Order map keys alphabetically (#270)

This makes sure we have a stable output when marshaling
maps.

Fixes #268

marshal.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 	"reflect"
+	"sort"
 	"strconv"
 	"strings"
 	"time"
@@ -329,7 +330,26 @@ func (e *Encoder) valueToTree(mtype reflect.Type, mval reflect.Value) (*Tree, er
 			}
 		}
 	case reflect.Map:
-		for _, key := range mval.MapKeys() {
+		keys := mval.MapKeys()
+		if e.order == OrderPreserve && len(keys) > 0 {
+			// Sorting []reflect.Value is not straight forward.
+			//
+			// OrderPreserve will support deterministic results when string is used
+			// as the key to maps.
+			typ := keys[0].Type()
+			kind := keys[0].Kind()
+			if kind == reflect.String {
+				ikeys := make([]string, len(keys))
+				for i := range keys {
+					ikeys[i] = keys[i].Interface().(string)
+				}
+				sort.Strings(ikeys)
+				for i := range ikeys {
+					keys[i] = reflect.ValueOf(ikeys[i]).Convert(typ)
+				}
+			}
+		}
+		for _, key := range keys {
 			mvalf := mval.MapIndex(key)
 			val, err := e.valueToToml(mtype.Elem(), mvalf)
 			if err != nil {

marshal_OrderPreserve_Map_test.toml

@@ -0,0 +1,17 @@
+title = "TOML Marshal Testing"
+
+[basic_map]
+  one = "one"
+  two = "two"
+
+[long_map]
+  a7 = "1"
+  b3 = "2"
+  c8 = "3"
+  d4 = "4"
+  e6 = "5"
+  f5 = "6"
+  g10 = "7"
+  h1 = "8"
+  i2 = "9"
+  j9 = "10"

marshal_test.go

@@ -126,6 +126,12 @@ type testDoc struct {
 	Unexported2 int               `toml:"-"`
 }
 
+type testMapDoc struct {
+	Title    string            `toml:"title"`
+	BasicMap map[string]string `toml:"basic_map"`
+	LongMap  map[string]string `toml:"long_map"`
+}
+
 type testDocBasics struct {
 	Uint       uint      `toml:"uint"`
 	Bool       bool      `toml:"bool"`
@@ -200,6 +206,26 @@ var docData = testDoc{
 	SubDocPtrs: []*testSubDoc{&subdoc},
 }
 
+var mapTestDoc = testMapDoc{
+	Title: "TOML Marshal Testing",
+	BasicMap: map[string]string{
+		"one": "one",
+		"two": "two",
+	},
+	LongMap: map[string]string{
+		"h1":  "8",
+		"i2":  "9",
+		"b3":  "2",
+		"d4":  "4",
+		"f5":  "6",
+		"e6":  "5",
+		"a7":  "1",
+		"c8":  "3",
+		"j9":  "10",
+		"g10": "7",
+	},
+}
+
 func TestDocMarshal(t *testing.T) {
 	result, err := Marshal(docData)
 	if err != nil {
@@ -223,6 +249,29 @@ func TestDocMarshalOrdered(t *testing.T) {
 	}
 }
 
+func TestDocMarshalMaps(t *testing.T) {
+	result, err := Marshal(mapTestDoc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected, _ := ioutil.ReadFile("marshal_OrderPreserve_Map_test.toml")
+	if !bytes.Equal(result, expected) {
+		t.Errorf("Bad marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, result)
+	}
+}
+
+func TestDocMarshalOrderedMaps(t *testing.T) {
+	var result bytes.Buffer
+	err := NewEncoder(&result).Order(OrderPreserve).Encode(mapTestDoc)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected, _ := ioutil.ReadFile("marshal_OrderPreserve_Map_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 {