Add Encoder opt to emit arrays on multiple lines

A new Encoder option emits arrays with more than one line on multiple lines.
This is off by default and toggled with `ArraysWithOneElementPerLine`.

For example:

```
A = [1,2,3]
```

Becomes:

```
A = [
  1,
  2,
  3
]
```

Fixes #200

marshal.go

@@ -21,6 +21,7 @@ type tomlOpts struct {
 
 type encOpts struct {
 	quoteMapKeys bool
+	arraysOneElementPerLine bool
 }
 
 var encOptsDefaults = encOpts{
@@ -174,6 +175,25 @@ func (e *Encoder) QuoteMapKeys(v bool) *Encoder {
 	return e
 }
 
+// ArraysWithOneElementPerLine sets up the encoder to encode arrays
+// with more than one element on multiple lines instead of one.
+//
+// For example:
+//
+//   A = [1,2,3]
+//
+// Becomes
+//
+//   A = [
+//     1,
+//     2,
+//     3
+//   ]
+func (e *Encoder) ArraysWithOneElementPerLine(v bool) *Encoder {
+	e.arraysOneElementPerLine = v
+	return e
+}
+
 func (e *Encoder) marshal(v interface{}) ([]byte, error) {
 	mtype := reflect.TypeOf(v)
 	if mtype.Kind() != reflect.Struct {
@@ -187,8 +207,11 @@ func (e *Encoder) marshal(v interface{}) ([]byte, error) {
 	if err != nil {
 		return []byte{}, err
 	}
-	s, err := t.ToTomlString()
-	return []byte(s), err
+
+	var buf bytes.Buffer
+	_, err = t.writeTo(&buf, "", "", 0, e.arraysOneElementPerLine)
+
+	return buf.Bytes(), err
 }
 
 // Convert given marshal struct or map value to toml tree
@@ -218,7 +241,7 @@ func (e *Encoder) valueToTree(mtype reflect.Type, mval reflect.Value) (*Tree, er
 				return nil, err
 			}
 			if e.quoteMapKeys {
-				keyStr, err := tomlValueStringRepresentation(key.String())
+				keyStr, err := tomlValueStringRepresentation(key.String(), "", e.arraysOneElementPerLine)
 				if err != nil {
 					return nil, err
 				}

marshal_test.go

@@ -721,6 +721,7 @@ func TestEncodeQuotedMapKeys(t *testing.T) {
 		t.Errorf("Bad maps marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, result)
 	}
 }
+
 func TestDecodeQuotedMapKeys(t *testing.T) {
 	result := mapsTestStruct{}
 	err := NewDecoder(bytes.NewBuffer(mapsTestToml)).Decode(&result)
@@ -732,3 +733,73 @@ func TestDecodeQuotedMapKeys(t *testing.T) {
 		t.Errorf("Bad maps unmarshal: expected %v, got %v", expected, result)
 	}
 }
+
+type structArrayNoTag struct {
+	A struct {
+		B []int64
+		C []int64
+	}
+}
+
+func TestMarshalArray(t *testing.T) {
+	expected := []byte(`
+[A]
+  B = [1,2,3]
+  C = [1]
+`)
+
+	m := structArrayNoTag{
+		A: struct{
+			B []int64
+			C []int64}{
+			B: []int64{1, 2, 3},
+			C: []int64{1},
+		},
+	}
+
+	b, err := Marshal(m)
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !bytes.Equal(b, expected) {
+		t.Errorf("Bad arrays marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, b)
+	}
+}
+
+func TestMarshalArrayOnePerLine(t *testing.T) {
+	expected := []byte(`
+[A]
+  B = [
+    1,
+    2,
+    3
+  ]
+  C = [1]
+`)
+
+	m := structArrayNoTag{
+		A: struct{
+			B []int64
+			C []int64
+			}{
+			B: []int64{1, 2, 3},
+			C: []int64{1},
+		},
+	}
+
+	var buf bytes.Buffer
+	encoder := NewEncoder(&buf).ArraysWithOneElementPerLine(true)
+	err := encoder.Encode(m)
+
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	b := buf.Bytes()
+
+	if !bytes.Equal(b, expected) {
+		t.Errorf("Bad arrays marshal: expected\n-----\n%s\n-----\ngot\n-----\n%s\n-----\n", expected, b)
+	}
+}

parser_test.go

@@ -652,7 +652,7 @@ func TestTomlValueStringRepresentation(t *testing.T) {
 			"[\"gamma\",\"delta\"]"},
 		{nil, ""},
 	} {
-		result, err := tomlValueStringRepresentation(item.Value)
+		result, err := tomlValueStringRepresentation(item.Value, "",false)
 		if err != nil {
 			t.Errorf("Test %d - unexpected error: %s", idx, err)
 		}

tomltree_write.go

@@ -44,7 +44,7 @@ func encodeTomlString(value string) string {
 	return b.String()
 }
 
-func tomlValueStringRepresentation(v interface{}) (string, error) {
+func tomlValueStringRepresentation(v interface{}, indent string, arraysOneElementPerLine bool) (string, error) {
 	switch value := v.(type) {
 	case uint64:
 		return strconv.FormatUint(value, 10), nil
@@ -61,7 +61,7 @@ func tomlValueStringRepresentation(v interface{}) (string, error) {
 		return "\"" + encodeTomlString(value) + "\"", nil
 	case []byte:
 		b, _ := v.([]byte)
-		return tomlValueStringRepresentation(string(b))
+		return tomlValueStringRepresentation(string(b), indent, arraysOneElementPerLine)
 	case bool:
 		if value {
 			return "true", nil
@@ -76,21 +76,40 @@ func tomlValueStringRepresentation(v interface{}) (string, error) {
 	rv := reflect.ValueOf(v)
 
 	if rv.Kind() == reflect.Slice {
-		values := []string{}
+		var values []string
 		for i := 0; i < rv.Len(); i++ {
 			item := rv.Index(i).Interface()
-			itemRepr, err := tomlValueStringRepresentation(item)
+			itemRepr, err := tomlValueStringRepresentation(item, indent, arraysOneElementPerLine)
 			if err != nil {
 				return "", err
 			}
 			values = append(values, itemRepr)
 		}
+		if arraysOneElementPerLine && len(values) > 1 {
+			stringBuffer := bytes.Buffer{}
+			valueIndent := indent + `  ` // TODO: move that to a shared encoder state
+
+			stringBuffer.WriteString("[\n")
+
+			for i, value := range values {
+				stringBuffer.WriteString(valueIndent)
+				stringBuffer.WriteString(value)
+				if i != len(values) - 1 {
+					stringBuffer.WriteString(`,`)
+				}
+				stringBuffer.WriteString("\n")
+			}
+
+			stringBuffer.WriteString(indent+"]")
+
+			return stringBuffer.String(), nil
+		}
 		return "[" + strings.Join(values, ",") + "]", nil
 	}
 	return "", fmt.Errorf("unsupported value type %T: %v", v, v)
 }
 
-func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (int64, error) {
+func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64, arraysOneElementPerLine bool) (int64, error) {
 	simpleValuesKeys := make([]string, 0)
 	complexValuesKeys := make([]string, 0)
 
@@ -113,7 +132,7 @@ func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (
 			return bytesCount, fmt.Errorf("invalid value type at %s: %T", k, t.values[k])
 		}
 
-		repr, err := tomlValueStringRepresentation(v.value)
+		repr, err := tomlValueStringRepresentation(v.value, indent, arraysOneElementPerLine)
 		if err != nil {
 			return bytesCount, err
 		}
@@ -178,7 +197,7 @@ func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (
 			if err != nil {
 				return bytesCount, err
 			}
-			bytesCount, err = node.writeTo(w, indent+"  ", combinedKey, bytesCount)
+			bytesCount, err = node.writeTo(w, indent+"  ", combinedKey, bytesCount, arraysOneElementPerLine)
 			if err != nil {
 				return bytesCount, err
 			}
@@ -190,7 +209,7 @@ func (t *Tree) writeTo(w io.Writer, indent, keyspace string, bytesCount int64) (
 					return bytesCount, err
 				}
 
-				bytesCount, err = subTree.writeTo(w, indent+"  ", combinedKey, bytesCount)
+				bytesCount, err = subTree.writeTo(w, indent+"  ", combinedKey, bytesCount, arraysOneElementPerLine)
 				if err != nil {
 					return bytesCount, err
 				}
@@ -216,7 +235,7 @@ func writeStrings(w io.Writer, s ...string) (int, error) {
 // WriteTo encode the Tree as Toml and writes it to the writer w.
 // Returns the number of bytes written in case of success, or an error if anything happened.
 func (t *Tree) WriteTo(w io.Writer) (int64, error) {
-	return t.writeTo(w, "", "", 0)
+	return t.writeTo(w, "", "", 0, false)
 }
 
 // ToTomlString generates a human-readable representation of the current tree.