// Package robot is the human-like computer opponent. It substitutes for a missing
// human in two-player auto-match: a pool of durable accounts (one robot identity
// each) is provisioned at startup, and a background driver makes their moves with
// human-like timing, a night sleep window and nudge behaviour
// (docs/ARCHITECTURE.md §7).
//
// The robot consumes the public game API as an ordinary seated player and works
// on decoded values only, so it never imports the solver (only internal/engine
// does). All of a robot's per-game and per-turn choices are derived
// deterministically from the game's bag seed (see strategy.go), so the driver
// holds no per-game state and is restart-safe.
package robot

import (
	"context"
	"errors"
	"fmt"
	"math/rand/v2"
	"sync"
	"time"

	"github.com/google/uuid"
	"go.opentelemetry.io/otel/metric"
	"go.opentelemetry.io/otel/metric/noop"
	"go.uber.org/zap"

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

// ErrNoRobotAvailable is returned by Pick when the pool is empty (EnsurePool has
// not run or failed).
var ErrNoRobotAvailable = errors.New("robot: no robot available in the pool")

// GameDriver is the slice of the game domain the robot needs: scanning its active
// games, reading a turn's candidates and state, and making moves as a seated
// player. game.Service satisfies it.
type GameDriver interface {
	RobotTurns(ctx context.Context, robotIDs []uuid.UUID) ([]game.RobotTurn, error)
	Participants(ctx context.Context, gameID uuid.UUID) ([]uuid.UUID, int, string, error)
	Candidates(ctx context.Context, gameID, accountID uuid.UUID) ([]engine.MoveRecord, error)
	GameState(ctx context.Context, gameID, accountID uuid.UUID) (game.StateView, error)
	SubmitPlay(ctx context.Context, gameID, accountID uuid.UUID, dir engine.Direction, tiles []engine.TileRecord) (game.MoveResult, error)
	Pass(ctx context.Context, gameID, accountID uuid.UUID) (game.MoveResult, error)
	Exchange(ctx context.Context, gameID, accountID uuid.UUID, tiles []string) (game.MoveResult, error)
}

// Nudger is the slice of the social domain the robot needs: sending a proactive
// nudge and reading the opponent's last nudge to answer it. social.Service
// satisfies it.
type Nudger interface {
	Nudge(ctx context.Context, gameID, senderID uuid.UUID) (social.Message, error)
	LastNudgeAt(ctx context.Context, gameID, senderID uuid.UUID) (time.Time, bool, error)
}

// Config configures the robot subsystem.
type Config struct {
	// DriveInterval is how often the driver scans for robot turns. Sourced from
	// BACKEND_ROBOT_DRIVE_INTERVAL.
	DriveInterval time.Duration
}

// DefaultConfig returns the robot configuration defaults.
func DefaultConfig() Config {
	return Config{DriveInterval: 30 * time.Second}
}

// Validate reports whether the configuration is usable.
func (c Config) Validate() error {
	if c.DriveInterval <= 0 {
		return fmt.Errorf("robot: drive interval must be positive, got %s", c.DriveInterval)
	}
	return nil
}

// Service owns the robot pool and the move driver. It is safe for concurrent use.
type Service struct {
	games    GameDriver
	accounts *account.Store
	social   Nudger
	finished metric.Int64Counter
	clock    func() time.Time
	log      *zap.Logger

	mu     sync.RWMutex
	poolEN []uuid.UUID
	poolRU []uuid.UUID
}

// NewService constructs a robot Service. games and social are the domain seams it
// drives; accounts provisions the pool and resolves opponent timezones; meter
// records the balance counter; log carries driver diagnostics.
func NewService(games GameDriver, accounts *account.Store, soc Nudger, meter metric.Meter, log *zap.Logger) *Service {
	if log == nil {
		log = zap.NewNop()
	}
	counter, err := meter.Int64Counter(
		"robot_games_finished_total",
		metric.WithDescription("Robot games finished, labelled by result from the robot's view (win/loss/draw)."),
	)
	if err != nil {
		log.Warn("robot: create finished counter", zap.Error(err))
		counter, _ = noop.NewMeterProvider().Meter("robot").Int64Counter("robot_games_finished_total")
	}
	return &Service{
		games:    games,
		accounts: accounts,
		social:   soc,
		finished: counter,
		clock:    func() time.Time { return time.Now().UTC() },
		log:      log,
	}
}

// EnsurePool idempotently provisions the robot accounts (one per slot of each
// language's composed name pool) and records their ids. Each robot is a durable
// account bound to a stable, index-keyed robot identity, with chat and friend
// requests blocked so it never engages socially (docs/ARCHITECTURE.md §7). It is a
// startup dependency, like the dictionary registry: a failure fails the boot.
func (s *Service) EnsurePool(ctx context.Context) error {
	en, err := s.provisionPool(ctx, "en", robotDisplayNamesEN())
	if err != nil {
		return err
	}
	ru, err := s.provisionPool(ctx, "ru", robotDisplayNamesRU())
	if err != nil {
		return err
	}
	s.mu.Lock()
	s.poolEN, s.poolRU = en, ru
	s.mu.Unlock()
	return nil
}

// provisionPool provisions one durable robot account per name and returns their ids
// in order. The identity is keyed by language and slot index (stable across restarts
// and independent of the composed display name); account.ProvisionRobot sets the
// display name and social blocks and is idempotent, so EnsurePool can run every boot.
func (s *Service) provisionPool(ctx context.Context, lang string, names []string) ([]uuid.UUID, error) {
	ids := make([]uuid.UUID, 0, len(names))
	for i, name := range names {
		acc, err := s.accounts.ProvisionRobot(ctx, fmt.Sprintf("robot-%s-%d", lang, i), name)
		if err != nil {
			return nil, fmt.Errorf("robot: provision %s #%d (%q): %w", lang, i, name, err)
		}
		ids = append(ids, acc.ID)
	}
	return ids, nil
}

// Pick returns a random robot account for the matchmaker to substitute into an
// auto-match of the given variant. An English game draws from the Latin pool; a
// Russian game (Russian Scrabble or Эрудит) draws from the Russian pool, mixing in a
// Latin name about latinShareInRussian% of the time; either side falls back to the
// other when its pool is empty. It satisfies lobby.RobotProvider.
func (s *Service) Pick(variant engine.Variant) (uuid.UUID, error) {
	s.mu.RLock()
	defer s.mu.RUnlock()
	primary, secondary := s.poolEN, s.poolRU
	if variant == engine.VariantRussianScrabble || variant == engine.VariantErudit {
		primary, secondary = s.poolRU, s.poolEN
		if len(primary) > 0 && len(secondary) > 0 && rand.IntN(100) < latinShareInRussian {
			primary, secondary = secondary, primary
		}
	}
	if len(primary) == 0 {
		primary = secondary
	}
	if len(primary) == 0 {
		return uuid.Nil, ErrNoRobotAvailable
	}
	return primary[rand.IntN(len(primary))], nil
}

// poolIDs returns a snapshot of the whole pool (both languages) for the driver scan,
// which is variant-agnostic — it acts on every robot's active games.
func (s *Service) poolIDs() []uuid.UUID {
	s.mu.RLock()
	defer s.mu.RUnlock()
	ids := make([]uuid.UUID, 0, len(s.poolEN)+len(s.poolRU))
	ids = append(ids, s.poolEN...)
	ids = append(ids, s.poolRU...)
	return ids
}