internal/cache/cache.go

// Package cache provides multi-tier caching implementation with local LRU and Redis
package cache

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"sync"
	"time"

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

// MultiTierCache implements a two-tier caching system with local LRU and Redis
type MultiTierCache struct {
	local      *LRUCache
	redis      *redis.Client
	ttl        time.Duration
	metrics    *CacheMetrics
}

// CacheMetrics tracks cache performance
type CacheMetrics struct {
	mu          sync.RWMutex
	LocalHits   int64
	LocalMisses int64
	RedisHits   int64
	RedisMisses int64
}

// Config for cache initialization
type Config struct {
	RedisURL   string
	LocalSize  int
	TTL        time.Duration
	MaxRetries int
	PoolSize   int
}

// New creates a new multi-tier cache
func New(cfg Config) (*MultiTierCache, error) {
	// Parse Redis URL
	opt, err := redis.ParseURL(cfg.RedisURL)
	if err != nil {
		return nil, err
	}
	opt.MaxRetries = cfg.MaxRetries
	opt.PoolSize = cfg.PoolSize

	redisClient := redis.NewClient(opt)

	// Test Redis connection
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	if err := redisClient.Ping(ctx).Err(); err != nil {
		// Redis not available, continue with local cache only
		redisClient = nil
	}

	return &MultiTierCache{
		local:   NewLRUCache(cfg.LocalSize),
		redis:   redisClient,
		ttl:     cfg.TTL,
		metrics: &CacheMetrics{},
	}, nil
}

// Get retrieves a value from cache, checking local first then Redis
func (c *MultiTierCache) Get(ctx context.Context, key string) ([]byte, error) {
	hashedKey := c.hashKey(key)

	// Check local cache first (sub-millisecond)
	if value, found := c.local.Get(hashedKey); found {
		c.metrics.mu.Lock()
		c.metrics.LocalHits++
		c.metrics.mu.Unlock()
		return value, nil
	}
	c.metrics.mu.Lock()
	c.metrics.LocalMisses++
	c.metrics.mu.Unlock()

	// Check Redis if available (1-5ms)
	if c.redis != nil {
		value, err := c.redis.Get(ctx, hashedKey).Bytes()
		if err == nil {
			c.metrics.mu.Lock()
			c.metrics.RedisHits++
			c.metrics.mu.Unlock()
			// Backfill local cache
			c.local.Set(hashedKey, value)
			return value, nil
		}
		if err != redis.Nil {
			// Log error but continue
		}
		c.metrics.mu.Lock()
		c.metrics.RedisMisses++
		c.metrics.mu.Unlock()
	}

	return nil, ErrCacheMiss
}

// Set stores a value in both cache tiers
func (c *MultiTierCache) Set(ctx context.Context, key string, value []byte, ttlSeconds int) error {
	hashedKey := c.hashKey(key)
	ttl := time.Duration(ttlSeconds) * time.Second
	if ttlSeconds == 0 {
		ttl = c.ttl
	}

	// Set in local cache
	c.local.Set(hashedKey, value)

	// Set in Redis if available
	if c.redis != nil {
		if err := c.redis.Set(ctx, hashedKey, value, ttl).Err(); err != nil {
			// Log error but don't fail - local cache is still valid
			return nil
		}
	}

	return nil
}

// Delete removes a value from both cache tiers
func (c *MultiTierCache) Delete(ctx context.Context, key string) error {
	hashedKey := c.hashKey(key)

	// Delete from local cache
	c.local.Delete(hashedKey)

	// Delete from Redis if available
	if c.redis != nil {
		if err := c.redis.Del(ctx, hashedKey).Err(); err != nil {
			return err
		}
	}

	return nil
}

// GetJSON retrieves and unmarshals a JSON value from cache
func (c *MultiTierCache) GetJSON(ctx context.Context, key string, v interface{}) error {
	data, err := c.Get(ctx, key)
	if err != nil {
		return err
	}
	return json.Unmarshal(data, v)
}

// SetJSON marshals and stores a JSON value in cache
func (c *MultiTierCache) SetJSON(ctx context.Context, key string, v interface{}, ttlSeconds int) error {
	data, err := json.Marshal(v)
	if err != nil {
		return err
	}
	return c.Set(ctx, key, data, ttlSeconds)
}

// GetMetrics returns cache performance metrics
func (c *MultiTierCache) GetMetrics() CacheMetrics {
	c.metrics.mu.RLock()
	defer c.metrics.mu.RUnlock()
	return CacheMetrics{
		LocalHits:   c.metrics.LocalHits,
		LocalMisses: c.metrics.LocalMisses,
		RedisHits:   c.metrics.RedisHits,
		RedisMisses: c.metrics.RedisMisses,
	}
}

// Close closes the cache connections
func (c *MultiTierCache) Close() error {
	if c.redis != nil {
		return c.redis.Close()
	}
	return nil
}

func (c *MultiTierCache) hashKey(key string) string {
	hash := sha256.Sum256([]byte(key))
	return "amani:" + hex.EncodeToString(hash[:16])
}

// ErrCacheMiss indicates the key was not found in cache
var ErrCacheMiss = &CacheMissError{}

// CacheMissError represents a cache miss
type CacheMissError struct{}

func (e *CacheMissError) Error() string {
	return "cache miss"
}

// LRUCache implements a simple LRU cache
type LRUCache struct {
	mu       sync.RWMutex
	capacity int
	items    map[string]*lruItem
	head     *lruItem
	tail     *lruItem
}

type lruItem struct {
	key   string
	value []byte
	prev  *lruItem
	next  *lruItem
}

// NewLRUCache creates a new LRU cache with the given capacity
func NewLRUCache(capacity int) *LRUCache {
	return &LRUCache{
		capacity: capacity,
		items:    make(map[string]*lruItem),
	}
}

// Get retrieves a value from the LRU cache
func (c *LRUCache) Get(key string) ([]byte, bool) {
	c.mu.Lock()
	defer c.mu.Unlock()

	item, found := c.items[key]
	if !found {
		return nil, false
	}

	// Move to front (most recently used)
	c.moveToFront(item)
	return item.value, true
}

// Set stores a value in the LRU cache
func (c *LRUCache) Set(key string, value []byte) {
	c.mu.Lock()
	defer c.mu.Unlock()

	// Check if item exists
	if item, found := c.items[key]; found {
		item.value = value
		c.moveToFront(item)
		return
	}

	// Create new item
	item := &lruItem{key: key, value: value}
	c.items[key] = item
	c.addToFront(item)

	// Evict if over capacity
	if len(c.items) > c.capacity {
		c.evictLRU()
	}
}

// Delete removes a value from the LRU cache
func (c *LRUCache) Delete(key string) {
	c.mu.Lock()
	defer c.mu.Unlock()

	if item, found := c.items[key]; found {
		c.removeItem(item)
		delete(c.items, key)
	}
}

func (c *LRUCache) moveToFront(item *lruItem) {
	if item == c.head {
		return
	}
	c.removeItem(item)
	c.addToFront(item)
}

func (c *LRUCache) addToFront(item *lruItem) {
	item.prev = nil
	item.next = c.head
	if c.head != nil {
		c.head.prev = item
	}
	c.head = item
	if c.tail == nil {
		c.tail = item
	}
}

func (c *LRUCache) removeItem(item *lruItem) {
	if item.prev != nil {
		item.prev.next = item.next
	} else {
		c.head = item.next
	}
	if item.next != nil {
		item.next.prev = item.prev
	} else {
		c.tail = item.prev
	}
}

func (c *LRUCache) evictLRU() {
	if c.tail == nil {
		return
	}
	delete(c.items, c.tail.key)
	c.removeItem(c.tail)
}