big: gobs for big rats

Fixes #1926.

R=r
CC=golang-dev
https://golang.org/cl/4550122

src/pkg/big/int.go

@@ -816,13 +816,13 @@ func (z *Int) Not(x *Int) *Int {
 
 
 // Gob codec version. Permits backward-compatible changes to the encoding.
-const version byte = 1
+const intGobVersion byte = 1
 
 // GobEncode implements the gob.GobEncoder interface.
 func (z *Int) GobEncode() ([]byte, os.Error) {
-	buf := make([]byte, len(z.abs)*_S+1) // extra byte for version and sign bit
+	buf := make([]byte, 1+len(z.abs)*_S) // extra byte for version and sign bit
 	i := z.abs.bytes(buf) - 1            // i >= 0
-	b := version << 1                    // make space for sign bit
+	b := intGobVersion << 1              // make space for sign bit
 	if z.neg {
 		b |= 1
 	}
@@ -837,7 +837,7 @@ func (z *Int) GobDecode(buf []byte) os.Error {
 		return os.ErrorString("Int.GobDecode: no data")
 	}
 	b := buf[0]
-	if b>>1 != version {
+	if b>>1 != intGobVersion {
 		return os.ErrorString(fmt.Sprintf("Int.GobDecode: encoding version %d not supported", b>>1))
 	}
 	z.neg = b&1 != 0

src/pkg/big/int_test.go

@@ -1303,6 +1303,7 @@ func TestModInverse(t *testing.T) {
 }
 
 
+// used by TestIntGobEncoding and TestRatGobEncoding
 var gobEncodingTests = []string{
 	"0",
 	"1",
@@ -1313,7 +1314,7 @@ var gobEncodingTests = []string{
 	"298472983472983471903246121093472394872319615612417471234712061",
 }
 
-func TestGobEncoding(t *testing.T) {
+func TestIntGobEncoding(t *testing.T) {
 	var medium bytes.Buffer
 	enc := gob.NewEncoder(&medium)
 	dec := gob.NewDecoder(&medium)
@@ -1321,7 +1322,8 @@ func TestGobEncoding(t *testing.T) {
 		for j := 0; j < 2; j++ {
 			medium.Reset() // empty buffer for each test case (in case of failures)
 			stest := test
-			if j == 0 {
+			if j != 0 {
+				// negative numbers
 				stest = "-" + test
 			}
 			var tx Int

src/pkg/big/rat.go

@@ -7,6 +7,7 @@
 package big
 
 import (
+	"encoding/binary"
 	"fmt"
 	"os"
 	"strings"
@@ -354,3 +355,45 @@ func (z *Rat) FloatString(prec int) string {
 
 	return s
 }
+
+
+// Gob codec version. Permits backward-compatible changes to the encoding.
+const ratGobVersion byte = 1
+
+// GobEncode implements the gob.GobEncoder interface.
+func (z *Rat) GobEncode() ([]byte, os.Error) {
+	buf := make([]byte, 1+4+(len(z.a.abs)+len(z.b))*_S) // extra bytes for version and sign bit (1), and numerator length (4)
+	i := z.b.bytes(buf)
+	j := z.a.abs.bytes(buf[0:i])
+	n := i - j
+	if int(uint32(n)) != n {
+		// this should never happen
+		return nil, os.ErrorString("Rat.GobEncode: numerator too large")
+	}
+	binary.BigEndian.PutUint32(buf[j-4:j], uint32(n))
+	j -= 1 + 4
+	b := ratGobVersion << 1 // make space for sign bit
+	if z.a.neg {
+		b |= 1
+	}
+	buf[j] = b
+	return buf[j:], nil
+}
+
+
+// GobDecode implements the gob.GobDecoder interface.
+func (z *Rat) GobDecode(buf []byte) os.Error {
+	if len(buf) == 0 {
+		return os.ErrorString("Rat.GobDecode: no data")
+	}
+	b := buf[0]
+	if b>>1 != ratGobVersion {
+		return os.ErrorString(fmt.Sprintf("Rat.GobDecode: encoding version %d not supported", b>>1))
+	}
+	const j = 1 + 4
+	i := j + binary.BigEndian.Uint32(buf[j-4:j])
+	z.a.neg = b&1 != 0
+	z.a.abs = z.a.abs.setBytes(buf[j:i])
+	z.b = z.b.setBytes(buf[i:])
+	return nil
+}

src/pkg/big/rat_test.go

@@ -7,6 +7,7 @@ package big
 import (
 	"bytes"
 	"fmt"
+	"gob"
 	"testing"
 )
 
@@ -308,3 +309,36 @@ func TestRatSetFrac64Rat(t *testing.T) {
 		}
 	}
 }
+
+
+func TestRatGobEncoding(t *testing.T) {
+	var medium bytes.Buffer
+	enc := gob.NewEncoder(&medium)
+	dec := gob.NewDecoder(&medium)
+	for i, test := range gobEncodingTests {
+		for j := 0; j < 4; j++ {
+			medium.Reset() // empty buffer for each test case (in case of failures)
+			stest := test
+			if j&1 != 0 {
+				// negative numbers
+				stest = "-" + test
+			}
+			if j%2 != 0 {
+				// fractions
+				stest = stest + "." + test
+			}
+			var tx Rat
+			tx.SetString(stest)
+			if err := enc.Encode(&tx); err != nil {
+				t.Errorf("#%d%c: encoding failed: %s", i, 'a'+j, err)
+			}
+			var rx Rat
+			if err := dec.Decode(&rx); err != nil {
+				t.Errorf("#%d%c: decoding failed: %s", i, 'a'+j, err)
+			}
+			if rx.Cmp(&tx) != 0 {
+				t.Errorf("#%d%c: transmission failed: got %s want %s", i, 'a'+j, &rx, &tx)
+			}
+		}
+	}
+}