package providerhealth

import (
	"context"
	"encoding/json"
	"errors"
	"log/slog"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"github.com/redis/go-redis/v9"
)

const (
	defaultHistoryStream    = "aigw:provider-health:events"
	defaultHistoryQueueSize = 4096
	defaultHistoryMaxEvents = 20_000
	defaultHistoryTTL       = 15 * time.Minute
	historyPublishBatchSize = 64
	historyCommandTimeout   = 500 * time.Millisecond
	historyRetryCooldown    = 5 * time.Second
)

type HistoryOptions struct {
	Enabled   bool
	RedisURL  string
	Stream    string
	Instance  string
	QueueSize int
	MaxEvents int64
	TTL       time.Duration
	Logger    *slog.Logger
	Client    *redis.Client
	Metrics   SharedHistoryMetrics
}

type SharedHistoryMetrics interface {
	ProviderHealthSharedPublished()
	ProviderHealthSharedImported()
	ProviderHealthSharedDropped()
	ProviderHealthSharedRedisFailure()
	ProviderHealthSharedConnected(bool)
}

type RedisHistory struct {
	redis       *redis.Client
	ownedClient bool
	stream      string
	instance    string
	queue       chan Event
	maxEvents   int64
	ttl         time.Duration
	logger      *slog.Logger
	metrics     SharedHistoryMetrics
	retryAt     atomic.Int64
	dropped     atomic.Uint64
	closed      atomic.Bool
	failureSeen atomic.Bool
}

func NewRedisHistory(options HistoryOptions) *RedisHistory {
	if !options.Enabled || (strings.TrimSpace(options.RedisURL) == "" && options.Client == nil) {
		return nil
	}
	if options.Logger == nil {
		options.Logger = slog.Default()
	}
	if options.Stream == "" {
		options.Stream = defaultHistoryStream
	}
	if options.QueueSize <= 0 {
		options.QueueSize = defaultHistoryQueueSize
	}
	if options.MaxEvents <= 0 {
		options.MaxEvents = defaultHistoryMaxEvents
	}
	if options.TTL <= 0 {
		options.TTL = defaultHistoryTTL
	}
	client := options.Client
	ownedClient := false
	if client == nil {
		redisOptions, err := redis.ParseURL(options.RedisURL)
		if err != nil {
			options.Logger.Warn("provider_health_redis_config_invalid", "error", err, "fallback", "local")
			return nil
		}
		redisOptions.MaxRetries = -1
		redisOptions.DialerRetries = 1
		redisOptions.DialTimeout = historyCommandTimeout
		redisOptions.ReadTimeout = historyCommandTimeout
		redisOptions.WriteTimeout = historyCommandTimeout
		redisOptions.PoolTimeout = historyCommandTimeout
		client = redis.NewClient(redisOptions)
		ownedClient = true
	}
	return &RedisHistory{redis: client, ownedClient: ownedClient, stream: options.Stream, instance: options.Instance,
		queue: make(chan Event, options.QueueSize), maxEvents: options.MaxEvents, ttl: options.TTL,
		logger: options.Logger, metrics: options.Metrics}
}

func (h *RedisHistory) Enqueue(event Event) {
	if h == nil || h.closed.Load() {
		return
	}
	select {
	case h.queue <- event:
	default:
		if h.recordDropped(1) == 1 {
			h.logger.Warn("provider_health_share_queue_full", "fallback", "local")
		}
	}
}

func (h *RedisHistory) Run(ctx context.Context, tracker *Tracker) {
	if h == nil || tracker == nil {
		return
	}
	var workers sync.WaitGroup
	workers.Add(2)
	go func() {
		defer workers.Done()
		h.runPublisher(ctx)
	}()
	go func() {
		defer workers.Done()
		h.runReader(ctx, tracker)
	}()
	workers.Wait()
}

func (h *RedisHistory) runPublisher(ctx context.Context) {
	batch := make([]Event, 0, historyPublishBatchSize)
	for {
		select {
		case <-ctx.Done():
			return
		case event := <-h.queue:
			batch = append(batch[:0], event)
		}
		for len(batch) < historyPublishBatchSize {
			select {
			case event := <-h.queue:
				batch = append(batch, event)
			default:
				h.publishBatch(ctx, batch)
				batch = batch[:0]
				goto nextBatch
			}
		}
		h.publishBatch(ctx, batch)
		batch = batch[:0]
	nextBatch:
	}
}

func (h *RedisHistory) runReader(ctx context.Context, tracker *Tracker) {
	lastID := h.importRecent(ctx, tracker)
	if lastID == "" {
		lastID = "0-0"
	}
	for ctx.Err() == nil {
		if time.Now().UnixNano() < h.retryAt.Load() {
			if !waitHistoryRetry(ctx, 100*time.Millisecond) {
				return
			}
			continue
		}
		lastID = h.read(ctx, tracker, lastID)
	}
}

func (h *RedisHistory) Close() error {
	if h == nil || !h.closed.CompareAndSwap(false, true) || !h.ownedClient {
		return nil
	}
	return h.redis.Close()
}

func (h *RedisHistory) Dropped() uint64 {
	if h == nil {
		return 0
	}
	return h.dropped.Load()
}

func (h *RedisHistory) publishBatch(parent context.Context, events []Event) {
	if len(events) == 0 {
		return
	}
	if time.Now().UnixNano() < h.retryAt.Load() {
		h.recordDropped(uint64(len(events)))
		return
	}
	ctx, cancel := context.WithTimeout(parent, historyCommandTimeout)
	defer cancel()
	pipe := h.redis.Pipeline()
	published := 0
	for _, event := range events {
		payload, err := json.Marshal(event)
		if err != nil {
			h.recordDropped(1)
			continue
		}
		pipe.XAdd(ctx, &redis.XAddArgs{Stream: h.stream, MaxLen: h.maxEvents, Approx: true,
			Values: map[string]any{"instance": h.instance, "event": string(payload)}})
		published++
	}
	if published == 0 {
		pipe.Discard()
		return
	}
	pipe.Expire(ctx, h.stream, h.ttl)
	if _, err := pipe.Exec(ctx); err != nil {
		h.recordDropped(uint64(published))
		h.fail(err)
		return
	}
	h.recovered()
	for range published {
		if h.metrics != nil {
			h.metrics.ProviderHealthSharedPublished()
		}
	}
}

func (h *RedisHistory) importRecent(parent context.Context, tracker *Tracker) string {
	ctx, cancel := context.WithTimeout(parent, historyCommandTimeout)
	defer cancel()
	messages, err := h.redis.XRevRangeN(ctx, h.stream, "+", "-", h.maxEvents).Result()
	if err != nil {
		if !errors.Is(err, redis.Nil) {
			h.fail(err)
		}
		return ""
	}
	if len(messages) == 0 {
		h.recovered()
		return ""
	}
	lastID := messages[0].ID
	for index := len(messages) - 1; index >= 0; index-- {
		h.applyMessage(tracker, messages[index])
	}
	h.recovered()
	return lastID
}

func (h *RedisHistory) read(parent context.Context, tracker *Tracker, lastID string) string {
	if time.Now().UnixNano() < h.retryAt.Load() {
		return lastID
	}
	ctx, cancel := context.WithTimeout(parent, historyCommandTimeout)
	defer cancel()
	streams, err := h.redis.XRead(ctx, &redis.XReadArgs{Streams: []string{h.stream, lastID}, Count: 512, Block: 200 * time.Millisecond}).Result()
	if errors.Is(err, redis.Nil) {
		h.recovered()
		return lastID
	}
	if err != nil {
		h.fail(err)
		return lastID
	}
	h.recovered()
	for _, stream := range streams {
		for _, message := range stream.Messages {
			h.applyMessage(tracker, message)
			lastID = message.ID
		}
	}
	return lastID
}

func (h *RedisHistory) applyMessage(tracker *Tracker, message redis.XMessage) {
	if asString(message.Values["instance"]) == h.instance && h.instance != "" {
		return
	}
	payload := asString(message.Values["event"])
	var event Event
	if payload == "" || json.Unmarshal([]byte(payload), &event) != nil {
		return
	}
	if event.ObservedAt.Before(time.Now().Add(-h.ttl)) {
		return
	}
	tracker.ApplyShared(event)
	if h.metrics != nil {
		h.metrics.ProviderHealthSharedImported()
	}
}

func (h *RedisHistory) fail(err error) {
	h.retryAt.Store(time.Now().Add(historyRetryCooldown).UnixNano())
	if h.metrics != nil {
		h.metrics.ProviderHealthSharedConnected(false)
	}
	if h.failureSeen.CompareAndSwap(false, true) {
		if h.metrics != nil {
			h.metrics.ProviderHealthSharedRedisFailure()
		}
		h.logger.Warn("provider_health_redis_unavailable", "error", err, "fallback", "local", "retry_after", historyRetryCooldown)
	}
}

func (h *RedisHistory) recovered() {
	h.retryAt.Store(0)
	if h.metrics != nil {
		h.metrics.ProviderHealthSharedConnected(true)
	}
	if h.failureSeen.Swap(false) {
		h.logger.Info("provider_health_redis_recovered")
	}
}

func (h *RedisHistory) recordDropped(count uint64) uint64 {
	total := h.dropped.Add(count)
	if h.metrics != nil {
		for range count {
			h.metrics.ProviderHealthSharedDropped()
		}
	}
	return total
}

func waitHistoryRetry(ctx context.Context, delay time.Duration) bool {
	timer := time.NewTimer(delay)
	defer timer.Stop()
	select {
	case <-ctx.Done():
		return false
	case <-timer.C:
		return true
	}
}

func asString(value any) string {
	switch item := value.(type) {
	case string:
		return item
	case []byte:
		return string(item)
	default:
		return ""
	}
}