package runtime

import (
	"context"
	"errors"
	"sync"
	"time"

	"galaxy/backend/internal/dockerclient"
	"galaxy/cronutil"

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

// Scheduler runs one goroutine per running game. Each goroutine holds
// a `cronutil.Schedule` parsed from `runtime_records.turn_schedule`
// and invokes `engineclient.Turn` on every tick (or when
// `skip_next_tick=true` short-circuits the timer).
//
// Implements `app.Component` so main.go can register the bookkeeper
// component alongside the worker pool and reconciler. Run blocks on
// ctx; per-game goroutines tear down when their game leaves the cache
// (stopGame is called) or when ctx is cancelled.
type Scheduler struct {
	svc *Service

	mu       sync.Mutex
	tickers  map[uuid.UUID]*scheduledGame
	parent   context.Context
	stopping bool
}

type scheduledGame struct {
	cancel context.CancelFunc
	done   chan struct{}
}

// NewScheduler builds a Scheduler. The svc reference is held for the
// life of the Scheduler.
func NewScheduler(svc *Service) *Scheduler {
	return &Scheduler{
		svc:     svc,
		tickers: make(map[uuid.UUID]*scheduledGame),
	}
}

// Run installs ctx as the parent context and re-attaches scheduler
// goroutines for every active runtime record at startup. Blocks on
// ctx.
func (sch *Scheduler) Run(ctx context.Context) error {
	if sch == nil {
		return nil
	}
	sch.mu.Lock()
	sch.parent = ctx
	sch.stopping = false
	sch.mu.Unlock()

	// Re-attach schedulers for every running record.
	for _, rec := range sch.svc.deps.Cache.ActiveRuntimes() {
		if rec.Status != RuntimeStatusRunning {
			continue
		}
		sch.startGame(rec)
	}

	<-ctx.Done()
	return nil
}

// Shutdown cancels every per-game goroutine and waits for them to
// drain. The provided context bounds the wait.
func (sch *Scheduler) Shutdown(ctx context.Context) error {
	if sch == nil {
		return nil
	}
	sch.mu.Lock()
	sch.stopping = true
	games := make([]*scheduledGame, 0, len(sch.tickers))
	for _, g := range sch.tickers {
		games = append(games, g)
	}
	sch.tickers = make(map[uuid.UUID]*scheduledGame)
	sch.mu.Unlock()

	for _, g := range games {
		g.cancel()
	}
	for _, g := range games {
		select {
		case <-g.done:
		case <-ctx.Done():
			return ctx.Err()
		}
	}
	return nil
}

// startGame attaches a per-game scheduler goroutine. Idempotent: a
// repeated call replaces the old goroutine with a fresh one bound to
// the supplied record.
func (sch *Scheduler) startGame(rec RuntimeRecord) {
	if sch == nil {
		return
	}
	sch.mu.Lock()
	if sch.stopping || sch.parent == nil {
		sch.mu.Unlock()
		return
	}
	if existing, ok := sch.tickers[rec.GameID]; ok {
		existing.cancel()
		sch.mu.Unlock()
		<-existing.done
		sch.mu.Lock()
	}
	parent := sch.parent
	if parent == nil {
		sch.mu.Unlock()
		return
	}
	gameCtx, cancel := context.WithCancel(parent)
	g := &scheduledGame{cancel: cancel, done: make(chan struct{})}
	sch.tickers[rec.GameID] = g
	sch.mu.Unlock()

	go sch.loop(gameCtx, rec, g.done)
}

// stopGame cancels the goroutine tied to gameID. Idempotent.
func (sch *Scheduler) stopGame(gameID uuid.UUID) {
	if sch == nil {
		return
	}
	sch.mu.Lock()
	g, ok := sch.tickers[gameID]
	if ok {
		delete(sch.tickers, gameID)
	}
	sch.mu.Unlock()
	if !ok {
		return
	}
	g.cancel()
	<-g.done
}

// activeCount reports how many games currently have a scheduler
// goroutine. Used by tests.
func (sch *Scheduler) activeCount() int {
	sch.mu.Lock()
	defer sch.mu.Unlock()
	return len(sch.tickers)
}

// tickInterval computes the wait for the next scheduler firing. When
// the cron schedule fails to parse the loop falls back to a one-hour
// safety interval and logs a warning so operators notice.
func (sch *Scheduler) loop(ctx context.Context, rec RuntimeRecord, done chan struct{}) {
	defer close(done)
	logger := sch.svc.deps.Logger.With(zap.String("game_id", rec.GameID.String()))

	schedule, err := cronutil.Parse(rec.TurnSchedule)
	if err != nil {
		logger.Warn("invalid turn_schedule, scheduler stopping",
			zap.String("turn_schedule", rec.TurnSchedule),
			zap.Error(err))
		return
	}

	for {
		latest, ok := sch.svc.deps.Cache.GetRuntime(rec.GameID)
		if !ok {
			return
		}
		if latest.Status != RuntimeStatusRunning {
			return
		}
		now := sch.svc.deps.Now().UTC()
		next := schedule.Next(now)
		wait := next.Sub(now)
		if latest.SkipNextTick {
			wait = 0
		}
		if wait < 0 {
			wait = 0
		}

		timer := time.NewTimer(wait)
		select {
		case <-ctx.Done():
			timer.Stop()
			return
		case <-timer.C:
		}
		// Fresh fetch in case of pause / status change while waiting.
		current, ok := sch.svc.deps.Cache.GetRuntime(rec.GameID)
		if !ok {
			return
		}
		if current.Status != RuntimeStatusRunning {
			return
		}
		if current.Paused {
			continue
		}
		if err := sch.tick(ctx, current); err != nil {
			logger.Warn("scheduler tick failed", zap.Error(err))
		}
	}
}

// tick runs one engine /admin/turn call under the per-game mutex,
// publishes the resulting snapshot, and clears `skip_next_tick`.
func (sch *Scheduler) tick(ctx context.Context, rec RuntimeRecord) error {
	mu := sch.svc.gameLock(rec.GameID)
	if !mu.TryLock() {
		return nil // another op is in flight; skip this tick
	}
	defer mu.Unlock()

	op, err := sch.svc.beginOperation(ctx, rec.GameID, OpTurn, OpSourceScheduler)
	if err != nil {
		return err
	}
	state, err := sch.svc.deps.Engine.Turn(ctx, rec.EngineEndpoint)
	if err != nil {
		sch.svc.completeOperation(ctx, op, err)
		_, _ = sch.svc.transitionRuntimeStatus(ctx, rec.GameID, RuntimeStatusEngineUnreachable, "")
		// On engine unreachable, also clear skip_next_tick so the next
		// real tick can start fresh.
		_ = sch.clearSkipFlag(ctx, rec.GameID)
		// Best-effort: ask Docker whether the container is still
		// alive; if it's gone we mark the runtime row as removed.
		if rec.CurrentContainerID != "" {
			if _, inspErr := sch.svc.deps.Docker.InspectContainer(ctx, rec.CurrentContainerID); errors.Is(inspErr, dockerclient.ErrContainerNotFound) {
				_, _ = sch.svc.transitionRuntimeStatus(ctx, rec.GameID, RuntimeStatusRemoved, "")
			}
		}
		return err
	}
	if err := sch.svc.publishSnapshot(ctx, rec.GameID, state); err != nil {
		sch.svc.completeOperation(ctx, op, err)
		return err
	}
	sch.svc.completeOperation(ctx, op, nil)
	_ = sch.clearSkipFlag(ctx, rec.GameID)
	return nil
}

func (sch *Scheduler) clearSkipFlag(ctx context.Context, gameID uuid.UUID) error {
	rec, ok := sch.svc.deps.Cache.GetRuntime(gameID)
	if !ok || !rec.SkipNextTick {
		return nil
	}
	skip := false
	now := sch.svc.deps.Now().UTC()
	updated, err := sch.svc.deps.Store.UpdateRuntimeRecord(ctx, gameID, runtimeRecordUpdate{SkipNextTick: &skip}, now)
	if err != nil {
		return err
	}
	sch.svc.deps.Cache.PutRuntime(updated)
	return nil
}