From: Rob Pike <r@golang.org>
Date: Wed, 29 Jul 2009 00:20:19 +0000 (-0700)
Subject: change the encoding of uints to simplify overflow checking and to make them
X-Git-Tag: weekly.2009-11-06~1038
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=b85147cd70b2e4d3efe33c2cb9ea65bed1af99e8;p=gostls13.git

change the encoding of uints to simplify overflow checking and to make them
easier and faster to read.  they are now either a one-byte value or a n-byte value
preceded by a byte holding -n.

R=rsc
DELTA=150  (45 added, 7 deleted, 98 changed)
OCL=32381
CL=32387
---

diff --git a/src/pkg/gob/Makefile b/src/pkg/gob/Makefile
index 54bcf813c0..3534430c53 100644
--- a/src/pkg/gob/Makefile
+++ b/src/pkg/gob/Makefile
@@ -36,15 +36,17 @@ O1=\
 	type.$O\
 
 O2=\
-	decode.$O\
 	encode.$O\
 
 O3=\
-	decoder.$O\
+	decode.$O\
 	encoder.$O\
 
+O4=\
+	decoder.$O\
+
 
-phases: a1 a2 a3
+phases: a1 a2 a3 a4
 _obj$D/gob.a: phases
 
 a1: $(O1)
@@ -52,13 +54,17 @@ a1: $(O1)
 	rm -f $(O1)
 
 a2: $(O2)
-	$(AR) grc _obj$D/gob.a decode.$O encode.$O
+	$(AR) grc _obj$D/gob.a encode.$O
 	rm -f $(O2)
 
 a3: $(O3)
-	$(AR) grc _obj$D/gob.a decoder.$O encoder.$O
+	$(AR) grc _obj$D/gob.a decode.$O encoder.$O
 	rm -f $(O3)
 
+a4: $(O4)
+	$(AR) grc _obj$D/gob.a decoder.$O
+	rm -f $(O4)
+
 
 newpkg: clean
 	mkdir -p _obj$D
@@ -68,6 +74,7 @@ $(O1): newpkg
 $(O2): a1
 $(O3): a2
 $(O4): a3
+$(O5): a4
 
 nuke: clean
 	rm -f $(GOROOT)/pkg/$(GOOS)_$(GOARCH)$D/gob.a
diff --git a/src/pkg/gob/codec_test.go b/src/pkg/gob/codec_test.go
index 66d6b01ec5..8263a9286c 100644
--- a/src/pkg/gob/codec_test.go
+++ b/src/pkg/gob/codec_test.go
@@ -21,20 +21,20 @@ type EncodeT struct {
 	b	[]byte;
 }
 var encodeT = []EncodeT {
-	EncodeT{ 0x00,	[]byte{0x80} },
-	EncodeT{ 0x0f,	[]byte{0x8f} },
-	EncodeT{ 0xff,	[]byte{0x7f, 0x81} },
-	EncodeT{ 0xffff,	[]byte{0x7f, 0x7f, 0x83} },
-	EncodeT{ 0xffffff,	[]byte{0x7f, 0x7f, 0x7f, 0x87} },
-	EncodeT{ 0xffffffff,	[]byte{0x7f, 0x7f, 0x7f, 0x7f, 0x8f} },
-	EncodeT{ 0xffffffffff,	[]byte{0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x9f} },
-	EncodeT{ 0xffffffffffff,	[]byte{0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0xbf} },
-	EncodeT{ 0xffffffffffffff,	[]byte{0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0xff} },
-	EncodeT{ 0xffffffffffffffff,	[]byte{0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x7f, 0x81} },
-	EncodeT{ 0x1111,	[]byte{0x11, 0xa2} },
-	EncodeT{ 0x1111111111111111,	[]byte{0x11, 0x22, 0x44, 0x08, 0x11, 0x22, 0x44, 0x08, 0x91} },
-	EncodeT{ 0x8888888888888888,	[]byte{0x08, 0x11, 0x22, 0x44, 0x08, 0x11, 0x22, 0x44, 0x08, 0x81} },
-	EncodeT{ 1<<63,	[]byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x81} },
+	EncodeT{ 0x00,	[]byte{0x00} },
+	EncodeT{ 0x0F,	[]byte{0x0F} },
+	EncodeT{ 0xFF,	[]byte{0xFF, 0xFF} },
+	EncodeT{ 0xFFFF,	[]byte{0xFE, 0xFF, 0xFF} },
+	EncodeT{ 0xFFFFFF,	[]byte{0xFD, 0xFF, 0xFF, 0xFF} },
+	EncodeT{ 0xFFFFFFFF,	[]byte{0xFC, 0xFF, 0xFF, 0xFF, 0xFF} },
+	EncodeT{ 0xFFFFFFFFFF,	[]byte{0xFB, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF} },
+	EncodeT{ 0xFFFFFFFFFFFF,	[]byte{0xFA, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF} },
+	EncodeT{ 0xFFFFFFFFFFFFFF,	[]byte{0xF9, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF} },
+	EncodeT{ 0xFFFFFFFFFFFFFFFF,	[]byte{0xF8, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF} },
+	EncodeT{ 0x1111,	[]byte{0xFE, 0x11, 0x11} },
+	EncodeT{ 0x1111111111111111,	[]byte{0xF8, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11} },
+	EncodeT{ 0x8888888888888888,	[]byte{0xF8, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88, 0x88} },
+	EncodeT{ 1<<63,	[]byte{0xF8, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00} },
 }
 
 
@@ -50,11 +50,10 @@ func TestUintCodec(t *testing.T) {
 			t.Error("encodeUint:", tt.x, encState.err)
 		}
 		if !bytes.Equal(tt.b, b.Data()) {
-			t.Errorf("encodeUint: expected % x got % x", tt.b, b.Data())
+			t.Errorf("encodeUint: %#x encode: expected % x got % x", tt.x, tt.b, b.Data())
 		}
 	}
-	decState := new(decodeState);
-	decState.b = b;
+	decState := newDecodeState(b);
 	for u := uint64(0); ; u = (u+1) * 7 {
 		b.Reset();
 		encodeUint(encState, u);
@@ -82,8 +81,8 @@ func verifyInt(i int64, t *testing.T) {
 	if encState.err != nil {
 		t.Error("encodeInt:", i, encState.err)
 	}
-	decState := new(decodeState);
-	decState.b = b;
+	decState := newDecodeState(b);
+	decState.buf = make([]byte, 8);
 	j := decodeInt(decState);
 	if decState.err != nil {
 		t.Error("DecodeInt:", i, decState.err)
@@ -109,14 +108,14 @@ func TestIntCodec(t *testing.T) {
 	verifyInt(-1<<63, t);	// a tricky case
 }
 
-// The result of encoding a true boolean with field number 6
-var boolResult = []byte{0x87, 0x81}
-// The result of encoding a number 17 with field number 6
-var signedResult = []byte{0x87, 0xa2}
-var unsignedResult = []byte{0x87, 0x91}
-var floatResult = []byte{0x87, 0x40, 0xe2}
+// The result of encoding a true boolean with field number 7
+var boolResult = []byte{0x07, 0x01}
+// The result of encoding a number 17 with field number 7
+var signedResult = []byte{0x07, 2*17}
+var unsignedResult = []byte{0x07, 17}
+var floatResult = []byte{0x07, 0xFE, 0x31, 0x40}
 // The result of encoding "hello" with field number 6
-var bytesResult = []byte{0x87, 0x85, 'h', 'e', 'l', 'l', 'o'}
+var bytesResult = []byte{0x07, 0x05, 'h', 'e', 'l', 'l', 'o'}
 
 func newencoderState(b *bytes.Buffer) *encoderState {
 	b.Reset();
@@ -338,9 +337,8 @@ func execDec(typ string, instr *decInstr, state *decodeState, t *testing.T, p un
 	state.fieldnum = 6;
 }
 
-func newdecodeState(data []byte) *decodeState {
-	state := new(decodeState);
-	state.b = bytes.NewBuffer(data);
+func newDecodeStateFromData(data []byte) *decodeState {
+	state := newDecodeState(bytes.NewBuffer(data));
 	state.fieldnum = -1;
 	return state;
 }
@@ -354,7 +352,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a bool };
 		instr := &decInstr{ decBool, 6, 0, 0, ovfl };
-		state := newdecodeState(boolResult);
+		state := newDecodeStateFromData(boolResult);
 		execDec("bool", instr, state, t, unsafe.Pointer(&data));
 		if data.a != true {
 			t.Errorf("bool a = %v not true", data.a)
@@ -364,7 +362,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a int };
 		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
-		state := newdecodeState(signedResult);
+		state := newDecodeStateFromData(signedResult);
 		execDec("int", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("int a = %v not 17", data.a)
@@ -375,7 +373,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a uint };
 		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
-		state := newdecodeState(unsignedResult);
+		state := newDecodeStateFromData(unsignedResult);
 		execDec("uint", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("uint a = %v not 17", data.a)
@@ -386,7 +384,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a int8 };
 		instr := &decInstr{ decInt8, 6, 0, 0, ovfl };
-		state := newdecodeState(signedResult);
+		state := newDecodeStateFromData(signedResult);
 		execDec("int8", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("int8 a = %v not 17", data.a)
@@ -397,7 +395,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a uint8 };
 		instr := &decInstr{ decUint8, 6, 0, 0, ovfl };
-		state := newdecodeState(unsignedResult);
+		state := newDecodeStateFromData(unsignedResult);
 		execDec("uint8", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("uint8 a = %v not 17", data.a)
@@ -408,7 +406,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a int16 };
 		instr := &decInstr{ decInt16, 6, 0, 0, ovfl };
-		state := newdecodeState(signedResult);
+		state := newDecodeStateFromData(signedResult);
 		execDec("int16", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("int16 a = %v not 17", data.a)
@@ -419,7 +417,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a uint16 };
 		instr := &decInstr{ decUint16, 6, 0, 0, ovfl };
-		state := newdecodeState(unsignedResult);
+		state := newDecodeStateFromData(unsignedResult);
 		execDec("uint16", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("uint16 a = %v not 17", data.a)
@@ -430,7 +428,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a int32 };
 		instr := &decInstr{ decInt32, 6, 0, 0, ovfl };
-		state := newdecodeState(signedResult);
+		state := newDecodeStateFromData(signedResult);
 		execDec("int32", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("int32 a = %v not 17", data.a)
@@ -441,7 +439,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a uint32 };
 		instr := &decInstr{ decUint32, 6, 0, 0, ovfl };
-		state := newdecodeState(unsignedResult);
+		state := newDecodeStateFromData(unsignedResult);
 		execDec("uint32", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("uint32 a = %v not 17", data.a)
@@ -452,7 +450,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a uintptr };
 		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
-		state := newdecodeState(unsignedResult);
+		state := newDecodeStateFromData(unsignedResult);
 		execDec("uintptr", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("uintptr a = %v not 17", data.a)
@@ -463,7 +461,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a int64 };
 		instr := &decInstr{ decInt64, 6, 0, 0, ovfl };
-		state := newdecodeState(signedResult);
+		state := newDecodeStateFromData(signedResult);
 		execDec("int64", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("int64 a = %v not 17", data.a)
@@ -474,7 +472,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a uint64 };
 		instr := &decInstr{ decUint64, 6, 0, 0, ovfl };
-		state := newdecodeState(unsignedResult);
+		state := newDecodeStateFromData(unsignedResult);
 		execDec("uint64", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("uint64 a = %v not 17", data.a)
@@ -485,7 +483,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a float };
 		instr := &decInstr{ decOpMap[valueKind(data.a)], 6, 0, 0, ovfl };
-		state := newdecodeState(floatResult);
+		state := newDecodeStateFromData(floatResult);
 		execDec("float", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("float a = %v not 17", data.a)
@@ -496,7 +494,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a float32 };
 		instr := &decInstr{ decFloat32, 6, 0, 0, ovfl };
-		state := newdecodeState(floatResult);
+		state := newDecodeStateFromData(floatResult);
 		execDec("float32", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("float32 a = %v not 17", data.a)
@@ -507,7 +505,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a float64 };
 		instr := &decInstr{ decFloat64, 6, 0, 0, ovfl };
-		state := newdecodeState(floatResult);
+		state := newDecodeStateFromData(floatResult);
 		execDec("float64", instr, state, t, unsafe.Pointer(&data));
 		if data.a != 17 {
 			t.Errorf("float64 a = %v not 17", data.a)
@@ -518,7 +516,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a []byte };
 		instr := &decInstr{ decUint8Array, 6, 0, 0, ovfl };
-		state := newdecodeState(bytesResult);
+		state := newDecodeStateFromData(bytesResult);
 		execDec("bytes", instr, state, t, unsafe.Pointer(&data));
 		if string(data.a) != "hello" {
 			t.Errorf(`bytes a = %q not "hello"`, string(data.a))
@@ -529,7 +527,7 @@ func TestScalarDecInstructions(t *testing.T) {
 	{
 		var data struct { a string };
 		instr := &decInstr{ decString, 6, 0, 0, ovfl };
-		state := newdecodeState(bytesResult);
+		state := newDecodeStateFromData(bytesResult);
 		execDec("bytes", instr, state, t, unsafe.Pointer(&data));
 		if data.a != "hello" {
 			t.Errorf(`bytes a = %q not "hello"`, data.a)
diff --git a/src/pkg/gob/decode.go b/src/pkg/gob/decode.go
index 4469089c45..9cd2387369 100644
--- a/src/pkg/gob/decode.go
+++ b/src/pkg/gob/decode.go
@@ -18,6 +18,7 @@ import (
 )
 
 var (
+	errBadUint = os.ErrorString("gob: encoded unsigned integer out of range");
 	errRange = os.ErrorString("gob: internal error: field numbers out of bounds");
 	errNotStruct = os.ErrorString("gob: TODO: can only handle structs")
 )
@@ -27,6 +28,14 @@ type decodeState struct {
 	b	*bytes.Buffer;
 	err	os.Error;
 	fieldnum	int;	// the last field number read.
+	buf	[]byte;
+}
+
+func newDecodeState(b *bytes.Buffer) *decodeState {
+	d := new(decodeState);
+	d.b = b;
+	d.buf = make([]byte, uint64Size);
+	return d;
 }
 
 func overflow(name string) os.ErrorString {
@@ -35,21 +44,34 @@ func overflow(name string) os.ErrorString {
 
 // decodeUintReader reads an encoded unsigned integer from an io.Reader.
 // Used only by the Decoder to read the message length.
-func decodeUintReader(r io.Reader, oneByte []byte) (x uint64, err os.Error) {
-	for shift := uint(0);; shift += 7 {
-		var n int;
-		n, err = r.Read(oneByte);
-		if err != nil {
-			return 0, err
-		}
-		b := oneByte[0];
-		x |= uint64(b) << shift;
-		if b&0x80 != 0 {
-			x &^= 0x80 << shift;
-			break
+func decodeUintReader(r io.Reader, buf []byte) (x uint64, err os.Error) {
+	n1, err := r.Read(buf[0:1]);
+	if err != nil {
+		return
+	}
+	b := buf[0];
+	if b <= 0x7f {
+		return uint64(b), nil
+	}
+	nb := -int(int8(b));
+	if nb > uint64Size {
+		err = errBadUint;
+		return;
+	}
+	var n int;
+	n, err = io.ReadFull(r, buf[0:nb]);
+	if err != nil {
+		if err == os.EOF {
+			err = io.ErrUnexpectedEOF
 		}
+		return
+	}
+	// Could check that the high byte is zero but it's not worth it.
+	for i := 0; i < n; i++ {
+		x <<= 8;
+		x |= uint64(buf[i]);
 	}
-	return x, nil;
+	return
 }
 
 // decodeUint reads an encoded unsigned integer from state.r.
@@ -59,17 +81,23 @@ func decodeUint(state *decodeState) (x uint64) {
 	if state.err != nil {
 		return
 	}
-	for shift := uint(0);; shift += 7 {
-		var b uint8;
-		b, state.err = state.b.ReadByte();
-		if state.err != nil {
-			return 0
-		}
-		x |= uint64(b) << shift;
-		if b&0x80 != 0 {
-			x &^= 0x80 << shift;
-			break
-		}
+	var b uint8;
+	b, state.err = state.b.ReadByte();
+	if b <= 0x7f {	// includes state.err != nil
+		return uint64(b)
+	}
+	nb := -int(int8(b));
+	if nb > uint64Size {
+		state.err = errBadUint;
+		return;
+	}
+	var n int;
+	n, state.err = state.b.Read(state.buf[0:nb]);
+	// Don't need to check error; it's safe to loop regardless.
+	// Could check that the high byte is zero but it's not worth it.
+	for i := 0; i < n; i++ {
+		x <<= 8;
+		x |= uint64(state.buf[i]);
 	}
 	return x;
 }
@@ -338,8 +366,7 @@ func decodeStruct(engine *decEngine, rtyp *reflect.StructType, b *bytes.Buffer,
 		}
 		p = *(*uintptr)(up);
 	}
-	state := new(decodeState);
-	state.b = b;
+	state := newDecodeState(b);
 	state.fieldnum = -1;
 	basep := p;
 	for state.err == nil {
@@ -368,8 +395,7 @@ func decodeStruct(engine *decEngine, rtyp *reflect.StructType, b *bytes.Buffer,
 }
 
 func ignoreStruct(engine *decEngine, b *bytes.Buffer) os.Error {
-	state := new(decodeState);
-	state.b = b;
+	state := newDecodeState(b);
 	state.fieldnum = -1;
 	for state.err == nil {
 		delta := int(decodeUint(state));
diff --git a/src/pkg/gob/decoder.go b/src/pkg/gob/decoder.go
index 7dd99a0762..91bfcbbb8e 100644
--- a/src/pkg/gob/decoder.go
+++ b/src/pkg/gob/decoder.go
@@ -30,7 +30,7 @@ func NewDecoder(r io.Reader) *Decoder {
 	dec := new(Decoder);
 	dec.r = r;
 	dec.seen = make(map[typeId] *wireType);
-	dec.state = new(decodeState);	// buffer set in Decode(); rest is unimportant
+	dec.state = newDecodeState(nil);	// buffer set in Decode(); rest is unimportant
 	dec.oneByte = make([]byte, 1);
 
 	return dec;
diff --git a/src/pkg/gob/encode.go b/src/pkg/gob/encode.go
index 0589f38632..be3599770f 100644
--- a/src/pkg/gob/encode.go
+++ b/src/pkg/gob/encode.go
@@ -15,6 +15,8 @@ import (
 	"unsafe";
 )
 
+const uint64Size = unsafe.Sizeof(uint64(0))
+
 // The global execution state of an instance of the encoder.
 // Field numbers are delta encoded and always increase. The field
 // number is initialized to -1 so 0 comes out as delta(1). A delta of
@@ -23,27 +25,33 @@ type encoderState struct {
 	b	*bytes.Buffer;
 	err	os.Error;	// error encountered during encoding;
 	fieldnum	int;	// the last field number written.
-	buf [16]byte;	// buffer used by the encoder; here to avoid allocation.
+	buf [1+uint64Size]byte;	// buffer used by the encoder; here to avoid allocation.
 }
 
-// Integers encode as a variant of Google's protocol buffer varint (varvarint?).
-// The variant is that the continuation bytes have a zero top bit instead of a one.
-// That way there's only one bit to clear and the value is a little easier to see if
-// you're the unfortunate sort of person who must read the hex to debug.
+// Unsigned integers have a two-state encoding.  If the number is less
+// than 128 (0 through 0x7F), its value is written directly.
+// Otherwise the value is written in big-endian byte order preceded
+// by the byte length, negated.
 
 // encodeUint writes an encoded unsigned integer to state.b.  Sets state.err.
 // If state.err is already non-nil, it does nothing.
 func encodeUint(state *encoderState, x uint64) {
-	var n int;
 	if state.err != nil {
 		return
 	}
-	for n = 0; x > 0x7F; n++ {
-		state.buf[n] = uint8(x & 0x7F);
-		x >>= 7;
+	if x <= 0x7F {
+		state.err = state.b.WriteByte(uint8(x));
+		return;
+	}
+	var n, m int;
+	m = uint64Size;
+	for n = 1; x > 0; n++ {
+		state.buf[m] = uint8(x & 0xFF);
+		x >>= 8;
+		m--;
 	}
-	state.buf[n] = 0x80 | uint8(x);
-	n, state.err = state.b.Write(state.buf[0:n+1]);
+	state.buf[m] = uint8(-(n-1));
+	n, state.err = state.b.Write(state.buf[m:uint64Size+1]);
 }
 
 // encodeInt writes an encoded signed integer to state.w.
diff --git a/src/pkg/gob/encoder.go b/src/pkg/gob/encoder.go
index 1a401eb1c5..3d8f3928cb 100644
--- a/src/pkg/gob/encoder.go
+++ b/src/pkg/gob/encoder.go
@@ -78,11 +78,11 @@
 	The rest of this comment documents the encoding, details that are not important
 	for most users.  Details are presented bottom-up.
 
-	An unsigned integer is encoded as an arbitrary-precision, variable-length sequence
-	of bytes.  It is sent in little-endian order (low bits first), with seven bits per
-	byte.  The high bit of each byte is zero, except that the high bit of the final
-	(highest precision) byte of the encoding will be set.  Thus 0 is transmitted as
-	(80), 7 is transmitted as (87) and 256=2*128 is transmitted as (00 82).
+	An unsigned integer is sent one of two ways.  If it is less than 128, it is sent
+	as a byte with that value.  Otherwise it is sent as a minimal-length big-endian
+	(high byte first) byte stream holding the value, preceded by one byte holding the
+	byte count, negated.  Thus 0 is transmitted as (00), 7 is transmitted as (07) and
+	256 is transmitted as (FE 01 00).
 
 	A boolean is encoded within an unsigned integer: 0 for false, 1 for true.
 
diff --git a/src/pkg/gob/encoder_test.go b/src/pkg/gob/encoder_test.go
index a7e66a57e3..3e82d8f76e 100644
--- a/src/pkg/gob/encoder_test.go
+++ b/src/pkg/gob/encoder_test.go
@@ -50,8 +50,7 @@ func TestBasicEncoder(t *testing.T) {
 	}
 
 	// Decode the result by hand to verify;
-	state := new(decodeState);
-	state.b = b;
+	state := newDecodeState(b);
 	// The output should be:
 	// 0) The length, 38.
 	length := decodeUint(state);