup loop: for each cue, CueAlbumSplitter.splitIfNeeded(cueURL:) returns a SplitOutcome { backingFileToExclude, trackFiles }:

1. shouldPerformSplit compares the deterministic expected output paths (computeOutputPath(..., checkFilesystem: false)) against disk. All present → idempotent skip, returns the backing file + existing track files. Any missing → split.
2. Per-album subdirectory: outputs go into <cueDir>/<Artist - Album>/ — named from the source file's own ALBUM/ARTIST tags read via ffprobe (sourceTags), falling back to the cue's PERFORMER/TITLE, then the cue filename. (The cue's TITLE from the Stream Ripper is the video/show title — the track name — not the album, so the file tags are preferred.) sourceTags is deterministic, so expectedOutputPaths and performSplit compute the same folder → idempotency holds.
3. Output is always re-encoded FLAC (-c:a flac, not -c copy — copy isn't sample-accurate at cut points) for both lossless and lossy sources, as NN - <sanitized title>.flac.
4. Filenames/folder are sanitized via sanitizeFilenameComponent: replace / \ : * ? " < > | + control chars with _, collapse whitespace, trim leading/trailing spaces+dots, NFC-normalize, truncate ~200 UTF-8 bytes; track filenames also de-dup with (2), (3).
5. ffmpeg args (mirroring StreamRipper's Process/[String] pattern — never a shell string): -ss/-to (last track omits -to), -map_metadata 0 (inherit the source's date/genre/cover-art tags), then override per-track title + track=N/total, and artist/album/album_artist from the source tags (sourceTags), falling back to cue values, only writing non-empty values so an inherited field is never blanked. Cover art is conditional — the -map 0:v:0 -c:v copy -disposition:v attached_pic group is added only if ffprobe finds a video/attached-pic stream.
6. Skip+warn when ffmpeg is absent, or on any write/permission/space failure: do not split, do not exclude the original, post a one-time notice. The original is excluded iff its split tracks actually exist.

MediaLibrary then: (a) removes any backing file in the "successfully split" set from tracks/tracksByPath/store and from audioMetadataTasks (critical — the enrichment pass would otherwise re-insert the backing under its own tags), and (b) adds the returned trackFiles to the library in-scan (they're written into a subdir created after discovery enumerated audio, so discovery didn't see them). Cue files are never upserted as tracks or playlists.

Reading FLAC/M4A tags (MediaLibrary.parseMetadata): AVFoundation exposes title/artist/album via common keys, but album-artist and track/disc number are not common keys — they were only read from ID3 (TPE2/TRCK/TPOS), so FLAC/M4A came back nil. parseMetadata now scans all metadata formats for Vorbis ALBUMARTIST/TRACKNUMBER/DISCNUMBER (and iTunes aART/trkn/disk), parsing the leading int from "1/10". Without these, cue-split FLAC albums fragmented by per-track artist and lost their track ordering.

Gotchas

• Idempotency seeding: performSplit seeds the de-dup set with the cue's full deterministic canonical path set up front, so an already-split track on a partial re-run is recognized as our own output and reused — never re-encoded into a (2) duplicate. (Trade-off: two different cues in the same folder producing identically-named tracks will overwrite rather than (2); idempotency is the priority.)
• Outputs land in a subdir of the cue's directory (<cueDir>/<Artist - Album>/), never relative to the backing file — there is no write-side path traversal.
• MAS sandbox: writing split files next to a scanned .cue needs the containing folder covered by a writable security-scoped bookmark. If watch folders aren't writable under the sandbox, Part B no-ops via skip+warn. Confirm before relying on Part B in the MAS build, or treat it as DMG-only.
• Sources/NullPlayerCore/Models/Track.swift is NOT modified — the cue fields live only on the Data/Models Track used by AudioEngine.playlist.
• Out of scope: embedded FLAC CUESHEET/Vorbis tags; multiple FILE entries per cue (first only); INDEX 00 pregap beyond the start-time fallback.