feat: add lru+ttl subtitle cache

internal/playback/handler/subtitle_cache_test.go

@@ -0,0 +1,46 @@
+package handler
+
+import (
+	"testing"
+	"time"
+)
+
+func TestSubtitleCache_TTLExpiry(t *testing.T) {
+	c := newSubtitleCache(50*time.Millisecond, 10)
+	now := time.Unix(100, 0)
+
+	c.Set("k", []byte("v"), "text/plain", now)
+
+	if _, _, ok := c.Get("k", now.Add(49*time.Millisecond)); !ok {
+		t.Fatalf("expected cache hit before expiry")
+	}
+	if _, _, ok := c.Get("k", now.Add(51*time.Millisecond)); ok {
+		t.Fatalf("expected cache miss after expiry")
+	}
+}
+
+func TestSubtitleCache_LRUEviction(t *testing.T) {
+	c := newSubtitleCache(time.Minute, 2)
+	now := time.Unix(200, 0)
+
+	c.Set("a", []byte("a"), "text/plain", now)
+	c.Set("b", []byte("b"), "text/plain", now)
+
+	// Make "a" most recently used; "b" should be evicted next.
+	if _, _, ok := c.Get("a", now); !ok {
+		t.Fatalf("expected hit for a")
+	}
+
+	c.Set("c", []byte("c"), "text/plain", now)
+
+	if _, _, ok := c.Get("b", now); ok {
+		t.Fatalf("expected b to be evicted")
+	}
+	if _, _, ok := c.Get("a", now); !ok {
+		t.Fatalf("expected a to remain")
+	}
+	if _, _, ok := c.Get("c", now); !ok {
+		t.Fatalf("expected c to exist")
+	}
+}
+