Option to keep fields ordered when marshal struct (#266)

Adds a new `Order()` option to preserve order of struct fields when
marshaling.

marshal_test.go

@@ -12,10 +12,10 @@ import (
 )
 
 type basicMarshalTestStruct struct {
-	String     string                      `toml:"string"`
-	StringList []string                    `toml:"strlist"`
-	Sub        basicMarshalTestSubStruct   `toml:"subdoc"`
-	SubList    []basicMarshalTestSubStruct `toml:"sublist"`
+	String     string                      `toml:"Zstring"`
+	StringList []string                    `toml:"Ystrlist"`
+	Sub        basicMarshalTestSubStruct   `toml:"Xsubdoc"`
+	SubList    []basicMarshalTestSubStruct `toml:"Wsublist"`
 }
 
 type basicMarshalTestSubStruct struct {
@@ -29,16 +29,29 @@ var basicTestData = basicMarshalTestStruct{
 	SubList:    []basicMarshalTestSubStruct{{"Two"}, {"Three"}},
 }
 
-var basicTestToml = []byte(`string = "Hello"
-strlist = ["Howdy","Hey There"]
+var basicTestToml = []byte(`Ystrlist = ["Howdy","Hey There"]
+Zstring = "Hello"
 
-[subdoc]
-  String2 = "One"
-
-[[sublist]]
+[[Wsublist]]
   String2 = "Two"
 
-[[sublist]]
+[[Wsublist]]
+  String2 = "Three"
+
+[Xsubdoc]
+  String2 = "One"
+`)
+
+var basicTestTomlOrdered = []byte(`Zstring = "Hello"
+Ystrlist = ["Howdy","Hey There"]
+
+[Xsubdoc]
+  String2 = "One"
+
+[[Wsublist]]
+  String2 = "Two"
+
+[[Wsublist]]
   String2 = "Three"
 `)
 
@@ -53,6 +66,18 @@ func TestBasicMarshal(t *testing.T) {
 	}
 }
 
+func TestBasicMarshalOrdered(t *testing.T) {
+	var result bytes.Buffer
+	err := NewEncoder(&result).Order(OrderPreserve).Encode(basicTestData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := basicTestTomlOrdered
+	if !bytes.Equal(result.Bytes(), expected) {
+		t.Errorf("Bad marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, result.Bytes())
+	}
+}
+
 func TestBasicMarshalWithPointer(t *testing.T) {
 	result, err := Marshal(&basicTestData)
 	if err != nil {
@@ -64,6 +89,18 @@ func TestBasicMarshalWithPointer(t *testing.T) {
 	}
 }
 
+func TestBasicMarshalOrderedWithPointer(t *testing.T) {
+	var result bytes.Buffer
+	err := NewEncoder(&result).Order(OrderPreserve).Encode(&basicTestData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := basicTestTomlOrdered
+	if !bytes.Equal(result.Bytes(), expected) {
+		t.Errorf("Bad marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, result.Bytes())
+	}
+}
+
 func TestBasicUnmarshal(t *testing.T) {
 	result := basicMarshalTestStruct{}
 	err := Unmarshal(basicTestToml, &result)
@@ -78,39 +115,39 @@ func TestBasicUnmarshal(t *testing.T) {
 
 type testDoc struct {
 	Title       string            `toml:"title"`
-	Basics      testDocBasics     `toml:"basic"`
 	BasicLists  testDocBasicLists `toml:"basic_lists"`
+	SubDocPtrs  []*testSubDoc     `toml:"subdocptrs"`
 	BasicMap    map[string]string `toml:"basic_map"`
 	Subdocs     testDocSubs       `toml:"subdoc"`
+	Basics      testDocBasics     `toml:"basic"`
 	SubDocList  []testSubDoc      `toml:"subdoclist"`
-	SubDocPtrs  []*testSubDoc     `toml:"subdocptrs"`
 	err         int               `toml:"shouldntBeHere"`
 	unexported  int               `toml:"shouldntBeHere"`
 	Unexported2 int               `toml:"-"`
 }
 
 type testDocBasics struct {
+	Uint       uint      `toml:"uint"`
 	Bool       bool      `toml:"bool"`
-	Date       time.Time `toml:"date"`
 	Float      float32   `toml:"float"`
 	Int        int       `toml:"int"`
-	Uint       uint      `toml:"uint"`
 	String     *string   `toml:"string"`
+	Date       time.Time `toml:"date"`
 	unexported int       `toml:"shouldntBeHere"`
 }
 
 type testDocBasicLists struct {
+	Floats  []*float32  `toml:"floats"`
 	Bools   []bool      `toml:"bools"`
 	Dates   []time.Time `toml:"dates"`
-	Floats  []*float32  `toml:"floats"`
 	Ints    []int       `toml:"ints"`
-	Strings []string    `toml:"strings"`
 	UInts   []uint      `toml:"uints"`
+	Strings []string    `toml:"strings"`
 }
 
 type testDocSubs struct {
-	First  testSubDoc  `toml:"first"`
 	Second *testSubDoc `toml:"second"`
+	First  testSubDoc  `toml:"first"`
 }
 
 type testSubDoc struct {
@@ -174,6 +211,18 @@ func TestDocMarshal(t *testing.T) {
 	}
 }
 
+func TestDocMarshalOrdered(t *testing.T) {
+	var result bytes.Buffer
+	err := NewEncoder(&result).Order(OrderPreserve).Encode(docData)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected, _ := ioutil.ReadFile("marshal_OrderPreserve_test.toml")
+	if !bytes.Equal(result.Bytes(), expected) {
+		t.Errorf("Bad marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, result.Bytes())
+	}
+}
+
 func TestDocMarshalPointer(t *testing.T) {
 	result, err := Marshal(&docData)
 	if err != nil {