package game

import (
	"context"
	crand "crypto/rand"
	"encoding/binary"
	"errors"
	"fmt"
	"slices"
	"strings"
	"time"

	"github.com/google/uuid"
	"go.uber.org/zap"

	"scrabble/backend/internal/account"
	"scrabble/backend/internal/engine"
)

// Service is the game domain: it drives the engine over a single match, persists
// the event-sourced journal, keeps live games warm in a cache, serves hints and
// the word-check tool, exports GCG and runs the turn-timeout sweeper. It is the
// only writer of the game tables and is safe for concurrent use (per-game
// serialised by an internal keyed mutex).
type Service struct {
	store    *Store
	accounts *account.Store
	registry *engine.Registry
	cache    *gameCache
	locks    *keyedMutex
	version  string
	clock    func() time.Time
	rng      func() int64
	log      *zap.Logger
}

// NewService constructs a Service. store and accounts wrap the same pool;
// registry holds the resident dictionaries; cfg supplies the pinned version and
// the cache idle window; log is used by the background sweeper.
func NewService(store *Store, accounts *account.Store, registry *engine.Registry, cfg Config, log *zap.Logger) *Service {
	clock := func() time.Time { return time.Now().UTC() }
	return &Service{
		store:    store,
		accounts: accounts,
		registry: registry,
		cache:    newGameCache(cfg.CacheTTL, clock),
		locks:    newKeyedMutex(),
		version:  cfg.DictVersion,
		clock:    clock,
		rng:      randomSeed,
		log:      log,
	}
}

// Create starts and persists a new game seating the given accounts in turn order
// (seat 0 first), deals the racks, and warms the live-game cache. It validates
// the player count (2–4), the move clock, the hint allowance and that every seat
// is a distinct existing account.
func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, error) {
	if n := len(params.Seats); n < 2 || n > 4 {
		return Game{}, fmt.Errorf("%w: need 2-4 players, got %d", ErrInvalidConfig, n)
	}
	if params.HintsPerPlayer < 0 {
		return Game{}, fmt.Errorf("%w: hints per player must be >= 0", ErrInvalidConfig)
	}
	timeout := params.TurnTimeout
	if timeout == 0 {
		timeout = DefaultTurnTimeout
	}
	if !allowedTimeout(timeout) {
		return Game{}, fmt.Errorf("%w: turn timeout %s not allowed", ErrInvalidConfig, timeout)
	}
	seen := make(map[uuid.UUID]bool, len(params.Seats))
	for _, id := range params.Seats {
		if seen[id] {
			return Game{}, fmt.Errorf("%w: account %s seated twice", ErrInvalidConfig, id)
		}
		seen[id] = true
		if _, err := svc.accounts.GetByID(ctx, id); err != nil {
			if errors.Is(err, account.ErrNotFound) {
				return Game{}, fmt.Errorf("%w: account %s not found", ErrInvalidConfig, id)
			}
			return Game{}, err
		}
	}

	seed := params.Seed
	if seed == 0 {
		seed = svc.rng()
	}
	g, err := engine.New(svc.registry, engine.Options{
		Variant:      params.Variant,
		Version:      svc.version,
		Players:      len(params.Seats),
		Seed:         seed,
		DropoutTiles: params.DropoutTiles,
	})
	if err != nil {
		if errors.Is(err, engine.ErrUnknownVariant) || errors.Is(err, engine.ErrUnknownVersion) {
			return Game{}, fmt.Errorf("%w: %v", ErrInvalidConfig, err)
		}
		return Game{}, err
	}

	id, err := uuid.NewV7()
	if err != nil {
		return Game{}, fmt.Errorf("game: new id: %w", err)
	}
	ins := gameInsert{
		id:              id,
		variant:         params.Variant.String(),
		dictVersion:     svc.version,
		seed:            seed,
		players:         len(params.Seats),
		turnTimeoutSecs: int(timeout / time.Second),
		hintsAllowed:    params.HintsAllowed,
		hintsPerPlayer:  params.HintsPerPlayer,
		dropoutTiles:    params.DropoutTiles.String(),
	}
	if err := svc.store.CreateGame(ctx, ins, params.Seats); err != nil {
		return Game{}, err
	}
	svc.cache.put(id, g)
	return svc.store.GetGame(ctx, id)
}

// engineOp applies one transition to the live game, returning the decoded record
// and, for an exchange, the swapped tiles.
type engineOp func(g *engine.Game) (engine.MoveRecord, []string, error)

// SubmitPlay validates, scores and commits the player's placement.
func (svc *Service) SubmitPlay(ctx context.Context, gameID, accountID uuid.UUID, dir engine.Direction, tiles []engine.TileRecord) (MoveResult, error) {
	return svc.transition(ctx, gameID, accountID, func(g *engine.Game) (engine.MoveRecord, []string, error) {
		rec, err := g.SubmitPlay(dir, tiles)
		return rec, nil, err
	})
}

// Pass commits a forfeited turn.
func (svc *Service) Pass(ctx context.Context, gameID, accountID uuid.UUID) (MoveResult, error) {
	return svc.transition(ctx, gameID, accountID, func(g *engine.Game) (engine.MoveRecord, []string, error) {
		rec, err := g.Pass()
		return rec, nil, err
	})
}

// Exchange swaps the named tiles ("?" for a blank) and commits the turn.
func (svc *Service) Exchange(ctx context.Context, gameID, accountID uuid.UUID, tiles []string) (MoveResult, error) {
	return svc.transition(ctx, gameID, accountID, func(g *engine.Game) (engine.MoveRecord, []string, error) {
		rec, err := g.SubmitExchange(tiles)
		return rec, tiles, err
	})
}

// Resign ends the game on the player's turn; the remaining player wins.
func (svc *Service) Resign(ctx context.Context, gameID, accountID uuid.UUID) (MoveResult, error) {
	return svc.transition(ctx, gameID, accountID, func(g *engine.Game) (engine.MoveRecord, []string, error) {
		rec, err := g.Resign()
		return rec, nil, err
	})
}

// transition validates the actor and turn, applies op under the per-game lock and
// commits the result.
func (svc *Service) transition(ctx context.Context, gameID, accountID uuid.UUID, op engineOp) (MoveResult, error) {
	pre, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return MoveResult{}, err
	}
	seat, ok := pre.seatOf(accountID)
	if !ok {
		return MoveResult{}, ErrNotAPlayer
	}
	if pre.Status != StatusActive {
		return MoveResult{}, ErrFinished
	}
	if pre.ToMove != seat {
		return MoveResult{}, ErrNotYourTurn
	}

	unlock := svc.locks.lock(gameID)
	defer unlock()

	g, err := svc.liveGame(ctx, pre)
	if err != nil {
		return MoveResult{}, err
	}
	if g.Over() {
		return MoveResult{}, ErrFinished
	}
	if g.ToMove() != seat {
		return MoveResult{}, ErrNotYourTurn
	}

	rackBefore := g.Hand(seat)
	rec, exchanged, err := op(g)
	if err != nil {
		return MoveResult{}, err
	}
	post, err := svc.commit(ctx, gameID, g, rec, rec.Action.String(), rackBefore, exchanged, pre.Seats)
	if err != nil {
		return MoveResult{}, err
	}
	return MoveResult{Move: rec, Game: post}, nil
}

// commit persists a just-applied transition: the journal row, the post-move turn
// cursor and scores, and on a game-ending move the finish stamp and statistics.
// On a persistence failure it evicts the now-divergent live game so the next
// access rebuilds from the journal.
func (svc *Service) commit(ctx context.Context, gameID uuid.UUID, g *engine.Game, rec engine.MoveRecord, action string, rackBefore, exchanged []string, seats []Seat) (Game, error) {
	now := svc.clock()
	logLen := len(g.Log())
	scores := make([]int, g.Players())
	for i := range scores {
		scores[i] = g.Score(i)
	}
	c := commit{
		gameID:        gameID,
		seq:           logLen - 1,
		seat:          rec.Player,
		action:        action,
		score:         rec.Score,
		runningTotal:  rec.Total,
		exchanged:     exchanged,
		rec:           rec,
		rackBefore:    rackBefore,
		toMove:        g.ToMove(),
		turnStartedAt: now,
		moveCount:     logLen,
		scores:        scores,
		now:           now,
	}
	if g.Over() {
		c.finished = true
		c.finishedAt = now
		c.endReason = g.Reason().String()
		if action == "timeout" {
			c.endReason = "timeout"
		}
		c.winner = g.Result().Winner
		c.stats = buildStats(g, seats)
	}
	if err := svc.store.CommitMove(ctx, c); err != nil {
		svc.cache.remove(gameID)
		return Game{}, err
	}
	if c.finished {
		svc.cache.remove(gameID)
	}
	return svc.store.GetGame(ctx, gameID)
}

// timeoutGame auto-resigns the to-move player of an overdue game. It re-checks,
// under the per-game lock, that the game is still active and still past the
// effective deadline (so a move made since the sweep is not clobbered), records
// the move as a timeout, and reports whether it timed the game out.
func (svc *Service) timeoutGame(ctx context.Context, gameID uuid.UUID, now time.Time) (bool, error) {
	unlock := svc.locks.lock(gameID)
	defer unlock()

	cur, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return false, err
	}
	if cur.Status != StatusActive {
		return false, nil
	}
	seat := cur.ToMove
	if seat < 0 || seat >= len(cur.Seats) {
		return false, nil
	}
	acc, err := svc.accounts.GetByID(ctx, cur.Seats[seat].AccountID)
	if err != nil {
		return false, err
	}
	deadline := effectiveDeadline(cur.TurnStartedAt, cur.TurnTimeout, loadLocation(acc.TimeZone), minutesOfDay(acc.AwayStart), minutesOfDay(acc.AwayEnd))
	if now.Before(deadline) {
		return false, nil
	}

	g, err := svc.liveGame(ctx, cur)
	if err != nil {
		return false, err
	}
	if g.Over() {
		return false, nil
	}
	rackBefore := g.Hand(g.ToMove())
	rec, err := g.Resign()
	if err != nil {
		return false, err
	}
	if _, err := svc.commit(ctx, gameID, g, rec, "timeout", rackBefore, nil, cur.Seats); err != nil {
		return false, err
	}
	return true, nil
}

// EvaluatePlay previews a tentative play for a seated player against the current
// board without committing it: whether it is legal and what it would score.
func (svc *Service) EvaluatePlay(ctx context.Context, gameID, accountID uuid.UUID, dir engine.Direction, tiles []engine.TileRecord) (EvalResult, error) {
	pre, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return EvalResult{}, err
	}
	if _, ok := pre.seatOf(accountID); !ok {
		return EvalResult{}, ErrNotAPlayer
	}
	if pre.Status != StatusActive {
		return EvalResult{}, ErrFinished
	}

	unlock := svc.locks.lock(gameID)
	defer unlock()
	g, err := svc.liveGame(ctx, pre)
	if err != nil {
		return EvalResult{}, err
	}
	rec, err := g.EvaluatePlay(dir, tiles)
	if err != nil {
		if errors.Is(err, engine.ErrIllegalPlay) {
			return EvalResult{Valid: false}, nil
		}
		return EvalResult{}, err
	}
	return EvalResult{Valid: true, Score: rec.Score, Words: rec.Words}, nil
}

// CheckWord reports whether word is in the game's pinned dictionary. It is the
// unlimited word-check tool; an input outside the variant's alphabet is simply
// not a word.
func (svc *Service) CheckWord(ctx context.Context, gameID uuid.UUID, word string) (bool, error) {
	pre, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return false, err
	}
	return svc.lookupWord(pre.Variant, pre.DictVersion, word)
}

// FileComplaint records a word-check complaint against the game's dictionary for
// later admin review, stamping the disputed lookup result.
func (svc *Service) FileComplaint(ctx context.Context, gameID, accountID uuid.UUID, word, note string) (Complaint, error) {
	pre, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return Complaint{}, err
	}
	if _, ok := pre.seatOf(accountID); !ok {
		return Complaint{}, ErrNotAPlayer
	}
	normalized := normalizeWord(word)
	valid, err := svc.lookupWord(pre.Variant, pre.DictVersion, normalized)
	if err != nil {
		return Complaint{}, err
	}
	return svc.store.FileComplaint(ctx, Complaint{
		ComplainantID: accountID,
		GameID:        gameID,
		Variant:       pre.Variant,
		DictVersion:   pre.DictVersion,
		Word:          normalized,
		WasValid:      valid,
		Note:          note,
	})
}

// Hint reveals the top-scoring legal play for the requesting player on their
// turn, spending one hint from their per-game allowance and then their profile
// wallet. It returns ErrHintsDisabled, ErrNoHintsLeft or ErrNoHintAvailable as
// appropriate.
func (svc *Service) Hint(ctx context.Context, gameID, accountID uuid.UUID) (HintResult, error) {
	pre, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return HintResult{}, err
	}
	seat, ok := pre.seatOf(accountID)
	if !ok {
		return HintResult{}, ErrNotAPlayer
	}
	if pre.Status != StatusActive {
		return HintResult{}, ErrFinished
	}
	if pre.ToMove != seat {
		return HintResult{}, ErrNotYourTurn
	}
	if !pre.HintsAllowed {
		return HintResult{}, ErrHintsDisabled
	}
	acc, err := svc.accounts.GetByID(ctx, accountID)
	if err != nil {
		return HintResult{}, err
	}
	used := pre.Seats[seat].HintsUsed
	fromAllowance := used < pre.HintsPerPlayer
	if !fromAllowance && acc.HintBalance <= 0 {
		return HintResult{}, ErrNoHintsLeft
	}

	unlock := svc.locks.lock(gameID)
	defer unlock()
	g, err := svc.liveGame(ctx, pre)
	if err != nil {
		return HintResult{}, err
	}
	move, ok := g.HintView()
	if !ok {
		return HintResult{}, ErrNoHintAvailable
	}

	walletAfter := acc.HintBalance
	if fromAllowance {
		if err := svc.store.SpendHintAllowance(ctx, gameID, seat); err != nil {
			return HintResult{}, err
		}
		used++
	} else {
		spent, err := svc.accounts.SpendHint(ctx, accountID)
		if err != nil {
			return HintResult{}, err
		}
		if !spent {
			return HintResult{}, ErrNoHintsLeft
		}
		walletAfter--
	}
	return HintResult{Move: move, HintsRemaining: hintsRemaining(pre.HintsPerPlayer, used, walletAfter)}, nil
}

// GameState returns a seated player's view of the game: the shared summary plus
// their private rack, the bag size and their remaining hint budget.
func (svc *Service) GameState(ctx context.Context, gameID, accountID uuid.UUID) (StateView, error) {
	pre, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return StateView{}, err
	}
	seat, ok := pre.seatOf(accountID)
	if !ok {
		return StateView{}, ErrNotAPlayer
	}
	acc, err := svc.accounts.GetByID(ctx, accountID)
	if err != nil {
		return StateView{}, err
	}

	unlock := svc.locks.lock(gameID)
	defer unlock()
	g, err := svc.liveGame(ctx, pre)
	if err != nil {
		return StateView{}, err
	}
	return StateView{
		Game:           pre,
		Seat:           seat,
		Rack:           g.Hand(seat),
		BagLen:         g.BagLen(),
		HintsRemaining: hintsRemaining(pre.HintsPerPlayer, pre.Seats[seat].HintsUsed, acc.HintBalance),
	}, nil
}

// Participants returns the seated account IDs in seat order, the seat index whose
// turn it is, and the game status. It is a snapshot read (no engine, no lock) that
// lets the social package gate per-game chat and nudges without importing the
// engine or the game's private state.
func (svc *Service) Participants(ctx context.Context, gameID uuid.UUID) ([]uuid.UUID, int, string, error) {
	g, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return nil, 0, "", err
	}
	seats := make([]uuid.UUID, len(g.Seats))
	for _, s := range g.Seats {
		seats[s.Seat] = s.AccountID
	}
	return seats, g.ToMove, g.Status, nil
}

// History returns a game's full, dictionary-independent move journal.
func (svc *Service) History(ctx context.Context, gameID uuid.UUID) (HistoryView, error) {
	g, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return HistoryView{}, err
	}
	moves, err := svc.store.GetJournal(ctx, gameID)
	if err != nil {
		return HistoryView{}, err
	}
	return HistoryView{Game: g, Moves: moves}, nil
}

// ExportGCG renders a game as GCG text from the journal alone (no dictionary).
func (svc *Service) ExportGCG(ctx context.Context, gameID uuid.UUID) (string, error) {
	g, err := svc.store.GetGame(ctx, gameID)
	if err != nil {
		return "", err
	}
	moves, err := svc.store.GetJournal(ctx, gameID)
	if err != nil {
		return "", err
	}
	return writeGCG(g, svc.seatNames(ctx, g), moves), nil
}

// liveGame returns the live engine.Game for pre, rebuilding it from the journal
// on a cache miss. Callers must hold the per-game lock.
func (svc *Service) liveGame(ctx context.Context, pre Game) (*engine.Game, error) {
	if g, ok := svc.cache.get(pre.ID); ok {
		return g, nil
	}
	g, err := svc.replay(ctx, pre)
	if err != nil {
		return nil, err
	}
	if !g.Over() {
		svc.cache.put(pre.ID, g)
	}
	return g, nil
}

// replay reconstructs an engine.Game by dealing from the pinned seed and
// re-applying every journalled move in order. The deterministic bag makes the
// reconstruction exact.
func (svc *Service) replay(ctx context.Context, pre Game) (*engine.Game, error) {
	seed, err := svc.store.GameSeed(ctx, pre.ID)
	if err != nil {
		return nil, err
	}
	g, err := engine.New(svc.registry, engine.Options{
		Variant:      pre.Variant,
		Version:      pre.DictVersion,
		Players:      pre.Players,
		Seed:         seed,
		DropoutTiles: pre.DropoutTiles,
	})
	if err != nil {
		return nil, err
	}
	moves, err := svc.store.GetJournal(ctx, pre.ID)
	if err != nil {
		return nil, err
	}
	for _, mv := range moves {
		if err := replayMove(g, mv); err != nil {
			return nil, fmt.Errorf("game: replay %s move %d: %w", pre.ID, mv.Seq, err)
		}
	}
	return g, nil
}

// replayMove re-applies one journalled move to g through the decoded engine API.
func replayMove(g *engine.Game, mv HistoryMove) error {
	switch mv.Action {
	case "play":
		dir := engine.Horizontal
		if mv.Dir == "V" {
			dir = engine.Vertical
		}
		_, err := g.SubmitPlay(dir, mv.Tiles)
		return err
	case "pass":
		_, err := g.Pass()
		return err
	case "exchange":
		_, err := g.SubmitExchange(mv.Exchanged)
		return err
	case "resign", "timeout":
		_, err := g.Resign()
		return err
	default:
		return fmt.Errorf("unknown action %q", mv.Action)
	}
}

// buildStats derives each seat's statistics contribution from a finished game:
// win/loss/draw from the (resignation-aware) winner, the final score, and the
// best single-move score from the log.
func buildStats(g *engine.Game, seats []Seat) []statDelta {
	res := g.Result()
	best := make(map[int]int)
	for _, rec := range g.Log() {
		if rec.Action == engine.ActionPlay && rec.Score > best[rec.Player] {
			best[rec.Player] = rec.Score
		}
	}
	out := make([]statDelta, 0, len(seats))
	for _, s := range seats {
		d := statDelta{accountID: s.AccountID, gamePoints: g.Score(s.Seat), wordPoints: best[s.Seat]}
		switch {
		case res.Winner < 0:
			d.draws = 1
		case res.Winner == s.Seat:
			d.wins = 1
		default:
			d.losses = 1
		}
		out = append(out, d)
	}
	return out
}

// seatNames resolves each seat's display name for GCG export.
func (svc *Service) seatNames(ctx context.Context, g Game) []string {
	names := make([]string, g.Players)
	for _, s := range g.Seats {
		if acc, err := svc.accounts.GetByID(ctx, s.AccountID); err == nil {
			names[s.Seat] = acc.DisplayName
		}
	}
	return names
}

// lookupWord checks word against a variant/version dictionary, treating an
// out-of-alphabet input as simply not a word (a real registry error still
// surfaces).
func (svc *Service) lookupWord(variant engine.Variant, version, word string) (bool, error) {
	present, err := svc.registry.Lookup(variant, version, normalizeWord(word))
	if err != nil {
		if errors.Is(err, engine.ErrUnknownVariant) || errors.Is(err, engine.ErrUnknownVersion) {
			return false, err
		}
		return false, nil
	}
	return present, nil
}

// hintsRemaining is a player's remaining hint budget: the unspent per-game
// allowance plus the profile wallet.
func hintsRemaining(allowance, used, wallet int) int {
	return max(0, allowance-used) + wallet
}

// allowedTimeout reports whether d is one of the offered move clocks.
func allowedTimeout(d time.Duration) bool {
	return slices.Contains(AllowedTurnTimeouts, d)
}

// normalizeWord lower-cases and trims a word-check input to the alphabet's form.
func normalizeWord(word string) string {
	return strings.ToLower(strings.TrimSpace(word))
}

// randomSeed returns an unpredictable bag seed, falling back to the clock if the
// system source fails.
func randomSeed() int64 {
	var b [8]byte
	if _, err := crand.Read(b[:]); err != nil {
		return time.Now().UnixNano()
	}
	return int64(binary.LittleEndian.Uint64(b[:]))
}