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Source file src/encoding/gob/decode.go

Documentation: encoding/gob

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  //go:generate go run decgen.go -output dec_helpers.go
  
  package gob
  
  import (
  	"encoding"
  	"errors"
  	"io"
  	"math"
  	"math/bits"
  	"reflect"
  )
  
  var (
  	errBadUint = errors.New("gob: encoded unsigned integer out of range")
  	errBadType = errors.New("gob: unknown type id or corrupted data")
  	errRange   = errors.New("gob: bad data: field numbers out of bounds")
  )
  
  type decHelper func(state *decoderState, v reflect.Value, length int, ovfl error) bool
  
  // decoderState is the execution state of an instance of the decoder. A new state
  // is created for nested objects.
  type decoderState struct {
  	dec *Decoder
  	// The buffer is stored with an extra indirection because it may be replaced
  	// if we load a type during decode (when reading an interface value).
  	b        *decBuffer
  	fieldnum int           // the last field number read.
  	next     *decoderState // for free list
  }
  
  // decBuffer is an extremely simple, fast implementation of a read-only byte buffer.
  // It is initialized by calling Size and then copying the data into the slice returned by Bytes().
  type decBuffer struct {
  	data   []byte
  	offset int // Read offset.
  }
  
  func (d *decBuffer) Read(p []byte) (int, error) {
  	n := copy(p, d.data[d.offset:])
  	if n == 0 && len(p) != 0 {
  		return 0, io.EOF
  	}
  	d.offset += n
  	return n, nil
  }
  
  func (d *decBuffer) Drop(n int) {
  	if n > d.Len() {
  		panic("drop")
  	}
  	d.offset += n
  }
  
  // Size grows the buffer to exactly n bytes, so d.Bytes() will
  // return a slice of length n. Existing data is first discarded.
  func (d *decBuffer) Size(n int) {
  	d.Reset()
  	if cap(d.data) < n {
  		d.data = make([]byte, n)
  	} else {
  		d.data = d.data[0:n]
  	}
  }
  
  func (d *decBuffer) ReadByte() (byte, error) {
  	if d.offset >= len(d.data) {
  		return 0, io.EOF
  	}
  	c := d.data[d.offset]
  	d.offset++
  	return c, nil
  }
  
  func (d *decBuffer) Len() int {
  	return len(d.data) - d.offset
  }
  
  func (d *decBuffer) Bytes() []byte {
  	return d.data[d.offset:]
  }
  
  func (d *decBuffer) Reset() {
  	d.data = d.data[0:0]
  	d.offset = 0
  }
  
  // We pass the bytes.Buffer separately for easier testing of the infrastructure
  // without requiring a full Decoder.
  func (dec *Decoder) newDecoderState(buf *decBuffer) *decoderState {
  	d := dec.freeList
  	if d == nil {
  		d = new(decoderState)
  		d.dec = dec
  	} else {
  		dec.freeList = d.next
  	}
  	d.b = buf
  	return d
  }
  
  func (dec *Decoder) freeDecoderState(d *decoderState) {
  	d.next = dec.freeList
  	dec.freeList = d
  }
  
  func overflow(name string) error {
  	return errors.New(`value for "` + name + `" out of range`)
  }
  
  // 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, buf []byte) (x uint64, width int, err error) {
  	width = 1
  	n, err := io.ReadFull(r, buf[0:width])
  	if n == 0 {
  		return
  	}
  	b := buf[0]
  	if b <= 0x7f {
  		return uint64(b), width, nil
  	}
  	n = -int(int8(b))
  	if n > uint64Size {
  		err = errBadUint
  		return
  	}
  	width, err = io.ReadFull(r, buf[0:n])
  	if err != nil {
  		if err == io.EOF {
  			err = io.ErrUnexpectedEOF
  		}
  		return
  	}
  	// Could check that the high byte is zero but it's not worth it.
  	for _, b := range buf[0:width] {
  		x = x<<8 | uint64(b)
  	}
  	width++ // +1 for length byte
  	return
  }
  
  // decodeUint reads an encoded unsigned integer from state.r.
  // Does not check for overflow.
  func (state *decoderState) decodeUint() (x uint64) {
  	b, err := state.b.ReadByte()
  	if err != nil {
  		error_(err)
  	}
  	if b <= 0x7f {
  		return uint64(b)
  	}
  	n := -int(int8(b))
  	if n > uint64Size {
  		error_(errBadUint)
  	}
  	buf := state.b.Bytes()
  	if len(buf) < n {
  		errorf("invalid uint data length %d: exceeds input size %d", n, len(buf))
  	}
  	// 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 _, b := range buf[0:n] {
  		x = x<<8 | uint64(b)
  	}
  	state.b.Drop(n)
  	return x
  }
  
  // decodeInt reads an encoded signed integer from state.r.
  // Does not check for overflow.
  func (state *decoderState) decodeInt() int64 {
  	x := state.decodeUint()
  	if x&1 != 0 {
  		return ^int64(x >> 1)
  	}
  	return int64(x >> 1)
  }
  
  // getLength decodes the next uint and makes sure it is a possible
  // size for a data item that follows, which means it must fit in a
  // non-negative int and fit in the buffer.
  func (state *decoderState) getLength() (int, bool) {
  	n := int(state.decodeUint())
  	if n < 0 || state.b.Len() < n || tooBig <= n {
  		return 0, false
  	}
  	return n, true
  }
  
  // decOp is the signature of a decoding operator for a given type.
  type decOp func(i *decInstr, state *decoderState, v reflect.Value)
  
  // The 'instructions' of the decoding machine
  type decInstr struct {
  	op    decOp
  	field int   // field number of the wire type
  	index []int // field access indices for destination type
  	ovfl  error // error message for overflow/underflow (for arrays, of the elements)
  }
  
  // ignoreUint discards a uint value with no destination.
  func ignoreUint(i *decInstr, state *decoderState, v reflect.Value) {
  	state.decodeUint()
  }
  
  // ignoreTwoUints discards a uint value with no destination. It's used to skip
  // complex values.
  func ignoreTwoUints(i *decInstr, state *decoderState, v reflect.Value) {
  	state.decodeUint()
  	state.decodeUint()
  }
  
  // Since the encoder writes no zeros, if we arrive at a decoder we have
  // a value to extract and store. The field number has already been read
  // (it's how we knew to call this decoder).
  // Each decoder is responsible for handling any indirections associated
  // with the data structure. If any pointer so reached is nil, allocation must
  // be done.
  
  // decAlloc takes a value and returns a settable value that can
  // be assigned to. If the value is a pointer, decAlloc guarantees it points to storage.
  // The callers to the individual decoders are expected to have used decAlloc.
  // The individual decoders don't need to it.
  func decAlloc(v reflect.Value) reflect.Value {
  	for v.Kind() == reflect.Ptr {
  		if v.IsNil() {
  			v.Set(reflect.New(v.Type().Elem()))
  		}
  		v = v.Elem()
  	}
  	return v
  }
  
  // decBool decodes a uint and stores it as a boolean in value.
  func decBool(i *decInstr, state *decoderState, value reflect.Value) {
  	value.SetBool(state.decodeUint() != 0)
  }
  
  // decInt8 decodes an integer and stores it as an int8 in value.
  func decInt8(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeInt()
  	if v < math.MinInt8 || math.MaxInt8 < v {
  		error_(i.ovfl)
  	}
  	value.SetInt(v)
  }
  
  // decUint8 decodes an unsigned integer and stores it as a uint8 in value.
  func decUint8(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeUint()
  	if math.MaxUint8 < v {
  		error_(i.ovfl)
  	}
  	value.SetUint(v)
  }
  
  // decInt16 decodes an integer and stores it as an int16 in value.
  func decInt16(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeInt()
  	if v < math.MinInt16 || math.MaxInt16 < v {
  		error_(i.ovfl)
  	}
  	value.SetInt(v)
  }
  
  // decUint16 decodes an unsigned integer and stores it as a uint16 in value.
  func decUint16(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeUint()
  	if math.MaxUint16 < v {
  		error_(i.ovfl)
  	}
  	value.SetUint(v)
  }
  
  // decInt32 decodes an integer and stores it as an int32 in value.
  func decInt32(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeInt()
  	if v < math.MinInt32 || math.MaxInt32 < v {
  		error_(i.ovfl)
  	}
  	value.SetInt(v)
  }
  
  // decUint32 decodes an unsigned integer and stores it as a uint32 in value.
  func decUint32(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeUint()
  	if math.MaxUint32 < v {
  		error_(i.ovfl)
  	}
  	value.SetUint(v)
  }
  
  // decInt64 decodes an integer and stores it as an int64 in value.
  func decInt64(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeInt()
  	value.SetInt(v)
  }
  
  // decUint64 decodes an unsigned integer and stores it as a uint64 in value.
  func decUint64(i *decInstr, state *decoderState, value reflect.Value) {
  	v := state.decodeUint()
  	value.SetUint(v)
  }
  
  // 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
  // unswizzling.
  func float64FromBits(u uint64) float64 {
  	v := bits.ReverseBytes64(u)
  	return math.Float64frombits(v)
  }
  
  // float32FromBits decodes an unsigned integer, treats it as a 32-bit floating-point
  // number, and returns it. It's a helper function for float32 and complex64.
  // It returns a float64 because that's what reflection needs, but its return
  // value is known to be accurately representable in a float32.
  func float32FromBits(u uint64, ovfl error) float64 {
  	v := float64FromBits(u)
  	av := v
  	if av < 0 {
  		av = -av
  	}
  	// +Inf is OK in both 32- and 64-bit floats. Underflow is always OK.
  	if math.MaxFloat32 < av && av <= math.MaxFloat64 {
  		error_(ovfl)
  	}
  	return v
  }
  
  // decFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
  // number, and stores it in value.
  func decFloat32(i *decInstr, state *decoderState, value reflect.Value) {
  	value.SetFloat(float32FromBits(state.decodeUint(), i.ovfl))
  }
  
  // decFloat64 decodes an unsigned integer, treats it as a 64-bit floating-point
  // number, and stores it in value.
  func decFloat64(i *decInstr, state *decoderState, value reflect.Value) {
  	value.SetFloat(float64FromBits(state.decodeUint()))
  }
  
  // decComplex64 decodes a pair of unsigned integers, treats them as a
  // pair of floating point numbers, and stores them as a complex64 in value.
  // The real part comes first.
  func decComplex64(i *decInstr, state *decoderState, value reflect.Value) {
  	real := float32FromBits(state.decodeUint(), i.ovfl)
  	imag := float32FromBits(state.decodeUint(), i.ovfl)
  	value.SetComplex(complex(real, imag))
  }
  
  // decComplex128 decodes a pair of unsigned integers, treats them as a
  // pair of floating point numbers, and stores them as a complex128 in value.
  // The real part comes first.
  func decComplex128(i *decInstr, state *decoderState, value reflect.Value) {
  	real := float64FromBits(state.decodeUint())
  	imag := float64FromBits(state.decodeUint())
  	value.SetComplex(complex(real, imag))
  }
  
  // decUint8Slice decodes a byte slice and stores in value a slice header
  // describing the data.
  // uint8 slices are encoded as an unsigned count followed by the raw bytes.
  func decUint8Slice(i *decInstr, state *decoderState, value reflect.Value) {
  	n, ok := state.getLength()
  	if !ok {
  		errorf("bad %s slice length: %d", value.Type(), n)
  	}
  	if value.Cap() < n {
  		value.Set(reflect.MakeSlice(value.Type(), n, n))
  	} else {
  		value.Set(value.Slice(0, n))
  	}
  	if _, err := state.b.Read(value.Bytes()); err != nil {
  		errorf("error decoding []byte: %s", err)
  	}
  }
  
  // decString decodes byte array and stores in value a string header
  // describing the data.
  // Strings are encoded as an unsigned count followed by the raw bytes.
  func decString(i *decInstr, state *decoderState, value reflect.Value) {
  	n, ok := state.getLength()
  	if !ok {
  		errorf("bad %s slice length: %d", value.Type(), n)
  	}
  	// Read the data.
  	data := state.b.Bytes()
  	if len(data) < n {
  		errorf("invalid string length %d: exceeds input size %d", n, len(data))
  	}
  	s := string(data[:n])
  	state.b.Drop(n)
  	value.SetString(s)
  }
  
  // ignoreUint8Array skips over the data for a byte slice value with no destination.
  func ignoreUint8Array(i *decInstr, state *decoderState, value reflect.Value) {
  	n, ok := state.getLength()
  	if !ok {
  		errorf("slice length too large")
  	}
  	bn := state.b.Len()
  	if bn < n {
  		errorf("invalid slice length %d: exceeds input size %d", n, bn)
  	}
  	state.b.Drop(n)
  }
  
  // Execution engine
  
  // The encoder engine is an array of instructions indexed by field number of the incoming
  // decoder. It is executed with random access according to field number.
  type decEngine struct {
  	instr    []decInstr
  	numInstr int // the number of active instructions
  }
  
  // decodeSingle decodes a top-level value that is not a struct and stores it in value.
  // Such values are preceded by a zero, making them have the memory layout of a
  // struct field (although with an illegal field number).
  func (dec *Decoder) decodeSingle(engine *decEngine, value reflect.Value) {
  	state := dec.newDecoderState(&dec.buf)
  	defer dec.freeDecoderState(state)
  	state.fieldnum = singletonField
  	if state.decodeUint() != 0 {
  		errorf("decode: corrupted data: non-zero delta for singleton")
  	}
  	instr := &engine.instr[singletonField]
  	instr.op(instr, state, value)
  }
  
  // decodeStruct decodes a top-level struct and stores it in value.
  // Indir is for the value, not the type. At the time of the call it may
  // differ from ut.indir, which was computed when the engine was built.
  // This state cannot arise for decodeSingle, which is called directly
  // from the user's value, not from the innards of an engine.
  func (dec *Decoder) decodeStruct(engine *decEngine, value reflect.Value) {
  	state := dec.newDecoderState(&dec.buf)
  	defer dec.freeDecoderState(state)
  	state.fieldnum = -1
  	for state.b.Len() > 0 {
  		delta := int(state.decodeUint())
  		if delta < 0 {
  			errorf("decode: corrupted data: negative delta")
  		}
  		if delta == 0 { // struct terminator is zero delta fieldnum
  			break
  		}
  		fieldnum := state.fieldnum + delta
  		if fieldnum >= len(engine.instr) {
  			error_(errRange)
  			break
  		}
  		instr := &engine.instr[fieldnum]
  		var field reflect.Value
  		if instr.index != nil {
  			// Otherwise the field is unknown to us and instr.op is an ignore op.
  			field = value.FieldByIndex(instr.index)
  			if field.Kind() == reflect.Ptr {
  				field = decAlloc(field)
  			}
  		}
  		instr.op(instr, state, field)
  		state.fieldnum = fieldnum
  	}
  }
  
  var noValue reflect.Value
  
  // ignoreStruct discards the data for a struct with no destination.
  func (dec *Decoder) ignoreStruct(engine *decEngine) {
  	state := dec.newDecoderState(&dec.buf)
  	defer dec.freeDecoderState(state)
  	state.fieldnum = -1
  	for state.b.Len() > 0 {
  		delta := int(state.decodeUint())
  		if delta < 0 {
  			errorf("ignore decode: corrupted data: negative delta")
  		}
  		if delta == 0 { // struct terminator is zero delta fieldnum
  			break
  		}
  		fieldnum := state.fieldnum + delta
  		if fieldnum >= len(engine.instr) {
  			error_(errRange)
  		}
  		instr := &engine.instr[fieldnum]
  		instr.op(instr, state, noValue)
  		state.fieldnum = fieldnum
  	}
  }
  
  // ignoreSingle discards the data for a top-level non-struct value with no
  // destination. It's used when calling Decode with a nil value.
  func (dec *Decoder) ignoreSingle(engine *decEngine) {
  	state := dec.newDecoderState(&dec.buf)
  	defer dec.freeDecoderState(state)
  	state.fieldnum = singletonField
  	delta := int(state.decodeUint())
  	if delta != 0 {
  		errorf("decode: corrupted data: non-zero delta for singleton")
  	}
  	instr := &engine.instr[singletonField]
  	instr.op(instr, state, noValue)
  }
  
  // decodeArrayHelper does the work for decoding arrays and slices.
  func (dec *Decoder) decodeArrayHelper(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
  	if helper != nil && helper(state, value, length, ovfl) {
  		return
  	}
  	instr := &decInstr{elemOp, 0, nil, ovfl}
  	isPtr := value.Type().Elem().Kind() == reflect.Ptr
  	for i := 0; i < length; i++ {
  		if state.b.Len() == 0 {
  			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
  		}
  		v := value.Index(i)
  		if isPtr {
  			v = decAlloc(v)
  		}
  		elemOp(instr, state, v)
  	}
  }
  
  // decodeArray decodes an array and stores it in value.
  // The length is an unsigned integer preceding the elements. Even though the length is redundant
  // (it's part of the type), it's a useful check and is included in the encoding.
  func (dec *Decoder) decodeArray(state *decoderState, value reflect.Value, elemOp decOp, length int, ovfl error, helper decHelper) {
  	if n := state.decodeUint(); n != uint64(length) {
  		errorf("length mismatch in decodeArray")
  	}
  	dec.decodeArrayHelper(state, value, elemOp, length, ovfl, helper)
  }
  
  // decodeIntoValue is a helper for map decoding.
  func decodeIntoValue(state *decoderState, op decOp, isPtr bool, value reflect.Value, instr *decInstr) reflect.Value {
  	v := value
  	if isPtr {
  		v = decAlloc(value)
  	}
  
  	op(instr, state, v)
  	return value
  }
  
  // decodeMap decodes a map and stores it in value.
  // Maps are encoded as a length followed by key:value pairs.
  // Because the internals of maps are not visible to us, we must
  // use reflection rather than pointer magic.
  func (dec *Decoder) decodeMap(mtyp reflect.Type, state *decoderState, value reflect.Value, keyOp, elemOp decOp, ovfl error) {
  	n := int(state.decodeUint())
  	if value.IsNil() {
  		value.Set(reflect.MakeMapWithSize(mtyp, n))
  	}
  	keyIsPtr := mtyp.Key().Kind() == reflect.Ptr
  	elemIsPtr := mtyp.Elem().Kind() == reflect.Ptr
  	keyInstr := &decInstr{keyOp, 0, nil, ovfl}
  	elemInstr := &decInstr{elemOp, 0, nil, ovfl}
  	keyP := reflect.New(mtyp.Key())
  	keyZ := reflect.Zero(mtyp.Key())
  	elemP := reflect.New(mtyp.Elem())
  	elemZ := reflect.Zero(mtyp.Elem())
  	for i := 0; i < n; i++ {
  		key := decodeIntoValue(state, keyOp, keyIsPtr, keyP.Elem(), keyInstr)
  		elem := decodeIntoValue(state, elemOp, elemIsPtr, elemP.Elem(), elemInstr)
  		value.SetMapIndex(key, elem)
  		keyP.Elem().Set(keyZ)
  		elemP.Elem().Set(elemZ)
  	}
  }
  
  // ignoreArrayHelper does the work for discarding arrays and slices.
  func (dec *Decoder) ignoreArrayHelper(state *decoderState, elemOp decOp, length int) {
  	instr := &decInstr{elemOp, 0, nil, errors.New("no error")}
  	for i := 0; i < length; i++ {
  		if state.b.Len() == 0 {
  			errorf("decoding array or slice: length exceeds input size (%d elements)", length)
  		}
  		elemOp(instr, state, noValue)
  	}
  }
  
  // ignoreArray discards the data for an array value with no destination.
  func (dec *Decoder) ignoreArray(state *decoderState, elemOp decOp, length int) {
  	if n := state.decodeUint(); n != uint64(length) {
  		errorf("length mismatch in ignoreArray")
  	}
  	dec.ignoreArrayHelper(state, elemOp, length)
  }
  
  // ignoreMap discards the data for a map value with no destination.
  func (dec *Decoder) ignoreMap(state *decoderState, keyOp, elemOp decOp) {
  	n := int(state.decodeUint())
  	keyInstr := &decInstr{keyOp, 0, nil, errors.New("no error")}
  	elemInstr := &decInstr{elemOp, 0, nil, errors.New("no error")}
  	for i := 0; i < n; i++ {
  		keyOp(keyInstr, state, noValue)
  		elemOp(elemInstr, state, noValue)
  	}
  }
  
  // decodeSlice decodes a slice and stores it in value.
  // Slices are encoded as an unsigned length followed by the elements.
  func (dec *Decoder) decodeSlice(state *decoderState, value reflect.Value, elemOp decOp, ovfl error, helper decHelper) {
  	u := state.decodeUint()
  	typ := value.Type()
  	size := uint64(typ.Elem().Size())
  	nBytes := u * size
  	n := int(u)
  	// Take care with overflow in this calculation.
  	if n < 0 || uint64(n) != u || nBytes > tooBig || (size > 0 && nBytes/size != u) {
  		// We don't check n against buffer length here because if it's a slice
  		// of interfaces, there will be buffer reloads.
  		errorf("%s slice too big: %d elements of %d bytes", typ.Elem(), u, size)
  	}
  	if value.Cap() < n {
  		value.Set(reflect.MakeSlice(typ, n, n))
  	} else {
  		value.Set(value.Slice(0, n))
  	}
  	dec.decodeArrayHelper(state, value, elemOp, n, ovfl, helper)
  }
  
  // ignoreSlice skips over the data for a slice value with no destination.
  func (dec *Decoder) ignoreSlice(state *decoderState, elemOp decOp) {
  	dec.ignoreArrayHelper(state, elemOp, int(state.decodeUint()))
  }
  
  // decodeInterface decodes an interface value and stores it in value.
  // Interfaces are encoded as the name of a concrete type followed by a value.
  // If the name is empty, the value is nil and no value is sent.
  func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, value reflect.Value) {
  	// Read the name of the concrete type.
  	nr := state.decodeUint()
  	if nr > 1<<31 { // zero is permissible for anonymous types
  		errorf("invalid type name length %d", nr)
  	}
  	if nr > uint64(state.b.Len()) {
  		errorf("invalid type name length %d: exceeds input size", nr)
  	}
  	n := int(nr)
  	name := state.b.Bytes()[:n]
  	state.b.Drop(n)
  	// Allocate the destination interface value.
  	if len(name) == 0 {
  		// Copy the nil interface value to the target.
  		value.Set(reflect.Zero(value.Type()))
  		return
  	}
  	if len(name) > 1024 {
  		errorf("name too long (%d bytes): %.20q...", len(name), name)
  	}
  	// The concrete type must be registered.
  	typi, ok := nameToConcreteType.Load(string(name))
  	if !ok {
  		errorf("name not registered for interface: %q", name)
  	}
  	typ := typi.(reflect.Type)
  
  	// Read the type id of the concrete value.
  	concreteId := dec.decodeTypeSequence(true)
  	if concreteId < 0 {
  		error_(dec.err)
  	}
  	// Byte count of value is next; we don't care what it is (it's there
  	// in case we want to ignore the value by skipping it completely).
  	state.decodeUint()
  	// Read the concrete value.
  	v := allocValue(typ)
  	dec.decodeValue(concreteId, v)
  	if dec.err != nil {
  		error_(dec.err)
  	}
  	// Assign the concrete value to the interface.
  	// Tread carefully; it might not satisfy the interface.
  	if !typ.AssignableTo(ityp) {
  		errorf("%s is not assignable to type %s", typ, ityp)
  	}
  	// Copy the interface value to the target.
  	value.Set(v)
  }
  
  // ignoreInterface discards the data for an interface value with no destination.
  func (dec *Decoder) ignoreInterface(state *decoderState) {
  	// Read the name of the concrete type.
  	n, ok := state.getLength()
  	if !ok {
  		errorf("bad interface encoding: name too large for buffer")
  	}
  	bn := state.b.Len()
  	if bn < n {
  		errorf("invalid interface value length %d: exceeds input size %d", n, bn)
  	}
  	state.b.Drop(n)
  	id := dec.decodeTypeSequence(true)
  	if id < 0 {
  		error_(dec.err)
  	}
  	// At this point, the decoder buffer contains a delimited value. Just toss it.
  	n, ok = state.getLength()
  	if !ok {
  		errorf("bad interface encoding: data length too large for buffer")
  	}
  	state.b.Drop(n)
  }
  
  // decodeGobDecoder decodes something implementing the GobDecoder interface.
  // The data is encoded as a byte slice.
  func (dec *Decoder) decodeGobDecoder(ut *userTypeInfo, state *decoderState, value reflect.Value) {
  	// Read the bytes for the value.
  	n, ok := state.getLength()
  	if !ok {
  		errorf("GobDecoder: length too large for buffer")
  	}
  	b := state.b.Bytes()
  	if len(b) < n {
  		errorf("GobDecoder: invalid data length %d: exceeds input size %d", n, len(b))
  	}
  	b = b[:n]
  	state.b.Drop(n)
  	var err error
  	// We know it's one of these.
  	switch ut.externalDec {
  	case xGob:
  		err = value.Interface().(GobDecoder).GobDecode(b)
  	case xBinary:
  		err = value.Interface().(encoding.BinaryUnmarshaler).UnmarshalBinary(b)
  	case xText:
  		err = value.Interface().(encoding.TextUnmarshaler).UnmarshalText(b)
  	}
  	if err != nil {
  		error_(err)
  	}
  }
  
  // ignoreGobDecoder discards the data for a GobDecoder value with no destination.
  func (dec *Decoder) ignoreGobDecoder(state *decoderState) {
  	// Read the bytes for the value.
  	n, ok := state.getLength()
  	if !ok {
  		errorf("GobDecoder: length too large for buffer")
  	}
  	bn := state.b.Len()
  	if bn < n {
  		errorf("GobDecoder: invalid data length %d: exceeds input size %d", n, bn)
  	}
  	state.b.Drop(n)
  }
  
  // Index by Go types.
  var decOpTable = [...]decOp{
  	reflect.Bool:       decBool,
  	reflect.Int8:       decInt8,
  	reflect.Int16:      decInt16,
  	reflect.Int32:      decInt32,
  	reflect.Int64:      decInt64,
  	reflect.Uint8:      decUint8,
  	reflect.Uint16:     decUint16,
  	reflect.Uint32:     decUint32,
  	reflect.Uint64:     decUint64,
  	reflect.Float32:    decFloat32,
  	reflect.Float64:    decFloat64,
  	reflect.Complex64:  decComplex64,
  	reflect.Complex128: decComplex128,
  	reflect.String:     decString,
  }
  
  // Indexed by gob types.  tComplex will be added during type.init().
  var decIgnoreOpMap = map[typeId]decOp{
  	tBool:    ignoreUint,
  	tInt:     ignoreUint,
  	tUint:    ignoreUint,
  	tFloat:   ignoreUint,
  	tBytes:   ignoreUint8Array,
  	tString:  ignoreUint8Array,
  	tComplex: ignoreTwoUints,
  }
  
  // decOpFor returns the decoding op for the base type under rt and
  // the indirection count to reach it.
  func (dec *Decoder) decOpFor(wireId typeId, rt reflect.Type, name string, inProgress map[reflect.Type]*decOp) *decOp {
  	ut := userType(rt)
  	// If the type implements GobEncoder, we handle it without further processing.
  	if ut.externalDec != 0 {
  		return dec.gobDecodeOpFor(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
  	}
  	typ := ut.base
  	var op decOp
  	k := typ.Kind()
  	if int(k) < len(decOpTable) {
  		op = decOpTable[k]
  	}
  	if op == nil {
  		inProgress[rt] = &op
  		// Special cases
  		switch t := typ; t.Kind() {
  		case reflect.Array:
  			name = "element of " + name
  			elemId := dec.wireType[wireId].ArrayT.Elem
  			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
  			ovfl := overflow(name)
  			helper := decArrayHelper[t.Elem().Kind()]
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.decodeArray(state, value, *elemOp, t.Len(), ovfl, helper)
  			}
  
  		case reflect.Map:
  			keyId := dec.wireType[wireId].MapT.Key
  			elemId := dec.wireType[wireId].MapT.Elem
  			keyOp := dec.decOpFor(keyId, t.Key(), "key of "+name, inProgress)
  			elemOp := dec.decOpFor(elemId, t.Elem(), "element of "+name, inProgress)
  			ovfl := overflow(name)
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.decodeMap(t, state, value, *keyOp, *elemOp, ovfl)
  			}
  
  		case reflect.Slice:
  			name = "element of " + name
  			if t.Elem().Kind() == reflect.Uint8 {
  				op = decUint8Slice
  				break
  			}
  			var elemId typeId
  			if tt, ok := builtinIdToType[wireId]; ok {
  				elemId = tt.(*sliceType).Elem
  			} else {
  				elemId = dec.wireType[wireId].SliceT.Elem
  			}
  			elemOp := dec.decOpFor(elemId, t.Elem(), name, inProgress)
  			ovfl := overflow(name)
  			helper := decSliceHelper[t.Elem().Kind()]
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.decodeSlice(state, value, *elemOp, ovfl, helper)
  			}
  
  		case reflect.Struct:
  			// Generate a closure that calls out to the engine for the nested type.
  			ut := userType(typ)
  			enginePtr, err := dec.getDecEnginePtr(wireId, ut)
  			if err != nil {
  				error_(err)
  			}
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				// indirect through enginePtr to delay evaluation for recursive structs.
  				dec.decodeStruct(*enginePtr, value)
  			}
  		case reflect.Interface:
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.decodeInterface(t, state, value)
  			}
  		}
  	}
  	if op == nil {
  		errorf("decode can't handle type %s", rt)
  	}
  	return &op
  }
  
  // decIgnoreOpFor returns the decoding op for a field that has no destination.
  func (dec *Decoder) decIgnoreOpFor(wireId typeId, inProgress map[typeId]*decOp) *decOp {
  	// 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[wireId]; opPtr != nil {
  		return opPtr
  	}
  	op, ok := decIgnoreOpMap[wireId]
  	if !ok {
  		inProgress[wireId] = &op
  		if wireId == tInterface {
  			// Special case because it's a method: the ignored item might
  			// define types and we need to record their state in the decoder.
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.ignoreInterface(state)
  			}
  			return &op
  		}
  		// Special cases
  		wire := dec.wireType[wireId]
  		switch {
  		case wire == nil:
  			errorf("bad data: undefined type %s", wireId.string())
  		case wire.ArrayT != nil:
  			elemId := wire.ArrayT.Elem
  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.ignoreArray(state, *elemOp, wire.ArrayT.Len)
  			}
  
  		case wire.MapT != nil:
  			keyId := dec.wireType[wireId].MapT.Key
  			elemId := dec.wireType[wireId].MapT.Elem
  			keyOp := dec.decIgnoreOpFor(keyId, inProgress)
  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.ignoreMap(state, *keyOp, *elemOp)
  			}
  
  		case wire.SliceT != nil:
  			elemId := wire.SliceT.Elem
  			elemOp := dec.decIgnoreOpFor(elemId, inProgress)
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.ignoreSlice(state, *elemOp)
  			}
  
  		case wire.StructT != nil:
  			// Generate a closure that calls out to the engine for the nested type.
  			enginePtr, err := dec.getIgnoreEnginePtr(wireId)
  			if err != nil {
  				error_(err)
  			}
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				// indirect through enginePtr to delay evaluation for recursive structs
  				state.dec.ignoreStruct(*enginePtr)
  			}
  
  		case wire.GobEncoderT != nil, wire.BinaryMarshalerT != nil, wire.TextMarshalerT != nil:
  			op = func(i *decInstr, state *decoderState, value reflect.Value) {
  				state.dec.ignoreGobDecoder(state)
  			}
  		}
  	}
  	if op == nil {
  		errorf("bad data: ignore can't handle type %s", wireId.string())
  	}
  	return &op
  }
  
  // gobDecodeOpFor returns the op for a type that is known to implement
  // GobDecoder.
  func (dec *Decoder) gobDecodeOpFor(ut *userTypeInfo) *decOp {
  	rcvrType := ut.user
  	if ut.decIndir == -1 {
  		rcvrType = reflect.PtrTo(rcvrType)
  	} else if ut.decIndir > 0 {
  		for i := int8(0); i < ut.decIndir; i++ {
  			rcvrType = rcvrType.Elem()
  		}
  	}
  	var op decOp
  	op = func(i *decInstr, state *decoderState, value reflect.Value) {
  		// We now have the base type. We need its address if the receiver is a pointer.
  		if value.Kind() != reflect.Ptr && rcvrType.Kind() == reflect.Ptr {
  			value = value.Addr()
  		}
  		state.dec.decodeGobDecoder(ut, state, value)
  	}
  	return &op
  }
  
  // compatibleType asks: Are these two gob Types compatible?
  // Answers the question for basic types, arrays, maps and slices, plus
  // GobEncoder/Decoder pairs.
  // Structs are considered ok; fields will be checked later.
  func (dec *Decoder) compatibleType(fr reflect.Type, fw typeId, inProgress map[reflect.Type]typeId) bool {
  	if rhs, ok := inProgress[fr]; ok {
  		return rhs == fw
  	}
  	inProgress[fr] = fw
  	ut := userType(fr)
  	wire, ok := dec.wireType[fw]
  	// If wire was encoded with an encoding method, fr must have that method.
  	// And if not, it must not.
  	// At most one of the booleans in ut is set.
  	// We could possibly relax this constraint in the future in order to
  	// choose the decoding method using the data in the wireType.
  	// The parentheses look odd but are correct.
  	if (ut.externalDec == xGob) != (ok && wire.GobEncoderT != nil) ||
  		(ut.externalDec == xBinary) != (ok && wire.BinaryMarshalerT != nil) ||
  		(ut.externalDec == xText) != (ok && wire.TextMarshalerT != nil) {
  		return false
  	}
  	if ut.externalDec != 0 { // This test trumps all others.
  		return true
  	}
  	switch t := ut.base; t.Kind() {
  	default:
  		// chan, etc: cannot handle.
  		return false
  	case reflect.Bool:
  		return fw == tBool
  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
  		return fw == tInt
  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
  		return fw == tUint
  	case reflect.Float32, reflect.Float64:
  		return fw == tFloat
  	case reflect.Complex64, reflect.Complex128:
  		return fw == tComplex
  	case reflect.String:
  		return fw == tString
  	case reflect.Interface:
  		return fw == tInterface
  	case reflect.Array:
  		if !ok || wire.ArrayT == nil {
  			return false
  		}
  		array := wire.ArrayT
  		return t.Len() == array.Len && dec.compatibleType(t.Elem(), array.Elem, inProgress)
  	case reflect.Map:
  		if !ok || wire.MapT == nil {
  			return false
  		}
  		MapType := wire.MapT
  		return dec.compatibleType(t.Key(), MapType.Key, inProgress) && dec.compatibleType(t.Elem(), MapType.Elem, inProgress)
  	case reflect.Slice:
  		// Is it an array of bytes?
  		if t.Elem().Kind() == reflect.Uint8 {
  			return fw == tBytes
  		}
  		// Extract and compare element types.
  		var sw *sliceType
  		if tt, ok := builtinIdToType[fw]; ok {
  			sw, _ = tt.(*sliceType)
  		} else if wire != nil {
  			sw = wire.SliceT
  		}
  		elem := userType(t.Elem()).base
  		return sw != nil && dec.compatibleType(elem, sw.Elem, inProgress)
  	case reflect.Struct:
  		return true
  	}
  }
  
  // typeString returns a human-readable description of the type identified by remoteId.
  func (dec *Decoder) typeString(remoteId typeId) string {
  	if t := idToType[remoteId]; t != nil {
  		// globally known type.
  		return t.string()
  	}
  	return dec.wireType[remoteId].string()
  }
  
  // compileSingle compiles the decoder engine for a non-struct top-level value, including
  // GobDecoders.
  func (dec *Decoder) compileSingle(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  	rt := ut.user
  	engine = new(decEngine)
  	engine.instr = make([]decInstr, 1) // one item
  	name := rt.String()                // best we can do
  	if !dec.compatibleType(rt, remoteId, make(map[reflect.Type]typeId)) {
  		remoteType := dec.typeString(remoteId)
  		// Common confusing case: local interface type, remote concrete type.
  		if ut.base.Kind() == reflect.Interface && remoteId != tInterface {
  			return nil, errors.New("gob: local interface type " + name + " can only be decoded from remote interface type; received concrete type " + remoteType)
  		}
  		return nil, errors.New("gob: decoding into local type " + name + ", received remote type " + remoteType)
  	}
  	op := dec.decOpFor(remoteId, rt, name, make(map[reflect.Type]*decOp))
  	ovfl := errors.New(`value for "` + name + `" out of range`)
  	engine.instr[singletonField] = decInstr{*op, singletonField, nil, ovfl}
  	engine.numInstr = 1
  	return
  }
  
  // compileIgnoreSingle compiles the decoder engine for a non-struct top-level value that will be discarded.
  func (dec *Decoder) compileIgnoreSingle(remoteId typeId) (engine *decEngine, err error) {
  	engine = new(decEngine)
  	engine.instr = make([]decInstr, 1) // one item
  	op := dec.decIgnoreOpFor(remoteId, make(map[typeId]*decOp))
  	ovfl := overflow(dec.typeString(remoteId))
  	engine.instr[0] = decInstr{*op, 0, nil, ovfl}
  	engine.numInstr = 1
  	return
  }
  
  // compileDec compiles the decoder engine for a value. If the value is not a struct,
  // it calls out to compileSingle.
  func (dec *Decoder) compileDec(remoteId typeId, ut *userTypeInfo) (engine *decEngine, err error) {
  	defer catchError(&err)
  	rt := ut.base
  	srt := rt
  	if srt.Kind() != reflect.Struct || ut.externalDec != 0 {
  		return dec.compileSingle(remoteId, ut)
  	}
  	var wireStruct *structType
  	// Builtin types can come from global pool; the rest must be defined by the decoder.
  	// Also we know we're decoding a struct now, so the client must have sent one.
  	if t, ok := builtinIdToType[remoteId]; ok {
  		wireStruct, _ = t.(*structType)
  	} else {
  		wire := dec.wireType[remoteId]
  		if wire == nil {
  			error_(errBadType)
  		}
  		wireStruct = wire.StructT
  	}
  	if wireStruct == nil {
  		errorf("type mismatch in decoder: want struct type %s; got non-struct", rt)
  	}
  	engine = new(decEngine)
  	engine.instr = make([]decInstr, len(wireStruct.Field))
  	seen := make(map[reflect.Type]*decOp)
  	// Loop over the fields of the wire type.
  	for fieldnum := 0; fieldnum < len(wireStruct.Field); fieldnum++ {
  		wireField := wireStruct.Field[fieldnum]
  		if wireField.Name == "" {
  			errorf("empty name for remote field of type %s", wireStruct.Name)
  		}
  		ovfl := overflow(wireField.Name)
  		// Find the field of the local type with the same name.
  		localField, present := srt.FieldByName(wireField.Name)
  		// TODO(r): anonymous names
  		if !present || !isExported(wireField.Name) {
  			op := dec.decIgnoreOpFor(wireField.Id, make(map[typeId]*decOp))
  			engine.instr[fieldnum] = decInstr{*op, fieldnum, nil, ovfl}
  			continue
  		}
  		if !dec.compatibleType(localField.Type, wireField.Id, make(map[reflect.Type]typeId)) {
  			errorf("wrong type (%s) for received field %s.%s", localField.Type, wireStruct.Name, wireField.Name)
  		}
  		op := dec.decOpFor(wireField.Id, localField.Type, localField.Name, seen)
  		engine.instr[fieldnum] = decInstr{*op, fieldnum, localField.Index, ovfl}
  		engine.numInstr++
  	}
  	return
  }
  
  // getDecEnginePtr returns the engine for the specified type.
  func (dec *Decoder) getDecEnginePtr(remoteId typeId, ut *userTypeInfo) (enginePtr **decEngine, err error) {
  	rt := ut.user
  	decoderMap, ok := dec.decoderCache[rt]
  	if !ok {
  		decoderMap = make(map[typeId]**decEngine)
  		dec.decoderCache[rt] = decoderMap
  	}
  	if enginePtr, ok = decoderMap[remoteId]; !ok {
  		// To handle recursive types, mark this engine as underway before compiling.
  		enginePtr = new(*decEngine)
  		decoderMap[remoteId] = enginePtr
  		*enginePtr, err = dec.compileDec(remoteId, ut)
  		if err != nil {
  			delete(decoderMap, remoteId)
  		}
  	}
  	return
  }
  
  // emptyStruct is the type we compile into when ignoring a struct value.
  type emptyStruct struct{}
  
  var emptyStructType = reflect.TypeOf(emptyStruct{})
  
  // getIgnoreEnginePtr returns the engine for the specified type when the value is to be discarded.
  func (dec *Decoder) getIgnoreEnginePtr(wireId typeId) (enginePtr **decEngine, err error) {
  	var ok bool
  	if enginePtr, ok = dec.ignorerCache[wireId]; !ok {
  		// To handle recursive types, mark this engine as underway before compiling.
  		enginePtr = new(*decEngine)
  		dec.ignorerCache[wireId] = enginePtr
  		wire := dec.wireType[wireId]
  		if wire != nil && wire.StructT != nil {
  			*enginePtr, err = dec.compileDec(wireId, userType(emptyStructType))
  		} else {
  			*enginePtr, err = dec.compileIgnoreSingle(wireId)
  		}
  		if err != nil {
  			delete(dec.ignorerCache, wireId)
  		}
  	}
  	return
  }
  
  // decodeValue decodes the data stream representing a value and stores it in value.
  func (dec *Decoder) decodeValue(wireId typeId, value reflect.Value) {
  	defer catchError(&dec.err)
  	// If the value is nil, it means we should just ignore this item.
  	if !value.IsValid() {
  		dec.decodeIgnoredValue(wireId)
  		return
  	}
  	// Dereference down to the underlying type.
  	ut := userType(value.Type())
  	base := ut.base
  	var enginePtr **decEngine
  	enginePtr, dec.err = dec.getDecEnginePtr(wireId, ut)
  	if dec.err != nil {
  		return
  	}
  	value = decAlloc(value)
  	engine := *enginePtr
  	if st := base; st.Kind() == reflect.Struct && ut.externalDec == 0 {
  		wt := dec.wireType[wireId]
  		if engine.numInstr == 0 && st.NumField() > 0 &&
  			wt != nil && len(wt.StructT.Field) > 0 {
  			name := base.Name()
  			errorf("type mismatch: no fields matched compiling decoder for %s", name)
  		}
  		dec.decodeStruct(engine, value)
  	} else {
  		dec.decodeSingle(engine, value)
  	}
  }
  
  // decodeIgnoredValue decodes the data stream representing a value of the specified type and discards it.
  func (dec *Decoder) decodeIgnoredValue(wireId typeId) {
  	var enginePtr **decEngine
  	enginePtr, dec.err = dec.getIgnoreEnginePtr(wireId)
  	if dec.err != nil {
  		return
  	}
  	wire := dec.wireType[wireId]
  	if wire != nil && wire.StructT != nil {
  		dec.ignoreStruct(*enginePtr)
  	} else {
  		dec.ignoreSingle(*enginePtr)
  	}
  }
  
  func init() {
  	var iop, uop decOp
  	switch reflect.TypeOf(int(0)).Bits() {
  	case 32:
  		iop = decInt32
  		uop = decUint32
  	case 64:
  		iop = decInt64
  		uop = decUint64
  	default:
  		panic("gob: unknown size of int/uint")
  	}
  	decOpTable[reflect.Int] = iop
  	decOpTable[reflect.Uint] = uop
  
  	// Finally uintptr
  	switch reflect.TypeOf(uintptr(0)).Bits() {
  	case 32:
  		uop = decUint32
  	case 64:
  		uop = decUint64
  	default:
  		panic("gob: unknown size of uintptr")
  	}
  	decOpTable[reflect.Uintptr] = uop
  }
  
  // Gob depends on being able to take the address
  // of zeroed Values it creates, so use this wrapper instead
  // of the standard reflect.Zero.
  // Each call allocates once.
  func allocValue(t reflect.Type) reflect.Value {
  	return reflect.New(t).Elem()
  }
  

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