// Package delivery manages asynchronous event delivery
// to configured targets.
package delivery

import (
	"context"
	"fmt"
	"log/slog"
	"net/http"
	"sync"
	"time"

	"go.uber.org/fx"
	"gorm.io/gorm"
	"sneak.berlin/go/webhooker/internal/database"
	"sneak.berlin/go/webhooker/internal/logger"
)

const (
	// deliveryChannelSize is the buffer size for the delivery
	// channel. New Tasks from the webhook handler are sent
	// here. Workers drain this channel. Sized large enough
	// that the webhook handler should never block under
	// normal load.
	deliveryChannelSize = 10000

	// retryChannelSize is the buffer size for the retry
	// channel. Timer-fired retries are sent here for
	// processing by workers.
	retryChannelSize = 10000

	// defaultWorkers is the number of worker goroutines in
	// the delivery engine pool. At most this many deliveries
	// are in-flight at any time, preventing goroutine
	// explosions regardless of queue depth.
	defaultWorkers = 10

	// retrySweepInterval is how often the periodic retry
	// sweep runs.
	retrySweepInterval = 60 * time.Second

	// MaxInlineBodySize is the maximum event body size that
	// will be carried inline in a Task through the channel.
	// Bodies at or above this size are left nil and fetched
	// from the per-webhook database on demand.
	MaxInlineBodySize = 16 * 1024

	// httpClientTimeout is the timeout for outbound HTTP
	// requests.
	httpClientTimeout = 30 * time.Second

	// maxBodyLog is the maximum response body length to
	// store in DeliveryResult.
	maxBodyLog = 4096

	// maxBackoffShift caps the exponential backoff shift to
	// avoid integer overflow in the 1<<shift expression.
	maxBackoffShift = 30

	// httpSuccessMin is the lower bound (inclusive) of the
	// HTTP success status code range.
	httpSuccessMin = 200

	// httpSuccessMax is the upper bound (exclusive) of the
	// HTTP success status code range.
	httpSuccessMax = 300
)

// Task contains everything needed to deliver an event to a
// single target.
type Task struct {
	DeliveryID   string
	EventID      string
	WebhookID    string
	EntrypointID string

	TargetID     string
	TargetName   string
	TargetType   database.TargetType
	TargetConfig string
	MaxRetries   int

	Method      string
	Headers     string
	ContentType string
	Body        *string

	AttemptNum int
}

// Notifier is the interface for notifying the delivery
// engine about new deliveries.
type Notifier interface {
	Notify(tasks []Task)
}

// EngineParams are the fx dependencies for the delivery
// engine.
type EngineParams struct {
	fx.In

	DB        *database.Database
	DBManager *database.WebhookDBManager
	Logger    *logger.Logger
}

// Engine processes queued deliveries in the background
// using a bounded worker pool architecture. It owns only
// the cross-target machinery: the worker pool, the queue
// and retry channels, restart recovery, and the persistence
// helpers and Scheduler that individual targets rely on.
// Each target type owns its own delivery, including retries,
// backoff, and circuit breaking.
type Engine struct {
	database   *database.Database
	dbManager  *database.WebhookDBManager
	log        *slog.Logger
	cancel     context.CancelFunc
	wg         sync.WaitGroup
	deliveryCh chan Task
	retryCh    chan Task
	workers    int

	// targets maps each target type to its implementation.
	targets map[database.TargetType]Target

	// httpTarget is retained so tests can reach the HTTP
	// target's shared client and circuit breakers.
	httpTarget *httpTarget
}

// New creates and registers the delivery engine with the
// fx lifecycle.
func New(
	lc fx.Lifecycle,
	params EngineParams,
) *Engine {
	e := &Engine{
		database:   params.DB,
		dbManager:  params.DBManager,
		log:        params.Logger.Get(),
		deliveryCh: make(chan Task, deliveryChannelSize),
		retryCh:    make(chan Task, retryChannelSize),
		workers:    defaultWorkers,
	}

	e.initTargets(&http.Client{
		Timeout:   httpClientTimeout,
		Transport: NewSSRFSafeTransport(),
	})

	lc.Append(fx.Hook{
		OnStart: func(ctx context.Context) error {
			e.start(ctx)

			return nil
		},
		OnStop: func(_ context.Context) error {
			e.stop()

			return nil
		},
	})

	return e
}

// Notify signals the delivery engine that new deliveries
// are ready.
func (e *Engine) Notify(tasks []Task) {
	for i := range tasks {
		select {
		case e.deliveryCh <- tasks[i]:
		default:
			e.log.Warn(
				"delivery channel full, "+
					"task will be recovered on restart",
				"delivery_id", tasks[i].DeliveryID,
				"event_id", tasks[i].EventID,
			)
		}
	}
}

// ScheduleRetry schedules a task to be re-enqueued onto the
// retry channel after delay. It implements the Scheduler
// interface the targets use to own their durable retries.
func (e *Engine) ScheduleRetry(
	task Task, delay time.Duration,
) {
	e.log.Debug(
		"scheduling delivery retry",
		"webhook_id", task.WebhookID,
		"delivery_id", task.DeliveryID,
		"delay", delay,
		"next_attempt", task.AttemptNum,
	)

	time.AfterFunc(delay, func() {
		select {
		case e.retryCh <- task:
		default:
			e.log.Warn(
				"retry channel full, delivery "+
					"will be recovered by periodic sweep",
				"delivery_id", task.DeliveryID,
				"webhook_id", task.WebhookID,
			)
		}
	})
}

func (e *Engine) start(ctx context.Context) {
	ctx, cancel := context.WithCancel(ctx)
	e.cancel = cancel

	for range e.workers {
		e.wg.Add(1)

		go e.worker(ctx)
	}

	e.wg.Add(1)

	go e.recoverPending(ctx)

	e.wg.Add(1)

	go e.retrySweep(ctx)

	e.log.Info(
		"delivery engine started",
		"workers", e.workers,
	)
}

func (e *Engine) stop() {
	e.log.Info("delivery engine stopping")
	e.cancel()
	e.wg.Wait()
	e.log.Info("delivery engine stopped")
}

func (e *Engine) worker(ctx context.Context) {
	defer e.wg.Done()

	for {
		select {
		case <-ctx.Done():
			return
		case task := <-e.deliveryCh:
			e.processNewTask(ctx, &task)
		case task := <-e.retryCh:
			e.processRetryTask(ctx, &task)
		}
	}
}

func (e *Engine) recoverPending(ctx context.Context) {
	defer e.wg.Done()

	e.recoverInFlight(ctx)
}

func (e *Engine) processNewTask(
	ctx context.Context, task *Task,
) {
	webhookDB, err := e.dbManager.GetDB(task.WebhookID)
	if err != nil {
		e.log.Error(
			"failed to get webhook database",
			"webhook_id", task.WebhookID,
			"error", err,
		)

		return
	}

	event := buildEventFromTask(task)

	event, err = e.resolveEventBody(
		webhookDB, event, task,
	)
	if err != nil {
		e.log.Error(
			"failed to fetch event body from database",
			"event_id", task.EventID,
			"error", err,
		)

		return
	}

	target := buildTargetFromTask(task)

	d := &database.Delivery{
		EventID:  task.EventID,
		TargetID: task.TargetID,
		Status:   database.DeliveryStatusPending,
		Event:    event,
		Target:   target,
	}
	d.ID = task.DeliveryID

	e.processDelivery(ctx, webhookDB, d, task)
}

func (e *Engine) processRetryTask(
	ctx context.Context, task *Task,
) {
	webhookDB, err := e.dbManager.GetDB(task.WebhookID)
	if err != nil {
		e.log.Error(
			"failed to get webhook database for retry",
			"webhook_id", task.WebhookID,
			"delivery_id", task.DeliveryID,
			"error", err,
		)

		return
	}

	d, err := e.loadRetryDelivery(
		webhookDB, task.DeliveryID,
	)
	if err != nil {
		e.log.Error(
			"failed to load delivery for retry",
			"delivery_id", task.DeliveryID,
			"error", err,
		)

		return
	}

	if d.Status != database.DeliveryStatusRetrying {
		e.log.Debug(
			"skipping retry for delivery "+
				"no longer in retrying status",
			"delivery_id", d.ID,
			"status", d.Status,
		)

		return
	}

	event := buildEventFromTask(task)

	event, err = e.resolveEventBody(
		webhookDB, event, task,
	)
	if err != nil {
		e.log.Error(
			"failed to fetch event body for retry",
			"event_id", task.EventID,
			"error", err,
		)

		return
	}

	target := buildTargetFromTask(task)
	d.EventID = task.EventID
	d.TargetID = task.TargetID
	d.Event = event
	d.Target = target

	e.processDelivery(ctx, webhookDB, d, task)
}

func (e *Engine) recoverInFlight(ctx context.Context) {
	var webhookIDs []string

	err := e.database.DB().
		Model(&database.Webhook{}).
		Pluck("id", &webhookIDs).Error
	if err != nil {
		e.log.Error(
			"failed to query webhook IDs for recovery",
			"error", err,
		)

		return
	}

	for _, webhookID := range webhookIDs {
		select {
		case <-ctx.Done():
			return
		default:
		}

		if !e.dbManager.DBExists(webhookID) {
			continue
		}

		e.recoverWebhookDeliveries(ctx, webhookID)
	}
}

func (e *Engine) recoverWebhookDeliveries(
	ctx context.Context, webhookID string,
) {
	webhookDB, err := e.dbManager.GetDB(webhookID)
	if err != nil {
		e.log.Error(
			"failed to get webhook database for recovery",
			"webhook_id", webhookID,
			"error", err,
		)

		return
	}

	e.recoverPendingDeliveries(
		ctx, webhookDB, webhookID,
	)

	e.recoverRetryingDeliveries(
		webhookDB, webhookID,
	)
}

func (e *Engine) recoverRetryingDeliveries(
	webhookDB *gorm.DB, webhookID string,
) {
	var retrying []database.Delivery

	err := webhookDB.
		Where(
			"status = ?",
			database.DeliveryStatusRetrying,
		).
		Find(&retrying).Error
	if err != nil {
		e.log.Error(
			"failed to query retrying deliveries "+
				"for recovery",
			"webhook_id", webhookID,
			"error", err,
		)

		return
	}

	for i := range retrying {
		e.recoverSingleRetry(
			webhookDB, webhookID, &retrying[i],
		)
	}
}

// recoverSingleRetry hands an orphaned retrying delivery back
// to its target to recompute the remaining backoff, then
// reschedules it. Targets that do not own durable retries
// (fire-and-forget) never produce retrying deliveries, so
// they are skipped.
func (e *Engine) recoverSingleRetry(
	webhookDB *gorm.DB,
	webhookID string,
	d *database.Delivery,
) {
	target, err := e.loadTarget(d.TargetID)
	if err != nil {
		e.log.Error(
			"failed to load target for retrying "+
				"delivery recovery",
			"delivery_id", d.ID,
			"target_id", d.TargetID,
			"error", err,
		)

		return
	}

	rs, ok := e.targets[target.Type].(rescheduler)
	if !ok {
		return
	}

	attemptNum := e.countAttempts(webhookDB, d.ID)
	remaining := rs.remainingBackoff(
		webhookDB, d.ID, attemptNum,
	)

	event, err := e.loadEvent(webhookDB, d.EventID)
	if err != nil {
		e.log.Error(
			"failed to load event for retrying "+
				"delivery recovery",
			"delivery_id", d.ID,
			"event_id", d.EventID,
			"error", err,
		)

		return
	}

	task := buildRecoveryTask(
		d, webhookID, &event, &target, attemptNum+1,
	)

	e.log.Info(
		"recovering retrying delivery",
		"webhook_id", webhookID,
		"delivery_id", d.ID,
		"attempt", attemptNum,
		"remaining_backoff", remaining,
	)

	e.ScheduleRetry(task, remaining)
}

func (e *Engine) recoverPendingDeliveries(
	ctx context.Context,
	webhookDB *gorm.DB,
	webhookID string,
) {
	var deliveries []database.Delivery

	result := webhookDB.
		Where(
			"status = ?",
			database.DeliveryStatusPending,
		).
		Preload("Event").
		Find(&deliveries)

	if result.Error != nil {
		e.log.Error(
			"failed to query pending deliveries",
			"webhook_id", webhookID,
			"error", result.Error,
		)

		return
	}

	if len(deliveries) == 0 {
		return
	}

	e.log.Info(
		"recovering pending deliveries",
		"webhook_id", webhookID,
		"count", len(deliveries),
	)

	targetMap := e.loadTargetMap(deliveries)

	e.sendRecoveredDeliveries(
		ctx, deliveries, webhookID, targetMap,
	)
}

func (e *Engine) retrySweep(ctx context.Context) {
	defer e.wg.Done()

	ticker := time.NewTicker(retrySweepInterval)
	defer ticker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
			e.sweepOrphanedRetries(ctx)
		}
	}
}

func (e *Engine) sweepOrphanedRetries(
	ctx context.Context,
) {
	var webhookIDs []string

	err := e.database.DB().
		Model(&database.Webhook{}).
		Pluck("id", &webhookIDs).Error
	if err != nil {
		e.log.Error(
			"retry sweep: failed to query webhook IDs",
			"error", err,
		)

		return
	}

	for _, webhookID := range webhookIDs {
		select {
		case <-ctx.Done():
			return
		default:
		}

		if !e.dbManager.DBExists(webhookID) {
			continue
		}

		e.sweepWebhookRetries(ctx, webhookID)
	}
}

func (e *Engine) sweepWebhookRetries(
	ctx context.Context, webhookID string,
) {
	webhookDB, err := e.dbManager.GetDB(webhookID)
	if err != nil {
		e.log.Error(
			"retry sweep: failed to get webhook database",
			"webhook_id", webhookID,
			"error", err,
		)

		return
	}

	var retrying []database.Delivery

	err = webhookDB.
		Where(
			"status = ?",
			database.DeliveryStatusRetrying,
		).
		Find(&retrying).Error
	if err != nil {
		e.log.Error(
			"retry sweep: "+
				"failed to query retrying deliveries",
			"webhook_id", webhookID,
			"error", err,
		)

		return
	}

	for i := range retrying {
		select {
		case <-ctx.Done():
			return
		default:
		}

		e.sweepSingleRetry(
			webhookDB, webhookID, &retrying[i],
		)
	}
}

// sweepSingleRetry re-enqueues an orphaned retrying delivery
// whose backoff window has elapsed, delegating the backoff
// decision to the delivery's target. Targets that do not own
// durable retries are skipped.
func (e *Engine) sweepSingleRetry(
	webhookDB *gorm.DB,
	webhookID string,
	d *database.Delivery,
) {
	target, err := e.loadTarget(d.TargetID)
	if err != nil {
		e.log.Error(
			"retry sweep: failed to load target",
			"delivery_id", d.ID,
			"target_id", d.TargetID,
			"error", err,
		)

		return
	}

	rs, ok := e.targets[target.Type].(rescheduler)
	if !ok {
		return
	}

	attemptNum := e.countAttempts(webhookDB, d.ID)

	if !rs.backoffElapsed(
		webhookDB, d.ID, attemptNum,
	) {
		return
	}

	event, err := e.loadEvent(webhookDB, d.EventID)
	if err != nil {
		e.log.Error(
			"retry sweep: failed to load event",
			"delivery_id", d.ID,
			"event_id", d.EventID,
			"error", err,
		)

		return
	}

	task := buildRecoveryTask(
		d, webhookID, &event, &target, attemptNum+1,
	)

	select {
	case e.retryCh <- task:
		e.log.Info(
			"retry sweep: "+
				"recovered orphaned retrying delivery",
			"delivery_id", d.ID,
			"webhook_id", webhookID,
			"attempt", attemptNum+1,
		)
	default:
	}
}

// processDelivery dispatches a delivery to the target that
// owns its type. Unknown target types fail the delivery.
func (e *Engine) processDelivery(
	ctx context.Context,
	webhookDB *gorm.DB,
	d *database.Delivery,
	task *Task,
) {
	target, ok := e.targets[d.Target.Type]
	if !ok {
		e.log.Error(
			"unknown target type",
			"target_id", d.TargetID,
			"type", d.Target.Type,
		)

		e.updateDeliveryStatus(
			webhookDB, d, database.DeliveryStatusFailed,
		)

		return
	}

	target.Deliver(ctx, webhookDB, d, task, e)
}

// recordResult persists a DeliveryResult row describing a
// single attempt. It is a cross-target helper the targets
// call.
func (e *Engine) recordResult(
	webhookDB *gorm.DB,
	d *database.Delivery,
	attemptNum int,
	success bool,
	statusCode int,
	respBody, errMsg string,
	durationMs int64,
) {
	result := &database.DeliveryResult{
		DeliveryID:   d.ID,
		AttemptNum:   attemptNum,
		Success:      success,
		StatusCode:   statusCode,
		ResponseBody: truncate(respBody, maxBodyLog),
		Error:        errMsg,
		Duration:     durationMs,
	}

	err := webhookDB.Create(result).Error
	if err != nil {
		e.log.Error(
			"failed to record delivery result",
			"delivery_id", d.ID,
			"error", err,
		)
	}
}

// updateDeliveryStatus persists a new status for a delivery.
// It is a cross-target helper the targets call.
func (e *Engine) updateDeliveryStatus(
	webhookDB *gorm.DB,
	d *database.Delivery,
	status database.DeliveryStatus,
) {
	err := webhookDB.Model(d).
		Update("status", status).Error
	if err != nil {
		e.log.Error(
			"failed to update delivery status",
			"delivery_id", d.ID,
			"status", status,
			"error", err,
		)
	}
}

func truncate(s string, maxLen int) string {
	if len(s) <= maxLen {
		return s
	}

	return s[:maxLen]
}

// --- Helper functions ---

func buildEventFromTask(task *Task) database.Event {
	event := database.Event{
		EntrypointID: task.EntrypointID,
		Method:       task.Method,
		Headers:      task.Headers,
		ContentType:  task.ContentType,
	}

	event.ID = task.EventID
	event.WebhookID = task.WebhookID

	return event
}

func buildTargetFromTask(task *Task) database.Target {
	target := database.Target{
		Name:       task.TargetName,
		Type:       task.TargetType,
		Config:     task.TargetConfig,
		MaxRetries: task.MaxRetries,
	}

	target.ID = task.TargetID

	return target
}

func (e *Engine) resolveEventBody(
	webhookDB *gorm.DB,
	event database.Event,
	task *Task,
) (database.Event, error) {
	if task.Body != nil {
		event.Body = *task.Body

		return event, nil
	}

	var dbEvent database.Event

	err := webhookDB.Select("body").
		First(&dbEvent, "id = ?", task.EventID).Error
	if err != nil {
		return event, fmt.Errorf(
			"fetching event body: %w", err,
		)
	}

	event.Body = dbEvent.Body

	return event, nil
}

func (e *Engine) loadRetryDelivery(
	webhookDB *gorm.DB, deliveryID string,
) (*database.Delivery, error) {
	var d database.Delivery

	err := webhookDB.Select("id", "status").
		First(&d, "id = ?", deliveryID).Error
	if err != nil {
		return nil, fmt.Errorf(
			"loading delivery: %w", err,
		)
	}

	return &d, nil
}

func (e *Engine) countAttempts(
	webhookDB *gorm.DB, deliveryID string,
) int {
	var resultCount int64

	webhookDB.Model(&database.DeliveryResult{}).
		Where("delivery_id = ?", deliveryID).
		Count(&resultCount)

	return int(resultCount)
}

func (e *Engine) loadEvent(
	webhookDB *gorm.DB, eventID string,
) (database.Event, error) {
	var event database.Event

	err := webhookDB.
		First(&event, "id = ?", eventID).Error
	if err != nil {
		return event, fmt.Errorf(
			"loading event: %w", err,
		)
	}

	return event, nil
}

func (e *Engine) loadTarget(
	targetID string,
) (database.Target, error) {
	var target database.Target

	err := e.database.DB().
		First(&target, "id = ?", targetID).Error
	if err != nil {
		return target, fmt.Errorf(
			"loading target: %w", err,
		)
	}

	return target, nil
}

func buildRecoveryTask(
	d *database.Delivery,
	webhookID string,
	event *database.Event,
	target *database.Target,
	attemptNum int,
) Task {
	var bodyPtr *string

	if len(event.Body) < MaxInlineBodySize {
		bodyStr := event.Body
		bodyPtr = &bodyStr
	}

	return Task{
		DeliveryID:   d.ID,
		EventID:      d.EventID,
		WebhookID:    webhookID,
		EntrypointID: event.EntrypointID,
		TargetID:     target.ID,
		TargetName:   target.Name,
		TargetType:   target.Type,
		TargetConfig: target.Config,
		MaxRetries:   target.MaxRetries,
		Method:       event.Method,
		Headers:      event.Headers,
		ContentType:  event.ContentType,
		Body:         bodyPtr,
		AttemptNum:   attemptNum,
	}
}

func (e *Engine) loadTargetMap(
	deliveries []database.Delivery,
) map[string]database.Target {
	seen := make(map[string]bool)

	targetIDs := make([]string, 0, len(deliveries))

	for _, d := range deliveries {
		if !seen[d.TargetID] {
			targetIDs = append(targetIDs, d.TargetID)
			seen[d.TargetID] = true
		}
	}

	var targets []database.Target

	err := e.database.DB().
		Where("id IN ?", targetIDs).
		Find(&targets).Error
	if err != nil {
		e.log.Error(
			"failed to load targets from main DB",
			"error", err,
		)

		return nil
	}

	targetMap := make(
		map[string]database.Target, len(targets),
	)

	for _, t := range targets {
		targetMap[t.ID] = t
	}

	return targetMap
}

func (e *Engine) sendRecoveredDeliveries(
	ctx context.Context,
	deliveries []database.Delivery,
	webhookID string,
	targetMap map[string]database.Target,
) {
	for i := range deliveries {
		select {
		case <-ctx.Done():
			return
		default:
		}

		target, ok := targetMap[deliveries[i].TargetID]
		if !ok {
			e.log.Error(
				"target not found for delivery",
				"delivery_id", deliveries[i].ID,
				"target_id", deliveries[i].TargetID,
			)

			continue
		}

		task := buildRecoveryTask(
			&deliveries[i], webhookID,
			&deliveries[i].Event, &target, 1,
		)

		select {
		case e.deliveryCh <- task:
		default:
			e.log.Warn(
				"delivery channel full during "+
					"recovery, remaining deliveries "+
					"will be recovered on next restart",
				"delivery_id", deliveries[i].ID,
			)

			return
		}
	}
}