package worker

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

	"galaxy/mail/internal/domain/attempt"
	"galaxy/mail/internal/domain/common"
	deliverydomain "galaxy/mail/internal/domain/delivery"
	"galaxy/mail/internal/logging"
	"galaxy/mail/internal/service/executeattempt"
)

const (
	defaultSchedulePollInterval = 250 * time.Millisecond
	defaultRecoveryInterval     = 30 * time.Second
	defaultRecoveryGrace        = 30 * time.Second
)

// AttemptExecutionStore describes the durable state operations used by the
// attempt scheduler.
type AttemptExecutionStore interface {
	// NextDueDeliveryIDs returns up to limit due delivery identifiers.
	NextDueDeliveryIDs(context.Context, time.Time, int64) ([]common.DeliveryID, error)

	// SendingDeliveryIDs returns every delivery currently indexed as sending.
	SendingDeliveryIDs(context.Context) ([]common.DeliveryID, error)

	// LoadWorkItem loads the current delivery and active attempt for deliveryID.
	LoadWorkItem(context.Context, common.DeliveryID) (executeattempt.WorkItem, bool, error)

	// ClaimDueAttempt atomically claims the due scheduled attempt for
	// deliveryID.
	ClaimDueAttempt(context.Context, common.DeliveryID, time.Time) (executeattempt.WorkItem, bool, error)

	// RemoveScheduledDelivery removes deliveryID from the attempt schedule set.
	RemoveScheduledDelivery(context.Context, common.DeliveryID) error
}

// AttemptPreparationService prepares queued template deliveries and recovers
// stale claimed attempts.
type AttemptPreparationService interface {
	// Prepare renders one queued template delivery when needed and reports
	// whether the scheduler may continue to claim the attempt.
	Prepare(context.Context, executeattempt.WorkItem) (bool, error)

	// RecoverExpired marks one stale in-progress attempt as timed out.
	RecoverExpired(context.Context, executeattempt.WorkItem) error
}

// SchedulerTelemetry records low-cardinality scheduler-side delivery
// transitions.
type SchedulerTelemetry interface {
	// RecordDeliveryStatusTransition records one durable delivery status
	// transition.
	RecordDeliveryStatusTransition(context.Context, string, string)
}

// SchedulerConfig stores the dependencies used by Scheduler.
type SchedulerConfig struct {
	// Store owns the durable scheduled and in-progress attempt state.
	Store AttemptExecutionStore

	// Service prepares queued template deliveries and recovers stale claims.
	Service AttemptPreparationService

	// WorkQueue stores the claimed attempt handoff channel consumed by the
	// attempt worker pool.
	WorkQueue chan<- executeattempt.WorkItem

	// Clock provides the scheduler wall clock.
	Clock Clock

	// AttemptTimeout stores the provider execution budget used to derive claim
	// recovery deadlines.
	AttemptTimeout time.Duration

	// Telemetry records scheduler-side delivery transitions.
	Telemetry SchedulerTelemetry

	// PollInterval overrides the default due-attempt polling interval when
	// positive.
	PollInterval time.Duration

	// RecoveryInterval overrides the default stale-claim recovery interval when
	// positive.
	RecoveryInterval time.Duration

	// RecoveryGrace overrides the default stale-claim grace window when
	// positive.
	RecoveryGrace time.Duration
}

// Scheduler polls due attempts, optionally renders queued template
// deliveries, atomically claims runnable work, and recovers stale in-progress
// ownership.
type Scheduler struct {
	store            AttemptExecutionStore
	service          AttemptPreparationService
	workQueue        chan<- executeattempt.WorkItem
	clock            Clock
	attemptTimeout   time.Duration
	telemetry        SchedulerTelemetry
	pollInterval     time.Duration
	recoveryInterval time.Duration
	recoveryGrace    time.Duration
	logger           *slog.Logger
}

// NewScheduler constructs one attempt scheduler.
func NewScheduler(cfg SchedulerConfig, logger *slog.Logger) (*Scheduler, error) {
	switch {
	case cfg.Store == nil:
		return nil, errors.New("new scheduler: nil attempt execution store")
	case cfg.Service == nil:
		return nil, errors.New("new scheduler: nil attempt preparation service")
	case cfg.WorkQueue == nil:
		return nil, errors.New("new scheduler: nil work queue")
	case cfg.Clock == nil:
		return nil, errors.New("new scheduler: nil clock")
	case cfg.AttemptTimeout <= 0:
		return nil, errors.New("new scheduler: non-positive attempt timeout")
	}
	if logger == nil {
		logger = slog.Default()
	}

	pollInterval := cfg.PollInterval
	if pollInterval <= 0 {
		pollInterval = defaultSchedulePollInterval
	}

	recoveryInterval := cfg.RecoveryInterval
	if recoveryInterval <= 0 {
		recoveryInterval = defaultRecoveryInterval
	}

	recoveryGrace := cfg.RecoveryGrace
	if recoveryGrace <= 0 {
		recoveryGrace = defaultRecoveryGrace
	}

	return &Scheduler{
		store:            cfg.Store,
		service:          cfg.Service,
		workQueue:        cfg.WorkQueue,
		clock:            cfg.Clock,
		attemptTimeout:   cfg.AttemptTimeout,
		telemetry:        cfg.Telemetry,
		pollInterval:     pollInterval,
		recoveryInterval: recoveryInterval,
		recoveryGrace:    recoveryGrace,
		logger: logger.With(
			"component", "scheduler",
			"poll_interval", pollInterval.String(),
			"recovery_interval", recoveryInterval.String(),
			"recovery_grace", recoveryGrace.String(),
		),
	}, nil
}

// Run starts the scheduler loop and blocks until ctx is canceled or one
// durable state operation fails.
func (scheduler *Scheduler) Run(ctx context.Context) error {
	if ctx == nil {
		return errors.New("run scheduler: nil context")
	}
	if err := ctx.Err(); err != nil {
		return err
	}
	if scheduler == nil {
		return errors.New("run scheduler: nil scheduler")
	}

	scheduler.logger.Info("scheduler started")
	defer scheduler.logger.Info("scheduler stopped")

	if err := scheduler.recoverExpired(ctx); err != nil {
		return err
	}

	pollTicker := time.NewTicker(scheduler.pollInterval)
	defer pollTicker.Stop()

	recoveryTicker := time.NewTicker(scheduler.recoveryInterval)
	defer recoveryTicker.Stop()

	for {
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-pollTicker.C:
			if err := scheduler.dispatchDueAttempts(ctx); err != nil {
				return err
			}
		case <-recoveryTicker.C:
			if err := scheduler.recoverExpired(ctx); err != nil {
				return err
			}
		}
	}
}

// Shutdown stops the scheduler within ctx. The scheduler does not own
// additional resources beyond its run loop.
func (scheduler *Scheduler) Shutdown(ctx context.Context) error {
	if ctx == nil {
		return errors.New("shutdown scheduler: nil context")
	}
	if scheduler == nil {
		return nil
	}

	return nil
}

func (scheduler *Scheduler) dispatchDueAttempts(ctx context.Context) error {
	for {
		now := scheduler.clock.Now().UTC().Truncate(time.Millisecond)
		deliveryIDs, err := scheduler.store.NextDueDeliveryIDs(ctx, now, 1)
		if err != nil {
			return fmt.Errorf("dispatch due attempts: %w", err)
		}
		if len(deliveryIDs) == 0 {
			return nil
		}

		if err := scheduler.dispatchOne(ctx, deliveryIDs[0], now); err != nil {
			return err
		}
	}
}

func (scheduler *Scheduler) dispatchOne(ctx context.Context, deliveryID common.DeliveryID, now time.Time) error {
	workItem, found, err := scheduler.store.LoadWorkItem(ctx, deliveryID)
	if err != nil {
		return fmt.Errorf("dispatch due delivery %q: load work item: %w", deliveryID, err)
	}
	if !found {
		if err := scheduler.store.RemoveScheduledDelivery(ctx, deliveryID); err != nil {
			return fmt.Errorf("dispatch due delivery %q: remove stale schedule: %w", deliveryID, err)
		}
		return nil
	}
	if !isSchedulable(workItem) {
		if err := scheduler.store.RemoveScheduledDelivery(ctx, deliveryID); err != nil {
			return fmt.Errorf("dispatch due delivery %q: remove unschedulable entry: %w", deliveryID, err)
		}
		return nil
	}

	ready, err := scheduler.service.Prepare(ctx, workItem)
	if err != nil {
		return fmt.Errorf("dispatch due delivery %q: prepare attempt: %w", deliveryID, err)
	}
	if !ready {
		return nil
	}

	claimed, found, err := scheduler.store.ClaimDueAttempt(ctx, deliveryID, now)
	if err != nil {
		return fmt.Errorf("dispatch due delivery %q: claim attempt: %w", deliveryID, err)
	}
	if !found {
		return nil
	}
	scheduler.recordStatusTransition(ctx, claimed.Delivery)

	select {
	case <-ctx.Done():
		return ctx.Err()
	case scheduler.workQueue <- claimed:
		logArgs := logging.DeliveryAttemptAttrs(claimed.Delivery, claimed.Attempt)
		logArgs = append(logArgs, logging.TraceAttrsFromContext(ctx)...)
		scheduler.logger.Debug("attempt claimed", logArgs...)
		return nil
	}
}

func (scheduler *Scheduler) recoverExpired(ctx context.Context) error {
	now := scheduler.clock.Now().UTC().Truncate(time.Millisecond)
	deadline := now.Add(-(scheduler.attemptTimeout + scheduler.recoveryGrace))

	deliveryIDs, err := scheduler.store.SendingDeliveryIDs(ctx)
	if err != nil {
		return fmt.Errorf("recover expired attempts: %w", err)
	}

	for _, deliveryID := range deliveryIDs {
		workItem, found, err := scheduler.store.LoadWorkItem(ctx, deliveryID)
		if err != nil {
			return fmt.Errorf("recover expired delivery %q: load work item: %w", deliveryID, err)
		}
		if !found || !isRecoverable(workItem) || workItem.Attempt.StartedAt == nil {
			continue
		}
		if workItem.Attempt.StartedAt.After(deadline) {
			continue
		}

		if err := scheduler.service.RecoverExpired(ctx, workItem); err != nil {
			return fmt.Errorf("recover expired delivery %q: %w", deliveryID, err)
		}

		logArgs := logging.DeliveryAttemptAttrs(workItem.Delivery, workItem.Attempt)
		logArgs = append(logArgs, "started_at", workItem.Attempt.StartedAt)
		logArgs = append(logArgs, logging.TraceAttrsFromContext(ctx)...)
		scheduler.logger.Warn("attempt claim expired", logArgs...)
	}

	return nil
}

func (scheduler *Scheduler) recordStatusTransition(ctx context.Context, record deliverydomain.Delivery) {
	if scheduler == nil || scheduler.telemetry == nil {
		return
	}

	scheduler.telemetry.RecordDeliveryStatusTransition(ctx, string(record.Status), string(record.Source))
}

func isSchedulable(item executeattempt.WorkItem) bool {
	if item.Delivery.AttemptCount != item.Attempt.AttemptNo {
		return false
	}
	switch item.Delivery.Status {
	case deliverydomain.StatusQueued, deliverydomain.StatusRendered:
	default:
		return false
	}

	return item.Attempt.Status == attempt.StatusScheduled
}

func isRecoverable(item executeattempt.WorkItem) bool {
	if item.Delivery.AttemptCount != item.Attempt.AttemptNo {
		return false
	}
	if item.Delivery.Status != deliverydomain.StatusSending {
		return false
	}

	return item.Attempt.Status == attempt.StatusInProgress
}