package library

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"log/slog"
	"sort"
	"strings"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgtype"
	"github.com/jackc/pgx/v5/pgxpool"

	"git.fabledsword.com/bvandeusen/minstrel/internal/audit"
	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	"git.fabledsword.com/bvandeusen/minstrel/internal/lidarr"
	"git.fabledsword.com/bvandeusen/minstrel/internal/notifications"
	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
)

// Duplicate resolver (M498 #5435, #5437).
//
// The sweep proposes duplicate groups; the resolver decides what each one is
// and settles the ones that are safe to settle without asking. Safety is
// Lidarr's to define: Lidarr maps one file to each track of the release it
// monitors, and re-downloads any mapped file that disappears. So:
//
//   - A copy Lidarr maps is never removed.
//   - On one album, copies Lidarr does not map are merged into the copy it does
//     (or, when it maps none, into the best by ProposeSurvivor).
//   - When Lidarr maps every copy on one album, the release it monitors lists
//     the song twice: the Humanz box set, 14 vinyl sides then the digital
//     medium. The resolver moves Lidarr to a release of that album that lists
//     each song once. Lidarr then rescans the folder, maps one copy and leaves
//     the rest unmapped, and the next pass merges them.
//
// The release change has a fixed point (lesson #4183): it happens only while
// the monitored release repeats a title, and the release it chooses repeats
// none, so the next pass finds nothing to change.

// LidarrLibrary is the part of Lidarr's API the resolver uses. *lidarr.Client
// satisfies it.
type LidarrLibrary interface {
	ListUnmappedTrackFiles(ctx context.Context) ([]lidarr.TrackFile, error)
	LookupAlbumByMBID(ctx context.Context, mbid string) (lidarr.LidarrAlbum, error)
	ListAlbumTracks(ctx context.Context, albumID int) ([]lidarr.ReleaseTrack, error)
	GetAlbumReleases(ctx context.Context, albumID int) ([]lidarr.AlbumRelease, error)
	ListReleaseTracks(ctx context.Context, releaseID int) ([]lidarr.ReleaseTrack, error)
	SetMonitoredRelease(ctx context.Context, albumID, releaseID int) error
	SearchAlbums(ctx context.Context, albumIDs []int) error
}

const (
	// resolveMergeCap bounds the merges one pass makes. The first pass after
	// deploy meets the whole backlog (433 groups measured on 2026-10-09); a
	// cap spreads it over a few hours, each pass short.
	resolveMergeCap = 200
	// resolveReleaseCap bounds the Lidarr release changes one pass makes. Each
	// makes Lidarr rescan an artist folder.
	resolveReleaseCap = 10
	// releaseChangeSettle is how long after a release change the resolver
	// leaves that album's groups alone. Lidarr unmaps every file of the album
	// and rescans; until the rescan lands, every copy reads as unmapped, and a
	// merge then could remove the very file Lidarr is about to map.
	releaseChangeSettle = 24 * time.Hour
	// duplicateResolveTick is how often the worker runs a pass.
	duplicateResolveTick = time.Hour
)

const (
	lidarrStateTracked  = "tracked"
	lidarrStateUnmapped = "unmapped"
)

// ReleaseChange is one Lidarr release the resolver switched.
type ReleaseChange struct {
	AlbumTitle string
	ArtistName string
	From, To   lidarr.AlbumRelease
}

// DuplicateResolveResult tallies one pass.
type DuplicateResolveResult struct {
	Groups          int
	Classes         map[DuplicateClass]int
	LidarrConsulted bool
	Merged          int
	MergeFailed     int
	ReleaseChanges  []ReleaseChange
	// SongsLinked counts cross-release groups newly linked as one song.
	SongsLinked int
	// HeldBack counts tracks newly held back from radio and the mixes as video
	// rips; Released counts held-back tracks whose name no longer says so.
	HeldBack, Released int64
	// RipSearches counts albums Lidarr was asked to search for a better copy
	// of a rip that has no clean copy (#5447).
	RipSearches int
}

// LidarrPathKey is the part of a path Minstrel and Lidarr agree on: the last
// three components (artist folder, album folder, file). Both see the library
// under their own mount; on the operator's deploy both happen to say /music,
// and the key keeps the match from depending on that.
func LidarrPathKey(p string) string {
	parts := strings.Split(strings.Trim(p, "/"), "/")
	if len(parts) > 3 {
		parts = parts[len(parts)-3:]
	}
	return strings.Join(parts, "/")
}

// resolveGroup is one pending group and its present copies.
type resolveGroup struct {
	id      pgtype.UUID
	tier    string
	members []dbq.ListDuplicateGroupsForResolveRow
	class   DuplicateClass
	note    string
	states  []string // per member: tracked, unmapped, or "" when Lidarr was not asked
}

// autoMergeable says whether the resolver may merge the group on its own: when
// every copy it would remove is one Lidarr does not map (D-a rule 1). On one
// album that is any group with at most one mapped copy. Across releases each
// mapped copy fulfils its own release (D-b), so only a group with exactly one
// mapped copy qualifies: the others fulfil nothing — a rip beside the clean
// copy on another release, or a wrong-file import Lidarr holds unmapped
// (#5439) — and the mapped copy is the clear one to keep.
func autoMergeable(g *resolveGroup) bool {
	switch g.class {
	case ClassSameRelease:
		return g.tracked() <= 1
	case ClassCrossRelease, ClassMismatch:
		return g.tracked() == 1
	default:
		return false
	}
}

// settling says whether any of the group's albums had its Lidarr release
// changed recently, so Lidarr may still be remapping its files.
func (g *resolveGroup) settling(albums map[string]bool) bool {
	for _, m := range g.members {
		if albums[syncpkg.FormatUUID(m.AlbumID)] {
			return true
		}
	}
	return false
}

func (g *resolveGroup) tracked() int {
	n := 0
	for _, s := range g.states {
		if s == lidarrStateTracked {
			n++
		}
	}
	return n
}

// ResolveDuplicates runs one pass. lid is nil when Lidarr is disabled; act is
// the operator's auto-resolve setting. Without Lidarr, or with act off, the
// pass only classifies: nothing is removed when Lidarr cannot say what is safe.
func ResolveDuplicates(
	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string, lid LidarrLibrary, act bool,
) (DuplicateResolveResult, error) {
	if logger == nil {
		logger = slog.Default()
	}
	q := dbq.New(pool)
	res := DuplicateResolveResult{Classes: map[DuplicateClass]int{}}

	rows, err := q.ListDuplicateGroupsForResolve(ctx)
	if err != nil {
		return res, fmt.Errorf("list duplicate groups: %w", err)
	}
	groups := foldResolveGroups(rows)
	res.Groups = len(groups)

	var unmapped map[string]bool
	if lid != nil {
		files, err := lid.ListUnmappedTrackFiles(ctx)
		if err != nil {
			logger.Warn("duplicate resolve: Lidarr's unmapped files unavailable; classifying only", "err", err)
		} else {
			unmapped = make(map[string]bool, len(files))
			for _, f := range files {
				unmapped[LidarrPathKey(f.Path)] = true
			}
			res.LidarrConsulted = true
		}
	}

	for _, g := range groups {
		cm := make([]ClassifyMember, len(g.members))
		g.states = make([]string, len(g.members))
		for i, m := range g.members {
			cm[i] = ClassifyMember{AlbumID: syncpkg.FormatUUID(m.AlbumID), Title: m.Title, ArtistName: m.ArtistName}
			if res.LidarrConsulted {
				if unmapped[LidarrPathKey(m.FilePath)] {
					g.states[i] = lidarrStateUnmapped
				} else {
					g.states[i] = lidarrStateTracked
				}
			}
		}
		g.class = ClassifyDuplicateGroup(g.tier, cm)
		g.note = resolveNote(g)
		res.Classes[g.class]++
	}
	if err := writeResolveVerdicts(ctx, q, groups); err != nil {
		return res, err
	}

	if !act {
		return res, nil
	}

	// Holding rips back and linking songs remove nothing and need nothing from
	// Lidarr.
	if res.HeldBack, err = q.FlagSuspectSourceTracks(ctx, SuspectSourcePattern); err != nil {
		return res, fmt.Errorf("hold back video rips: %w", err)
	}
	if res.Released, err = q.ClearStaleSuspectSourceTracks(ctx, SuspectSourcePattern); err != nil {
		return res, fmt.Errorf("release renamed tracks: %w", err)
	}
	linked, err := linkCrossRelease(ctx, pool, logger, groups)
	res.SongsLinked = linked
	if err != nil {
		return res, err
	}

	if !res.LidarrConsulted {
		return res, nil
	}

	settling, err := recentlyChangedAlbums(ctx, q, time.Now())
	if err != nil {
		return res, err
	}

	// Merges first, on the state Lidarr reported at the start of the pass.
	removable := func(p string) bool { return unmapped[LidarrPathKey(p)] }
	for _, g := range groups {
		if res.Merged >= resolveMergeCap {
			break
		}
		if ctx.Err() != nil {
			return res, ctx.Err()
		}
		if !autoMergeable(g) || g.settling(settling) {
			continue
		}
		if err := autoMerge(ctx, pool, logger, dataDir, g, removable); err != nil {
			res.MergeFailed++
			logger.Warn("duplicate resolve: merge failed", "group_id", syncpkg.FormatUUID(g.id), "err", err)
			continue
		}
		res.Merged++
	}

	// Then release changes, for albums where Lidarr maps every copy.
	changes, err := changeRepeatingReleases(ctx, q, pool, logger, lid, groups, settling)
	res.ReleaseChanges = changes
	if err != nil {
		return res, err
	}

	// Last, rips with no clean copy anywhere: ask Lidarr for something better.
	searched, err := searchSoleCopyRips(ctx, q, pool, logger, lid)
	res.RipSearches = searched
	if err != nil {
		logger.Warn("duplicate resolve: rip search failed", "err", err)
	}

	if res.Merged > 0 || len(changes) > 0 {
		notifyAdmins(ctx, notifications.KindDuplicatesResolved, notifications.Payload{
			Count:  int64(res.Merged + len(changes)),
			Detail: resolvedDetail(res.Merged, changes),
		})
	}
	return res, nil
}

// linkCrossRelease links each cross-release group's copies as one song
// (#5438): a like on one is a like on all, and the Liked list, shuffle and the
// mixes count the song once. Linking an already-linked group changes nothing,
// so this runs every pass; likes are shared only when a link is new, since a
// like made after it reaches every copy as it is made.
func linkCrossRelease(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, groups []*resolveGroup) (int, error) {
	linked := 0
	for _, g := range groups {
		if g.class != ClassCrossRelease {
			continue
		}
		if ctx.Err() != nil {
			return linked, ctx.Err()
		}
		ids := make([]pgtype.UUID, len(g.members))
		for i, m := range g.members {
			ids[i] = m.TrackID
		}
		moved, err := linkSong(ctx, pool, ids)
		if err != nil {
			logger.Warn("duplicate resolve: song link failed", "group_id", syncpkg.FormatUUID(g.id), "err", err)
			continue
		}
		if moved {
			linked++
		}
	}
	return linked, nil
}

// linkSong links the tracks as one song and, when that changed anything,
// shares their likes across the song and logs each added like for sync.
func linkSong(ctx context.Context, pool *pgxpool.Pool, ids []pgtype.UUID) (bool, error) {
	tx, err := pool.Begin(ctx)
	if err != nil {
		return false, err
	}
	defer func() { _ = tx.Rollback(ctx) }()
	tq := dbq.New(tx)
	link, err := tq.LinkTracksAsSong(ctx, ids)
	if err != nil {
		return false, fmt.Errorf("link: %w", err)
	}
	if link.Moved == 0 {
		return false, nil
	}
	added, err := tq.ShareSongLikes(ctx, link.SongKey)
	if err != nil {
		return false, fmt.Errorf("share likes: %w", err)
	}
	likeIDs := make([]string, len(added))
	for i, a := range added {
		likeIDs[i] = syncpkg.EncodeLikeID(syncpkg.FormatUUID(a.UserID), syncpkg.FormatUUID(a.TrackID))
	}
	if err := syncpkg.LogChanges(ctx, tx, syncpkg.EntityLikeTrack, likeIDs, syncpkg.OpUpsert); err != nil {
		return false, fmt.Errorf("log shared likes: %w", err)
	}
	return true, tx.Commit(ctx)
}

// ripSearchCap bounds the albums one pass asks Lidarr to search, so the first
// pass after deploy does not queue the whole backlog of rips at once.
const ripSearchCap = 10

// searchSoleCopyRips asks Lidarr to search each album holding a video rip that
// is the only copy of its song (#5447, operator's choice: option 2). While
// Lidarr maps the rip it counts the track fulfilled, so nothing else asks for
// better. An album search grabs only when the quality profile allows an
// upgrade, so this removes nothing and may change nothing: the rip stays held
// back either way. An album is searched at most once a week (the query reads
// the audit log), and an album Lidarr does not hold is skipped.
func searchSoleCopyRips(
	ctx context.Context, q *dbq.Queries, pool *pgxpool.Pool, logger *slog.Logger, lid LidarrLibrary,
) (int, error) {
	albums, err := q.ListAlbumsWithSoleCopyRips(ctx, ripSearchCap)
	if err != nil {
		return 0, fmt.Errorf("list albums with sole-copy rips: %w", err)
	}
	type found struct {
		row      dbq.ListAlbumsWithSoleCopyRipsRow
		lidarrID int
	}
	var targets []found
	for _, a := range albums {
		la, err := lid.LookupAlbumByMBID(ctx, a.ReleaseGroupMbid)
		if errors.Is(err, lidarr.ErrNotFound) {
			continue
		}
		if err != nil {
			return 0, fmt.Errorf("look up %s: %w", a.ReleaseGroupMbid, err)
		}
		targets = append(targets, found{row: a, lidarrID: la.ID})
	}
	if len(targets) == 0 {
		return 0, nil
	}
	ids := make([]int, len(targets))
	for i, t := range targets {
		ids[i] = t.lidarrID
	}
	if err := lid.SearchAlbums(ctx, ids); err != nil {
		return 0, fmt.Errorf("search albums: %w", err)
	}
	for _, t := range targets {
		audit.WriteOrLog(ctx, pool, logger, pgtype.UUID{}, pgtype.UUID{}, audit.ActionLidarrRipSearch, map[string]any{
			"album_id":        syncpkg.FormatUUID(t.row.AlbumID),
			"album_title":     t.row.AlbumTitle,
			"artist_name":     t.row.ArtistName,
			"lidarr_album_id": t.lidarrID,
			"rips":            t.row.Rips,
		})
	}
	return len(targets), nil
}

func foldResolveGroups(rows []dbq.ListDuplicateGroupsForResolveRow) []*resolveGroup {
	var groups []*resolveGroup
	for _, r := range rows {
		if n := len(groups); n == 0 || groups[n-1].id != r.GroupID {
			groups = append(groups, &resolveGroup{id: r.GroupID, tier: r.Tier})
		}
		g := groups[len(groups)-1]
		g.members = append(g.members, r)
	}
	// A group whose other copies went missing has nothing left to resolve.
	out := groups[:0]
	for _, g := range groups {
		if len(g.members) >= 2 {
			out = append(out, g)
		}
	}
	return out
}

// resolveNote is what the report says beside a group the resolver leaves for
// the operator, when there is something to say.
func resolveNote(g *resolveGroup) string {
	switch g.class {
	case ClassSameRelease:
		if g.tracked() > 1 {
			return "Lidarr tracks every copy as its own track, so removing one would make Lidarr download it again."
		}
	case ClassCrossRelease:
		return "Each copy belongs to its own release, and Lidarr keeps each one. Nothing is removed; they count as one song."
	case ClassMismatch:
		return "Identical audio filed under different titles: one file carries another song's tags."
	}
	return ""
}

func writeResolveVerdicts(ctx context.Context, q *dbq.Queries, groups []*resolveGroup) error {
	classes := dbq.SetDuplicateGroupClassesParams{}
	states := dbq.SetDuplicateMemberLidarrStatesParams{}
	for _, g := range groups {
		classes.GroupIds = append(classes.GroupIds, g.id)
		classes.Classes = append(classes.Classes, string(g.class))
		classes.Notes = append(classes.Notes, g.note)
		for i, m := range g.members {
			states.GroupIds = append(states.GroupIds, g.id)
			states.TrackIds = append(states.TrackIds, m.TrackID)
			states.States = append(states.States, g.states[i])
		}
	}
	if len(classes.GroupIds) == 0 {
		return nil
	}
	if err := q.SetDuplicateGroupClasses(ctx, classes); err != nil {
		return fmt.Errorf("write duplicate classes: %w", err)
	}
	if err := q.SetDuplicateMemberLidarrStates(ctx, states); err != nil {
		return fmt.Errorf("write lidarr states: %w", err)
	}
	return nil
}

// survivorCandidates builds what ProposeSurvivor weighs for a group's copies.
func survivorCandidates(g *resolveGroup) []SurvivorCandidate {
	cands := make([]SurvivorCandidate, len(g.members))
	for i, m := range g.members {
		cands[i] = SurvivorCandidate{
			TrackID:       syncpkg.FormatUUID(m.TrackID),
			FileFormat:    m.FileFormat,
			FileSize:      m.FileSize,
			AddedAt:       m.AddedAt.Time,
			LidarrTracked: g.states[i] == lidarrStateTracked,
			TagFit:        TagFitScore(m.TrackNumber != nil, m.PositionClash, m.FilePath, m.HasMbid),
		}
	}
	return cands
}

// autoMerge merges one group into the copy ProposeSurvivor picks, which is the
// copy Lidarr tracks when there is one. removable is the guard: a copy Lidarr
// maps can never be among those removed, whatever the survivor rule said.
func autoMerge(
	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string, g *resolveGroup, removable RemovableFunc,
) error {
	survivorKey, reason := ProposeSurvivor(survivorCandidates(g))
	var survivorID pgtype.UUID
	if err := survivorID.Scan(survivorKey); err != nil {
		return fmt.Errorf("parse survivor id: %w", err)
	}
	res, err := MergeDuplicateGroupGuarded(ctx, pool, logger, dataDir, g.id, survivorID, removable)
	if err != nil {
		return err
	}
	if err := dbq.New(pool).MarkDuplicateGroupResolvedAutomatically(ctx, g.id); err != nil {
		logger.Warn("duplicate resolve: marking the merge automatic failed", "group_id", syncpkg.FormatUUID(g.id), "err", err)
	}
	removed := make([]map[string]string, 0, len(res.Removed))
	for _, c := range res.Removed {
		removed = append(removed, map[string]string{"track_id": syncpkg.FormatUUID(c.TrackID), "file_path": c.FilePath})
	}
	audit.WriteOrLog(ctx, pool, logger, pgtype.UUID{}, pgtype.UUID{}, audit.ActionDuplicateMerge, map[string]any{
		"automatic":         true,
		"group_id":          syncpkg.FormatUUID(g.id),
		"tier":              res.Tier,
		"class":             string(g.class),
		"survivor_track_id": syncpkg.FormatUUID(res.Survivor.TrackID),
		"survivor_path":     res.Survivor.FilePath,
		"survivor_reason":   reason,
		"removed":           removed,
		"moved": map[string]any{
			"play_events": res.PlayEvents, "skip_events": res.SkipEvents,
			"likes": res.Likes, "playlist_entries": res.PlaylistEntries,
		},
	})
	return nil
}

// recentlyChangedAlbums is the Minstrel albums whose Lidarr release the
// resolver changed within releaseChangeSettle, read from the audit log so a
// restart does not forget them.
func recentlyChangedAlbums(ctx context.Context, q *dbq.Queries, now time.Time) (map[string]bool, error) {
	rows, err := q.ListAuditLogByActions(ctx, dbq.ListAuditLogByActionsParams{
		Actions: []string{string(audit.ActionLidarrReleaseChange)}, SystemOnly: true,
		PageLimit: 200, PageOffset: 0,
	})
	if err != nil {
		return nil, fmt.Errorf("read recent release changes: %w", err)
	}
	out := map[string]bool{}
	for _, r := range rows {
		if now.Sub(r.CreatedAt.Time) > releaseChangeSettle {
			break // newest first
		}
		var meta struct {
			AlbumID string `json:"album_id"`
		}
		if json.Unmarshal(r.Metadata, &meta) == nil && meta.AlbumID != "" {
			out[meta.AlbumID] = true
		}
	}
	return out, nil
}

// repeatingAlbum is a Minstrel album where Lidarr maps more than one copy of a
// song, and the groups that showed it.
type repeatingAlbum struct {
	albumID          pgtype.UUID
	releaseGroupMbid string
	title, artist    string
	groups           []*resolveGroup
}

func changeRepeatingReleases(
	ctx context.Context, q *dbq.Queries, pool *pgxpool.Pool, logger *slog.Logger,
	lid LidarrLibrary, groups []*resolveGroup, settling map[string]bool,
) ([]ReleaseChange, error) {
	albums := map[string]*repeatingAlbum{}
	for _, g := range groups {
		if g.class != ClassSameRelease || g.tracked() < 2 {
			continue
		}
		m := g.members[0]
		key := syncpkg.FormatUUID(m.AlbumID)
		if settling[key] || m.ReleaseGroupMbid == nil || *m.ReleaseGroupMbid == "" {
			continue
		}
		a := albums[key]
		if a == nil {
			a = &repeatingAlbum{albumID: m.AlbumID, releaseGroupMbid: *m.ReleaseGroupMbid, title: m.AlbumTitle, artist: m.ArtistName}
			albums[key] = a
		}
		a.groups = append(a.groups, g)
	}
	keys := make([]string, 0, len(albums))
	for k := range albums {
		keys = append(keys, k)
	}
	sort.Strings(keys)

	var changes []ReleaseChange
	for _, k := range keys {
		if len(changes) >= resolveReleaseCap {
			break
		}
		if ctx.Err() != nil {
			return changes, ctx.Err()
		}
		a := albums[k]
		change, note, err := changeRelease(ctx, q, lid, a)
		if err != nil {
			logger.Warn("duplicate resolve: Lidarr release check failed", "album", a.title, "err", err)
			continue
		}
		if change != nil {
			changes = append(changes, *change)
			audit.WriteOrLog(ctx, pool, logger, pgtype.UUID{}, pgtype.UUID{}, audit.ActionLidarrReleaseChange, map[string]any{
				"automatic":    true,
				"album_id":     k,
				"album_title":  a.title,
				"artist_name":  a.artist,
				"from_release": releaseLabel(change.From),
				"to_release":   releaseLabel(change.To),
			})
		}
		if note != "" {
			if err := setGroupNotes(ctx, q, a.groups, note); err != nil {
				return changes, err
			}
		}
	}
	return changes, nil
}

// changeRelease checks one album's monitored release and, when it lists a
// song twice, moves Lidarr to the release that lists each song once and best
// covers what is on disk. It returns the change made (nil when none) and the
// note for the album's groups.
func changeRelease(
	ctx context.Context, q *dbq.Queries, lid LidarrLibrary, a *repeatingAlbum,
) (*ReleaseChange, string, error) {
	la, err := lid.LookupAlbumByMBID(ctx, a.releaseGroupMbid)
	if errors.Is(err, lidarr.ErrNotFound) {
		return nil, "Lidarr does not hold this album, so its copies are left for you.", nil
	}
	if err != nil {
		return nil, "", err
	}
	current, err := lid.ListAlbumTracks(ctx, la.ID)
	if err != nil {
		return nil, "", err
	}
	if !releaseRepeats(current) {
		return nil, "Lidarr maps each copy to a different track of this album, so none can go without a download.", nil
	}
	releases, err := lid.GetAlbumReleases(ctx, la.ID)
	if err != nil {
		return nil, "", err
	}
	titles, err := q.ListAlbumPresentTrackTitles(ctx, a.albumID)
	if err != nil {
		return nil, "", fmt.Errorf("list album titles: %w", err)
	}
	onDisk := map[string]bool{}
	for _, t := range titles {
		if k := ReleaseTitleKey(t); k != "" {
			onDisk[k] = true
		}
	}

	var from lidarr.AlbumRelease
	var candidates []releaseCandidate
	for _, r := range releases {
		if r.Monitored {
			from = r
			continue
		}
		tracks, err := lid.ListReleaseTracks(ctx, r.ID)
		if err != nil {
			return nil, "", err
		}
		if len(tracks) == 0 || releaseRepeats(tracks) {
			continue
		}
		candidates = append(candidates, releaseCandidate{release: r, coverage: coverage(tracks, onDisk)})
	}
	best, ok := pickRelease(candidates, coverage(current, onDisk))
	if !ok {
		return nil, "Lidarr's release of this album lists songs twice, and no other release lists each once while keeping what is on disk.", nil
	}
	if err := lid.SetMonitoredRelease(ctx, la.ID, best.ID); err != nil {
		return nil, "", err
	}
	note := fmt.Sprintf("Lidarr now monitors the %s release, which lists each song once. The extra copies are merged once Lidarr has rescanned.", releaseLabel(best))
	return &ReleaseChange{AlbumTitle: a.title, ArtistName: a.artist, From: from, To: best}, note, nil
}

type releaseCandidate struct {
	release  lidarr.AlbumRelease
	coverage int
}

// pickRelease chooses among releases that list each song once: the one
// covering most of the titles on disk, then the fewest tracks (the plainest
// edition that holds them), then a digital one, then the lowest id so the
// choice is stable. It refuses a release covering fewer titles on disk than
// the current one does: the change must not drop a song Lidarr now keeps.
func pickRelease(cands []releaseCandidate, currentCoverage int) (lidarr.AlbumRelease, bool) {
	if len(cands) == 0 {
		return lidarr.AlbumRelease{}, false
	}
	sort.SliceStable(cands, func(i, j int) bool {
		a, b := cands[i], cands[j]
		if a.coverage != b.coverage {
			return a.coverage > b.coverage
		}
		if a.release.TrackCount != b.release.TrackCount {
			return a.release.TrackCount < b.release.TrackCount
		}
		if da, db := isDigital(a.release), isDigital(b.release); da != db {
			return da
		}
		return a.release.ID < b.release.ID
	})
	if cands[0].coverage < currentCoverage {
		return lidarr.AlbumRelease{}, false
	}
	return cands[0].release, true
}

func isDigital(r lidarr.AlbumRelease) bool {
	return strings.Contains(strings.ToLower(r.Format), "digital")
}

// releaseRepeats reports whether a release lists one title more than once.
func releaseRepeats(tracks []lidarr.ReleaseTrack) bool {
	seen := map[string]bool{}
	for _, t := range tracks {
		k := ReleaseTitleKey(t.Title)
		if k == "" {
			continue
		}
		if seen[k] {
			return true
		}
		seen[k] = true
	}
	return false
}

// coverage counts the distinct titles on disk that the release lists.
func coverage(tracks []lidarr.ReleaseTrack, onDisk map[string]bool) int {
	hit := map[string]bool{}
	for _, t := range tracks {
		if k := ReleaseTitleKey(t.Title); onDisk[k] {
			hit[k] = true
		}
	}
	return len(hit)
}

func releaseLabel(r lidarr.AlbumRelease) string {
	label := r.Format
	if label == "" {
		label = r.Title
	}
	if r.Disambiguation != "" {
		label = r.Disambiguation + ", " + label
	}
	return fmt.Sprintf("%s (%d tracks)", label, r.TrackCount)
}

func setGroupNotes(ctx context.Context, q *dbq.Queries, groups []*resolveGroup, note string) error {
	p := dbq.SetDuplicateGroupClassesParams{}
	for _, g := range groups {
		g.note = note
		p.GroupIds = append(p.GroupIds, g.id)
		p.Classes = append(p.Classes, string(g.class))
		p.Notes = append(p.Notes, note)
	}
	if err := q.SetDuplicateGroupClasses(ctx, p); err != nil {
		return fmt.Errorf("write duplicate notes: %w", err)
	}
	return nil
}

func resolvedDetail(merged int, changes []ReleaseChange) string {
	var parts []string
	if merged > 0 {
		parts = append(parts, fmt.Sprintf("Merged %d %s Lidarr does not need.", merged, pluralWord(merged, "copy", "copies")))
	}
	switch len(changes) {
	case 0:
	case 1:
		c := changes[0]
		parts = append(parts, fmt.Sprintf("Lidarr now monitors the %s release of %s by %s, which lists each song once.",
			releaseLabel(c.To), c.AlbumTitle, c.ArtistName))
	default:
		parts = append(parts, fmt.Sprintf("Lidarr now monitors a release that lists each song once for %d albums.", len(changes)))
	}
	return strings.Join(parts, " ")
}

func pluralWord(n int, one, many string) string {
	if n == 1 {
		return one
	}
	return many
}

// DuplicateResolveWorker runs a resolver pass every hour.
type DuplicateResolveWorker struct {
	pool     *pgxpool.Pool
	logger   *slog.Logger
	dataDir  string
	settings *FingerprintSettingsService
	lidarr   func() LidarrLibrary
	tick     time.Duration
}

// NewDuplicateResolveWorker builds a worker with the production cadence.
// lidarrFn is asked each pass, so a Lidarr setting saved in admin takes effect
// without a restart; it returns nil while Lidarr is disabled.
func NewDuplicateResolveWorker(
	pool *pgxpool.Pool, logger *slog.Logger, dataDir string,
	settings *FingerprintSettingsService, lidarrFn func() LidarrLibrary,
) *DuplicateResolveWorker {
	return &DuplicateResolveWorker{
		pool: pool, logger: logger, dataDir: dataDir, settings: settings, lidarr: lidarrFn, tick: duplicateResolveTick,
	}
}

// Run blocks until ctx is cancelled, running a pass at start and then each tick.
func (w *DuplicateResolveWorker) Run(ctx context.Context) {
	w.tickOnce(ctx)
	t := time.NewTicker(w.tick)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			w.tickOnce(ctx)
		}
	}
}

// tickOnce contains one pass so nothing it does can stop the next tick (rule 157).
func (w *DuplicateResolveWorker) tickOnce(ctx context.Context) {
	defer func() {
		if r := recover(); r != nil {
			w.logger.Error("duplicate resolve: tick panicked", "panic", r)
		}
	}()
	// A sweep in flight is rewriting the groups; the next tick sees its result.
	_, err := dbq.New(w.pool).GetInFlightDuplicateSweep(ctx)
	switch {
	case err == nil:
		return
	case !errors.Is(err, pgx.ErrNoRows):
		if ctx.Err() == nil {
			w.logger.Warn("duplicate resolve: sweep check failed", "err", err)
		}
		return
	}
	var lid LidarrLibrary
	if w.lidarr != nil {
		lid = w.lidarr()
	}
	res, err := ResolveDuplicates(ctx, w.pool, w.logger, w.dataDir, lid, w.settings.Get().AutoResolve)
	if err != nil && ctx.Err() == nil {
		w.logger.Warn("duplicate resolve: pass failed", "err", err)
	}
	w.logger.Info("duplicate resolve complete",
		"groups", res.Groups, "lidarr", res.LidarrConsulted, "merged", res.Merged,
		"merge_failed", res.MergeFailed, "release_changes", len(res.ReleaseChanges), "songs_linked", res.SongsLinked,
		"held_back", res.HeldBack, "released", res.Released, "rip_searches", res.RipSearches)
}