Add SetNx (Set if not exist) method to cache

- addressng the issue : https://github.com/patrickmn/go-cache/issues/59

README.md

@@ -75,6 +75,12 @@ func main() {
 		foo := x.(*MyStruct)
 			// ...
 	}
+
+	// Set new value if the item is missing or expired
+	item, found := c.SetNx("foo", cache.DefaultExpiration, func(s string) (interface{}, error) {
+            // return value to set if cache miss
+            return "bar", nil
+        })
 }
 ```
 

cache.go

@@ -135,6 +135,25 @@ func (c *cache) Get(k string) (interface{}, bool) {
 	return item.Object, true
 }
 
+func (c *cache) SetNx(k string, expiry time.Duration, funcToCall func(string) (interface{}, error)) (interface{},
+bool) {
+	c.mu.RLock()
+	item, found := c.items[k]
+	if !found || (item.Expiration > 0 && time.Now().UnixNano() > item.Expiration){
+		val, err := funcToCall(k)
+		if err != nil {
+			c.mu.RUnlock()
+			return nil, false
+		}
+
+		c.set(k, val, expiry)
+		c.mu.RUnlock()
+		return val, true
+	}
+	c.mu.RUnlock()
+	return item.Object, true
+}
+
 // GetWithExpiration returns an item and its expiration time from the cache.
 // It returns the item or nil, the expiration time if one is set (if the item
 // never expires a zero value for time.Time is returned), and a bool indicating

cache_test.go

@@ -8,6 +8,8 @@ import (
 	"sync"
 	"testing"
 	"time"
+	"github.com/stretchr/testify/assert"
+	"github.com/pkg/errors"
 )
 
 type TestStruct struct {
@@ -1769,3 +1771,51 @@ func TestGetWithExpiration(t *testing.T) {
 		t.Error("expiration for e is in the past")
 	}
 }
+
+func TestSetNx(t *testing.T) {
+	tc := New(DefaultExpiration, 0)
+	val1, isPresent := tc.SetNx("key1", 10 * time.Second, func(s string) (interface{}, error) {
+		return "a for apple", nil
+	})
+
+	assert.True(t, isPresent)
+	assert.Equal(t, "a for apple", val1)
+
+	val2, isPresent := tc.SetNx("key2", 10 * time.Second, func(s string) (interface{}, error) {
+		return 10, nil
+	})
+
+	assert.True(t, isPresent)
+	assert.Equal(t, 10, val2)
+
+	tc.set("existingKey", []int{1,2,3}, 10 * time.Second)
+	val3, isPresent := tc.SetNx("existingKey", 10 * time.Second, func(s string) (interface{}, error) {
+		return []int{4}, nil
+	})
+
+	assert.True(t, isPresent)
+	assert.Equal(t, []int{1,2,3}, val3)
+
+	tc.set("expiredKey", "expiredValue", 10 * time.Millisecond)
+	time.Sleep(20 * time.Millisecond)
+	val4, isPresent := tc.SetNx("expiredKey", 10 * time.Second, func(s string) (interface{}, error) {
+		return "newValue", nil
+	})
+
+	assert.True(t, isPresent)
+	assert.Equal(t, "newValue", val4)
+
+	val5, isPresent := tc.SetNx("errorKey", 10 * time.Second, func(s string) (interface{}, error) {
+		return "doesn't matter return value", errors.New("some error")
+	})
+
+	assert.False(t, isPresent)
+	assert.Nil(t, val5)
+
+	val6, isPresent := tc.SetNx("errorKey", 10 * time.Second, func(s string) (interface{}, error) {
+		return nil, nil
+	})
+
+	assert.True(t, isPresent)
+	assert.Nil(t, val6)
+}