minstrel/internal/library/duplicate_classify.go

package library

import (
	"regexp"
	"strings"
	"unicode"
)

// Duplicate classification (M498 #5435; settled further 2026-10-19).
//
// A duplicate group is one recording found more than once. What should happen
// to it depends on where the copies sit:
//
//   - same_release: every copy is on one album under one title. An album holds
//     a song once, so the extra copies are the ones to fold away.
//   - cross_release: the copies are on different albums. A single and the
//     album it came from, a deluxe edition and the standard one, or one
//     recording a later release retitled ("The Ragtime Cat", "F.O.D. (live)",
//     a romanized title): each copy belongs to its own release, so none is
//     deleted (unless it is a video rip; the resolver decides). The measured
//     library (2026-10-19) had 451 such groups waiting on the operator under
//     different titles, and they were retitles, not different songs.
//   - distinct: one album, two versions of a song that sound alike, named as
//     such ("Lovesick", "Lovesick (Instrumental)"). Not duplicates.
//   - mismatch: one album holds the same audio under different song titles:
//     one file filling several track slots, a broken download. Which slot is
//     right is the resolver's to decide from MusicBrainz's track lengths.
//   - review: what none of these fits. The operator decides.

// DuplicateClass is a group's shape. The values are duplicate_groups.class.
type DuplicateClass string

const (
	ClassSameRelease  DuplicateClass = "same_release"
	ClassCrossRelease DuplicateClass = "cross_release"
	ClassDistinct     DuplicateClass = "distinct"
	ClassMismatch     DuplicateClass = "mismatch"
	ClassReview       DuplicateClass = "review"
)

// ClassifyMember is what classifying needs to know about one copy.
type ClassifyMember struct {
	AlbumID    string
	Title      string
	ArtistName string
}

// ClassifyDuplicateGroup decides a group's shape from its tier ("exact" or
// "acoustic") and its copies.
//
// Titles are compared by MatchTitleKey, which drops what a release or an upload
// adds to a title (featuring credits, remaster notes, video markers) and keeps
// what names a different recording (live, demo, remix, instrumental). On one
// album, titles that differ only by such a word are two versions (distinct)
// unless the bytes are identical, which is one file twice: the #3885 case of
// "WWW" and "WWW (instrumental)" holding the same file is a wrong slot.
func ClassifyDuplicateGroup(tier string, members []ClassifyMember) DuplicateClass {
	if len(members) < 2 {
		return ClassReview
	}
	oneAlbum, oneTitle, oneSong := true, true, true
	key := MatchTitleKey(members[0].Title, members[0].ArtistName)
	song := songKey(members[0].Title, members[0].ArtistName)
	for _, m := range members[1:] {
		if m.AlbumID != members[0].AlbumID {
			oneAlbum = false
		}
		if MatchTitleKey(m.Title, m.ArtistName) != key {
			oneTitle = false
		}
		if songKey(m.Title, m.ArtistName) != song {
			oneSong = false
		}
	}
	exact := tier == string(tierExact)
	switch {
	case !oneAlbum:
		return ClassCrossRelease
	case oneTitle:
		return ClassSameRelease
	case oneSong && !exact:
		return ClassDistinct
	default:
		return ClassMismatch
	}
}

// versionSuffix is a version named after a dash rather than in brackets:
// "Goodbye Yesterday - Instrumental Version", "Song - Radio Edit".
var versionSuffix = regexp.MustCompile(
	`(?i)\s-\s[^-]*\b(version|instrumental|remix|mix|live|acoustic|demo|edit|reprise|rework|vip)\b[^-]*$`)

// songKey is MatchTitleKey with every bracketed segment and a dashed version
// suffix dropped: the song a title names, whatever version of it.
// "Lovesick (Instrumental)", "Lovesick - Instrumental Version" and "Lovesick"
// share one.
func songKey(title, artist string) string {
	t := versionSuffix.ReplaceAllString(bracketed.ReplaceAllString(title, " "), "")
	if key := MatchTitleKey(t, artist); key != "" {
		return key
	}
	return MatchTitleKey(title, artist)
}

var (
	// A bracketed credit: "(feat. Panda Bear)", "[ft. X]", "(featuring Y)".
	featBracket = regexp.MustCompile(`(?i)[(\[][^)\]]*\b(feat\.?|ft\.?|featuring)\s[^)\]]*[)\]]`)
	// A credit trailing the title without brackets: "Doin' It Right ft. Panda Bear".
	featTrailing = regexp.MustCompile(`(?i)\s(feat\.?|ft\.?|featuring)\s.*$`)
	// "(2011 Remaster)", "[Remastered]", "- Remastered 2009".
	remasterBracket = regexp.MustCompile(`(?i)[(\[][^)\]]*remaster[^)\]]*[)\]]`)
	remasterSuffix  = regexp.MustCompile(`(?i)\s-\s[^-]*remaster.*$`)
	// Any bracketed segment, to test each against the video-rip markers.
	bracketed = regexp.MustCompile(`[(\[][^)\]]*[)\]]`)
)

// MatchTitleKey reduces a title to what identifies the song, for deciding
// whether two copies carry the same one.
//
// Dropped: a leading "Artist - " (a video upload's title), bracketed featuring
// credits and remaster notes, and bracketed segments carrying a video-rip marker
// ("(Official Video)", "[HD]"). Kept: every other bracketed word, so "(live)",
// "(demo)", "(acoustic)", "(remix)" and "(instrumental)" still tell recordings
// apart. Then case, "&" against "and", and punctuation and spacing are ignored.
func MatchTitleKey(title, artist string) string {
	t := strings.TrimSpace(title)
	if a := strings.TrimSpace(artist); a != "" {
		if prefix := a + " - "; len(t) > len(prefix) && strings.EqualFold(t[:len(prefix)], prefix) {
			t = t[len(prefix):]
		}
	}
	t = featBracket.ReplaceAllString(t, " ")
	t = remasterBracket.ReplaceAllString(t, " ")
	t = bracketed.ReplaceAllStringFunc(t, func(seg string) string {
		for _, re := range suspectSourceRegexps {
			if re.MatchString(seg) {
				return " "
			}
		}
		return seg
	})
	t = remasterSuffix.ReplaceAllString(t, "")
	t = featTrailing.ReplaceAllString(t, "")
	if key := alnumKey(t); key != "" {
		return key
	}
	return alnumKey(title)
}

// alnumKey lowercases, reads "&" as "and", and keeps only letters and digits.
func alnumKey(s string) string {
	s = strings.ReplaceAll(strings.ToLower(s), "&", " and ")
	var b strings.Builder
	for _, r := range s {
		if unicode.IsLetter(r) || unicode.IsDigit(r) {
			b.WriteRune(r)
		}
	}
	return b.String()
}