feat: add lru+ttl subtitle cache

internal/playback/handler/subtitle_cache.go

@@ -0,0 +1,95 @@
+package handler
+
+import (
+	"container/list"
+	"sync"
+	"time"
+)
+
+type subtitleCacheEntry struct {
+	key         string
+	data        []byte
+	contentType string
+	expiresAt   time.Time
+}
+
+// subtitleCache is a small TTL+LRU cache to avoid unbounded memory usage when
+// proxying subtitles.
+type subtitleCache struct {
+	mu         sync.Mutex
+	ttl        time.Duration
+	maxEntries int
+
+	entries map[string]*list.Element
+	lru     *list.List
+}
+
+func newSubtitleCache(ttl time.Duration, maxEntries int) *subtitleCache {
+	if ttl <= 0 {
+		ttl = 10 * time.Minute
+	}
+	if maxEntries <= 0 {
+		maxEntries = 256
+	}
+	return &subtitleCache{
+		ttl:        ttl,
+		maxEntries: maxEntries,
+		entries:    make(map[string]*list.Element, maxEntries),
+		lru:        list.New(),
+	}
+}
+
+func (c *subtitleCache) Get(key string, now time.Time) (data []byte, contentType string, ok bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	el := c.entries[key]
+	if el == nil {
+		return nil, "", false
+	}
+	entry := el.Value.(subtitleCacheEntry)
+	if !entry.expiresAt.IsZero() && now.After(entry.expiresAt) {
+		c.removeElement(el)
+		return nil, "", false
+	}
+	c.lru.MoveToFront(el)
+	return entry.data, entry.contentType, true
+}
+
+func (c *subtitleCache) Set(key string, data []byte, contentType string, now time.Time) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	if el := c.entries[key]; el != nil {
+		entry := el.Value.(subtitleCacheEntry)
+		entry.data = data
+		entry.contentType = contentType
+		entry.expiresAt = now.Add(c.ttl)
+		el.Value = entry
+		c.lru.MoveToFront(el)
+		return
+	}
+
+	entry := subtitleCacheEntry{
+		key:         key,
+		data:        data,
+		contentType: contentType,
+		expiresAt:   now.Add(c.ttl),
+	}
+	el := c.lru.PushFront(entry)
+	c.entries[key] = el
+
+	for len(c.entries) > c.maxEntries {
+		back := c.lru.Back()
+		if back == nil {
+			break
+		}
+		c.removeElement(back)
+	}
+}
+
+func (c *subtitleCache) removeElement(el *list.Element) {
+	entry := el.Value.(subtitleCacheEntry)
+	delete(c.entries, entry.key)
+	c.lru.Remove(el)
+}