internal/godoc/codec/codec.go - pkgsite - Git at Google

 // Copyright 2020 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 codec

 import (
 	"encoding/binary"
 	"fmt"
 	"math"
 	"reflect"
 )

 // An Encoder encodes Go values into a sequence of bytes.
 // To use an Encoder:
 // - Create one with NewEncoder.
 // - Call the Encode method one or more times.
 // - Retrieve the resulting bytes by calling Bytes.
 type Encoder struct {
 	buf      []byte
 	typeNums map[reflect.Type]int
 	seen     map[any]uint64 // for references; see StartStruct
 }

 // NewEncoder returns an Encoder.
 func NewEncoder() *Encoder {
 	return &Encoder{
 		typeNums: map[reflect.Type]int{},
 		seen:     map[any]uint64{},
 	}
 }

 // Encode encodes x.
 func (e *Encoder) Encode(x any) (err error) {
 	defer handlePanic(&err)
 	e.EncodeAny(x)
 	return nil
 }

 func handlePanic(errp *error) {
 	r := recover()
 	if r == nil {
 		// No panic; do nothing.
 		return
 	}
 	// If the panic is not from this package, re-panic.
 	cerr, ok := r.(codecError)
 	if !ok {
 		panic(r)
 	}
 	// Otherwise, set errp.
 	*errp = cerr.err
 }

 // codecError wraps errors from this package so handlePanic
 // can distinguish them.
 type codecError struct {
 	err error
 }

 func fail(err error) {
 	panic(codecError{err})
 }

 func failf(format string, args ...any) {
 	fail(fmt.Errorf(format, args...))
 }

 // Bytes returns the encoded byte slice.
 func (e *Encoder) Bytes() []byte {
 	data := e.buf                 // remember the data
 	e.buf = nil                   // start with a fresh buffer
 	e.encodeInitial()             // encode metadata
 	return append(e.buf, data...) // concatenate metadata and data
 }

 // A Decoder decodes a Go value encoded by an Encoder.
 // To use a Decoder:
 // - Pass NewDecoder the return value of Encoder.Bytes.
 // - Call the Decode method once for each call to Encoder.Encode.
 type Decoder struct {
 	buf       []byte
 	i         int
 	typeInfos []*typeInfo
 	refs      []any // list of struct pointers, in the order seen
 }

 // NewDecoder returns a Decoder for the given bytes.
 func NewDecoder(data []byte) *Decoder {
 	return &Decoder{buf: data, i: 0}
 }

 // Decode decodes a value encoded with Encoder.Encode.
 func (d *Decoder) Decode() (_ any, err error) {
 	defer handlePanic(&err)
 	if d.typeInfos == nil {
 		d.decodeInitial()
 	}
 	return d.DecodeAny(), nil
 }

 func badcode(c byte) {
 	failf("bad code: %d", c)
 }

 //////////////// Reading From and Writing To the Buffer

 func (e *Encoder) writeByte(b byte) {
 	e.buf = append(e.buf, b)
 }

 func (d *Decoder) readByte() byte {
 	b := d.curByte()
 	d.i++
 	return b
 }

 // curByte returns the next byte to be read
 // without actually consuming it.
 func (d *Decoder) curByte() byte {
 	return d.buf[d.i]
 }

 func (e *Encoder) writeBytes(b []byte) {
 	e.buf = append(e.buf, b...)
 }

 // readBytes reads and returns the given number of bytes.
 // It fails if there are not enough bytes in the input.
 func (d *Decoder) readBytes(n int) []byte {
 	d.i += n
 	return d.buf[d.i-n : d.i]
 }

 func (e *Encoder) writeString(s string) {
 	e.buf = append(e.buf, s...)
 }

 func (d *Decoder) readString(len int) string {
 	return string(d.readBytes(len))
 }

 func (e *Encoder) writeUint32(u uint32) {
 	var buf [4]byte
 	binary.LittleEndian.PutUint32(buf[:], u)
 	e.writeBytes(buf[:])
 }

 func (d *Decoder) readUint32() uint32 {
 	return binary.LittleEndian.Uint32(d.readBytes(4))
 }

 func (e *Encoder) writeUint64(u uint64) {
 	var buf [8]byte
 	binary.LittleEndian.PutUint64(buf[:], u)
 	e.writeBytes(buf[:])
 }

 func (d *Decoder) readUint64() uint64 {
 	return binary.LittleEndian.Uint64(d.readBytes(8))
 }

 //////////////// Encoding Scheme

 // Byte codes that begin each encoded value.
 // See the package doc for their descriptions.
 const (
 	nilCode = 255 - iota // a nil value
 	// reserve a few values for future use
 	reserved1
 	reserved2
 	reserved3
 	reserved4
 	reserved5
 	reserved6
 	reserved7
 	nBytesCode  // uint n follows, then n bytes
 	nValuesCode // uint n follows, then n values
 	refCode     // uint n follows, referring to a previous value
 	startCode   // start of a value of indeterminate length
 	endCode     // end of a value that began with start
 	// Bytes less than endCode represent themselves.
 )

 // EncodeUint encodes a uint64.
 func (e *Encoder) EncodeUint(u uint64) {
 	switch {
 	case u < endCode:
 		// u fits into the initial byte.
 		e.writeByte(byte(u))
 	case u <= math.MaxUint32:
 		// Encode as a sequence of 4 bytes, the little-endian representation of
 		// a uint32.
 		e.writeByte(nBytesCode)
 		e.writeByte(4)
 		e.writeUint32(uint32(u))
 	default:
 		// Encode as a sequence of 8 bytes, the little-endian representation of
 		// a uint64.
 		e.writeByte(nBytesCode)
 		e.writeByte(8)
 		e.writeUint64(u)
 	}
 }

 // DecodeUint decodes a uint64.
 func (d *Decoder) DecodeUint() uint64 {
 	b := d.readByte()
 	switch {
 	case b < endCode:
 		return uint64(b)
 	case b == nBytesCode:
 		switch n := d.readByte(); n {
 		case 4:
 			return uint64(d.readUint32())
 		case 8:
 			return d.readUint64()
 		default:
 			failf("DecodeUint: bad length %d", n)
 		}
 	default:
 		badcode(b)
 	}
 	return 0
 }

 // EncodeInt encodes a signed integer.
 func (e *Encoder) EncodeInt(i int64) {
 	// Encode small negative as well as small positive integers efficiently.
 	// Algorithm from gob; see "Encoding Details" at https://pkg.go.dev/encoding/gob.
 	var u uint64
 	if i < 0 {
 		u = (^uint64(i) << 1) | 1 // complement i, bit 0 is 1
 	} else {
 		u = (uint64(i) << 1) // do not complement i, bit 0 is 0
 	}
 	e.EncodeUint(u)
 }

 // DecodeInt decodes a signed integer.
 func (d *Decoder) DecodeInt() int64 {
 	u := d.DecodeUint()
 	if u&1 == 1 {
 		return int64(^(u >> 1))
 	}
 	return int64(u >> 1)
 }

 // encodeLen encodes the length of a byte sequence.
 func (e *Encoder) encodeLen(n int) {
 	e.writeByte(nBytesCode)
 	e.EncodeUint(uint64(n))
 }

 // decodeLen decodes the length of a byte sequence.
 func (d *Decoder) decodeLen() int {
 	if b := d.readByte(); b != nBytesCode {
 		badcode(b)
 	}
 	return int(d.DecodeUint())
 }

 // EncodeBytes encodes a byte slice.
 func (e *Encoder) EncodeBytes(b []byte) {
 	e.encodeLen(len(b))
 	e.writeBytes(b)
 }

 // DecodeBytes decodes a byte slice.
 // It does no copying.
 func (d *Decoder) DecodeBytes() []byte {
 	return d.readBytes(d.decodeLen())
 }

 // EncodeString encodes a string.
 func (e *Encoder) EncodeString(s string) {
 	e.encodeLen(len(s))
 	e.writeString(s)
 }

 // DecodeString decodes a string.
 func (d *Decoder) DecodeString() string {
 	return d.readString(d.decodeLen())
 }

 // EncodeBool encodes a bool.
 func (e *Encoder) EncodeBool(b bool) {
 	if b {
 		e.writeByte(1)
 	} else {
 		e.writeByte(0)
 	}
 }

 // DecodeBool decodes a bool.
 func (d *Decoder) DecodeBool() bool {
 	b := d.readByte()
 	switch b {
 	case 0:
 		return false
 	case 1:
 		return true
 	default:
 		failf("bad bool: %d", b)
 		return false
 	}
 }

 // EncodeFloat encodes a float64.
 func (e *Encoder) EncodeFloat(f float64) {
 	e.EncodeUint(math.Float64bits(f))
 }

 // DecodeFloat decodes a float64.
 func (d *Decoder) DecodeFloat() float64 {
 	return math.Float64frombits(d.DecodeUint())
 }

 func (e *Encoder) EncodeNil() {
 	e.writeByte(nilCode)
 }

 // StartList should be called before encoding any sequence of variable-length
 // values.
 func (e *Encoder) StartList(len int) {
 	e.writeByte(nValuesCode)
 	e.EncodeUint(uint64(len))
 }

 // StartList should be called before decoding any sequence of variable-length
 // values. It returns -1 if the encoded list was nil. Otherwise, it returns the
 // length of the sequence.
 func (d *Decoder) StartList() int {
 	switch b := d.readByte(); b {
 	case nilCode:
 		return -1
 	case nValuesCode:
 		return int(d.DecodeUint())
 	default:
 		badcode(b)
 		return 0
 	}
 }

 //////////////// Struct Support

 // StartStruct should be called before encoding a struct pointer. The isNil
 // argument says whether the pointer is nil. The p argument is the struct
 // pointer. If StartStruct returns false, encoding should not proceed.
 func (e *Encoder) StartStruct(isNil bool, p any) bool {
 	if isNil {
 		e.EncodeNil()
 		return false
 	}
 	if u, ok := e.seen[p]; ok {
 		// If we have already seen this struct pointer,
 		// encode a reference to it.
 		e.writeByte(refCode)
 		e.EncodeUint(u)
 		return false // Caller should not encode the struct.
 	}
 	// Note that we have seen this pointer, and assign it
 	// its position in the encoding.
 	e.seen[p] = uint64(len(e.seen))
 	e.writeByte(startCode)
 	return true
 }

 // StartStruct should be called before decoding a struct pointer. If it returns
 // false, decoding should not proceed. If it returns true and the second return
 // value is non-nil, it is a reference to a previous value and should be used
 // instead of proceeding with decoding.
 func (d *Decoder) StartStruct() (bool, any) {
 	b := d.readByte()
 	switch b {
 	case nilCode: // do not set the pointer
 		return false, nil
 	case refCode:
 		u := d.DecodeUint()
 		return true, d.refs[u]
 	case startCode:
 		return true, nil
 	default:
 		badcode(b)
 		return false, nil // unreached, needed for compiler
 	}
 }

 // StoreRef should be called by a struct decoder immediately after it allocates
 // a struct pointer.
 func (d *Decoder) StoreRef(p any) {
 	d.refs = append(d.refs, p)
 }

 // EndStruct should be called after encoding a struct.
 func (e *Encoder) EndStruct() {
 	e.writeByte(endCode)
 }

 // NextStructField should be called by a struct decoder in a loop.
 // It returns the field number of the next encoded field, or -1
 // if there are no more fields.
 func (d *Decoder) NextStructField() int {
 	if d.curByte() == endCode {
 		d.readByte() // consume the end byte
 		return -1
 	}
 	return int(d.DecodeUint())
 }

 // UnknownField should be called by a struct decoder
 // when it sees a field number that it doesn't know.
 func (d *Decoder) UnknownField(typeName string, num int) {
 	d.skip()
 }

 // skip reads past a value in the input.
 func (d *Decoder) skip() {
 	b := d.readByte()
 	if b < endCode {
 		// Small integers represent themselves in a single byte.
 		return
 	}
 	switch b {
 	case nilCode:
 		// Nothing follows.
 	case nBytesCode:
 		// A uint n and n bytes follow. It is efficient to call readBytes here
 		// because it does no allocation.
 		d.readBytes(int(d.DecodeUint()))
 	case nValuesCode:
 		// A uint n and n values follow.
 		n := int(d.DecodeUint())
 		for i := 0; i < n; i++ {
 			d.skip()
 		}
 	case refCode:
 		// A uint follows.
 		d.DecodeUint()
 	case startCode:
 		// Skip until we see endCode.
 		for d.curByte() != endCode {
 			d.skip()
 		}
 		d.readByte() // consume the endCode byte
 	default:
 		badcode(b)
 	}
 }

 //////////////// Encoding Arbitrary Values

 // EncodeAny encodes a Go type. The type must have
 // been registered with Register.
 func (e *Encoder) EncodeAny(x any) {
 	// Encode a nil interface value with a zero.
 	if x == nil {
 		e.writeByte(0)
 		return
 	}
 	// Find the typeInfo for the type, which has the encoder.
 	t := reflect.TypeOf(x)
 	ti := typeInfosByType[t]
 	if ti == nil {
 		failf("unregistered type %s", t)
 	}
 	// Assign a number to the type if we haven't already.
 	num, ok := e.typeNums[t]
 	if !ok {
 		num = len(e.typeNums)
 		e.typeNums[t] = num
 	}
 	// Encode a pair (2-element list) of the type number and the encoded value.
 	e.StartList(2)
 	e.EncodeUint(uint64(num))
 	ti.encode(e, x)
 }

 // DecodeAny decodes a value encoded by EncodeAny.
 func (d *Decoder) DecodeAny() any {
 	// If we're looking at a zero, this is a nil interface.
 	if d.curByte() == 0 {
 		d.readByte() // consume the byte
 		return nil
 	}
 	// Otherwise, we should have a two-item list: type number and value.
 	n := d.StartList()
 	if n != 2 {
 		failf("DecodeAny: bad list length %d", n)
 	}
 	num := d.DecodeUint()
 	if num >= uint64(len(d.typeInfos)) {
 		failf("type number %d out of range", num)
 	}
 	ti := d.typeInfos[num]
 	return ti.decode(d)
 }

 // encodeInitial encodes metadata that appears at the start of the
 // encoded byte slice.
 func (e *Encoder) encodeInitial() {
 	// Encode the list of type names we saw, in the order we
 	// assigned numbers to them.
 	names := make([]string, len(e.typeNums))
 	for t, num := range e.typeNums {
 		names[num] = typeName(t)
 	}
 	e.StartList(len(names))
 	for _, n := range names {
 		e.EncodeString(n)
 	}
 }

 // decodeInitial decodes metadata that appears at the start of the
 // encoded byte slice.
 func (d *Decoder) decodeInitial() {
 	// Decode the list of type names. The number of a type is its position in
 	// the list.
 	n := d.StartList()
 	d.typeInfos = make([]*typeInfo, n)
 	for num := 0; num < n; num++ {
 		name := d.DecodeString()
 		ti := typeInfosByName[name]
 		if ti == nil {
 			failf("unregistered type: %s", name)
 		}
 		d.typeInfos[num] = ti
 	}
 }

 //////////////// Type Registry

 // All types subject to encoding must be registered, even
 // builtin types.

 // A typeInfo describes how to encode and decode a type.
 type typeInfo struct {
 	name   string // e.g. "go/ast.File"
 	encode encodeFunc
 	decode decodeFunc
 }

 type (
 	encodeFunc func(*Encoder, any)
 	decodeFunc func(*Decoder) any
 )

 var (
 	typeInfosByName = map[string]*typeInfo{}
 	typeInfosByType = map[reflect.Type]*typeInfo{}
 )

 // Register records the type of x for use by Encoders and Decoders.
 func Register(x any, enc encodeFunc, dec decodeFunc) {
 	t := reflect.TypeOf(x)
 	tn := typeName(t)
 	if _, ok := typeInfosByName[tn]; ok {
 		panic(fmt.Sprintf("codec.Register: duplicate type %s (typeName=%q)", t, tn))
 	}
 	ti := &typeInfo{
 		name:   tn,
 		encode: enc,
 		decode: dec,
 	}
 	typeInfosByName[ti.name] = ti
 	typeInfosByType[t] = ti
 }

 // typeName returns the full, qualified name for a type.
 func typeName(t reflect.Type) string {
 	if t.PkgPath() == "" {
 		return t.String()
 	}
 	return t.PkgPath() + "." + t.Name()
 }

 var builtinTypes []reflect.Type

 func init() {
 	Register(int64(0),
 		func(e *Encoder, x any) { e.EncodeInt(x.(int64)) },
 		func(d *Decoder) any { return d.DecodeInt() })
 	Register(uint64(0),
 		func(e *Encoder, x any) { e.EncodeUint(x.(uint64)) },
 		func(d *Decoder) any { return d.DecodeUint() })
 	Register(int(0),
 		func(e *Encoder, x any) { e.EncodeInt(int64(x.(int))) },
 		func(d *Decoder) any { return int(d.DecodeInt()) })
 	Register(float64(0),
 		func(e *Encoder, x any) { e.EncodeFloat(x.(float64)) },
 		func(d *Decoder) any { return d.DecodeFloat() })
 	Register(false,
 		func(e *Encoder, x any) { e.EncodeBool(x.(bool)) },
 		func(d *Decoder) any { return d.DecodeBool() })
 	Register("",
 		func(e *Encoder, x any) { e.EncodeString(x.(string)) },
 		func(d *Decoder) any { return d.DecodeString() })
 	Register([]byte(nil),
 		func(e *Encoder, x any) { e.EncodeBytes(x.([]byte)) },
 		func(d *Decoder) any { return d.DecodeBytes() })

 	for t := range typeInfosByType {
 		builtinTypes = append(builtinTypes, t)
 	}
 }
	// Copyright 2020 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 codec

	import (
	"encoding/binary"
	"fmt"
	"math"
	"reflect"
	)

	// An Encoder encodes Go values into a sequence of bytes.
	// To use an Encoder:
	// - Create one with NewEncoder.
	// - Call the Encode method one or more times.
	// - Retrieve the resulting bytes by calling Bytes.
	type Encoder struct {
	buf []byte
	typeNums map[reflect.Type]int
	seen map[any]uint64 // for references; see StartStruct
	}

	// NewEncoder returns an Encoder.
	func NewEncoder() *Encoder {
	return &Encoder{
	typeNums: map[reflect.Type]int{},
	seen: map[any]uint64{},
	}
	}

	// Encode encodes x.
	func (e *Encoder) Encode(x any) (err error) {
	defer handlePanic(&err)
	e.EncodeAny(x)
	return nil
	}

	func handlePanic(errp *error) {
	r := recover()
	if r == nil {
	// No panic; do nothing.
	return
	}
	// If the panic is not from this package, re-panic.
	cerr, ok := r.(codecError)
	if !ok {
	panic(r)
	}
	// Otherwise, set errp.
	*errp = cerr.err
	}

	// codecError wraps errors from this package so handlePanic
	// can distinguish them.
	type codecError struct {
	err error
	}

	func fail(err error) {
	panic(codecError{err})
	}

	func failf(format string, args ...any) {
	fail(fmt.Errorf(format, args...))
	}

	// Bytes returns the encoded byte slice.
	func (e *Encoder) Bytes() []byte {
	data := e.buf // remember the data
	e.buf = nil // start with a fresh buffer
	e.encodeInitial() // encode metadata
	return append(e.buf, data...) // concatenate metadata and data
	}

	// A Decoder decodes a Go value encoded by an Encoder.
	// To use a Decoder:
	// - Pass NewDecoder the return value of Encoder.Bytes.
	// - Call the Decode method once for each call to Encoder.Encode.
	type Decoder struct {
	buf []byte
	i int
	typeInfos []*typeInfo
	refs []any // list of struct pointers, in the order seen
	}

	// NewDecoder returns a Decoder for the given bytes.
	func NewDecoder(data []byte) *Decoder {
	return &Decoder{buf: data, i: 0}
	}

	// Decode decodes a value encoded with Encoder.Encode.
	func (d *Decoder) Decode() (_ any, err error) {
	defer handlePanic(&err)
	if d.typeInfos == nil {
	d.decodeInitial()
	}
	return d.DecodeAny(), nil
	}

	func badcode(c byte) {
	failf("bad code: %d", c)
	}

	//////////////// Reading From and Writing To the Buffer

	func (e *Encoder) writeByte(b byte) {
	e.buf = append(e.buf, b)
	}

	func (d *Decoder) readByte() byte {
	b := d.curByte()
	d.i++
	return b
	}

	// curByte returns the next byte to be read
	// without actually consuming it.
	func (d *Decoder) curByte() byte {
	return d.buf[d.i]
	}

	func (e *Encoder) writeBytes(b []byte) {
	e.buf = append(e.buf, b...)
	}

	// readBytes reads and returns the given number of bytes.
	// It fails if there are not enough bytes in the input.
	func (d *Decoder) readBytes(n int) []byte {
	d.i += n
	return d.buf[d.i-n : d.i]
	}

	func (e *Encoder) writeString(s string) {
	e.buf = append(e.buf, s...)
	}

	func (d *Decoder) readString(len int) string {
	return string(d.readBytes(len))
	}

	func (e *Encoder) writeUint32(u uint32) {
	var buf [4]byte
	binary.LittleEndian.PutUint32(buf[:], u)
	e.writeBytes(buf[:])
	}

	func (d *Decoder) readUint32() uint32 {
	return binary.LittleEndian.Uint32(d.readBytes(4))
	}

	func (e *Encoder) writeUint64(u uint64) {
	var buf [8]byte
	binary.LittleEndian.PutUint64(buf[:], u)
	e.writeBytes(buf[:])
	}

	func (d *Decoder) readUint64() uint64 {
	return binary.LittleEndian.Uint64(d.readBytes(8))
	}

	//////////////// Encoding Scheme

	// Byte codes that begin each encoded value.
	// See the package doc for their descriptions.
	const (
	nilCode = 255 - iota // a nil value
	// reserve a few values for future use
	reserved1
	reserved2
	reserved3
	reserved4
	reserved5
	reserved6
	reserved7
	nBytesCode // uint n follows, then n bytes
	nValuesCode // uint n follows, then n values
	refCode // uint n follows, referring to a previous value
	startCode // start of a value of indeterminate length
	endCode // end of a value that began with start
	// Bytes less than endCode represent themselves.
	)

	// EncodeUint encodes a uint64.
	func (e *Encoder) EncodeUint(u uint64) {
	switch {
	case u < endCode:
	// u fits into the initial byte.
	e.writeByte(byte(u))
	case u <= math.MaxUint32:
	// Encode as a sequence of 4 bytes, the little-endian representation of
	// a uint32.
	e.writeByte(nBytesCode)
	e.writeByte(4)
	e.writeUint32(uint32(u))
	default:
	// Encode as a sequence of 8 bytes, the little-endian representation of
	// a uint64.
	e.writeByte(nBytesCode)
	e.writeByte(8)
	e.writeUint64(u)
	}
	}

	// DecodeUint decodes a uint64.
	func (d *Decoder) DecodeUint() uint64 {
	b := d.readByte()
	switch {
	case b < endCode:
	return uint64(b)
	case b == nBytesCode:
	switch n := d.readByte(); n {
	case 4:
	return uint64(d.readUint32())
	case 8:
	return d.readUint64()
	default:
	failf("DecodeUint: bad length %d", n)
	}
	default:
	badcode(b)
	}
	return 0
	}

	// EncodeInt encodes a signed integer.
	func (e *Encoder) EncodeInt(i int64) {
	// Encode small negative as well as small positive integers efficiently.
	// Algorithm from gob; see "Encoding Details" at https://pkg.go.dev/encoding/gob.
	var u uint64
	if i < 0 {
	u = (^uint64(i) << 1) \| 1 // complement i, bit 0 is 1
	} else {
	u = (uint64(i) << 1) // do not complement i, bit 0 is 0
	}
	e.EncodeUint(u)
	}

	// DecodeInt decodes a signed integer.
	func (d *Decoder) DecodeInt() int64 {
	u := d.DecodeUint()
	if u&1 == 1 {
	return int64(^(u >> 1))
	}
	return int64(u >> 1)
	}

	// encodeLen encodes the length of a byte sequence.
	func (e *Encoder) encodeLen(n int) {
	e.writeByte(nBytesCode)
	e.EncodeUint(uint64(n))
	}

	// decodeLen decodes the length of a byte sequence.
	func (d *Decoder) decodeLen() int {
	if b := d.readByte(); b != nBytesCode {
	badcode(b)
	}
	return int(d.DecodeUint())
	}

	// EncodeBytes encodes a byte slice.
	func (e *Encoder) EncodeBytes(b []byte) {
	e.encodeLen(len(b))
	e.writeBytes(b)
	}

	// DecodeBytes decodes a byte slice.
	// It does no copying.
	func (d *Decoder) DecodeBytes() []byte {
	return d.readBytes(d.decodeLen())
	}

	// EncodeString encodes a string.
	func (e *Encoder) EncodeString(s string) {
	e.encodeLen(len(s))
	e.writeString(s)
	}

	// DecodeString decodes a string.
	func (d *Decoder) DecodeString() string {
	return d.readString(d.decodeLen())
	}

	// EncodeBool encodes a bool.
	func (e *Encoder) EncodeBool(b bool) {
	if b {
	e.writeByte(1)
	} else {
	e.writeByte(0)
	}
	}

	// DecodeBool decodes a bool.
	func (d *Decoder) DecodeBool() bool {
	b := d.readByte()
	switch b {
	case 0:
	return false
	case 1:
	return true
	default:
	failf("bad bool: %d", b)
	return false
	}
	}

	// EncodeFloat encodes a float64.
	func (e *Encoder) EncodeFloat(f float64) {
	e.EncodeUint(math.Float64bits(f))
	}

	// DecodeFloat decodes a float64.
	func (d *Decoder) DecodeFloat() float64 {
	return math.Float64frombits(d.DecodeUint())
	}

	func (e *Encoder) EncodeNil() {
	e.writeByte(nilCode)
	}

	// StartList should be called before encoding any sequence of variable-length
	// values.
	func (e *Encoder) StartList(len int) {
	e.writeByte(nValuesCode)
	e.EncodeUint(uint64(len))
	}

	// StartList should be called before decoding any sequence of variable-length
	// values. It returns -1 if the encoded list was nil. Otherwise, it returns the
	// length of the sequence.
	func (d *Decoder) StartList() int {
	switch b := d.readByte(); b {
	case nilCode:
	return -1
	case nValuesCode:
	return int(d.DecodeUint())
	default:
	badcode(b)
	return 0
	}
	}

	//////////////// Struct Support

	// StartStruct should be called before encoding a struct pointer. The isNil
	// argument says whether the pointer is nil. The p argument is the struct
	// pointer. If StartStruct returns false, encoding should not proceed.
	func (e *Encoder) StartStruct(isNil bool, p any) bool {
	if isNil {
	e.EncodeNil()
	return false
	}
	if u, ok := e.seen[p]; ok {
	// If we have already seen this struct pointer,
	// encode a reference to it.
	e.writeByte(refCode)
	e.EncodeUint(u)
	return false // Caller should not encode the struct.
	}
	// Note that we have seen this pointer, and assign it
	// its position in the encoding.
	e.seen[p] = uint64(len(e.seen))
	e.writeByte(startCode)
	return true
	}

	// StartStruct should be called before decoding a struct pointer. If it returns
	// false, decoding should not proceed. If it returns true and the second return
	// value is non-nil, it is a reference to a previous value and should be used
	// instead of proceeding with decoding.
	func (d *Decoder) StartStruct() (bool, any) {
	b := d.readByte()
	switch b {
	case nilCode: // do not set the pointer
	return false, nil
	case refCode:
	u := d.DecodeUint()
	return true, d.refs[u]
	case startCode:
	return true, nil
	default:
	badcode(b)
	return false, nil // unreached, needed for compiler
	}
	}

	// StoreRef should be called by a struct decoder immediately after it allocates
	// a struct pointer.
	func (d *Decoder) StoreRef(p any) {
	d.refs = append(d.refs, p)
	}

	// EndStruct should be called after encoding a struct.
	func (e *Encoder) EndStruct() {
	e.writeByte(endCode)
	}

	// NextStructField should be called by a struct decoder in a loop.
	// It returns the field number of the next encoded field, or -1
	// if there are no more fields.
	func (d *Decoder) NextStructField() int {
	if d.curByte() == endCode {
	d.readByte() // consume the end byte
	return -1
	}
	return int(d.DecodeUint())
	}

	// UnknownField should be called by a struct decoder
	// when it sees a field number that it doesn't know.
	func (d *Decoder) UnknownField(typeName string, num int) {
	d.skip()
	}

	// skip reads past a value in the input.
	func (d *Decoder) skip() {
	b := d.readByte()
	if b < endCode {
	// Small integers represent themselves in a single byte.
	return
	}
	switch b {
	case nilCode:
	// Nothing follows.
	case nBytesCode:
	// A uint n and n bytes follow. It is efficient to call readBytes here
	// because it does no allocation.
	d.readBytes(int(d.DecodeUint()))
	case nValuesCode:
	// A uint n and n values follow.
	n := int(d.DecodeUint())
	for i := 0; i < n; i++ {
	d.skip()
	}
	case refCode:
	// A uint follows.
	d.DecodeUint()
	case startCode:
	// Skip until we see endCode.
	for d.curByte() != endCode {
	d.skip()
	}
	d.readByte() // consume the endCode byte
	default:
	badcode(b)
	}
	}

	//////////////// Encoding Arbitrary Values

	// EncodeAny encodes a Go type. The type must have
	// been registered with Register.
	func (e *Encoder) EncodeAny(x any) {
	// Encode a nil interface value with a zero.
	if x == nil {
	e.writeByte(0)
	return
	}
	// Find the typeInfo for the type, which has the encoder.
	t := reflect.TypeOf(x)
	ti := typeInfosByType[t]
	if ti == nil {
	failf("unregistered type %s", t)
	}
	// Assign a number to the type if we haven't already.
	num, ok := e.typeNums[t]
	if !ok {
	num = len(e.typeNums)
	e.typeNums[t] = num
	}
	// Encode a pair (2-element list) of the type number and the encoded value.
	e.StartList(2)
	e.EncodeUint(uint64(num))
	ti.encode(e, x)
	}

	// DecodeAny decodes a value encoded by EncodeAny.
	func (d *Decoder) DecodeAny() any {
	// If we're looking at a zero, this is a nil interface.
	if d.curByte() == 0 {
	d.readByte() // consume the byte
	return nil
	}
	// Otherwise, we should have a two-item list: type number and value.
	n := d.StartList()
	if n != 2 {
	failf("DecodeAny: bad list length %d", n)
	}
	num := d.DecodeUint()
	if num >= uint64(len(d.typeInfos)) {
	failf("type number %d out of range", num)
	}
	ti := d.typeInfos[num]
	return ti.decode(d)
	}

	// encodeInitial encodes metadata that appears at the start of the
	// encoded byte slice.
	func (e *Encoder) encodeInitial() {
	// Encode the list of type names we saw, in the order we
	// assigned numbers to them.
	names := make([]string, len(e.typeNums))
	for t, num := range e.typeNums {
	names[num] = typeName(t)
	}
	e.StartList(len(names))
	for _, n := range names {
	e.EncodeString(n)
	}
	}

	// decodeInitial decodes metadata that appears at the start of the
	// encoded byte slice.
	func (d *Decoder) decodeInitial() {
	// Decode the list of type names. The number of a type is its position in
	// the list.
	n := d.StartList()
	d.typeInfos = make([]*typeInfo, n)
	for num := 0; num < n; num++ {
	name := d.DecodeString()
	ti := typeInfosByName[name]
	if ti == nil {
	failf("unregistered type: %s", name)
	}
	d.typeInfos[num] = ti
	}
	}

	//////////////// Type Registry

	// All types subject to encoding must be registered, even
	// builtin types.

	// A typeInfo describes how to encode and decode a type.
	type typeInfo struct {
	name string // e.g. "go/ast.File"
	encode encodeFunc
	decode decodeFunc
	}

	type (
	encodeFunc func(*Encoder, any)
	decodeFunc func(*Decoder) any
	)

	var (
	typeInfosByName = map[string]*typeInfo{}
	typeInfosByType = map[reflect.Type]*typeInfo{}
	)

	// Register records the type of x for use by Encoders and Decoders.
	func Register(x any, enc encodeFunc, dec decodeFunc) {
	t := reflect.TypeOf(x)
	tn := typeName(t)
	if _, ok := typeInfosByName[tn]; ok {
	panic(fmt.Sprintf("codec.Register: duplicate type %s (typeName=%q)", t, tn))
	}
	ti := &typeInfo{
	name: tn,
	encode: enc,
	decode: dec,
	}
	typeInfosByName[ti.name] = ti
	typeInfosByType[t] = ti
	}

	// typeName returns the full, qualified name for a type.
	func typeName(t reflect.Type) string {
	if t.PkgPath() == "" {
	return t.String()
	}
	return t.PkgPath() + "." + t.Name()
	}

	var builtinTypes []reflect.Type

	func init() {
	Register(int64(0),
	func(e *Encoder, x any) { e.EncodeInt(x.(int64)) },
	func(d *Decoder) any { return d.DecodeInt() })
	Register(uint64(0),
	func(e *Encoder, x any) { e.EncodeUint(x.(uint64)) },
	func(d *Decoder) any { return d.DecodeUint() })
	Register(int(0),
	func(e *Encoder, x any) { e.EncodeInt(int64(x.(int))) },
	func(d *Decoder) any { return int(d.DecodeInt()) })
	Register(float64(0),
	func(e *Encoder, x any) { e.EncodeFloat(x.(float64)) },
	func(d *Decoder) any { return d.DecodeFloat() })
	Register(false,
	func(e *Encoder, x any) { e.EncodeBool(x.(bool)) },
	func(d *Decoder) any { return d.DecodeBool() })
	Register("",
	func(e *Encoder, x any) { e.EncodeString(x.(string)) },
	func(d *Decoder) any { return d.DecodeString() })
	Register([]byte(nil),
	func(e *Encoder, x any) { e.EncodeBytes(x.([]byte)) },
	func(d *Decoder) any { return d.DecodeBytes() })

	for t := range typeInfosByType {
	builtinTypes = append(builtinTypes, t)
	}
	}