libgo: update to go1.9

[official-gcc.git] / libgo / go / encoding / gob / decode.go

// 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()
}