package library

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"sort"

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

	"git.fabledsword.com/bvandeusen/minstrel/internal/coverart"
	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
)

// Duplicate merge (M400 #3911).

// ErrDuplicateGroupNotPending means the group was already merged or dismissed,
// no longer exists, or no longer has two members to merge.
var ErrDuplicateGroupNotPending = errors.New("library: duplicate group is not pending")

// ErrSurvivorNotInGroup means the copy chosen to keep is not a member of the group.
var ErrSurvivorNotInGroup = errors.New("library: survivor is not a member of the group")

// MergedCopy is one copy a merge kept or removed.
type MergedCopy struct {
	TrackID   pgtype.UUID
	FilePath  string
	TrackMbid *string
	AlbumMbid *string
}

// MergeResult says what a merge did.
type MergeResult struct {
	Tier     string
	Survivor MergedCopy
	Removed  []MergedCopy

	// What moved onto the survivor — reported so the operator, and the audit
	// log, can see that the history was kept rather than take it on trust.
	PlayEvents      int64
	SkipEvents      int64
	Likes           int // users whose like now sits on the survivor
	PlaylistEntries int

	DeletedAlbumIDs  []pgtype.UUID
	DeletedArtistIDs []pgtype.UUID
}

// MergeDuplicateGroup keeps one copy of a duplicate group and removes the rest,
// carrying everything the removed copies held onto the one kept.
//
// survivorID chooses the copy to keep; an invalid (zero) id takes the proposal
// from ProposeSurvivor.
//
// The danger this is built around: every table referencing tracks does so ON
// DELETE CASCADE, so deleting a duplicate's row outright silently destroys its
// likes, plays, playlist entries and tags. The merge moves all of that onto the
// survivor first, and only then deletes the now-empty row.
//
// It deletes the removed copies' FILES too, and first, before any row changes
// (#3918, note #3926). A merge that left the file behind would be undone by the
// next scan, which re-imports it as a new track with no history. If a file cannot
// be removed, the *FileRemoveError comes back and nothing in the database changes.
// With several copies to remove, one file may already be gone when a later one
// fails; that copy's row keeps all its history and is marked missing by the next
// scan, and retrying the merge picks up where it stopped.
//
// Everything else happens in one transaction, which holds a lock on the group so
// two merges of it cannot run at once. Sync changes for clients' caches are logged
// inside it, the way the playlists service logs its own.
func MergeDuplicateGroup(
	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, dataDir string,
	groupID, survivorID pgtype.UUID,
) (MergeResult, error) {
	if logger == nil {
		logger = slog.Default()
	}
	tx, err := pool.Begin(ctx)
	if err != nil {
		return MergeResult{}, fmt.Errorf("begin merge: %w", err)
	}
	defer func() { _ = tx.Rollback(ctx) }()
	tq := dbq.New(tx)

	group, err := tq.LockDuplicateGroupForMerge(ctx, groupID)
	if errors.Is(err, pgx.ErrNoRows) {
		return MergeResult{}, ErrDuplicateGroupNotPending
	}
	if err != nil {
		return MergeResult{}, fmt.Errorf("lock duplicate group: %w", err)
	}
	if group.Status != "pending" {
		return MergeResult{}, ErrDuplicateGroupNotPending
	}

	members, err := tq.ListDuplicateGroupMergeMembers(ctx, groupID)
	if err != nil {
		return MergeResult{}, fmt.Errorf("list group members: %w", err)
	}
	if len(members) < 2 {
		return MergeResult{}, ErrDuplicateGroupNotPending
	}
	survivor, losers, err := splitSurvivor(members, survivorID)
	if err != nil {
		return MergeResult{}, err
	}

	for _, l := range losers {
		if err := removeTrackFileOnDisk(l.FilePath); err != nil {
			return MergeResult{}, err
		}
	}

	res := MergeResult{Tier: group.Tier, Survivor: mergedCopyOf(survivor)}
	likers := map[string]struct{}{}
	changes := mergeChanges{}
	survivorKey := syncpkg.FormatUUID(survivor.ID)

	for _, l := range losers {
		ids := struct{ survivor, loser pgtype.UUID }{survivor.ID, l.ID}
		loserKey := syncpkg.FormatUUID(l.ID)

		n, err := tq.MergeRepointPlayEvents(ctx, dbq.MergeRepointPlayEventsParams{SurvivorID: ids.survivor, LoserID: ids.loser})
		if err != nil {
			return MergeResult{}, fmt.Errorf("move play events: %w", err)
		}
		res.PlayEvents += n
		n, err = tq.MergeRepointSkipEvents(ctx, dbq.MergeRepointSkipEventsParams{SurvivorID: ids.survivor, LoserID: ids.loser})
		if err != nil {
			return MergeResult{}, fmt.Errorf("move skip events: %w", err)
		}
		res.SkipEvents += n
		if _, err := tq.MergeRepointContextualLikes(ctx, dbq.MergeRepointContextualLikesParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
			return MergeResult{}, fmt.Errorf("move contextual likes: %w", err)
		}
		if _, err := tq.MergeRepointPlaybackErrors(ctx, dbq.MergeRepointPlaybackErrorsParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
			return MergeResult{}, fmt.Errorf("move playback errors: %w", err)
		}
		if _, err := tq.MergeRepointLidarrRequests(ctx, dbq.MergeRepointLidarrRequestsParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
			return MergeResult{}, fmt.Errorf("move lidarr request matches: %w", err)
		}

		playlists, err := tq.MergeRepointPlaylistTracks(ctx, dbq.MergeRepointPlaylistTracksParams{SurvivorID: ids.survivor, LoserID: ids.loser})
		if err != nil {
			return MergeResult{}, fmt.Errorf("move playlist entries: %w", err)
		}
		res.PlaylistEntries += len(playlists)
		for _, pl := range playlists {
			plKey := syncpkg.FormatUUID(pl)
			changes.playlistDelete(syncpkg.EncodePlaylistTrackID(plKey, loserKey))
			changes.playlistUpsert(syncpkg.EncodePlaylistTrackID(plKey, survivorKey))
		}

		users, err := tq.MergeCopyGeneralLikes(ctx, dbq.MergeCopyGeneralLikesParams{SurvivorID: ids.survivor, LoserID: ids.loser})
		if err != nil {
			return MergeResult{}, fmt.Errorf("move likes: %w", err)
		}
		for _, u := range users {
			userKey := syncpkg.FormatUUID(u)
			likers[userKey] = struct{}{}
			changes.likeDelete(syncpkg.EncodeLikeID(userKey, loserKey))
			changes.likeUpsert(syncpkg.EncodeLikeID(userKey, survivorKey))
		}

		if _, err := tq.MergeCopyTrackTags(ctx, dbq.MergeCopyTrackTagsParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
			return MergeResult{}, fmt.Errorf("merge tags: %w", err)
		}
		if _, err := tq.MergeCopyTrackSimilarity(ctx, dbq.MergeCopyTrackSimilarityParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
			return MergeResult{}, fmt.Errorf("merge similarity: %w", err)
		}
		if err := tq.MergeInheritTrackMbid(ctx, dbq.MergeInheritTrackMbidParams{SurvivorID: ids.survivor, LoserID: ids.loser}); err != nil {
			return MergeResult{}, fmt.Errorf("inherit recording mbid: %w", err)
		}

		// Everything the loser carried now sits on the survivor, so the CASCADE
		// this delete sets off has nothing left to destroy.
		deleted, err := tq.DeleteTrack(ctx, l.ID)
		if err != nil {
			return MergeResult{}, fmt.Errorf("delete merged copy: %w", err)
		}
		tidied, err := tidyEmptiedAlbum(ctx, tq, deleted.AlbumID)
		if err != nil {
			return MergeResult{}, err
		}
		if tidied.AlbumID != nil {
			res.DeletedAlbumIDs = append(res.DeletedAlbumIDs, *tidied.AlbumID)
		}
		if tidied.ArtistID != nil {
			res.DeletedArtistIDs = append(res.DeletedArtistIDs, *tidied.ArtistID)
		}
		res.Removed = append(res.Removed, mergedCopyOf(l))
		changes.trackDelete(loserKey)
	}
	res.Likes = len(likers)

	marked, err := tq.MarkDuplicateGroupMerged(ctx, groupID)
	if err != nil {
		return MergeResult{}, fmt.Errorf("mark group merged: %w", err)
	}
	if marked != 1 {
		return MergeResult{}, ErrDuplicateGroupNotPending
	}
	if err := changes.log(ctx, tx); err != nil {
		return MergeResult{}, err
	}
	if err := tx.Commit(ctx); err != nil {
		return MergeResult{}, fmt.Errorf("commit merge: %w", err)
	}

	// After commit, like DeleteTrackFile: a leftover art directory is only disk.
	if dataDir != "" {
		for _, artistID := range res.DeletedArtistIDs {
			if err := coverart.CleanupArtistArt(dataDir, artistID); err != nil {
				logger.Warn("duplicate merge: artist-art cleanup failed",
					"artist_id", syncpkg.FormatUUID(artistID), "err", err)
			}
		}
	}
	return res, nil
}

// splitSurvivor separates the copy to keep from the copies to remove. An
// invalid survivorID takes ProposeSurvivor's choice.
func splitSurvivor(
	members []dbq.ListDuplicateGroupMergeMembersRow, survivorID pgtype.UUID,
) (dbq.ListDuplicateGroupMergeMembersRow, []dbq.ListDuplicateGroupMergeMembersRow, error) {
	want := ""
	if survivorID.Valid {
		want = syncpkg.FormatUUID(survivorID)
	} else {
		cands := make([]SurvivorCandidate, len(members))
		for i, m := range members {
			cands[i] = SurvivorCandidate{
				TrackID: syncpkg.FormatUUID(m.ID), FileFormat: m.FileFormat, FileSize: m.FileSize, AddedAt: m.AddedAt.Time,
			}
		}
		want, _ = ProposeSurvivor(cands)
	}

	var survivor dbq.ListDuplicateGroupMergeMembersRow
	found := false
	var losers []dbq.ListDuplicateGroupMergeMembersRow
	for _, m := range members {
		if syncpkg.FormatUUID(m.ID) == want {
			survivor, found = m, true
			continue
		}
		losers = append(losers, m)
	}
	if !found {
		return dbq.ListDuplicateGroupMergeMembersRow{}, nil, ErrSurvivorNotInGroup
	}
	return survivor, losers, nil
}

func mergedCopyOf(m dbq.ListDuplicateGroupMergeMembersRow) MergedCopy {
	return MergedCopy{TrackID: m.ID, FilePath: m.FilePath, TrackMbid: m.Mbid, AlbumMbid: m.AlbumMbid}
}

// mergeChanges collects the sync-log entries a merge owes clients' caches, each
// once: a user who liked two removed copies still gets one upsert for the
// survivor.
type mergeChanges struct {
	tracks, likeDeletes, likeUpserts, playlistDeletes, playlistUpserts map[string]struct{}
}

func addTo(set *map[string]struct{}, id string) {
	if *set == nil {
		*set = map[string]struct{}{}
	}
	(*set)[id] = struct{}{}
}

func (c *mergeChanges) trackDelete(id string)    { addTo(&c.tracks, id) }
func (c *mergeChanges) likeDelete(id string)     { addTo(&c.likeDeletes, id) }
func (c *mergeChanges) likeUpsert(id string)     { addTo(&c.likeUpserts, id) }
func (c *mergeChanges) playlistDelete(id string) { addTo(&c.playlistDeletes, id) }
func (c *mergeChanges) playlistUpsert(id string) { addTo(&c.playlistUpserts, id) }

func (c *mergeChanges) log(ctx context.Context, tx pgx.Tx) error {
	for _, entry := range []struct {
		kind syncpkg.EntityType
		ids  map[string]struct{}
		op   syncpkg.Op
	}{
		{syncpkg.EntityTrack, c.tracks, syncpkg.OpDelete},
		{syncpkg.EntityLikeTrack, c.likeDeletes, syncpkg.OpDelete},
		{syncpkg.EntityLikeTrack, c.likeUpserts, syncpkg.OpUpsert},
		{syncpkg.EntityPlaylistTrack, c.playlistDeletes, syncpkg.OpDelete},
		{syncpkg.EntityPlaylistTrack, c.playlistUpserts, syncpkg.OpUpsert},
	} {
		if len(entry.ids) == 0 {
			continue
		}
		ids := make([]string, 0, len(entry.ids))
		for id := range entry.ids {
			ids = append(ids, id)
		}
		sort.Strings(ids)
		if err := syncpkg.LogChanges(ctx, tx, entry.kind, ids, entry.op); err != nil {
			return fmt.Errorf("log merge changes: %w", err)
		}
	}
	return nil
}