add attribute sort support.

The Element SortAttrs lexicographically sorts an element's attributes
by key.

etree.go

@@ -13,6 +13,7 @@ import (
 	"errors"
 	"io"
 	"os"
+	"sort"
 	"strings"
 )
 
@@ -781,6 +782,29 @@ func (e *Element) RemoveAttr(key string) *Attr {
 	return nil
 }
 
+// SortAttrs sorts the element's attributes lexicographically by key.
+func (e *Element) SortAttrs() {
+	sort.Sort(byAttr(e.Attr))
+}
+
+type byAttr []Attr
+
+func (a byAttr) Len() int {
+	return len(a)
+}
+
+func (a byAttr) Swap(i, j int) {
+	a[i], a[j] = a[j], a[i]
+}
+
+func (a byAttr) Less(i, j int) bool {
+	sp := strings.Compare(a[i].Space, a[j].Space)
+	if sp == 0 {
+		return strings.Compare(a[i].Key, a[j].Key) < 0
+	}
+	return sp < 0
+}
+
 var xmlReplacerNormal = strings.NewReplacer(
 	"&", "&amp;",
 	"<", "&lt;",

etree_test.go

@@ -485,3 +485,17 @@ func TestSetRoot(t *testing.T) {
 	s5, _ := doc.WriteToString()
 	checkEq(t, s5, expected5)
 }
+
+func TestSortAttrs(t *testing.T) {
+	testdoc := `<el foo='5' Foo='2' aaa='4' สวัสดี='7' AAA='1' a01='3' z='6' a:ZZZ='9' a:AAA='8'/>`
+	doc := NewDocument()
+	err := doc.ReadFromString(testdoc)
+	if err != nil {
+		t.Fatal("etree ReadFromString: " + err.Error())
+	}
+
+	doc.Root().SortAttrs()
+	doc.Indent(2)
+	out, _ := doc.WriteToString()
+	checkEq(t, out, `<el AAA="1" Foo="2" a01="3" aaa="4" foo="5" z="6" สวัสดี="7" a:AAA="8" a:ZZZ="9"/>`+"\n")
+}