// Package wanted is what Minstrel is looking for (M502 #5520): followed
// artists, each on a release-type profile, and wanted release groups. A want
// comes from an approved request, a followed artist's new release, or an
// album whose files went missing; it is held once the library has a full
// copy. The library's own albums were put on the list as held once (#5567),
// so the list is the record of what the library keeps. Searching for open
// wants is #5521's.
package wanted

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

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

	"git.fabledsword.com/bvandeusen/minstrel/internal/catalogue"
	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
	"git.fabledsword.com/bvandeusen/minstrel/internal/musicbrainz"
	"git.fabledsword.com/bvandeusen/minstrel/internal/tags"
)

// Reasons a release group is wanted, matching wants_reason_check.
const (
	ReasonRequest = "request"
	ReasonFollow  = "follow"
	ReasonMissing = "missing"
)

// variousArtists is MusicBrainz's Various Artists. Its "discography" is every
// compilation ever catalogued; nobody follows it.
const variousArtists = "89ad4ac3-39f7-470e-963a-56509c546377"

// ErrNotFollowable: an artist nobody follows (Various Artists, or one of
// MusicBrainz's bracketed placeholders such as [unknown]).
var ErrNotFollowable = errors.New("wanted: not an artist anyone follows")

// Service owns follows, wants and the worker that keeps them current.
type Service struct {
	pool   *pgxpool.Pool
	cat    *catalogue.Service
	logger *slog.Logger
	now    func() time.Time
	// releaseGroup names the group of a MusicBrainz release id, to move a
	// want recorded under a release id (#5241) to its group. A field so tests
	// stand in for MusicBrainz.
	releaseGroup func(ctx context.Context, releaseMBID string) (string, error)
}

// New makes a Service reading discographies and editions through cat.
func New(pool *pgxpool.Pool, cat *catalogue.Service, logger *slog.Logger) *Service {
	if logger == nil {
		logger = slog.Default()
	}
	return &Service{pool: pool, cat: cat, logger: logger, now: time.Now, releaseGroup: tags.ReleaseGroupForRelease}
}

// Followable reports whether an artist is one anyone would follow.
func Followable(mbid, name string) bool {
	if mbid == "" || mbid == variousArtists {
		return false
	}
	return len(name) < 2 || name[0] != '[' || name[len(name)-1] != ']'
}

// FollowParams is a follow as the caller asks for it. A zero ProfileID means
// the default profile. WantExisting also wants what the artist has already
// released that the profile passes and the library lacks.
type FollowParams struct {
	ArtistMBID   string
	ArtistName   string
	ProfileID    pgtype.UUID
	By           pgtype.UUID
	WantExisting bool
}

// Follow starts or changes a follow. The worker reads the discography.
func (s *Service) Follow(ctx context.Context, p FollowParams) (dbq.Follow, error) {
	return FollowWith(ctx, dbq.New(s.pool), p)
}

// FollowWith is Follow inside the caller's transaction (request approval).
func FollowWith(ctx context.Context, q *dbq.Queries, p FollowParams) (dbq.Follow, error) {
	if !Followable(p.ArtistMBID, p.ArtistName) {
		return dbq.Follow{}, ErrNotFollowable
	}
	profile, err := profileOrDefault(ctx, q, p.ProfileID)
	if err != nil {
		return dbq.Follow{}, err
	}
	f, err := q.UpsertFollow(ctx, dbq.UpsertFollowParams{
		ArtistMbid: p.ArtistMBID, ArtistName: p.ArtistName, ProfileID: profile.ID,
		FollowedBy: p.By, WantExisting: p.WantExisting,
	})
	if err != nil {
		return dbq.Follow{}, fmt.Errorf("wanted: follow: %w", err)
	}
	return f, nil
}

// Unfollow stops following. Wants already recorded stay.
func (s *Service) Unfollow(ctx context.Context, artistMBID string) (bool, error) {
	n, err := dbq.New(s.pool).DeleteFollow(ctx, artistMBID)
	if err != nil {
		return false, fmt.Errorf("wanted: unfollow: %w", err)
	}
	return n > 0, nil
}

// WantParams is a release group to want.
type WantParams struct {
	GroupMBID        string
	ArtistMBID       string
	ArtistName       string
	Title            string
	PrimaryType      string
	FirstReleaseDate string
	Reason           string
	CreatedBy        pgtype.UUID
}

// WantWith records a want inside the caller's transaction, or returns the
// want already recorded for the group.
func WantWith(ctx context.Context, q *dbq.Queries, p WantParams) (dbq.Want, error) {
	if p.GroupMBID == "" || p.ArtistMBID == "" {
		return dbq.Want{}, errors.New("wanted: a release group and its artist are required")
	}
	w, err := q.WantReleaseGroup(ctx, dbq.WantReleaseGroupParams{
		ReleaseGroupMbid: p.GroupMBID, ArtistMbid: p.ArtistMBID, ArtistName: p.ArtistName,
		Title: p.Title, PrimaryType: p.PrimaryType, FirstReleaseDate: p.FirstReleaseDate,
		Reason: p.Reason, CreatedBy: p.CreatedBy,
	})
	if err != nil {
		return dbq.Want{}, fmt.Errorf("wanted: want: %w", err)
	}
	return w, nil
}

// RemoveWant drops a want, open or held.
func (s *Service) RemoveWant(ctx context.Context, id pgtype.UUID) (bool, error) {
	n, err := dbq.New(s.pool).DeleteWant(ctx, id)
	if err != nil {
		return false, fmt.Errorf("wanted: remove want: %w", err)
	}
	return n > 0, nil
}

const (
	// FollowRefreshAge is how often a followed artist's discography is read
	// for new releases: daily, as Lidarr refreshed its artists.
	FollowRefreshAge = 24 * time.Hour
	// NewReleaseGrace: a group first seen after the baseline is wanted only
	// if it came out no earlier than this before the follow began. MusicBrainz
	// gains old releases all the time; a single from 1998 catalogued today is
	// not a new release.
	NewReleaseGrace = 30 * 24 * time.Hour
	// Interval is how often the worker looks for work; followsPerPass and
	// wantsPerPass bound one pass so neither starves the other.
	Interval       = 5 * time.Minute
	followsPerPass = 60
	wantsPerPass   = 500
)

// Run applies follows and settles wants every Interval until ctx ends.
func (s *Service) Run(ctx context.Context) {
	ticker := time.NewTicker(Interval)
	defer ticker.Stop()
	for {
		res, err := s.Pass(ctx)
		switch {
		case err != nil && ctx.Err() == nil:
			s.logger.Warn("wanted: pass stopped", "err", err, "follows", res.Follows, "wanted", res.Wanted, "held", res.Held)
		case res.Wanted > 0 || res.Held > 0 || res.Baselined > 0 || res.Listed > 0:
			s.logger.Info("wanted: pass complete", "follows", res.Follows, "baselined", res.Baselined,
				"wanted", res.Wanted, "held", res.Held, "listed", res.Listed)
		}
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
		}
	}
}

// PassResult is what one pass did.
type PassResult struct {
	Follows   int // follows whose discography was applied
	Baselined int // of those, first reads
	Wanted    int // wants recorded from follows
	Held      int // wants the library now answers
	Listed    int // library albums put on the list as held (#5567)
}

// Pass applies the follows due and settles open wants. An unreachable
// MusicBrainz stops the follows (they need it) but not the settling, which
// falls back to what is held.
func (s *Service) Pass(ctx context.Context) (PassResult, error) {
	var res PassResult
	ferr := s.applyFollows(ctx, &res)
	if ctx.Err() != nil {
		return res, ctx.Err()
	}
	// The one-time listing of the library (#5567); a failure waits for the
	// next pass and does not hold back settling.
	if err := s.listLibrary(ctx, &res); err != nil && ctx.Err() == nil {
		s.logger.Warn("wanted: list the library", "err", err)
	}
	if err := s.settleWants(ctx, &res); err != nil {
		return res, err
	}
	return res, ferr
}

func (s *Service) applyFollows(ctx context.Context, res *PassResult) error {
	q := dbq.New(s.pool)
	due, err := q.ListFollowsDue(ctx, dbq.ListFollowsDueParams{
		Before:  pgtype.Timestamptz{Time: s.now().Add(-FollowRefreshAge), Valid: true},
		MaxRows: followsPerPass,
	})
	if err != nil {
		return fmt.Errorf("wanted: list follows due: %w", err)
	}
	for _, f := range due {
		n, err := s.applyFollow(ctx, q, f)
		if err != nil {
			if errors.Is(err, musicbrainz.ErrUnreachable) || ctx.Err() != nil {
				return err
			}
			s.logger.Warn("wanted: apply follow", "artist", f.ArtistName, "mbid", f.ArtistMbid, "err", err)
			continue
		}
		res.Follows++
		res.Wanted += n
		if !f.BaselineAt.Valid {
			res.Baselined++
		}
	}
	return nil
}

// applyFollow reads the artist's discography and judges each release group
// the follow has not judged before. On the first read (the baseline) they
// are only marked seen, unless the follow asked to want what already exists.
// After it, a group is wanted when the profile passes it, it is recent, and
// the library does not hold it.
func (s *Service) applyFollow(ctx context.Context, q *dbq.Queries, f dbq.Follow) (int, error) {
	d, err := s.cat.DiscographyWithin(ctx, f.ArtistMbid, FollowRefreshAge)
	if err == nil && d.Stale {
		// The held copy, served because MusicBrainz could not be asked:
		// applying it would mark the follow checked without having looked.
		return 0, musicbrainz.ErrUnreachable
	}
	if err != nil {
		if !errors.Is(err, catalogue.ErrNotFound) {
			return 0, err
		}
		// Merged away or removed from MusicBrainz: nothing to read. Marked
		// checked so it is asked again tomorrow, not every pass.
		s.logger.Info("wanted: followed artist not in MusicBrainz", "artist", f.ArtistName, "mbid", f.ArtistMbid)
		return 0, q.MarkFollowChecked(ctx, f.ArtistMbid)
	}
	profile, err := q.GetReleaseTypeProfile(ctx, f.ProfileID)
	if err != nil {
		return 0, fmt.Errorf("read profile: %w", err)
	}
	groups, err := q.ListFollowGroups(ctx, f.ArtistMbid)
	if err != nil {
		return 0, fmt.Errorf("read discography: %w", err)
	}
	baseline := !f.BaselineAt.Valid
	wanted := 0
	for _, g := range groups {
		judge := (baseline && f.WantExisting) || (!baseline && !g.Seen && s.recent(g.FirstReleaseDate, f.BaselineAt.Time))
		if judge && Passes(profile, g.PrimaryType, g.SecondaryTypes) {
			held, err := q.LibraryHoldsReleaseGroup(ctx, dbq.LibraryHoldsReleaseGroupParams{
				GroupMbid: g.Mbid, Title: g.Title, ArtistMbid: f.ArtistMbid,
			})
			if err != nil {
				return wanted, fmt.Errorf("library check: %w", err)
			}
			if !held {
				w, err := WantWith(ctx, q, WantParams{
					GroupMBID: g.Mbid, ArtistMBID: f.ArtistMbid, ArtistName: f.ArtistName, Title: g.Title,
					PrimaryType: g.PrimaryType, FirstReleaseDate: g.FirstReleaseDate,
					Reason: ReasonFollow, CreatedBy: f.FollowedBy,
				})
				if err != nil {
					return wanted, err
				}
				if !w.FulfilledAt.Valid {
					wanted++
				}
			}
		}
		if !g.Seen {
			if err := q.MarkGroupSeen(ctx, dbq.MarkGroupSeenParams{ArtistMbid: f.ArtistMbid, ReleaseGroupMbid: g.Mbid}); err != nil {
				return wanted, fmt.Errorf("mark seen: %w", err)
			}
		}
	}
	return wanted, q.MarkFollowChecked(ctx, f.ArtistMbid)
}

// recent reports whether a release dated date (MusicBrainz's partial date:
// "2026", "2026-10", "2026-10-09", or empty when not yet known) came out no
// earlier than NewReleaseGrace before the follow's baseline. A partial date
// is read as the last day it could mean, so a year-only date is not
// dismissed for being "January".
func (s *Service) recent(date string, baseline time.Time) bool {
	t, ok := latestDate(date)
	if !ok {
		return true
	}
	return !t.Before(baseline.Add(-NewReleaseGrace))
}

func latestDate(date string) (time.Time, bool) {
	if t, err := time.Parse("2006-01-02", date); err == nil {
		return t, true
	}
	if t, err := time.Parse("2006-01", date); err == nil {
		return t.AddDate(0, 1, -1), true
	}
	if t, err := time.Parse("2006", date); err == nil {
		return t.AddDate(1, 0, -1), true
	}
	return time.Time{}, false
}

// settleWants marks open wants held when the library answers them. Each
// want's editions come from the catalogue, read from MusicBrainz the first
// time; once MusicBrainz is unreachable the rest use only what is held.
func (s *Service) settleWants(ctx context.Context, res *PassResult) error {
	q := dbq.New(s.pool)
	open, err := q.ListOpenWants(ctx, wantsPerPass)
	if err != nil {
		return fmt.Errorf("wanted: list open wants: %w", err)
	}
	mbDown := false
	for _, w := range open {
		if ctx.Err() != nil {
			return ctx.Err()
		}
		shortest, moved, err := s.shortestEdition(ctx, q, w, &mbDown)
		if err != nil {
			s.logger.Warn("wanted: read editions", "want", w.Title, "group", w.ReleaseGroupMbid, "err", err)
		}
		if moved {
			continue // settled next pass, under its group
		}
		album, ok, err := s.answeredBy(ctx, q, w, shortest)
		if err != nil {
			return err
		}
		if !ok {
			continue
		}
		if err := q.FulfillWant(ctx, dbq.FulfillWantParams{ID: w.ID, AlbumID: album}); err != nil {
			return fmt.Errorf("wanted: fulfil: %w", err)
		}
		res.Held++
	}
	return nil
}

// shortestEdition is the track count of the group's shortest official
// edition (any edition when none is official), or 0 when the catalogue
// cannot say. moved reports that the want was recorded under a release id
// and has been moved to its group.
func (s *Service) shortestEdition(ctx context.Context, q *dbq.Queries, w dbq.Want, mbDown *bool) (int, bool, error) {
	var releases []dbq.MbRelease
	if *mbDown {
		held, err := q.ListMBReleasesForGroup(ctx, w.ReleaseGroupMbid)
		if err != nil {
			return 0, false, err
		}
		releases = held
	} else {
		ed, err := s.cat.Releases(ctx, w.ReleaseGroupMbid)
		switch {
		case err == nil:
			for _, e := range ed.Editions {
				releases = append(releases, e.Release)
			}
		case errors.Is(err, catalogue.ErrNotFound):
			moved, merr := s.moveToGroup(ctx, w)
			return 0, moved, merr
		case errors.Is(err, musicbrainz.ErrUnreachable):
			*mbDown = true
			return s.shortestEdition(ctx, q, w, mbDown)
		default:
			return 0, false, err
		}
	}
	return Shortest(releases), false, nil
}

// Shortest is the track count of the shortest official release with tracks,
// or of the shortest release with tracks when none is official, or 0.
func Shortest(releases []dbq.MbRelease) int {
	best, bestAny := 0, 0
	for _, r := range releases {
		n := int(r.TrackCount)
		if n <= 0 {
			continue
		}
		if bestAny == 0 || n < bestAny {
			bestAny = n
		}
		if r.Status == "Official" && (best == 0 || n < best) {
			best = n
		}
	}
	if best > 0 {
		return best
	}
	return bestAny
}

// moveToGroup handles a want MusicBrainz has no release group under: one
// recorded under a release id before #5241. It is moved to the release's
// group, with the requests that name it; when the group is already wanted,
// this copy is dropped.
func (s *Service) moveToGroup(ctx context.Context, w dbq.Want) (bool, error) {
	group, err := s.releaseGroup(ctx, w.ReleaseGroupMbid)
	if err != nil {
		if errors.Is(err, tags.ErrNotFound) {
			return false, nil
		}
		return false, fmt.Errorf("release group of %s: %w", w.ReleaseGroupMbid, err)
	}
	if group == "" || group == w.ReleaseGroupMbid {
		return false, nil
	}
	err = pgx.BeginFunc(ctx, s.pool, func(tx pgx.Tx) error {
		tq := dbq.New(tx)
		if _, gerr := tq.GetWantByReleaseGroup(ctx, group); gerr == nil {
			if _, derr := tq.DeleteWant(ctx, w.ID); derr != nil {
				return derr
			}
		} else if errors.Is(gerr, pgx.ErrNoRows) {
			if rerr := tq.RepointWant(ctx, dbq.RepointWantParams{ID: w.ID, ReleaseGroupMbid: group}); rerr != nil {
				return rerr
			}
		} else {
			return gerr
		}
		return tq.RepointRequestsAlbumMbid(ctx, dbq.RepointRequestsAlbumMbidParams{
			NewMbid: &group, OldMbid: &w.ReleaseGroupMbid,
		})
	})
	if err != nil {
		return false, fmt.Errorf("move want to its group: %w", err)
	}
	s.logger.Info("wanted: want moved from a release id to its group",
		"want", w.Title, "release_mbid", w.ReleaseGroupMbid, "release_group_mbid", group)
	return true, nil
}

// answeredBy finds the library album that answers a want, if any:
//
//   - for a want about an album whose files went missing, that album once no
//     missing track is left without a track on disk in its place;
//   - otherwise an album of the group with at least as many tracks on disk as
//     the shortest official edition;
//   - and when the catalogue cannot say how long the group is, the test from
//     #5263: a track arrived since the want, or nothing missing before it is
//     still missing.
//
// Only albums with a track on disk count.
func (s *Service) answeredBy(ctx context.Context, q *dbq.Queries, w dbq.Want, shortest int) (pgtype.UUID, bool, error) {
	holdings, err := q.WantHoldings(ctx, dbq.WantHoldingsParams{
		Since: w.WantedAt, GroupMbid: w.ReleaseGroupMbid, AlbumID: w.AlbumID,
	})
	if err != nil {
		return pgtype.UUID{}, false, fmt.Errorf("wanted: read holdings: %w", err)
	}
	for _, h := range holdings {
		if Answers(w, h, shortest) {
			return h.AlbumID, true, nil
		}
	}
	return pgtype.UUID{}, false, nil
}

// Answers is answeredBy's verdict on one album.
func Answers(w dbq.Want, h dbq.WantHoldingsRow, shortest int) bool {
	if h.Present == 0 {
		return false
	}
	if w.Reason == ReasonMissing && w.AlbumID.Valid && h.AlbumID == w.AlbumID {
		return h.UncoveredMissing == 0
	}
	if shortest > 0 {
		return int(h.Present) >= shortest
	}
	return h.ArrivedSince || !h.MissingBefore
}