Add sortkeys (#2118)

* added Args.SortKeys

* add test for SortKeys

* fix comment

args.go

@@ -147,6 +147,15 @@ func (a *Args) Sort(f func(x, y []byte) int) {
 	})
 }
 
+// SortKeys sorts Args by key only using 'f' as comparison function.
+//
+// For example args.SortKeys(bytes.Compare).
+func (a *Args) SortKeys(f func(x, y []byte) int) {
+	sort.SliceStable(a.args, func(i, j int) bool {
+		return f(a.args[i].key, a.args[j].key) == -1
+	})
+}
+
 // AppendBytes appends query string to dst and returns the extended dst.
 func (a *Args) AppendBytes(dst []byte) []byte {
 	for i, n := 0, len(a.args); i < n; i++ {

args_test.go

@@ -101,6 +101,22 @@ func TestArgsAdd(t *testing.T) {
 	}
 }
 
+func TestArgsSortKeys(t *testing.T) {
+	t.Parallel()
+
+	var a Args
+	a.Add("a", "789")
+	a.Add("b", "456")
+	a.Add("a", "123")
+	a.SortKeys(bytes.Compare)
+
+	s := a.String()
+	expectedS := "a=789&a=123&b=456"
+	if s != expectedS {
+		t.Fatalf("unexpected result: %q. Expecting %q", s, expectedS)
+	}
+}
+
 func TestArgsAcquireReleaseSequential(t *testing.T) {
 	testArgsAcquireRelease(t)
 }