minstrel/internal/library/duplicate_survivor.go

package library

import (
	"sort"
	"strings"
	"time"
)

// SurvivorCandidate is what choosing which copy to keep needs to know about one
// member of a duplicate group.
type SurvivorCandidate struct {
	TrackID    string
	FileFormat string
	FileSize   int64
	AddedAt    time.Time

	// TagFit is TagFitScore: how well the copy's tags and name fit the album.
	TagFit int
}

// TagFitScore counts the signs that a copy belongs where it is filed, one
// point each:
//   - it has a track number that no other track on its album also claims
//   - its file name carries no video-rip marker (SourceMarkersFor)
//   - it has a MusicBrainz recording id
//
// Two copies of one song differ in these far more often than in anything a
// listener hears: rips of one song can differ by a few bytes, which makes file
// size a poor guide.
func TagFitScore(hasTrackNumber, positionClash bool, filePath string, hasMBID bool) int {
	score := 0
	if hasTrackNumber && !positionClash {
		score++
	}
	if len(SourceMarkersFor(filePath)) == 0 {
		score++
	}
	if hasMBID {
		score++
	}
	return score
}

// losslessFormats are the scanned extensions that are lossless by definition.
// m4a is left out on purpose: it holds either ALAC or AAC, and the scanner
// records only the extension, so calling it lossless would sometimes prefer an
// AAC copy over a FLAC one.
var losslessFormats = map[string]bool{"flac": true, "wav": true}

// ProposeSurvivor picks which copy of a duplicate group to keep, and gives the
// reason in words an admin reads beside it. It is a default, not a verdict:
// the report shows it and the merge lets an admin choose another.
//
// In order:
//  1. lossless over lossy — the one difference no later step can recover. It
//     comes before tag fit: tags can be fixed, a deleted FLAC cannot
//  2. the copy whose tags fit the album best (TagFitScore). It is zero for
//     every copy when the caller does not know it, and the order falls through
//     to the rules below
//  3. the larger file — for one recording at one duration that is the higher
//     bitrate. The scanner does not record bitrate (tracks.bitrate is never
//     filled), so file size is the signal that actually exists
//  4. the copy in the library longest — the one most likely to carry the play
//     history and likes, so the merge moves the least
//  5. the lowest track id, so the choice is stable between page loads
func ProposeSurvivor(cands []SurvivorCandidate) (trackID, reason string) {
	if len(cands) == 0 {
		return "", ""
	}
	ranked := append([]SurvivorCandidate(nil), cands...)
	sort.SliceStable(ranked, func(i, j int) bool { return survivorBefore(ranked[i], ranked[j]) })
	best := ranked[0]
	if len(ranked) == 1 {
		return best.TrackID, "the only copy"
	}

	// The reason names the first rule that separated the best copy from the
	// runner-up — the rule that actually decided, not every rule it passed.
	next := ranked[1]
	switch {
	case isLossless(best) != isLossless(next):
		return best.TrackID, "lossless (" + strings.ToLower(best.FileFormat) + ")"
	case best.TagFit != next.TagFit:
		return best.TrackID, "tags fit the album"
	case best.FileSize != next.FileSize:
		return best.TrackID, "largest file"
	case !best.AddedAt.Equal(next.AddedAt):
		return best.TrackID, "in the library longest"
	default:
		return best.TrackID, "copies are otherwise identical"
	}
}

func survivorBefore(a, b SurvivorCandidate) bool {
	if isLossless(a) != isLossless(b) {
		return isLossless(a)
	}
	if a.TagFit != b.TagFit {
		return a.TagFit > b.TagFit
	}
	if a.FileSize != b.FileSize {
		return a.FileSize > b.FileSize
	}
	if !a.AddedAt.Equal(b.AddedAt) {
		return a.AddedAt.Before(b.AddedAt)
	}
	return a.TrackID < b.TrackID
}

func isLossless(c SurvivorCandidate) bool {
	return losslessFormats[strings.ToLower(c.FileFormat)]
}