minstrel/internal/playlists/system.go

// Package playlists' system.go implements the system-generated mix
// builder (M7 #352 slice 2). The cron loop and lazy fallback both call
// BuildSystemPlaylists; the helpers in this file (pickSeedArtists,
// pickRepresentativeTrack, tieBreakHash) are the pure parts.
package playlists

import (
	"context"
	"crypto/sha256"
	"encoding/binary"
	"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/recommendation"
)

// seedArtistRow mirrors the sqlc-generated PickSeedArtistsRow shape.
// Defined locally so unit tests don't need a real DB.
type seedArtistRow struct {
	ArtistID pgtype.UUID
	Score    int64
}

// pickSeedArtistsFromRows projects sqlc rows into the seed list. The
// SQL already orders by score DESC + artist_id and LIMIT 3, so this is
// just a column projection — but pulling it into a function keeps the
// call-site readable and makes the post-fetch path testable without
// a database.
func pickSeedArtistsFromRows(rows []seedArtistRow) []pgtype.UUID {
	out := make([]pgtype.UUID, 0, len(rows))
	for _, r := range rows {
		out = append(out, r.ArtistID)
	}
	return out
}

// tieBreakHash returns a deterministic 64-bit hash of (track_id, date_str).
// Used to break score ties and to drive the For-You tail sample's
// daily-deterministic ordering. Same inputs always produce the same
// output; different inputs almost always differ.
//
// Switched from FNV-1a (May 2026) because two date strings differing
// only in the last character (e.g. "2026-05-07" vs "2026-05-08") gave
// hash values whose RELATIVE ORDERING across our small candidate pools
// was identical — the dateStr-driven divergence concentrated in low
// bits that lost out to high-bit ordering during sort. SHA-256
// truncated to 8 bytes has full avalanche, so any single-byte input
// change roughly half-flips the output bits and reorders meaningfully.
// The hash isn't security-load-bearing; we just want strong avalanche
// for tiny input deltas.
func tieBreakHash(trackID pgtype.UUID, dateStr string) uint64 {
	h := sha256.New()
	if trackID.Valid {
		_, _ = h.Write(trackID.Bytes[:])
	}
	_, _ = h.Write([]byte(dateStr))
	sum := h.Sum(nil)
	return binary.BigEndian.Uint64(sum[:8])
}

// rankedCandidate is a (track_id, score) pair used during in-memory
// sorting before insert into playlist_tracks. T5 fills these from
// recommendation.Candidate scores.
type rankedCandidate struct {
	TrackID pgtype.UUID
	Score   float64
}

const systemMixLength = 25

// systemMixWeights are the fixed scoring weights used by the cron worker.
// JitterMagnitude is 0 because daily determinism comes from tieBreakHash;
// any randomness would defeat the within-day stability invariant.
var systemMixWeights = recommendation.ScoringWeights{
	BaseWeight:       1.0,
	LikeBoost:        2.0,
	RecencyWeight:    1.0,
	SkipPenalty:      2.0,
	JitterMagnitude:  0.0,
	ContextWeight:    0.5,
	SimilarityWeight: 1.5,
}

// noopRNG returns 0 for any call. Combined with JitterMagnitude=0 it makes
// recommendation.Score fully deterministic.
func noopRNG() float64 { return 0 }

// forYouHeadN is the number of top-scored tracks that anchor the For-You
// playlist. forYouTailN is the number of diversity picks sampled from the
// tail of the score-sorted pool (positions 2*forYouHeadN onward), injected
// after the head to give users a daily-deterministic surprise without
// compromising quality. Songs-like-X keeps simple pickTopN (the seed-artist
// context already frames the "you'll like this" promise).
const (
	forYouHeadN = 20
	forYouTailN = 5
)

// scoreAndSortCandidates scores every candidate with recommendation.Score
// and returns a new slice sorted by score DESC (ties broken by
// tieBreakHash). Pure — no truncation, no cap.
func scoreAndSortCandidates(cands []recommendation.Candidate, dateStr string, now time.Time) []recommendation.Candidate {
	type scored struct {
		c     recommendation.Candidate
		score float64
	}
	pairs := make([]scored, len(cands))
	for i, c := range cands {
		pairs[i] = scored{c: c, score: recommendation.Score(c.Inputs, systemMixWeights, now, noopRNG)}
	}
	sort.SliceStable(pairs, func(i, j int) bool {
		if pairs[i].score != pairs[j].score {
			return pairs[i].score > pairs[j].score
		}
		return tieBreakHash(pairs[i].c.Track.ID, dateStr) < tieBreakHash(pairs[j].c.Track.ID, dateStr)
	})
	out := make([]recommendation.Candidate, len(pairs))
	for i, p := range pairs {
		out[i] = p.c
	}
	return out
}

// capCandidatesByAlbumAndArtist trims a candidate list so no single
// album appears more than discoverMaxTracksPerAlbum times and no
// single artist appears more than discoverMaxTracksPerArtist times.
// Mirrors capByAlbumAndArtist (which operates on []discoverTrack);
// here the input/output is []recommendation.Candidate so For-You and
// Songs-like-X can apply the same diversity caps as Discover.
//
// Preserves input order. Reuses the discoverMax* constants —
// playlists across all three system variants get consistent
// diversity behavior.
func capCandidatesByAlbumAndArtist(cands []recommendation.Candidate) []recommendation.Candidate {
	albumCount := map[pgtype.UUID]int{}
	artistCount := map[pgtype.UUID]int{}
	out := make([]recommendation.Candidate, 0, len(cands))
	for _, c := range cands {
		if albumCount[c.Track.AlbumID] >= discoverMaxTracksPerAlbum {
			continue
		}
		if artistCount[c.Track.ArtistID] >= discoverMaxTracksPerArtist {
			continue
		}
		albumCount[c.Track.AlbumID]++
		artistCount[c.Track.ArtistID]++
		out = append(out, c)
	}
	return out
}

// BuildSystemPlaylists builds the user's daily system mixes (one For-You +
// up to 3 Songs-like-{seed} mixes). Atomic-replace inside one tx;
// concurrency-guarded via system_playlist_runs.in_flight; deterministic
// within a day via tieBreakHash(track_id, now.UTC().Format("2006-01-02")).
//
// Callable from both the cron loop (system_cron.go) and the lazy fallback.
// Returns nil when the user has no library activity to build from. Errors
// are captured in system_playlist_runs.last_error before returning.
func BuildSystemPlaylists(ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger, userID pgtype.UUID, now time.Time, dataDir string) error {
	q := dbq.New(pool)
	dateStr := now.UTC().Format("2006-01-02")
	dateOnly := pgtype.Date{Time: now.UTC(), Valid: true}

	// Concurrency guard: try to claim the run row.
	if _, err := q.TryClaimSystemPlaylistRun(ctx, userID); err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			logger.Debug("system playlist build: another run is in flight",
				"user_id", uuidStringPL(userID))
			return nil
		}
		return fmt.Errorf("claim run: %w", err)
	}

	// On any return path, mark the run finished. Failure capture is
	// best-effort; success is updated in the happy path.
	var buildErr error
	defer func() {
		if buildErr == nil {
			if err := q.FinishSystemPlaylistRun(ctx, dbq.FinishSystemPlaylistRunParams{
				UserID:      userID,
				LastRunAt:   pgtype.Timestamptz{Time: now.UTC(), Valid: true},
				LastRunDate: dateOnly,
			}); err != nil {
				logger.Warn("system playlist: failed to mark run finished; in_flight may be stuck",
					"user_id", uuidStringPL(userID), "err", err)
			}
		} else {
			errStr := buildErr.Error()
			if err := q.FailSystemPlaylistRun(ctx, dbq.FailSystemPlaylistRunParams{
				UserID:    userID,
				LastError: &errStr,
			}); err != nil {
				logger.Warn("system playlist: failed to record build failure",
					"user_id", uuidStringPL(userID), "err", err)
			}
		}
	}()

	// 1. For-You: synth seed from top recently-played track, pull candidates.
	forYouSeed, err := q.PickTopPlayedTrackForUser(ctx, userID)
	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
		buildErr = fmt.Errorf("pick for-you seed: %w", err)
		return buildErr
	}
	var forYouTracks []rankedCandidate
	if err == nil && forYouSeed.Valid {
		zeroVec := recommendation.SessionVector{Seed: true}
		cands, cerr := recommendation.LoadCandidatesFromSimilarity(
			ctx, q, userID, forYouSeed,
			1, // recentlyPlayedHours — small to avoid filtering the seed's recent neighbourhood
			zeroVec,
			[]pgtype.UUID{forYouSeed},
			recommendation.DefaultCandidateSourceLimits(),
		)
		if cerr == nil {
			// For-You uses head+tail composition: forYouHeadN top-similarity
			// tracks + forYouTailN tail-sampled to inject daily-deterministic
			// surprise. Songs-like-X keeps pickTopN (top-25 with caps) since
			// the seed-artist context already provides the "you'll like this"
			// framing.
			forYouTracks = pickHeadAndTail(cands, dateStr, now, forYouHeadN, forYouTailN)
		} else {
			logger.Warn("system playlist: for-you candidates load failed; skipping",
				"user_id", uuidStringPL(userID), "err", cerr)
		}
	}

	// 2. Seed artists for "Songs like {X}".
	seedRows, err := q.PickSeedArtists(ctx, userID)
	if err != nil {
		buildErr = fmt.Errorf("pick seed artists: %w", err)
		return buildErr
	}
	seedRowsLocal := make([]seedArtistRow, 0, len(seedRows))
	for _, r := range seedRows {
		seedRowsLocal = append(seedRowsLocal, seedArtistRow{
			ArtistID: r.ArtistID, Score: r.Score,
		})
	}
	seeds := pickSeedArtistsFromRows(seedRowsLocal)

	type seedMix struct {
		ArtistID   pgtype.UUID
		ArtistName string
		Tracks     []rankedCandidate
	}
	seedMixes := make([]seedMix, 0, len(seeds))
	for _, artistID := range seeds {
		artistRow, aerr := q.GetArtistByID(ctx, artistID)
		if aerr != nil {
			logger.Warn("system playlist: seed artist load failed; skipping",
				"artist_id", uuidStringPL(artistID), "err", aerr)
			continue
		}
		seedTrack, terr := q.PickTopPlayedTrackForArtistByUser(ctx, dbq.PickTopPlayedTrackForArtistByUserParams{
			UserID: userID, ArtistID: artistID,
		})
		if terr != nil || !seedTrack.Valid {
			continue
		}
		zeroVec := recommendation.SessionVector{Seed: true}
		cands, cerr := recommendation.LoadCandidatesFromSimilarity(
			ctx, q, userID, seedTrack, 1, zeroVec, []pgtype.UUID{seedTrack},
			recommendation.DefaultCandidateSourceLimits(),
		)
		if cerr != nil {
			logger.Warn("system playlist: seed candidates load failed; skipping",
				"artist_id", uuidStringPL(artistID), "err", cerr)
			continue
		}
		// Keep only tracks NOT by the seed artist (we want "songs like X",
		// not X's own songs).
		filtered := make([]recommendation.Candidate, 0, len(cands))
		for _, c := range cands {
			if !pgtypeUUIDEqual(c.Track.ArtistID, artistID) {
				filtered = append(filtered, c)
			}
		}
		tracks := pickTopN(filtered, dateStr, now, systemMixLength)
		seedMixes = append(seedMixes, seedMix{
			ArtistID:   artistID,
			ArtistName: artistRow.Name,
			Tracks:     tracks,
		})
	}

	// 4. Discover: surface unheard tracks, biased toward dormant artists.
	discoverTracks, derr := buildDiscoverCandidates(ctx, q, userID, dateStr)
	if derr != nil {
		logger.Warn("system playlist: discover candidates load failed; skipping",
			"user_id", uuidStringPL(userID), "err", derr)
		discoverTracks = nil
	}

	// 3. Atomic replace inside a transaction.
	tx, err := pool.Begin(ctx)
	if err != nil {
		buildErr = fmt.Errorf("begin tx: %w", err)
		return buildErr
	}
	defer func() { _ = tx.Rollback(ctx) }()
	qtx := dbq.New(tx)

	if err := qtx.DeleteSystemPlaylistsForUser(ctx, userID); err != nil {
		buildErr = fmt.Errorf("delete old system playlists: %w", err)
		return buildErr
	}

	// Track the playlist IDs we create so we can generate collage covers
	// for each one after the transaction commits. Collage generation
	// reads the playlist's tracks via a separate query that won't see
	// uncommitted rows, so the work happens post-commit.
	createdIDs := make([]pgtype.UUID, 0, 1+len(seedMixes))

	if len(forYouTracks) > 0 {
		id, err := insertSystemPlaylist(ctx, qtx, userID, "For You", "for_you",
			pgtype.UUID{}, forYouTracks)
		if err != nil {
			buildErr = fmt.Errorf("insert for-you: %w", err)
			return buildErr
		}
		createdIDs = append(createdIDs, id)
	}

	for _, m := range seedMixes {
		if len(m.Tracks) == 0 {
			continue
		}
		name := fmt.Sprintf("Songs like %s", m.ArtistName)
		id, err := insertSystemPlaylist(ctx, qtx, userID, name, "songs_like_artist",
			m.ArtistID, m.Tracks)
		if err != nil {
			buildErr = fmt.Errorf("insert songs-like %s: %w", uuidStringPL(m.ArtistID), err)
			return buildErr
		}
		createdIDs = append(createdIDs, id)
	}

	if len(discoverTracks) > 0 {
		id, err := insertSystemPlaylist(ctx, qtx, userID, "Discover", "discover",
			pgtype.UUID{}, discoverTracks)
		if err != nil {
			buildErr = fmt.Errorf("insert discover: %w", err)
			return buildErr
		}
		createdIDs = append(createdIDs, id)
	}

	if err := tx.Commit(ctx); err != nil {
		buildErr = fmt.Errorf("commit tx: %w", err)
		return buildErr
	}

	// Post-commit: generate a 4-cell collage cover for each system
	// playlist from the contributing tracks' album covers. The collage
	// gracefully falls back to the FabledSword glyph in cells whose
	// album lacks cover art, so a partially-covered library still
	// produces a sensible visual rather than an empty placeholder.
	// Errors are non-fatal — the playlist is already committed; a
	// missing collage just leaves cover_path NULL until next build.
	if dataDir != "" {
		for _, plID := range createdIDs {
			if _, cerr := GenerateCollage(ctx, pool, plID, dataDir); cerr != nil {
				logger.Warn("system playlist: collage generation failed",
					"playlist_id", uuidStringPL(plID), "err", cerr)
			}
		}
	}
	return nil
}

// pickTopN sorts candidates by score DESC, applies per-album (<=2) /
// per-artist (<=3) diversity caps matching Discover's behavior, then
// truncates to n. Used by Songs-like-X (and as the fallback inside
// pickHeadAndTail for small pools).
func pickTopN(cands []recommendation.Candidate, dateStr string, now time.Time, n int) []rankedCandidate {
	sorted := scoreAndSortCandidates(cands, dateStr, now)
	capped := capCandidatesByAlbumAndArtist(sorted)
	if len(capped) > n {
		capped = capped[:n]
	}
	out := make([]rankedCandidate, len(capped))
	for i, c := range capped {
		out[i] = rankedCandidate{
			TrackID: c.Track.ID,
			Score:   recommendation.Score(c.Inputs, systemMixWeights, now, noopRNG),
		}
	}
	return out
}

// pickHeadAndTail picks headN from the score-sorted head plus tailN from
// positions 2*headN onward (the tail), with the tail sampled
// daily-deterministically via tieBreakHash. Caps applied before the
// head/tail split. Used by For-You only.
//
// The "tail" — candidates ranked beyond 2*headN — is still similarity-
// related (every candidate passed the similarity filter) but isn't among
// the obvious top hits. Sampling from there gives the user variety they'll
// probably enjoy without resorting to genuinely random unrelated tracks.
//
// Falls back to standard pickTopN behavior when the candidate pool is too
// small to support a meaningful head/tail split (capped pool <=
// headN+tailN, or no candidates at or beyond position 2*headN).
func pickHeadAndTail(cands []recommendation.Candidate, dateStr string, now time.Time, headN, tailN int) []rankedCandidate {
	sorted := scoreAndSortCandidates(cands, dateStr, now)
	capped := capCandidatesByAlbumAndArtist(sorted)

	total := headN + tailN
	if len(capped) <= total {
		// Pool too small for a head/tail split — return up to total entries.
		if len(capped) < total {
			total = len(capped)
		}
		out := make([]rankedCandidate, total)
		for i := 0; i < total; i++ {
			out[i] = rankedCandidate{
				TrackID: capped[i].Track.ID,
				Score:   recommendation.Score(capped[i].Inputs, systemMixWeights, now, noopRNG),
			}
		}
		return out
	}

	head := capped[:headN]

	tailStart := 2 * headN
	if tailStart >= len(capped) {
		tailStart = headN
	}

	// Defensive copy so that sorting the tail pool does not mutate capped.
	tailPool := append([]recommendation.Candidate{}, capped[tailStart:]...)

	// Sort tail pool by tieBreakHash (daily-deterministic), take tailN.
	// Sample is stable across requests within a day but varies across days.
	sort.SliceStable(tailPool, func(i, j int) bool {
		return tieBreakHash(tailPool[i].Track.ID, dateStr) < tieBreakHash(tailPool[j].Track.ID, dateStr)
	})
	tail := tailPool
	if len(tail) > tailN {
		tail = tail[:tailN]
	}

	// Combine: head order preserved (score-sorted), tail in tieBreakHash
	// order. "First similar, then surprise" reads naturally in playback.
	combined := make([]recommendation.Candidate, 0, len(head)+len(tail))
	combined = append(combined, head...)
	combined = append(combined, tail...)

	out := make([]rankedCandidate, len(combined))
	for i, c := range combined {
		out[i] = rankedCandidate{
			TrackID: c.Track.ID,
			Score:   recommendation.Score(c.Inputs, systemMixWeights, now, noopRNG),
		}
	}
	return out
}

// insertSystemPlaylist inserts a kind='system' playlists row plus its
// playlist_tracks via AppendPlaylistTrack (slice 1's existing query, which
// does the snapshot lookup from tracks/albums/artists). Then recomputes
// rollups. Returns the new playlist's ID so the caller can drive
// post-commit collage generation.
//
// Pass seed_artist_id with Valid=false (zero pgtype.UUID) for the For-You
// variant; the CreateSystemPlaylist query persists NULL.
//
// cover_path is always inserted as NULL — the post-commit collage step
// in BuildSystemPlaylists overrides it with a 4-cell composite of the
// contributing tracks' album art.
func insertSystemPlaylist(ctx context.Context, qtx *dbq.Queries, userID pgtype.UUID,
	name, variant string, seedArtistID pgtype.UUID,
	tracks []rankedCandidate) (pgtype.UUID, error) {
	variantStr := variant
	p, err := qtx.CreateSystemPlaylist(ctx, dbq.CreateSystemPlaylistParams{
		UserID:        userID,
		Name:          name,
		SystemVariant: &variantStr,
		SeedArtistID:  seedArtistID,
		CoverPath:     nil,
	})
	if err != nil {
		return pgtype.UUID{}, fmt.Errorf("insert playlist row: %w", err)
	}

	for _, t := range tracks {
		if _, err := qtx.AppendPlaylistTrack(ctx, dbq.AppendPlaylistTrackParams{
			PlaylistID: p.ID,
			TrackID:    t.TrackID,
		}); err != nil {
			// Track may have been deleted between candidate-load and insert;
			// skip silently rather than failing the whole build.
			if errors.Is(err, pgx.ErrNoRows) {
				continue
			}
			return pgtype.UUID{}, fmt.Errorf("append track %s: %w", uuidStringPL(t.TrackID), err)
		}
	}

	if err := qtx.UpdatePlaylistRollups(ctx, p.ID); err != nil {
		return pgtype.UUID{}, fmt.Errorf("update rollups: %w", err)
	}
	return p.ID, nil
}

// uuidStringPL renders a pgtype.UUID as the canonical 8-4-4-4-12 form.
// (pgtype.UUID's String method exists on some pgx versions but not others;
// also the playlists package needs this independent of test helpers.)
// Suffix `PL` distinguishes it from any test helper named uuidString.
func uuidStringPL(u pgtype.UUID) string {
	if !u.Valid {
		return "<nil>"
	}
	return fmt.Sprintf("%x-%x-%x-%x-%x",
		u.Bytes[0:4], u.Bytes[4:6], u.Bytes[6:8], u.Bytes[8:10], u.Bytes[10:16])
}