Source: encode.go in package encoding/gob

go:generate go run encgen.go -output enc_helpers.go


package gob

import (
	"encoding"
	"encoding/binary"
	"math"
	"math/bits"
	"reflect"
	"sync"
)

const uint64Size = 8

type encHelper func(state *encoderState, v reflect.Value) bool

encoderState is 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 0 terminates the structure.

type encoderState struct {
	enc      *Encoder
	b        *encBuffer
	sendZero bool                 // encoding an array element or map key/value pair; send zero values
	fieldnum int                  // the last field number written.
	buf      [1 + uint64Size]byte // buffer used by the encoder; here to avoid allocation.
	next     *encoderState        // for free list
}

encBuffer is an extremely simple, fast implementation of a write-only byte buffer. It never returns a non-nil error, but Write returns an error value so it matches io.Writer.

type encBuffer struct {
	data    []byte
	scratch [64]byte
}

var encBufferPool = sync.Pool{
	New: func() interface{} {
		e := new(encBuffer)
		e.data = e.scratch[0:0]
		return e
	},
}

func (e *encBuffer) writeByte(c byte) {
	e.data = append(e.data, c)
}

func (e *encBuffer) Write(p []byte) (int, error) {
	e.data = append(e.data, p...)
	return len(p), nil
}

func (e *encBuffer) WriteString(s string) {
	e.data = append(e.data, s...)
}

func (e *encBuffer) Len() int {
	return len(e.data)
}

func (e *encBuffer) Bytes() []byte {
	return e.data
}

func (e *encBuffer) Reset() {
	if len(e.data) >= tooBig {
		e.data = e.scratch[0:0]
	} else {
		e.data = e.data[0:0]
	}
}

func (enc *Encoder) newEncoderState(b *encBuffer) *encoderState {
	e := enc.freeList
	if e == nil {
		e = new(encoderState)
		e.enc = enc
	} else {
		enc.freeList = e.next
	}
	e.sendZero = false
	e.fieldnum = 0
	e.b = b
	if len(b.data) == 0 {
		b.data = b.scratch[0:0]
	}
	return e
}

func (enc *Encoder) freeEncoderState(e *encoderState) {
	e.next = enc.freeList
	enc.freeList = e
}

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.

func (state *encoderState) encodeUint(x uint64) {
	if x <= 0x7F {
		state.b.writeByte(uint8(x))
		return
	}

	binary.BigEndian.PutUint64(state.buf[1:], x)
	bc := bits.LeadingZeros64(x) >> 3      // 8 - bytelen(x)
	state.buf[bc] = uint8(bc - uint64Size) // and then we subtract 8 to get -bytelen(x)

	state.b.Write(state.buf[bc : uint64Size+1])
}

encodeInt writes an encoded signed integer to state.w. The low bit of the encoding says whether to bit complement the (other bits of the) uint to recover the int.

func (state *encoderState) encodeInt(i int64) {
	var x uint64
	if i < 0 {
		x = uint64(^i<<1) | 1
	} else {
		x = uint64(i << 1)
	}
	state.encodeUint(x)
}

encOp is the signature of an encoding operator for a given type.

type encOp func(i *encInstr, state *encoderState, v reflect.Value)

The 'instructions' of the encoding machine

type encInstr struct {
	op    encOp
	field int   // field number in input
	index []int // struct index
	indir int   // how many pointer indirections to reach the value in the struct
}

update emits a field number and updates the state to record its value for delta encoding. If the instruction pointer is nil, it does nothing

func (state *encoderState) update(instr *encInstr) {
	if instr != nil {
		state.encodeUint(uint64(instr.field - state.fieldnum))
		state.fieldnum = instr.field
	}
}

Each encoder for a composite is responsible for handling any indirections associated with the elements of the data structure. If any pointer so reached is nil, no bytes are written. If the data item is zero, no bytes are written. Single values - ints, strings etc. - are indirected before calling their encoders. Otherwise, the output (for a scalar) is the field number, as an encoded integer, followed by the field data in its appropriate format.

encIndirect dereferences pv indir times and returns the result.

func encIndirect(pv reflect.Value, indir int) reflect.Value {
	for ; indir > 0; indir-- {
		if pv.IsNil() {
			break
		}
		pv = pv.Elem()
	}
	return pv
}

encBool encodes the bool referenced by v as an unsigned 0 or 1.

func encBool(i *encInstr, state *encoderState, v reflect.Value) {
	b := v.Bool()
	if b || state.sendZero {
		state.update(i)
		if b {
			state.encodeUint(1)
		} else {
			state.encodeUint(0)
		}
	}
}

encInt encodes the signed integer (int int8 int16 int32 int64) referenced by v.

func encInt(i *encInstr, state *encoderState, v reflect.Value) {
	value := v.Int()
	if value != 0 || state.sendZero {
		state.update(i)
		state.encodeInt(value)
	}
}

encUint encodes the unsigned integer (uint uint8 uint16 uint32 uint64 uintptr) referenced by v.

func encUint(i *encInstr, state *encoderState, v reflect.Value) {
	value := v.Uint()
	if value != 0 || state.sendZero {
		state.update(i)
		state.encodeUint(value)
	}
}

floatBits returns a uint64 holding the bits of a floating-point number. Floating-point numbers are transmitted as uint64s holding the bits of the underlying representation. They are sent byte-reversed, with the exponent end coming out first, so integer floating point numbers (for example) transmit more compactly. This routine does the swizzling.

func floatBits(f float64) uint64 {
	u := math.Float64bits(f)
	return bits.ReverseBytes64(u)
}

encFloat encodes the floating point value (float32 float64) referenced by v.

func encFloat(i *encInstr, state *encoderState, v reflect.Value) {
	f := v.Float()
	if f != 0 || state.sendZero {
		bits := floatBits(f)
		state.update(i)
		state.encodeUint(bits)
	}
}

encComplex encodes the complex value (complex64 complex128) referenced by v. Complex numbers are just a pair of floating-point numbers, real part first.

func encComplex(i *encInstr, state *encoderState, v reflect.Value) {
	c := v.Complex()
	if c != 0+0i || state.sendZero {
		rpart := floatBits(real(c))
		ipart := floatBits(imag(c))
		state.update(i)
		state.encodeUint(rpart)
		state.encodeUint(ipart)
	}
}

encUint8Array encodes the byte array referenced by v. Byte arrays are encoded as an unsigned count followed by the raw bytes.

func encUint8Array(i *encInstr, state *encoderState, v reflect.Value) {
	b := v.Bytes()
	if len(b) > 0 || state.sendZero {
		state.update(i)
		state.encodeUint(uint64(len(b)))
		state.b.Write(b)
	}
}

encString encodes the string referenced by v. Strings are encoded as an unsigned count followed by the raw bytes.

func encString(i *encInstr, state *encoderState, v reflect.Value) {
	s := v.String()
	if len(s) > 0 || state.sendZero {
		state.update(i)
		state.encodeUint(uint64(len(s)))
		state.b.WriteString(s)
	}
}

encStructTerminator encodes the end of an encoded struct as delta field number of 0.

func encStructTerminator(i *encInstr, state *encoderState, v reflect.Value) {
	state.encodeUint(0)
}

Execution engine

encEngine an array of instructions indexed by field number of the encoding data, typically a struct. It is executed top to bottom, walking the struct.

type encEngine struct {
	instr []encInstr
}

const singletonField = 0

valid reports whether the value is valid and a non-nil pointer. (Slices, maps, and chans take care of themselves.)

func valid(v reflect.Value) bool {
	switch v.Kind() {
	case reflect.Invalid:
		return false
	case reflect.Ptr:
		return !v.IsNil()
	}
	return true
}

encodeSingle encodes a single top-level non-struct value.

func (enc *Encoder) encodeSingle(b *encBuffer, engine *encEngine, value reflect.Value) {
	state := enc.newEncoderState(b)
	defer enc.freeEncoderState(state)

There is no surrounding struct to frame the transmission, so we must generate data even if the item is zero. To do this, set sendZero.

	state.sendZero = true
	instr := &engine.instr[singletonField]
	if instr.indir > 0 {
		value = encIndirect(value, instr.indir)
	}
	if valid(value) {
		instr.op(instr, state, value)
	}
}

encodeStruct encodes a single struct value.

func (enc *Encoder) encodeStruct(b *encBuffer, engine *encEngine, value reflect.Value) {
	if !valid(value) {
		return
	}
	state := enc.newEncoderState(b)
	defer enc.freeEncoderState(state)
	state.fieldnum = -1
	for i := 0; i < len(engine.instr); i++ {
		instr := &engine.instr[i]

encStructTerminator

			instr.op(instr, state, reflect.Value{})
			break
		}
		field := value.FieldByIndex(instr.index)
		if instr.indir > 0 {

TODO: Is field guaranteed valid? If so we could avoid this check.

			if !valid(field) {
				continue
			}
		}
		instr.op(instr, state, field)
	}
}

encodeArray encodes an array.

func (enc *Encoder) encodeArray(b *encBuffer, value reflect.Value, op encOp, elemIndir int, length int, helper encHelper) {
	state := enc.newEncoderState(b)
	defer enc.freeEncoderState(state)
	state.fieldnum = -1
	state.sendZero = true
	state.encodeUint(uint64(length))
	if helper != nil && helper(state, value) {
		return
	}
	for i := 0; i < length; i++ {
		elem := value.Index(i)
		if elemIndir > 0 {

TODO: Is elem guaranteed valid? If so we could avoid this check.

			if !valid(elem) {
				errorf("encodeArray: nil element")
			}
		}
		op(nil, state, elem)
	}
}

encodeReflectValue is a helper for maps. It encodes the value v.

func encodeReflectValue(state *encoderState, v reflect.Value, op encOp, indir int) {
	for i := 0; i < indir && v.IsValid(); i++ {
		v = reflect.Indirect(v)
	}
	if !v.IsValid() {
		errorf("encodeReflectValue: nil element")
	}
	op(nil, state, v)
}

encodeMap encodes a map as unsigned count followed by key:value pairs.

func (enc *Encoder) encodeMap(b *encBuffer, mv reflect.Value, keyOp, elemOp encOp, keyIndir, elemIndir int) {
	state := enc.newEncoderState(b)
	state.fieldnum = -1
	state.sendZero = true
	keys := mv.MapKeys()
	state.encodeUint(uint64(len(keys)))
	for _, key := range keys {
		encodeReflectValue(state, key, keyOp, keyIndir)
		encodeReflectValue(state, mv.MapIndex(key), elemOp, elemIndir)
	}
	enc.freeEncoderState(state)
}

encodeInterface encodes the interface value iv. To send an interface, we send a string identifying the concrete type, followed by the type identifier (which might require defining that type right now), followed by the concrete value. A nil value gets sent as the empty string for the name, followed by no value.

Gobs can encode nil interface values but not typed interface values holding nil pointers, since nil pointers point to no value.

	elem := iv.Elem()
	if elem.Kind() == reflect.Ptr && elem.IsNil() {
		errorf("gob: cannot encode nil pointer of type %s inside interface", iv.Elem().Type())
	}
	state := enc.newEncoderState(b)
	state.fieldnum = -1
	state.sendZero = true
	if iv.IsNil() {
		state.encodeUint(0)
		return
	}

	ut := userType(iv.Elem().Type())
	namei, ok := concreteTypeToName.Load(ut.base)
	if !ok {
		errorf("type not registered for interface: %s", ut.base)
	}
	name := namei.(string)

Send the name.

	state.encodeUint(uint64(len(name)))

Define the type id if necessary.

Send the type id.

Encode the value into a new buffer. Any nested type definitions should be written to b, before the encoded value.

	enc.pushWriter(b)
	data := encBufferPool.Get().(*encBuffer)
	data.Write(spaceForLength)
	enc.encode(data, elem, ut)
	if enc.err != nil {
		error_(enc.err)
	}
	enc.popWriter()
	enc.writeMessage(b, data)
	data.Reset()
	encBufferPool.Put(data)
	if enc.err != nil {
		error_(enc.err)
	}
	enc.freeEncoderState(state)
}

isZero reports whether the value is the zero of its type.

func isZero(val reflect.Value) bool {
	switch val.Kind() {
	case reflect.Array:
		for i := 0; i < val.Len(); i++ {
			if !isZero(val.Index(i)) {
				return false
			}
		}
		return true
	case reflect.Map, reflect.Slice, reflect.String:
		return val.Len() == 0
	case reflect.Bool:
		return !val.Bool()
	case reflect.Complex64, reflect.Complex128:
		return val.Complex() == 0
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Ptr:
		return val.IsNil()
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return val.Int() == 0
	case reflect.Float32, reflect.Float64:
		return val.Float() == 0
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
		return val.Uint() == 0
	case reflect.Struct:
		for i := 0; i < val.NumField(); i++ {
			if !isZero(val.Field(i)) {
				return false
			}
		}
		return true
	}
	panic("unknown type in isZero " + val.Type().String())
}

encGobEncoder encodes a value that implements the GobEncoder interface. The data is sent as a byte array.

TODO: should we catch panics from the called method?


	var data []byte

We know it's one of these.

	switch ut.externalEnc {
	case xGob:
		data, err = v.Interface().(GobEncoder).GobEncode()
	case xBinary:
		data, err = v.Interface().(encoding.BinaryMarshaler).MarshalBinary()
	case xText:
		data, err = v.Interface().(encoding.TextMarshaler).MarshalText()
	}
	if err != nil {
		error_(err)
	}
	state := enc.newEncoderState(b)
	state.fieldnum = -1
	state.encodeUint(uint64(len(data)))
	state.b.Write(data)
	enc.freeEncoderState(state)
}

var encOpTable = [...]encOp{
	reflect.Bool:       encBool,
	reflect.Int:        encInt,
	reflect.Int8:       encInt,
	reflect.Int16:      encInt,
	reflect.Int32:      encInt,
	reflect.Int64:      encInt,
	reflect.Uint:       encUint,
	reflect.Uint8:      encUint,
	reflect.Uint16:     encUint,
	reflect.Uint32:     encUint,
	reflect.Uint64:     encUint,
	reflect.Uintptr:    encUint,
	reflect.Float32:    encFloat,
	reflect.Float64:    encFloat,
	reflect.Complex64:  encComplex,
	reflect.Complex128: encComplex,
	reflect.String:     encString,
}

encOpFor returns (a pointer to) the encoding op for the base type under rt and the indirection count to reach it.

func encOpFor(rt reflect.Type, inProgress map[reflect.Type]*encOp, building map[*typeInfo]bool) (*encOp, int) {

If the type implements GobEncoder, we handle it without further processing.

	if ut.externalEnc != 0 {
		return gobEncodeOpFor(ut)

If this type is already in progress, it's a recursive type (e.g. map[string]*T). Return the pointer to the op we're already building.

	if opPtr := inProgress[rt]; opPtr != nil {
		return opPtr, ut.indir
	}
	typ := ut.base
	indir := ut.indir
	k := typ.Kind()
	var op encOp
	if int(k) < len(encOpTable) {
		op = encOpTable[k]
	}
	if op == nil {

Special cases

		switch t := typ; t.Kind() {
		case reflect.Slice:
			if t.Elem().Kind() == reflect.Uint8 {
				op = encUint8Array
				break

Slices have a header; we decode it to find the underlying array.

			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
			helper := encSliceHelper[t.Elem().Kind()]
			op = func(i *encInstr, state *encoderState, slice reflect.Value) {
				if !state.sendZero && slice.Len() == 0 {
					return
				}
				state.update(i)
				state.enc.encodeArray(state.b, slice, *elemOp, elemIndir, slice.Len(), helper)
			}

True arrays have size in the type.

			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)
			helper := encArrayHelper[t.Elem().Kind()]
			op = func(i *encInstr, state *encoderState, array reflect.Value) {
				state.update(i)
				state.enc.encodeArray(state.b, array, *elemOp, elemIndir, array.Len(), helper)
			}
		case reflect.Map:
			keyOp, keyIndir := encOpFor(t.Key(), inProgress, building)
			elemOp, elemIndir := encOpFor(t.Elem(), inProgress, building)

We send zero-length (but non-nil) maps because the receiver might want to use the map. (Maps don't use append.)

				if !state.sendZero && mv.IsNil() {
					return
				}
				state.update(i)
				state.enc.encodeMap(state.b, mv, *keyOp, *elemOp, keyIndir, elemIndir)
			}

Generate a closure that calls out to the engine for the nested type.

			getEncEngine(userType(typ), building)
			info := mustGetTypeInfo(typ)
			op = func(i *encInstr, state *encoderState, sv reflect.Value) {

indirect through info to delay evaluation for recursive structs

				enc := info.encoder.Load().(*encEngine)
				state.enc.encodeStruct(state.b, enc, sv)
			}
		case reflect.Interface:
			op = func(i *encInstr, state *encoderState, iv reflect.Value) {
				if !state.sendZero && (!iv.IsValid() || iv.IsNil()) {
					return
				}
				state.update(i)
				state.enc.encodeInterface(state.b, iv)
			}
		}
	}
	if op == nil {
		errorf("can't happen: encode type %s", rt)
	}
	return &op, indir
}

gobEncodeOpFor returns the op for a type that is known to implement GobEncoder.

func gobEncodeOpFor(ut *userTypeInfo) (*encOp, int) {
	rt := ut.user
	if ut.encIndir == -1 {
		rt = reflect.PtrTo(rt)
	} else if ut.encIndir > 0 {
		for i := int8(0); i < ut.encIndir; i++ {
			rt = rt.Elem()
		}
	}
	var op encOp
	op = func(i *encInstr, state *encoderState, v reflect.Value) {

Need to climb up one level to turn value into pointer.

			if !v.CanAddr() {
				errorf("unaddressable value of type %s", rt)
			}
			v = v.Addr()
		}
		if !state.sendZero && isZero(v) {
			return
		}
		state.update(i)
		state.enc.encodeGobEncoder(state.b, ut, v)
	}
	return &op, int(ut.encIndir) // encIndir: op will get called with p == address of receiver.
}

compileEnc returns the engine to compile the type.

func compileEnc(ut *userTypeInfo, building map[*typeInfo]bool) *encEngine {
	srt := ut.base
	engine := new(encEngine)
	seen := make(map[reflect.Type]*encOp)
	rt := ut.base
	if ut.externalEnc != 0 {
		rt = ut.user
	}
	if ut.externalEnc == 0 && srt.Kind() == reflect.Struct {
		for fieldNum, wireFieldNum := 0, 0; fieldNum < srt.NumField(); fieldNum++ {
			f := srt.Field(fieldNum)
			if !isSent(&f) {
				continue
			}
			op, indir := encOpFor(f.Type, seen, building)
			engine.instr = append(engine.instr, encInstr{*op, wireFieldNum, f.Index, indir})
			wireFieldNum++
		}
		if srt.NumField() > 0 && len(engine.instr) == 0 {
			errorf("type %s has no exported fields", rt)
		}
		engine.instr = append(engine.instr, encInstr{encStructTerminator, 0, nil, 0})
	} else {
		engine.instr = make([]encInstr, 1)
		op, indir := encOpFor(rt, seen, building)
		engine.instr[0] = encInstr{*op, singletonField, nil, indir}
	}
	return engine
}

getEncEngine returns the engine to compile the type.

func getEncEngine(ut *userTypeInfo, building map[*typeInfo]bool) *encEngine {
	info, err := getTypeInfo(ut)
	if err != nil {
		error_(err)
	}
	enc, ok := info.encoder.Load().(*encEngine)
	if !ok {
		enc = buildEncEngine(info, ut, building)
	}
	return enc
}

Check for recursive types.

	if building != nil && building[info] {
		return nil
	}
	info.encInit.Lock()
	defer info.encInit.Unlock()
	enc, ok := info.encoder.Load().(*encEngine)
	if !ok {
		if building == nil {
			building = make(map[*typeInfo]bool)
		}
		building[info] = true
		enc = compileEnc(ut, building)
		info.encoder.Store(enc)
	}
	return enc
}

func (enc *Encoder) encode(b *encBuffer, value reflect.Value, ut *userTypeInfo) {
	defer catchError(&enc.err)
	engine := getEncEngine(ut, nil)
	indir := ut.indir
	if ut.externalEnc != 0 {
		indir = int(ut.encIndir)
	}
	for i := 0; i < indir; i++ {
		value = reflect.Indirect(value)
	}
	if ut.externalEnc == 0 && value.Type().Kind() == reflect.Struct {
		enc.encodeStruct(b, engine, value)
	} else {
		enc.encodeSingle(b, engine, value)
	}