Fix toString float encoding (#172)

Ensure a round float does contain a decimal point. Otherwise
feeding the output back to the parser would convert to an integer.

Fixes #171

tomltree_write.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"math"
 	"reflect"
 	"sort"
 	"strconv"
@@ -50,6 +51,11 @@ func tomlValueStringRepresentation(v interface{}) (string, error) {
 	case int64:
 		return strconv.FormatInt(value, 10), nil
 	case float64:
+		// Ensure a round float does contain a decimal point. Otherwise feeding
+		// the output back to the parser would convert to an integer.
+		if math.Trunc(value) == value {
+			return strconv.FormatFloat(value, 'f', 1, 32), nil
+		}
 		return strconv.FormatFloat(value, 'f', -1, 32), nil
 	case string:
 		return "\"" + encodeTomlString(value) + "\"", nil

tomltree_write_test.go

@@ -294,6 +294,21 @@ func TestTreeWriteToMapWithArrayOfInlineTables(t *testing.T) {
 	testMaps(t, treeMap, expected)
 }
 
+func TestTreeWriteToFloat(t *testing.T) {
+	tree, err := Load(`a = 3.0`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	str, err := tree.ToTomlString()
+	if err != nil {
+		t.Fatal(err)
+	}
+	expected := `a = 3.0`
+	if strings.TrimSpace(str) != strings.TrimSpace(expected) {
+		t.Fatalf("Expected:\n%s\nGot:\n%s", expected, str)
+	}
+}
+
 func BenchmarkTreeToTomlString(b *testing.B) {
 	toml, err := Load(sampleHard)
 	if err != nil {