add method Sort in args (#505)

* add method StringSort QueryStringSort AppendBytesSort in args

* simplify code

* only Sort method

* format

* add method StringSort QueryStringSort AppendBytesSort in args

* simplify code

* only Sort method

* format

* merge and fix tests

* change sort into generic by having the sort function

* change sort into generic by having the sort function

* change comment

args.go

@@ -4,13 +4,14 @@ import (
 	"bytes"
 	"errors"
 	"io"
+	"sort"
 	"sync"
 
 	"github.com/valyala/bytebufferpool"
 )
 
 const (
-	argsNoValue = true
+	argsNoValue  = true
 	argsHasValue = false
 )
 
@@ -115,6 +116,19 @@ func (a *Args) QueryString() []byte {
 	return a.buf
 }
 
+// Sort sorts Args by key and then value using 'f' as comparison function.
+//
+// For example args.Sort(bytes.Compare)
+func (a *Args) Sort(f func(x, y []byte) int) {
+	sort.SliceStable(a.args, func(i, j int) bool {
+		n := f(a.args[i].key, a.args[j].key)
+		if n == 0 {
+			return f(a.args[i].value, a.args[j].value) == -1
+		}
+		return n == -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

@@ -1,6 +1,7 @@
 package fasthttp
 
 import (
+	"bytes"
 	"fmt"
 	"reflect"
 	"strings"
@@ -48,6 +49,13 @@ func TestArgsAdd(t *testing.T) {
 		t.Fatalf("unexpected result: %q. Expecting %q", s, expectedS)
 	}
 
+	a.Sort(bytes.Compare)
+	ss := a.String()
+	expectedSS := "ba=23&foo=&foo&foo=1&foo=bar&foo=baz"
+	if ss != expectedSS {
+		t.Fatalf("unexpected result: %q. Expecting %q", ss, expectedSS)
+	}
+
 	var a1 Args
 	a1.Parse(s)
 	if a1.Len() != 6 {