package dashboard

import (
	"context"
	"encoding/json"
	"fmt"
	"sync"
	"time"
	"tm/pkg/logger"
	"tm/pkg/redis"

	goredis "github.com/redis/go-redis/v9"
	"golang.org/x/sync/singleflight"
)

const (
	// widgetCacheTTL is the fresh window before a background reload is triggered.
	widgetCacheTTL = 60 * time.Second
	// widgetStaleGraceTTL is how long stale entries may be served while reloading.
	// Dashboard widgets have no write-side invalidation; expect up to this lag after
	// data changes (e.g. a new tender may not appear in cached widgets immediately).
	widgetStaleGraceTTL = 5 * time.Minute
	widgetReloadTimeout = 2 * time.Minute
)

type widgetLoader[T any] func(ctx context.Context) (T, error)

type widgetCache[T any] struct {
	mu         sync.Mutex
	entries    map[string]cacheEntry[T]
	group      singleflight.Group
	redis      redis.Client
	logger     logger.Logger
	keyPrefix  string
	ttl        time.Duration
	staleGrace time.Duration
}

func newWidgetCache[T any](redisClient redis.Client, log logger.Logger, keyPrefix string) *widgetCache[T] {
	return &widgetCache[T]{
		entries:    make(map[string]cacheEntry[T]),
		redis:      redisClient,
		logger:     log,
		keyPrefix:  keyPrefix,
		ttl:        widgetCacheTTL,
		staleGrace: widgetStaleGraceTTL,
	}
}

func (c *widgetCache[T]) Get(ctx context.Context, key string, load widgetLoader[T]) (T, error) {
	var zero T

	// Returned values are shared across concurrent callers via singleflight and stale
	// hits. Callers must treat them as read-only through JSON serialization.
	if value, ok := c.fresh(key); ok {
		return value, nil
	}
	if value, ok := c.stale(key); ok {
		go c.Reload(key, load)
		return value, nil
	}
	if value, ok := c.fromRedis(ctx, key); ok {
		c.store(key, value)
		go c.Reload(key, load)
		return value, nil
	}

	out, err, _ := c.group.Do(key, func() (interface{}, error) {
		if value, ok := c.fresh(key); ok {
			return value, nil
		}

		result, err := load(context.WithoutCancel(ctx))
		if err != nil {
			return zero, err
		}

		c.store(key, result)
		c.storeRedis(context.Background(), key, result)
		return result, nil
	})
	if err != nil {
		return zero, err
	}

	return out.(T), nil
}

func (c *widgetCache[T]) Reload(key string, load widgetLoader[T]) {
	ctx, cancel := context.WithTimeout(context.Background(), widgetReloadTimeout)
	defer cancel()

	_, _, _ = c.group.Do("reload:"+key, func() (interface{}, error) {
		result, err := load(ctx)
		if err != nil {
			if c.logger != nil {
				c.logger.Warn("Failed to refresh dashboard widget cache", map[string]interface{}{
					"widget": c.keyPrefix,
					"key":    key,
					"error":  err.Error(),
				})
			}
			return nil, err
		}

		c.store(key, result)
		c.storeRedis(context.Background(), key, result)
		return result, nil
	})
}

func (c *widgetCache[T]) WarmFromRedis(ctx context.Context, key string) bool {
	value, ok := c.fromRedis(ctx, key)
	if !ok {
		return false
	}
	c.store(key, value)
	return true
}

func (c *widgetCache[T]) fresh(key string) (T, bool) {
	var zero T
	now := time.Now()

	c.mu.Lock()
	defer c.mu.Unlock()

	entry, ok := c.entries[key]
	if !ok || now.After(entry.expiresAt) {
		return zero, false
	}

	return entry.value, true
}

func (c *widgetCache[T]) stale(key string) (T, bool) {
	var zero T
	now := time.Now()

	c.mu.Lock()
	defer c.mu.Unlock()

	entry, ok := c.entries[key]
	if !ok || now.After(entry.staleUntil) {
		if ok {
			delete(c.entries, key)
		}
		return zero, false
	}

	return entry.value, true
}

func (c *widgetCache[T]) store(key string, value T) {
	now := time.Now()

	c.mu.Lock()
	defer c.mu.Unlock()

	c.entries[key] = cacheEntry[T]{
		expiresAt:  now.Add(c.ttl),
		staleUntil: now.Add(c.ttl + c.staleGrace),
		value:      value,
	}
}

func (c *widgetCache[T]) redisKey(key string) string {
	return fmt.Sprintf("dashboard:%s:%s", c.keyPrefix, key)
}

func (c *widgetCache[T]) fromRedis(ctx context.Context, key string) (T, bool) {
	var zero T
	if c.redis == nil {
		return zero, false
	}

	raw, err := c.redis.Get(ctx, c.redisKey(key))
	if err != nil {
		if err != goredis.Nil && c.logger != nil {
			c.logger.Warn("Failed to read dashboard widget cache from Redis", map[string]interface{}{
				"widget": c.keyPrefix,
				"key":    key,
				"error":  err.Error(),
			})
		}
		return zero, false
	}

	var value T
	if err := json.Unmarshal([]byte(raw), &value); err != nil {
		if c.logger != nil {
			c.logger.Warn("Failed to decode dashboard widget cache from Redis", map[string]interface{}{
				"widget": c.keyPrefix,
				"key":    key,
				"error":  err.Error(),
			})
		}
		_ = c.redis.Del(ctx, c.redisKey(key))
		return zero, false
	}

	return value, true
}

func (c *widgetCache[T]) storeRedis(ctx context.Context, key string, value T) {
	if c.redis == nil {
		return
	}

	encoded, err := json.Marshal(value)
	if err != nil {
		if c.logger != nil {
			c.logger.Warn("Failed to encode dashboard widget cache for Redis", map[string]interface{}{
				"widget": c.keyPrefix,
				"key":    key,
				"error":  err.Error(),
			})
		}
		return
	}

	ttl := c.ttl + c.staleGrace
	if err := c.redis.Set(ctx, c.redisKey(key), string(encoded), ttl); err != nil && c.logger != nil {
		c.logger.Warn("Failed to store dashboard widget cache in Redis", map[string]interface{}{
			"widget": c.keyPrefix,
			"key":    key,
			"error":  err.Error(),
		})
	}
}