math/big: implement GobDecode/Encode for big.Float

Added GobEncode/Decode and a test for them.

Fixes #14593

Change-Id: Ic8d3efd24d0313a1a66f01da293c4c1fd39764a8
Reviewed-on: https://go-review.googlesource.com/21755Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/floatmarsh.go

@@ -6,7 +6,70 @@
 
 package big
 
-import "fmt"
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+// Gob codec version. Permits backward-compatible changes to the encoding.
+const floatGobVersion byte = 1
+
+// GobEncode implements the gob.GobEncoder interface.
+func (x *Float) GobEncode() ([]byte, error) {
+	if x == nil {
+		return nil, nil
+	}
+	sz := 1 + 1 + 4 // version + mode|acc|form|neg (3+2+2+1bit) + prec
+	if x.form == finite {
+		sz += 4 + int((x.prec+(_W-1))/_W)*_S // exp + mant
+	}
+	buf := make([]byte, sz)
+
+	buf[0] = floatGobVersion
+	b := byte(x.mode&7)<<5 | byte((x.acc+1)&3)<<3 | byte(x.form&3)<<1
+	if x.neg {
+		b |= 1
+	}
+	buf[1] = b
+	binary.BigEndian.PutUint32(buf[2:], x.prec)
+	if x.form == finite {
+		binary.BigEndian.PutUint32(buf[6:], uint32(x.exp))
+		x.mant.bytes(buf[10:])
+	}
+	return buf, nil
+}
+
+// GobDecode implements the gob.GobDecoder interface.
+func (z *Float) GobDecode(buf []byte) error {
+	if len(buf) == 0 {
+		// Other side sent a nil or default value.
+		*z = Float{}
+		return nil
+	}
+
+	if buf[0] != floatGobVersion {
+		return fmt.Errorf("Float.GobDecode: encoding version %d not supported", buf[0])
+	}
+
+	b := buf[1]
+	z.mode = RoundingMode((b >> 5) & 7)
+	z.acc = Accuracy((b>>3)&3) - 1
+	z.form = form((b >> 1) & 3)
+	z.neg = b&1 != 0
+
+	oldPrec := uint(z.prec)
+	z.prec = binary.BigEndian.Uint32(buf[2:])
+
+	if z.form == finite {
+		z.exp = int32(binary.BigEndian.Uint32(buf[6:]))
+		z.mant = z.mant.setBytes(buf[10:])
+	}
+
+	if oldPrec != 0 {
+		z.SetPrec(oldPrec)
+	}
+	return nil
+}
 
 // MarshalText implements the encoding.TextMarshaler interface.
 // Only the Float value is marshaled (in full precision), other

src/math/big/floatmarsh_test.go

@@ -5,7 +5,10 @@
 package big
 
 import (
+	"bytes"
+	"encoding/gob"
 	"encoding/json"
+	"io"
 	"testing"
 )
 
@@ -23,6 +26,66 @@ var floatVals = []string{
 	"Inf",
 }
 
+func TestFloatGobEncoding(t *testing.T) {
+	var medium bytes.Buffer
+	for _, test := range floatVals {
+		for _, sign := range []string{"", "+", "-"} {
+			for _, prec := range []uint{0, 1, 2, 10, 53, 64, 100, 1000} {
+				medium.Reset() // empty buffer for each test case (in case of failures)
+				enc := gob.NewEncoder(&medium)
+				dec := gob.NewDecoder(&medium)
+				x := sign + test
+				var tx Float
+				_, _, err := tx.SetPrec(prec).Parse(x, 0)
+				if err != nil {
+					t.Errorf("parsing of %s (prec = %d) failed (invalid test case): %v", x, prec, err)
+					continue
+				}
+				tx.SetMode(ToPositiveInf)
+				if err := enc.Encode(&tx); err != nil {
+					t.Errorf("encoding of %v (prec = %d) failed: %v", &tx, prec, err)
+					continue
+				}
+
+				var rx Float
+				if err := dec.Decode(&rx); err != nil {
+					t.Errorf("decoding of %v (prec = %d) failed: %v", &tx, prec, err)
+					continue
+				}
+
+				if rx.Cmp(&tx) != 0 {
+					t.Errorf("transmission of %s failed: got %s want %s", x, rx.String(), tx.String())
+					continue
+				}
+
+				if rx.Mode() != ToPositiveInf {
+					t.Errorf("transmission of %s's mode failed: got %s want %s", x, rx.Mode(), ToPositiveInf)
+				}
+			}
+		}
+	}
+}
+func TestFloatCorruptGob(t *testing.T) {
+	var buf bytes.Buffer
+	tx := NewFloat(4 / 3).SetPrec(1000).SetMode(ToPositiveInf)
+	if err := gob.NewEncoder(&buf).Encode(tx); err != nil {
+		t.Fatal(err)
+	}
+	b := buf.Bytes()
+	var rx Float
+	if err := gob.NewDecoder(bytes.NewReader(b)).Decode(&rx); err != nil {
+		t.Fatal(err)
+	}
+	var rx2 Float
+	if err := gob.NewDecoder(bytes.NewReader(b[:10])).Decode(&rx2); err != io.ErrUnexpectedEOF {
+		t.Errorf("expected io.ErrUnexpectedEOF, got %v", err)
+	}
+	b[1] = 0
+	if err := gob.NewDecoder(bytes.NewReader(b)).Decode(&rx); err == nil {
+		t.Fatal("expected a version error, got nil")
+	}
+
+}
 func TestFloatJSONEncoding(t *testing.T) {
 	for _, test := range floatVals {
 		for _, sign := range []string{"", "+", "-"} {