libgo: Merge from revision 18783:00cce3a34d7e of master library.

[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.

package gob

// TODO(rsc): When garbage collector changes, revisit
// the allocations in this file that use unsafe.Pointer.

import (
        "bytes"
        "encoding"
        "errors"
        "io"
        "math"
        "reflect"
        "unsafe"
)

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")
)

// 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        *bytes.Buffer
        fieldnum int // the last field number read.
        buf      []byte
        next     *decoderState // for free list
}

// We pass the bytes.Buffer separately for easier testing of the infrastructure
// without requiring a full Decoder.
func (dec *Decoder) newDecoderState(buf *bytes.Buffer) *decoderState {
        d := dec.freeList
        if d == nil {
                d = new(decoderState)
                d.dec = dec
                d.buf = make([]byte, uint64Size)
        } 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)
        }
        width, err := state.b.Read(state.buf[0:n])
        if err != nil {
                error_(err)
        }
        // 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 state.buf[0:width] {
                x = x<<8 | uint64(b)
        }
        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)
}

// decOp is the signature of a decoding operator for a given type.
type decOp func(i *decInstr, state *decoderState, p unsafe.Pointer)

// The 'instructions' of the decoding machine
type decInstr struct {
        op     decOp
        field  int     // field number of the wire type
        indir  int     // how many pointer indirections to reach the value in the struct
        offset uintptr // offset in the structure of the field to encode
        ovfl   error   // error message for overflow/underflow (for arrays, of the elements)
}

// 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.

// Walk the pointer hierarchy, allocating if we find a nil.  Stop one before the end.
func decIndirect(p unsafe.Pointer, indir int) unsafe.Pointer {
        for ; indir > 1; indir-- {
                if *(*unsafe.Pointer)(p) == nil {
                        // Allocation required
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(unsafe.Pointer))
                }
                p = *(*unsafe.Pointer)(p)
        }
        return p
}

// ignoreUint discards a uint value with no destination.
func ignoreUint(i *decInstr, state *decoderState, p unsafe.Pointer) {
        state.decodeUint()
}

// ignoreTwoUints discards a uint value with no destination. It's used to skip
// complex values.
func ignoreTwoUints(i *decInstr, state *decoderState, p unsafe.Pointer) {
        state.decodeUint()
        state.decodeUint()
}

// decBool decodes a uint and stores it as a boolean through p.
func decBool(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(bool))
                }
                p = *(*unsafe.Pointer)(p)
        }
        *(*bool)(p) = state.decodeUint() != 0
}

// decInt8 decodes an integer and stores it as an int8 through p.
func decInt8(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(int8))
                }
                p = *(*unsafe.Pointer)(p)
        }
        v := state.decodeInt()
        if v < math.MinInt8 || math.MaxInt8 < v {
                error_(i.ovfl)
        } else {
                *(*int8)(p) = int8(v)
        }
}

// decUint8 decodes an unsigned integer and stores it as a uint8 through p.
func decUint8(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint8))
                }
                p = *(*unsafe.Pointer)(p)
        }
        v := state.decodeUint()
        if math.MaxUint8 < v {
                error_(i.ovfl)
        } else {
                *(*uint8)(p) = uint8(v)
        }
}

// decInt16 decodes an integer and stores it as an int16 through p.
func decInt16(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(int16))
                }
                p = *(*unsafe.Pointer)(p)
        }
        v := state.decodeInt()
        if v < math.MinInt16 || math.MaxInt16 < v {
                error_(i.ovfl)
        } else {
                *(*int16)(p) = int16(v)
        }
}

// decUint16 decodes an unsigned integer and stores it as a uint16 through p.
func decUint16(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint16))
                }
                p = *(*unsafe.Pointer)(p)
        }
        v := state.decodeUint()
        if math.MaxUint16 < v {
                error_(i.ovfl)
        } else {
                *(*uint16)(p) = uint16(v)
        }
}

// decInt32 decodes an integer and stores it as an int32 through p.
func decInt32(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(int32))
                }
                p = *(*unsafe.Pointer)(p)
        }
        v := state.decodeInt()
        if v < math.MinInt32 || math.MaxInt32 < v {
                error_(i.ovfl)
        } else {
                *(*int32)(p) = int32(v)
        }
}

// decUint32 decodes an unsigned integer and stores it as a uint32 through p.
func decUint32(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint32))
                }
                p = *(*unsafe.Pointer)(p)
        }
        v := state.decodeUint()
        if math.MaxUint32 < v {
                error_(i.ovfl)
        } else {
                *(*uint32)(p) = uint32(v)
        }
}

// decInt64 decodes an integer and stores it as an int64 through p.
func decInt64(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(int64))
                }
                p = *(*unsafe.Pointer)(p)
        }
        *(*int64)(p) = int64(state.decodeInt())
}

// decUint64 decodes an unsigned integer and stores it as a uint64 through p.
func decUint64(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(uint64))
                }
                p = *(*unsafe.Pointer)(p)
        }
        *(*uint64)(p) = uint64(state.decodeUint())
}

// 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 floatFromBits(u uint64) float64 {
        var v uint64
        for i := 0; i < 8; i++ {
                v <<= 8
                v |= u & 0xFF
                u >>= 8
        }
        return math.Float64frombits(v)
}

// storeFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
// number, and stores it through p. It's a helper function for float32 and complex64.
func storeFloat32(i *decInstr, state *decoderState, p unsafe.Pointer) {
        v := floatFromBits(state.decodeUint())
        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_(i.ovfl)
        } else {
                *(*float32)(p) = float32(v)
        }
}

// decFloat32 decodes an unsigned integer, treats it as a 32-bit floating-point
// number, and stores it through p.
func decFloat32(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(float32))
                }
                p = *(*unsafe.Pointer)(p)
        }
        storeFloat32(i, state, p)
}

// decFloat64 decodes an unsigned integer, treats it as a 64-bit floating-point
// number, and stores it through p.
func decFloat64(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(float64))
                }
                p = *(*unsafe.Pointer)(p)
        }
        *(*float64)(p) = floatFromBits(uint64(state.decodeUint()))
}

// decComplex64 decodes a pair of unsigned integers, treats them as a
// pair of floating point numbers, and stores them as a complex64 through p.
// The real part comes first.
func decComplex64(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(complex64))
                }
                p = *(*unsafe.Pointer)(p)
        }
        storeFloat32(i, state, p)
        storeFloat32(i, state, unsafe.Pointer(uintptr(p)+unsafe.Sizeof(float32(0))))
}

// decComplex128 decodes a pair of unsigned integers, treats them as a
// pair of floating point numbers, and stores them as a complex128 through p.
// The real part comes first.
func decComplex128(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(complex128))
                }
                p = *(*unsafe.Pointer)(p)
        }
        real := floatFromBits(uint64(state.decodeUint()))
        imag := floatFromBits(uint64(state.decodeUint()))
        *(*complex128)(p) = complex(real, imag)
}

// decUint8Slice decodes a byte slice and stores through p 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, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new([]uint8))
                }
                p = *(*unsafe.Pointer)(p)
        }
        n := state.decodeUint()
        if n > uint64(state.b.Len()) {
                errorf("length of []byte exceeds input size (%d bytes)", n)
        }
        slice := (*[]uint8)(p)
        if uint64(cap(*slice)) < n {
                *slice = make([]uint8, n)
        } else {
                *slice = (*slice)[0:n]
        }
        if _, err := state.b.Read(*slice); err != nil {
                errorf("error decoding []byte: %s", err)
        }
}

// decString decodes byte array and stores through p a string header
// describing the data.
// Strings are encoded as an unsigned count followed by the raw bytes.
func decString(i *decInstr, state *decoderState, p unsafe.Pointer) {
        if i.indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new(string))
                }
                p = *(*unsafe.Pointer)(p)
        }
        n := state.decodeUint()
        if n > uint64(state.b.Len()) {
                errorf("string length exceeds input size (%d bytes)", n)
        }
        b := make([]byte, n)
        state.b.Read(b)
        // It would be a shame to do the obvious thing here,
        //      *(*string)(p) = string(b)
        // because we've already allocated the storage and this would
        // allocate again and copy.  So we do this ugly hack, which is even
        // even more unsafe than it looks as it depends the memory
        // representation of a string matching the beginning of the memory
        // representation of a byte slice (a byte slice is longer).
        *(*string)(p) = *(*string)(unsafe.Pointer(&b))
}

// ignoreUint8Array skips over the data for a byte slice value with no destination.
func ignoreUint8Array(i *decInstr, state *decoderState, p unsafe.Pointer) {
        b := make([]byte, state.decodeUint())
        state.b.Read(b)
}

// 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
}

// allocate makes sure storage is available for an object of underlying type rtyp
// that is indir levels of indirection through p.
func allocate(rtyp reflect.Type, p unsafe.Pointer, indir int) unsafe.Pointer {
        if indir == 0 {
                return p
        }
        up := p
        if indir > 1 {
                up = decIndirect(up, indir)
        }
        if *(*unsafe.Pointer)(up) == nil {
                // Allocate object.
                *(*unsafe.Pointer)(up) = unsafe.Pointer(reflect.New(rtyp).Pointer())
        }
        return *(*unsafe.Pointer)(up)
}

// decodeSingle decodes a top-level value that is not a struct and stores it through p.
// 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, ut *userTypeInfo, basep unsafe.Pointer) {
        state := dec.newDecoderState(&dec.buf)
        state.fieldnum = singletonField
        delta := int(state.decodeUint())
        if delta != 0 {
                errorf("decode: corrupted data: non-zero delta for singleton")
        }
        instr := &engine.instr[singletonField]
        if instr.indir != ut.indir {
                errorf("internal error: inconsistent indirection instr %d ut %d", instr.indir, ut.indir)
        }
        ptr := basep // offset will be zero
        if instr.indir > 1 {
                ptr = decIndirect(ptr, instr.indir)
        }
        instr.op(instr, state, ptr)
        dec.freeDecoderState(state)
}

// decodeStruct decodes a top-level struct and stores it through p.
// 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, ut *userTypeInfo, p unsafe.Pointer, indir int) {
        p = allocate(ut.base, p, indir)
        state := dec.newDecoderState(&dec.buf)
        state.fieldnum = -1
        basep := p
        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]
                p := unsafe.Pointer(uintptr(basep) + instr.offset)
                if instr.indir > 1 {
                        p = decIndirect(p, instr.indir)
                }
                instr.op(instr, state, p)
                state.fieldnum = fieldnum
        }
        dec.freeDecoderState(state)
}

// ignoreStruct discards the data for a struct with no destination.
func (dec *Decoder) ignoreStruct(engine *decEngine) {
        state := dec.newDecoderState(&dec.buf)
        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, unsafe.Pointer(nil))
                state.fieldnum = fieldnum
        }
        dec.freeDecoderState(state)
}

// 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)
        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, unsafe.Pointer(nil))
        dec.freeDecoderState(state)
}

// decodeArrayHelper does the work for decoding arrays and slices.
func (dec *Decoder) decodeArrayHelper(state *decoderState, p unsafe.Pointer, elemOp decOp, elemWid uintptr, length, elemIndir int, ovfl error) {
        instr := &decInstr{elemOp, 0, elemIndir, 0, ovfl}
        for i := 0; i < length; i++ {
                if state.b.Len() == 0 {
                        errorf("decoding array or slice: length exceeds input size (%d elements)", length)
                }
                up := p
                if elemIndir > 1 {
                        up = decIndirect(up, elemIndir)
                }
                elemOp(instr, state, up)
                p = unsafe.Pointer(uintptr(p) + elemWid)
        }
}

// decodeArray decodes an array and stores it through p, that is, p points to the zeroth element.
// 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(atyp reflect.Type, state *decoderState, p unsafe.Pointer, elemOp decOp, elemWid uintptr, length, indir, elemIndir int, ovfl error) {
        if indir > 0 {
                p = allocate(atyp, p, 1) // All but the last level has been allocated by dec.Indirect
        }
        if n := state.decodeUint(); n != uint64(length) {
                errorf("length mismatch in decodeArray")
        }
        dec.decodeArrayHelper(state, p, elemOp, elemWid, length, elemIndir, ovfl)
}

// decodeIntoValue is a helper for map decoding.  Since maps are decoded using reflection,
// unlike the other items we can't use a pointer directly.
func decodeIntoValue(state *decoderState, op decOp, indir int, v reflect.Value, ovfl error) reflect.Value {
        instr := &decInstr{op, 0, indir, 0, ovfl}
        up := unsafeAddr(v)
        if indir > 1 {
                up = decIndirect(up, indir)
        }
        op(instr, state, up)
        return v
}

// decodeMap decodes a map and stores its header through p.
// 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, p unsafe.Pointer, keyOp, elemOp decOp, indir, keyIndir, elemIndir int, ovfl error) {
        if indir > 0 {
                p = allocate(mtyp, p, 1) // All but the last level has been allocated by dec.Indirect
        }
        up := unsafe.Pointer(p)
        if *(*unsafe.Pointer)(up) == nil { // maps are represented as a pointer in the runtime
                // Allocate map.
                *(*unsafe.Pointer)(up) = unsafe.Pointer(reflect.MakeMap(mtyp).Pointer())
        }
        // Maps cannot be accessed by moving addresses around the way
        // that slices etc. can.  We must recover a full reflection value for
        // the iteration.
        v := reflect.NewAt(mtyp, unsafe.Pointer(p)).Elem()
        n := int(state.decodeUint())
        for i := 0; i < n; i++ {
                key := decodeIntoValue(state, keyOp, keyIndir, allocValue(mtyp.Key()), ovfl)
                elem := decodeIntoValue(state, elemOp, elemIndir, allocValue(mtyp.Elem()), ovfl)
                v.SetMapIndex(key, elem)
        }
}

// ignoreArrayHelper does the work for discarding arrays and slices.
func (dec *Decoder) ignoreArrayHelper(state *decoderState, elemOp decOp, length int) {
        instr := &decInstr{elemOp, 0, 0, 0, errors.New("no error")}
        for i := 0; i < length; i++ {
                elemOp(instr, state, nil)
        }
}

// 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, 0, 0, errors.New("no error")}
        elemInstr := &decInstr{elemOp, 0, 0, 0, errors.New("no error")}
        for i := 0; i < n; i++ {
                keyOp(keyInstr, state, nil)
                elemOp(elemInstr, state, nil)
        }
}

// decodeSlice decodes a slice and stores the slice header through p.
// Slices are encoded as an unsigned length followed by the elements.
func (dec *Decoder) decodeSlice(atyp reflect.Type, state *decoderState, p unsafe.Pointer, elemOp decOp, elemWid uintptr, indir, elemIndir int, ovfl error) {
        nr := state.decodeUint()
        n := int(nr)
        if indir > 0 {
                if *(*unsafe.Pointer)(p) == nil {
                        // Allocate the slice header.
                        *(*unsafe.Pointer)(p) = unsafe.Pointer(new([]unsafe.Pointer))
                }
                p = *(*unsafe.Pointer)(p)
        }
        // Allocate storage for the slice elements, that is, the underlying array,
        // if the existing slice does not have the capacity.
        // Always write a header at p.
        hdrp := (*reflect.SliceHeader)(p)
        if hdrp.Cap < n {
                hdrp.Data = reflect.MakeSlice(atyp, n, n).Pointer()
                hdrp.Cap = n
        }
        hdrp.Len = n
        dec.decodeArrayHelper(state, unsafe.Pointer(hdrp.Data), elemOp, elemWid, n, elemIndir, ovfl)
}

// 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()))
}

// setInterfaceValue sets an interface value to a concrete value,
// but first it checks that the assignment will succeed.
func setInterfaceValue(ivalue reflect.Value, value reflect.Value) {
        if !value.Type().AssignableTo(ivalue.Type()) {
                errorf("cannot assign value of type %s to %s", value.Type(), ivalue.Type())
        }
        ivalue.Set(value)
}

// decodeInterface decodes an interface value and stores it through p.
// 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, p unsafe.Pointer, indir int) {
        // Create a writable interface reflect.Value.  We need one even for the nil case.
        ivalue := allocValue(ityp)
        // Read the name of the concrete type.
        nr := state.decodeUint()
        if nr < 0 || nr > 1<<31 { // zero is permissible for anonymous types
                errorf("invalid type name length %d", nr)
        }
        b := make([]byte, nr)
        state.b.Read(b)
        name := string(b)
        if name == "" {
                // Copy the representation of the nil interface value to the target.
                // This is horribly unsafe and special.
                if indir > 0 {
                        p = allocate(ityp, p, 1) // All but the last level has been allocated by dec.Indirect
                }
                *(*[2]uintptr)(unsafe.Pointer(p)) = ivalue.InterfaceData()
                return
        }
        if len(name) > 1024 {
                errorf("name too long (%d bytes): %.20q...", len(name), name)
        }
        // The concrete type must be registered.
        registerLock.RLock()
        typ, ok := nameToConcreteType[name]
        registerLock.RUnlock()
        if !ok {
                errorf("name not registered for interface: %q", name)
        }
        // 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.
        value := allocValue(typ)
        dec.decodeValue(concreteId, value)
        if dec.err != nil {
                error_(dec.err)
        }
        // Allocate the destination interface value.
        if indir > 0 {
                p = allocate(ityp, p, 1) // All but the last level has been allocated by dec.Indirect
        }
        // Assign the concrete value to the interface.
        // Tread carefully; it might not satisfy the interface.
        setInterfaceValue(ivalue, value)
        // Copy the representation of the interface value to the target.
        // This is horribly unsafe and special.
        *(*[2]uintptr)(unsafe.Pointer(p)) = ivalue.InterfaceData()
}

// ignoreInterface discards the data for an interface value with no destination.
func (dec *Decoder) ignoreInterface(state *decoderState) {
        // Read the name of the concrete type.
        b := make([]byte, state.decodeUint())
        _, err := state.b.Read(b)
        if err != nil {
                error_(err)
        }
        id := dec.decodeTypeSequence(true)
        if id < 0 {
                error_(dec.err)
        }
        // At this point, the decoder buffer contains a delimited value. Just toss it.
        state.b.Next(int(state.decodeUint()))
}

// decodeGobDecoder decodes something implementing the GobDecoder interface.
// The data is encoded as a byte slice.
func (dec *Decoder) decodeGobDecoder(ut *userTypeInfo, state *decoderState, v reflect.Value) {
        // Read the bytes for the value.
        b := make([]byte, state.decodeUint())
        _, err := state.b.Read(b)
        if err != nil {
                error_(err)
        }
        // We know it's one of these.
        switch ut.externalDec {
        case xGob:
                err = v.Interface().(GobDecoder).GobDecode(b)
        case xBinary:
                err = v.Interface().(encoding.BinaryUnmarshaler).UnmarshalBinary(b)
        case xText:
                err = v.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.
        b := make([]byte, state.decodeUint())
        _, err := state.b.Read(b)
        if err != nil {
                error_(err)
        }
}

// 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, int) {
        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, ut.indir
        }
        typ := ut.base
        indir := ut.indir
        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, elemIndir := dec.decOpFor(elemId, t.Elem(), name, inProgress)
                        ovfl := overflow(name)
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                state.dec.decodeArray(t, state, p, *elemOp, t.Elem().Size(), t.Len(), i.indir, elemIndir, ovfl)
                        }

                case reflect.Map:
                        keyId := dec.wireType[wireId].MapT.Key
                        elemId := dec.wireType[wireId].MapT.Elem
                        keyOp, keyIndir := dec.decOpFor(keyId, t.Key(), "key of "+name, inProgress)
                        elemOp, elemIndir := dec.decOpFor(elemId, t.Elem(), "element of "+name, inProgress)
                        ovfl := overflow(name)
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                state.dec.decodeMap(t, state, p, *keyOp, *elemOp, i.indir, keyIndir, elemIndir, 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, elemIndir := dec.decOpFor(elemId, t.Elem(), name, inProgress)
                        ovfl := overflow(name)
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                state.dec.decodeSlice(t, state, p, *elemOp, t.Elem().Size(), i.indir, elemIndir, ovfl)
                        }

                case reflect.Struct:
                        // Generate a closure that calls out to the engine for the nested type.
                        enginePtr, err := dec.getDecEnginePtr(wireId, userType(typ))
                        if err != nil {
                                error_(err)
                        }
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                // indirect through enginePtr to delay evaluation for recursive structs.
                                dec.decodeStruct(*enginePtr, userType(typ), p, i.indir)
                        }
                case reflect.Interface:
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                state.dec.decodeInterface(t, state, p, i.indir)
                        }
                }
        }
        if op == nil {
                errorf("decode can't handle type %s", rt)
        }
        return &op, indir
}

// decIgnoreOpFor returns the decoding op for a field that has no destination.
func (dec *Decoder) decIgnoreOpFor(wireId typeId) decOp {
        op, ok := decIgnoreOpMap[wireId]
        if !ok {
                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, p unsafe.Pointer) {
                                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)
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                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)
                        elemOp := dec.decIgnoreOpFor(elemId)
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                state.dec.ignoreMap(state, keyOp, elemOp)
                        }

                case wire.SliceT != nil:
                        elemId := wire.SliceT.Elem
                        elemOp := dec.decIgnoreOpFor(elemId)
                        op = func(i *decInstr, state *decoderState, p unsafe.Pointer) {
                                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, p unsafe.Pointer) {
                                // 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, p unsafe.Pointer) {
                                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, int) {
        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, p unsafe.Pointer) {
                // Caller has gotten us to within one indirection of our value.
                if i.indir > 0 {
                        if *(*unsafe.Pointer)(p) == nil {
                                *(*unsafe.Pointer)(p) = unsafe.Pointer(reflect.New(ut.base).Pointer())
                        }
                }
                // Now p is a pointer to the base type.  Do we need to climb out to
                // get to the receiver type?
                var v reflect.Value
                if ut.decIndir == -1 {
                        v = reflect.NewAt(rcvrType, unsafe.Pointer(&p)).Elem()
                } else {
                        v = reflect.NewAt(rcvrType, p).Elem()
                }
                state.dec.decodeGobDecoder(ut, state, v)
        }
        return &op, int(ut.indir)

}

// 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, indir := 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, indir, 0, 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)
        ovfl := overflow(dec.typeString(remoteId))
        engine.instr[0] = decInstr{op, 0, 0, 0, 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) {
        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)
                        engine.instr[fieldnum] = decInstr{op, fieldnum, 0, 0, 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, indir := dec.decOpFor(wireField.Id, localField.Type, localField.Name, seen)
                engine.instr[fieldnum] = decInstr{*op, fieldnum, indir, uintptr(localField.Offset), 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{})

// getDecEnginePtr 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 val.
func (dec *Decoder) decodeValue(wireId typeId, val reflect.Value) {
        defer catchError(&dec.err)
        // If the value is nil, it means we should just ignore this item.
        if !val.IsValid() {
                dec.decodeIgnoredValue(wireId)
                return
        }
        // Dereference down to the underlying type.
        ut := userType(val.Type())
        base := ut.base
        var enginePtr **decEngine
        enginePtr, dec.err = dec.getDecEnginePtr(wireId, ut)
        if dec.err != nil {
                return
        }
        engine := *enginePtr
        if st := base; st.Kind() == reflect.Struct && ut.externalDec == 0 {
                if engine.numInstr == 0 && st.NumField() > 0 && len(dec.wireType[wireId].StructT.Field) > 0 {
                        name := base.Name()
                        errorf("type mismatch: no fields matched compiling decoder for %s", name)
                }
                dec.decodeStruct(engine, ut, unsafeAddr(val), ut.indir)
        } else {
                dec.decodeSingle(engine, ut, unsafeAddr(val))
        }
}

// 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 assumes it can call UnsafeAddr on any Value
// in order to get a pointer it can copy data from.
// Values that have just been created and do not point
// into existing structs or slices cannot be addressed,
// so simulate it by returning a pointer to a copy.
// Each call allocates once.
func unsafeAddr(v reflect.Value) unsafe.Pointer {
        if v.CanAddr() {
                return unsafe.Pointer(v.UnsafeAddr())
        }
        x := reflect.New(v.Type()).Elem()
        x.Set(v)
        return unsafe.Pointer(x.UnsafeAddr())
}

// 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()
}