encode: ensure floats have decimal point (#615)

Fixes #571

Co-authored-by: Sterling Hanenkamp <sterling@ziprecruiter.com>

marshaler.go

@@ -5,6 +5,7 @@ import (
 	"encoding"
 	"fmt"
 	"io"
+	"math"
 	"reflect"
 	"sort"
 	"strconv"
@@ -244,9 +245,17 @@ func (enc *Encoder) encode(b []byte, ctx encoderCtx, v reflect.Value) ([]byte, e
 	case reflect.String:
 		b = enc.encodeString(b, v.String(), ctx.options)
 	case reflect.Float32:
-		b = strconv.AppendFloat(b, v.Float(), 'f', -1, 32)
+		if math.Trunc(v.Float()) == v.Float() {
+			b = strconv.AppendFloat(b, v.Float(), 'f', 1, 32)
+		} else {
+			b = strconv.AppendFloat(b, v.Float(), 'f', -1, 32)
+		}
 	case reflect.Float64:
-		b = strconv.AppendFloat(b, v.Float(), 'f', -1, 64)
+		if math.Trunc(v.Float()) == v.Float() {
+			b = strconv.AppendFloat(b, v.Float(), 'f', 1, 64)
+		} else {
+			b = strconv.AppendFloat(b, v.Float(), 'f', -1, 64)
+		}
 	case reflect.Bool:
 		if v.Bool() {
 			b = append(b, "true"...)

marshaler_test.go

@@ -822,3 +822,26 @@ func ExampleMarshal() {
 	// Name = 'go-toml'
 	// Tags = ['go', 'toml']
 }
+
+func TestIssue571(t *testing.T) {
+	type Foo struct {
+		Float32 float32
+		Float64 float64
+	}
+
+	const closeEnough = 1e-9
+
+	foo := Foo{
+		Float32: 42,
+		Float64: 43,
+	}
+	b, err := toml.Marshal(foo)
+	require.NoError(t, err)
+
+	var foo2 Foo
+	err = toml.Unmarshal(b, &foo2)
+	require.NoError(t, err)
+
+	assert.InDelta(t, 42, foo2.Float32, closeEnough)
+	assert.InDelta(t, 43, foo2.Float64, closeEnough)
+}