Add a SetAndGet method to return previous value when setting a new one (

#101)

cache.go

@@ -117,6 +117,25 @@ func (cache *Cache) GetOrSet(key, value []byte, expireSeconds int) (retValue []b
 	return
 }
 
+// SetAndGet sets a key, value and expiration for a cache entry and stores it in the cache.
+// If the key is larger than 65535 or value is larger than 1/1024 of the cache size,
+// the entry will not be written to the cache. expireSeconds <= 0 means no expire,
+// but it can be evicted when cache is full.  Returns existing value if record exists
+// with a bool value to indicate whether an existing record was found
+func (cache *Cache) SetAndGet(key, value []byte, expireSeconds int) (retValue []byte, found bool, err error) {
+	hashVal := hashFunc(key)
+	segID := hashVal & segmentAndOpVal
+	cache.locks[segID].Lock()
+	defer cache.locks[segID].Unlock()
+
+	retValue, _, err = cache.segments[segID].get(key, nil, hashVal, false)
+	if err == nil {
+		found = true
+	}
+	err = cache.segments[segID].set(key, value, hashVal, expireSeconds)
+	return
+}
+
 // Peek returns the value or not found error, without updating access time or counters.
 func (cache *Cache) Peek(key []byte) (value []byte, err error) {
 	hashVal := hashFunc(key)

cache_test.go

@@ -824,3 +824,23 @@ func TestConcurrentGetTTL(t *testing.T) {
 		t.Fatalf("Failed to get the TTL with an error: %+v", err)
 	}
 }
+
+func TestSetAndGet(t *testing.T) {
+	cache := NewCache(1024)
+	key := []byte("abcd")
+	val1 := []byte("efgh")
+
+	_, found, _ := cache.SetAndGet(key, val1, 0)
+	if found == true {
+		t.Fatalf("SetAndGet unexpected found data")
+	}
+
+	val2 := []byte("ijkl")
+	rval, found, _ := cache.SetAndGet(key, val2, 0)
+	if found == false {
+		t.Fatalf("SetAndGet expected found data")
+	}
+	if string(val1) != string(rval) {
+		t.Fatalf("SetAndGet expected SetAndGet %s: got %s", string(val1), string(rval))
+	}
+}