math/big: Add text marshaller interface to Int

Fixes #7329

LGTM=gri
R=gri, bradfitz, mtj
CC=golang-codereviews
https://golang.org/cl/63710043

src/pkg/math/big/int.go

@@ -982,17 +982,29 @@ func (z *Int) GobDecode(buf []byte) error {
 }
 
 // MarshalJSON implements the json.Marshaler interface.
-func (x *Int) MarshalJSON() ([]byte, error) {
+func (z *Int) MarshalJSON() ([]byte, error) {
 	// TODO(gri): get rid of the []byte/string conversions
-	return []byte(x.String()), nil
+	return []byte(z.String()), nil
 }
 
 // UnmarshalJSON implements the json.Unmarshaler interface.
-func (z *Int) UnmarshalJSON(x []byte) error {
+func (z *Int) UnmarshalJSON(text []byte) error {
 	// TODO(gri): get rid of the []byte/string conversions
-	_, ok := z.SetString(string(x), 0)
-	if !ok {
-		return fmt.Errorf("math/big: cannot unmarshal %s into a *big.Int", x)
+	if _, ok := z.SetString(string(text), 0); !ok {
+		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Int", text)
+	}
+	return nil
+}
+
+// MarshalText implements the encoding.TextMarshaler interface
+func (z *Int) MarshalText() (text []byte, err error) {
+	return []byte(z.String()), nil
+}
+
+// UnmarshalText implements the encoding.TextUnmarshaler interface
+func (z *Int) UnmarshalText(text []byte) error {
+	if _, ok := z.SetString(string(text), 0); !ok {
+		return fmt.Errorf("math/big: cannot unmarshal %q into a *big.Int", text)
 	}
 	return nil
 }

src/pkg/math/big/int_test.go

@@ -9,6 +9,7 @@ import (
 	"encoding/gob"
 	"encoding/hex"
 	"encoding/json"
+	"encoding/xml"
 	"fmt"
 	"math/rand"
 	"testing"
@@ -1528,6 +1529,58 @@ func TestIntJSONEncoding(t *testing.T) {
 	}
 }
 
+var intVals = []string{
+	"-141592653589793238462643383279502884197169399375105820974944592307816406286",
+	"-1415926535897932384626433832795028841971",
+	"-141592653589793",
+	"-1",
+	"0",
+	"1",
+	"141592653589793",
+	"1415926535897932384626433832795028841971",
+	"141592653589793238462643383279502884197169399375105820974944592307816406286",
+}
+
+func TestIntJSONEncodingTextMarshaller(t *testing.T) {
+	for _, num := range intVals {
+		var tx Int
+		tx.SetString(num, 0)
+		b, err := json.Marshal(&tx)
+		if err != nil {
+			t.Errorf("marshaling of %s failed: %s", &tx, err)
+			continue
+		}
+		var rx Int
+		if err := json.Unmarshal(b, &rx); err != nil {
+			t.Errorf("unmarshaling of %s failed: %s", &tx, err)
+			continue
+		}
+		if rx.Cmp(&tx) != 0 {
+			t.Errorf("JSON encoding of %s failed: got %s want %s", &tx, &rx, &tx)
+		}
+	}
+}
+
+func TestIntXMLEncodingTextMarshaller(t *testing.T) {
+	for _, num := range intVals {
+		var tx Int
+		tx.SetString(num, 0)
+		b, err := xml.Marshal(&tx)
+		if err != nil {
+			t.Errorf("marshaling of %s failed: %s", &tx, err)
+			continue
+		}
+		var rx Int
+		if err := xml.Unmarshal(b, &rx); err != nil {
+			t.Errorf("unmarshaling of %s failed: %s", &tx, err)
+			continue
+		}
+		if rx.Cmp(&tx) != 0 {
+			t.Errorf("XML encoding of %s failed: got %s want %s", &tx, &rx, &tx)
+		}
+	}
+}
+
 func TestIssue2607(t *testing.T) {
 	// This code sequence used to hang.
 	n := NewInt(10)