// Package similarity owns the inbound ListenBrainz similarity ingest
// pipeline. A periodic worker queries the LB Labs API
// (labs.api.listenbrainz.org similar-recordings / similar-artists) for
// tracks the user has played.
//
// For recordings it keeps LB's whole answer in listenbrainz_similar_recordings
// and derives the track_similarity edges from it in SQL, against whatever is
// in the library at the time (#5296). Matching once at fetch time missed every
// recording that reached the library afterwards, through Lidarr or an MBID the
// AcoustID lookup filled in. For artists it stores the in-library matches in
// artist_similarity and the rest in artist_similarity_unmatched, as before.
//
// Every answer is recorded as a fetch, an empty one included, and the queue
// reads those records: freshness used to be read from the edges, so a seed
// that matched nothing was never fresh and held the head of the queue.
package similarity

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

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

	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	"git.fabledsword.com/bvandeusen/minstrel/internal/scrobble/listenbrainz"
)

// Worker drains played tracks and artists needing similarity. A transient
// failure is not recorded as a fetch, so the timer retries it next tick.
type Worker struct {
	pool   *pgxpool.Pool
	client *listenbrainz.Client
	logger *slog.Logger
	tick   time.Duration
	batch  int32
	topK   int // per artist, for each of the matched and unmatched stores
}

// NewWorker constructs a worker with production defaults: 1h tick,
// batch=25, topK=20. batch is tracks AND artists processed per tick, well
// under ListenBrainz rate limits (429s abort the tick). With never-fetched
// seeds first, a freshly played library is covered in days.
func NewWorker(pool *pgxpool.Pool, client *listenbrainz.Client, logger *slog.Logger) *Worker {
	return &Worker{
		pool:   pool,
		client: client,
		logger: logger,
		tick:   1 * time.Hour,
		batch:  25,
		topK:   20,
	}
}

// Run blocks until ctx is cancelled, ticking every w.tick.
func (w *Worker) Run(ctx context.Context) {
	t := time.NewTicker(w.tick)
	defer t.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-t.C:
			if err := w.tickOnce(ctx); err != nil {
				w.logger.Error("similarity: tick failed", "err", err)
			}
		}
	}
}

// tickOnce drains one batch of tracks and one batch of artists, then
// re-resolves every cached answer against the library. Per-row errors are
// logged and skipped. 429 aborts the fetching, not the resolve.
func (w *Worker) tickOnce(ctx context.Context) error {
	q := dbq.New(w.pool)
	if err := w.tickTracks(ctx, q); err != nil {
		return err
	}
	if err := w.tickArtists(ctx, q); err != nil {
		return err
	}
	// Links recordings that reached the library since their seed was fetched:
	// a scan, a Lidarr import, an MBID from the AcoustID lookup.
	if err := q.ResolveListenBrainzTrackEdges(ctx, nil); err != nil {
		return fmt.Errorf("resolve similarity edges: %w", err)
	}
	return nil
}

func (w *Worker) tickTracks(ctx context.Context, q *dbq.Queries) error {
	rows, err := q.ListPlayedTracksNeedingSimilarity(ctx, w.batch)
	if err != nil {
		return err
	}
	for _, r := range rows {
		if r.Mbid == nil {
			continue // defensive — query already filters NULL
		}
		results, err := w.client.SimilarRecordings(ctx, *r.Mbid, 100)
		switch {
		case err == nil:
		case isRateLimited(err):
			w.logger.Warn("similarity: 429 — aborting tick")
			return nil
		case errors.Is(err, listenbrainz.ErrPermanent):
			// LB refused this MBID outright; asking again next hour won't
			// change that. Recorded as an empty answer so it waits its turn.
			w.logger.Warn("similarity: similar-recordings refused", "track_id", r.ID, "err", err)
			results = nil
		default:
			w.logger.Warn("similarity: similar-recordings failed", "track_id", r.ID, "err", err)
			continue
		}
		if err := w.storeTrackSimilar(ctx, r.ID, results); err != nil {
			w.logger.Warn("similarity: store similar recordings", "track_id", r.ID, "err", err)
		}
	}
	return nil
}

func (w *Worker) tickArtists(ctx context.Context, q *dbq.Queries) error {
	rows, err := q.ListPlayedArtistsNeedingSimilarity(ctx, w.batch)
	if err != nil {
		return err
	}
	for _, r := range rows {
		if r.Mbid == nil {
			continue
		}
		results, err := w.client.SimilarArtists(ctx, *r.Mbid, 100)
		switch {
		case err == nil:
		case isRateLimited(err):
			w.logger.Warn("similarity: 429 — aborting tick")
			return nil
		case errors.Is(err, listenbrainz.ErrPermanent):
			w.logger.Warn("similarity: similar-artists refused", "artist_id", r.ID, "err", err)
			results = nil
		default:
			w.logger.Warn("similarity: similar-artists failed", "artist_id", r.ID, "err", err)
			continue
		}
		w.upsertArtistSimilar(ctx, q, r.ID, results)
		if err := q.RecordArtistSimilarityFetch(ctx, dbq.RecordArtistSimilarityFetchParams{
			ArtistID: r.ID, Returned: int32(len(results)),
		}); err != nil {
			w.logger.Warn("similarity: record artist fetch", "artist_id", r.ID, "err", err)
		}
	}
	return nil
}

func isRateLimited(err error) bool {
	var ra *listenbrainz.RetryAfterError
	return errors.As(err, &ra)
}

// storeTrackSimilar replaces the seed's cached answer with this one, records
// the fetch, and resolves the seed's edges, in one transaction so the queue
// never sees a fetch whose answer is not stored.
func (w *Worker) storeTrackSimilar(ctx context.Context, seedID pgtype.UUID, results []listenbrainz.SimilarRecording) error {
	mbids := make([]string, 0, len(results))
	scores := make([]float64, 0, len(results))
	for _, r := range results {
		mbids = append(mbids, r.MBID)
		scores = append(scores, r.Score)
	}
	return pgx.BeginFunc(ctx, w.pool, func(tx pgx.Tx) error {
		q := dbq.New(tx)
		if err := q.DeleteSimilarRecordingsForSeed(ctx, seedID); err != nil {
			return fmt.Errorf("clear cached answer: %w", err)
		}
		if len(mbids) > 0 {
			if err := q.InsertSimilarRecordings(ctx, dbq.InsertSimilarRecordingsParams{
				SeedTrackID: seedID, Mbids: mbids, Scores: scores,
			}); err != nil {
				return fmt.Errorf("cache answer: %w", err)
			}
		}
		if err := q.RecordTrackSimilarityFetch(ctx, dbq.RecordTrackSimilarityFetchParams{
			TrackID: seedID, Returned: int32(len(results)),
		}); err != nil {
			return fmt.Errorf("record fetch: %w", err)
		}
		if err := q.ResolveListenBrainzTrackEdges(ctx, []pgtype.UUID{seedID}); err != nil {
			return fmt.Errorf("resolve edges: %w", err)
		}
		return nil
	})
}

func (w *Worker) upsertArtistSimilar(ctx context.Context, q *dbq.Queries, artistAID pgtype.UUID, results []listenbrainz.SimilarArtist) {
	if len(results) == 0 {
		return
	}
	sort.Slice(results, func(i, j int) bool { return results[i].Score > results[j].Score })

	mbids := make([]string, 0, len(results))
	for _, r := range results {
		mbids = append(mbids, r.MBID)
	}
	rows, err := q.GetArtistsByMBIDs(ctx, mbids)
	if err != nil {
		w.logger.Warn("similarity: GetArtistsByMBIDs", "err", err)
		return
	}
	idByMBID := make(map[string]pgtype.UUID, len(rows))
	for _, r := range rows {
		if r.Mbid != nil {
			idByMBID[*r.Mbid] = r.ID
		}
	}

	// Matched: in-library similars → artist_similarity (existing path).
	takenMatched := 0
	for _, r := range results {
		if takenMatched >= w.topK {
			break
		}
		localID, ok := idByMBID[r.MBID]
		if !ok {
			continue
		}
		if localID == artistAID {
			continue
		}
		if uerr := q.UpsertArtistSimilarity(ctx, dbq.UpsertArtistSimilarityParams{
			ArtistAID: artistAID, ArtistBID: localID, Score: r.Score,
		}); uerr != nil {
			w.logger.Warn("similarity: UpsertArtistSimilarity", "err", uerr)
			continue
		}
		takenMatched++
	}

	// Unmatched: out-of-library similars → artist_similarity_unmatched (M5c).
	// Same top-K cap as the matched path. Skip rows missing a name — we can't
	// render a suggestion card without one.
	takenUnmatched := 0
	for _, r := range results {
		if takenUnmatched >= w.topK {
			break
		}
		if _, inLib := idByMBID[r.MBID]; inLib {
			continue
		}
		if r.Name == "" {
			w.logger.Debug("similarity: skipping unmatched similar with empty name", "mbid", r.MBID)
			continue
		}
		if uerr := q.UpsertArtistSimilarityUnmatched(ctx, dbq.UpsertArtistSimilarityUnmatchedParams{
			SeedArtistID:  artistAID,
			CandidateMbid: r.MBID,
			CandidateName: r.Name,
			Score:         r.Score,
			Source:        "listenbrainz",
		}); uerr != nil {
			w.logger.Warn("similarity: UpsertArtistSimilarityUnmatched", "err", uerr)
			continue
		}
		takenUnmatched++
	}
}