Add method GetAndAdd to LRU Cache

lru.go

@@ -53,6 +53,14 @@ func (c *Cache) Get(key interface{}) (value interface{}, ok bool) {
 	return value, ok
 }
 
+// GetAndAdd returns the previous key's value from the cache and adds a new one.
+func (c *Cache) GetAndAdd(key, value interface{}) (previous interface{}, ok, evicted bool) {
+	c.lock.Lock()
+	previous, ok, evicted = c.lru.GetAndAdd(key, value)
+	c.lock.Unlock()
+	return
+}
+
 // Contains checks if a key is in the cache, without updating the
 // recent-ness or deleting it for being stale.
 func (c *Cache) Contains(key interface{}) bool {

lru_test.go

@@ -152,6 +152,25 @@ func TestLRUAdd(t *testing.T) {
 	}
 }
 
+func TestLRU_GetAndAdd(t *testing.T) {
+	l, err := New(1)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if previous, ok, evicted := l.GetAndAdd(1, 1); ok || evicted {
+		t.Errorf("1 should not be found, eviction should not be: %v, %v, %v", previous, ok, evicted)
+	}
+	if previous, ok, evicted := l.GetAndAdd(1, 10); !ok || previous != 1 || evicted {
+		t.Errorf("1 should be returned as previous value, eviction should not be: %v, %v, %v", previous, ok, evicted)
+	}
+	if v, ok := l.Get(1); !ok || v != 10 {
+		t.Errorf("1 should be set to 10: %v, %v", v, ok)
+	}
+	if previous, ok, evicted := l.GetAndAdd(2, 2); ok || !evicted {
+		t.Errorf("2 should not be found, eviction should be: %v, %v, %v", previous, ok, evicted)
+	}
+}
+
 // test that Contains doesn't update recent-ness
 func TestLRUContains(t *testing.T) {
 	l, err := New(2)

simplelru/lru.go

@@ -47,26 +47,33 @@ func (c *LRU) Purge() {
 	c.evictList.Init()
 }
 
-// Add adds a value to the cache.  Returns true if an eviction occurred.
+// Add adds a value to the cache. Returns true if an eviction occurred.
 func (c *LRU) Add(key, value interface{}) (evicted bool) {
+	_, _, evicted = c.GetAndAdd(key, value)
+	return
+}
+
+// GetAndAdd returns the previous key's value from the cache and adds a new one.
+func (c *LRU) GetAndAdd(key, value interface{}) (previous interface{}, ok, evicted bool) {
 	// Check for existing item
 	if ent, ok := c.items[key]; ok {
 		c.evictList.MoveToFront(ent)
+		previous = ent.Value.(*entry).value
 		ent.Value.(*entry).value = value
-		return false
+		return previous, true, false
 	}
 
 	// Add new item
 	ent := &entry{key, value}
 	entry := c.evictList.PushFront(ent)
 	c.items[key] = entry
 
-	evict := c.evictList.Len() > c.size
+	evicted = c.evictList.Len() > c.size
 	// Verify size not exceeded
-	if evict {
+	if evicted {
 		c.removeOldest()
 	}
-	return evict
+	return nil, false, evicted
 }
 
 // Get looks up a key's value from the cache.

simplelru/lru_interface.go

@@ -11,6 +11,9 @@ type LRUCache interface {
 	// updates the "recently used"-ness of the key. #value, isFound
 	Get(key interface{}) (value interface{}, ok bool)
 
+	// Returns the previous key's value from the cache and adds a new one.
+	GetAndAdd(key, value interface{}) (previous interface{}, ok, evicted bool)
+
 	// Checks if a key exists in cache without updating the recent-ness.
 	Contains(key interface{}) (ok bool)
 

simplelru/lru_test.go

@@ -128,6 +128,25 @@ func TestLRU_Add(t *testing.T) {
 	}
 }
 
+func TestLRU_GetAndAdd(t *testing.T) {
+	l, err := NewLRU(1, nil)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+	if previous, ok, evicted := l.GetAndAdd(1, 1); ok || evicted {
+		t.Errorf("1 should not be found, eviction should not be: %v, %v, %v", previous, ok, evicted)
+	}
+	if previous, ok, evicted := l.GetAndAdd(1, 10); !ok || previous != 1 || evicted {
+		t.Errorf("1 should be returned as previous value, eviction should not be: %v, %v, %v", previous, ok, evicted)
+	}
+	if v, ok := l.Get(1); !ok || v != 10 {
+		t.Errorf("1 should be set to 10: %v, %v", v, ok)
+	}
+	if previous, ok, evicted := l.GetAndAdd(2, 2); ok || !evicted {
+		t.Errorf("2 should not be found, eviction should be: %v, %v, %v", previous, ok, evicted)
+	}
+}
+
 // Test that Contains doesn't update recent-ness
 func TestLRU_Contains(t *testing.T) {
 	l, err := NewLRU(2, nil)