2 files changed, 68 insertions, 0 deletions
diff --git a/internal/storage/replay.go b/internal/storage/replay.go
new file mode 100644
index 0000000..a2366a4
--- /dev/null
+++ b/internal/storage/replay.go
@@ -0,0 +1,32 @@
+package storage
+
+import (
+	"sync"
+	"time"
+)
+
+type ReplayCache struct {
+	ttl  time.Duration
+	mu   sync.Mutex
+	data map[string]time.Time
+}
+
+func NewReplayCache(ttl time.Duration) *ReplayCache {
+	return &ReplayCache{ttl: ttl, data: map[string]time.Time{}}
+}
+
+func (c *ReplayCache) CheckAndMark(hash string) (bool, error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	now := time.Now()
+	for k, ts := range c.data {
+		if now.Sub(ts) > c.ttl {
+			delete(c.data, k)
+		}
+	}
+	if _, exists := c.data[hash]; exists {
+		return true, nil
+	}
+	c.data[hash] = now
+	return false, nil
+}
diff --git a/internal/storage/replay_test.go b/internal/storage/replay_test.go
new file mode 100644
index 0000000..4d147b8
--- /dev/null
+++ b/internal/storage/replay_test.go
@@ -0,0 +1,36 @@
+package storage
+
+import (
+	"testing"
+	"time"
+)
+
+func TestReplayCacheCheckAndMarkLivesOnlyInMemory(t *testing.T) {
+	cache := NewReplayCache(40 * time.Millisecond)
+
+	replayed, err := cache.CheckAndMark("token-hash")
+	if err != nil {
+		t.Fatalf("CheckAndMark returned error: %v", err)
+	}
+	if replayed {
+		t.Fatal("first token use should not be marked as replayed")
+	}
+
+	replayed, err = cache.CheckAndMark("token-hash")
+	if err != nil {
+		t.Fatalf("CheckAndMark returned error on second use: %v", err)
+	}
+	if !replayed {
+		t.Fatal("second token use should be marked as replayed")
+	}
+
+	time.Sleep(60 * time.Millisecond)
+
+	replayed, err = cache.CheckAndMark("token-hash")
+	if err != nil {
+		t.Fatalf("CheckAndMark returned error after ttl expiry: %v", err)
+	}
+	if replayed {
+		t.Fatal("expired token entry should be forgotten from memory")
+	}
+}