path: root/rlp/decode.go

package rlp

import (
    "bufio"
    "bytes"
    "encoding/binary"
    "errors"
    "fmt"
    "io"
    "math/big"
    "reflect"
    "strings"
)

var (
    errNoPointer     = errors.New("rlp: interface given to Decode must be a pointer")
    errDecodeIntoNil = errors.New("rlp: pointer given to Decode must not be nil")
)

// Decoder is implemented by types that require custom RLP
// decoding rules or need to decode into private fields.
//
// The DecodeRLP method should read one value from the given
// Stream. It is not forbidden to read less or more, but it might
// be confusing.
type Decoder interface {
    DecodeRLP(*Stream) error
}

// Decode parses RLP-encoded data from r and stores the result in the
// value pointed to by val. Val must be a non-nil pointer. If r does
// not implement ByteReader, Decode will do its own buffering.
//
// Decode uses the following type-dependent decoding rules:
//
// If the type implements the Decoder interface, decode calls
// DecodeRLP.
//
// To decode into a pointer, Decode will decode into the value pointed
// to. If the pointer is nil, a new value of the pointer's element
// type is allocated. If the pointer is non-nil, the existing value
// will reused.
//
// To decode into a struct, Decode expects the input to be an RLP
// list. The decoded elements of the list are assigned to each public
// field in the order given by the struct's definition. The input list
// must contain an element for each decoded field. Decode returns an
// error if there are too few or too many elements.
//
// The decoding of struct fields honours one particular struct tag,
// "nil". This tag applies to pointer-typed fields and changes the
// decoding rules for the field such that input values of size zero
// decode as a nil pointer. This tag can be useful when decoding recursive
// types.
//
//     type StructWithEmptyOK struct {
//         Foo *[20]byte `rlp:"nil"`
//     }
//
// To decode into a slice, the input must be a list and the resulting
// slice will contain the input elements in order. For byte slices,
// the input must be an RLP string. Array types decode similarly, with
// the additional restriction that the number of input elements (or
// bytes) must match the array's length.
//
// To decode into a Go string, the input must be an RLP string. The
// input bytes are taken as-is and will not necessarily be valid UTF-8.
//
// To decode into an unsigned integer type, the input must also be an RLP
// string. The bytes are interpreted as a big endian representation of
// the integer. If the RLP string is larger than the bit size of the
// type, Decode will return an error. Decode also supports *big.Int.
// There is no size limit for big integers.
//
// To decode into an interface value, Decode stores one of these
// in the value:
//
//    []interface{}, for RLP lists
//    []byte, for RLP strings
//
// Non-empty interface types are not supported, nor are booleans,
// signed integers, floating point numbers, maps, channels and
// functions.
//
// Note that Decode does not set an input limit for all readers
// and may be vulnerable to panics cause by huge value sizes. If
// you need an input limit, use
//
//     NewStream(r, limit).Decode(val)
func Decode(r io.Reader, val interface{}) error {
    // TODO: this could use a Stream from a pool.
    return NewStream(r, 0).Decode(val)
}

// DecodeBytes parses RLP data from b into val.
// Please see the documentation of Decode for the decoding rules.
func DecodeBytes(b []byte, val interface{}) error {
    // TODO: this could use a Stream from a pool.
    return NewStream(bytes.NewReader(b), uint64(len(b))).Decode(val)
}

type decodeError struct {
    msg string
    typ reflect.Type
    ctx []string
}

func (err *decodeError) Error() string {
    ctx := ""
    if len(err.ctx) > 0 {
        ctx = ", decoding into "
        for i := len(err.ctx) - 1; i >= 0; i-- {
            ctx += err.ctx[i]
        }
    }
    return fmt.Sprintf("rlp: %s for %v%s", err.msg, err.typ, ctx)
}

func wrapStreamError(err error, typ reflect.Type) error {
    switch err {
    case ErrCanonInt:
        return &decodeError{msg: "non-canonical integer (leading zero bytes)", typ: typ}
    case ErrCanonSize:
        return &decodeError{msg: "non-canonical size information", typ: typ}
    case ErrExpectedList:
        return &decodeError{msg: "expected input list", typ: typ}
    case ErrExpectedString:
        return &decodeError{msg: "expected input string or byte", typ: typ}
    case errUintOverflow:
        return &decodeError{msg: "input string too long", typ: typ}
    case errNotAtEOL:
        return &decodeError{msg: "input list has too many elements", typ: typ}
    }
    return err
}

func addErrorContext(err error, ctx string) error {
    if decErr, ok := err.(*decodeError); ok {
        decErr.ctx = append(decErr.ctx, ctx)
    }
    return err
}

var (
    decoderInterface = reflect.TypeOf(new(Decoder)).Elem()
    bigInt           = reflect.TypeOf(big.Int{})
)

func makeDecoder(typ reflect.Type, tags tags) (dec decoder, err error) {
    kind := typ.Kind()
    switch {
    case typ.Implements(decoderInterface):
        return decodeDecoder, nil
    case kind != reflect.Ptr && reflect.PtrTo(typ).Implements(decoderInterface):
        return decodeDecoderNoPtr, nil
    case typ.AssignableTo(reflect.PtrTo(bigInt)):
        return decodeBigInt, nil
    case typ.AssignableTo(bigInt):
        return decodeBigIntNoPtr, nil
    case isUint(kind):
        return decodeUint, nil
    case kind == reflect.String:
        return decodeString, nil
    case kind == reflect.Slice || kind == reflect.Array:
        return makeListDecoder(typ)
    case kind == reflect.Struct:
        return makeStructDecoder(typ)
    case kind == reflect.Ptr:
        if tags.nilOK {
            return makeOptionalPtrDecoder(typ)
        }
        return makePtrDecoder(typ)
    case kind == reflect.Interface:
        return decodeInterface, nil
    default:
        return nil, fmt.Errorf("rlp: type %v is not RLP-serializable", typ)
    }
}

func decodeUint(s *Stream, val reflect.Value) error {
    typ := val.Type()
    num, err := s.uint(typ.Bits())
    if err != nil {
        return wrapStreamError(err, val.Type())
    }
    val.SetUint(num)
    return nil
}

func decodeString(s *Stream, val reflect.Value) error {
    b, err := s.Bytes()
    if err != nil {
        return wrapStreamError(err, val.Type())
    }
    val.SetString(string(b))
    return nil
}

func decodeBigIntNoPtr(s *Stream, val reflect.Value) error {
    return decodeBigInt(s, val.Addr())
}

func decodeBigInt(s *Stream, val reflect.Value) error {
    b, err := s.Bytes()
    if err != nil {
        return wrapStreamError(err, val.Type())
    }
    i := val.Interface().(*big.Int)
    if i == nil {
        i = new(big.Int)
        val.Set(reflect.ValueOf(i))
    }
    // Reject leading zero bytes
    if len(b) > 0 && b[0] == 0 {
        return wrapStreamError(ErrCanonInt, val.Type())
    }
    i.SetBytes(b)
    return nil
}

func makeListDecoder(typ reflect.Type) (decoder, error) {
    etype := typ.Elem()
    if etype.Kind() == reflect.Uint8 && !reflect.PtrTo(etype).Implements(decoderInterface) {
        if typ.Kind() == reflect.Array {
            return decodeByteArray, nil
        } else {
            return decodeByteSlice, nil
        }
    }
    etypeinfo, err := cachedTypeInfo1(etype, tags{})
    if err != nil {
        return nil, err
    }

    isArray := typ.Kind() == reflect.Array
    return func(s *Stream, val reflect.Value) error {
        if isArray {
            return decodeListArray(s, val, etypeinfo.decoder)
        } else {
            return decodeListSlice(s, val, etypeinfo.decoder)
        }
    }, nil
}

func decodeListSlice(s *Stream, val reflect.Value, elemdec decoder) error {
    size, err := s.List()
    if err != nil {
        return wrapStreamError(err, val.Type())
    }
    if size == 0 {
        val.Set(reflect.MakeSlice(val.Type(), 0, 0))
        return s.ListEnd()
    }

    i := 0
    for ; ; i++ {
        // grow slice if necessary
        if i >= val.Cap() {
            newcap := val.Cap() + val.Cap()/2
            if newcap < 4 {
                newcap = 4
            }
            newv := reflect.MakeSlice(val.Type(), val.Len(), newcap)
            reflect.Copy(newv, val)
            val.Set(newv)
        }
        if i >= val.Len() {
            val.SetLen(i + 1)
        }
        // decode into element
        if err := elemdec(s, val.Index(i)); err == EOL {
            break
        } else if err != nil {
            return addErrorContext(err, fmt.Sprint("[", i, "]"))
        }
    }
    if i < val.Len() {
        val.SetLen(i)
    }
    return s.ListEnd()
}

func decodeListArray(s *Stream, val reflect.Value, elemdec decoder) error {
    _, err := s.List()
    if err != nil {
        return wrapStreamError(err, val.Type())
    }
    vlen := val.Len()
    i := 0
    for ; i < vlen; i++ {
        if err := elemdec(s, val.Index(i)); err == EOL {
            break
        } else if err != nil {
            return addErrorContext(err, fmt.Sprint("[", i, "]"))
        }
    }
    if i < vlen {
        return &decodeError{msg: "input list has too few elements", typ: val.Type()}
    }
    return wrapStreamError(s.ListEnd(), val.Type())
}

func decodeByteSlice(s *Stream, val reflect.Value) error {
    b, err := s.Bytes()
    if err != nil {
        return wrapStreamError(err, val.Type())
    }
    val.SetBytes(b)
    return nil
}

func decodeByteArray(s *Stream, val reflect.Value) error {
    kind, size, err := s.Kind()
    if err != nil {
        return err
    }
    vlen := val.Len()
    switch kind {
    case Byte:
        if vlen == 0 {
            return &decodeError{msg: "input string too long", typ: val.Type()}
        }
        if vlen > 1 {
            return &decodeError{msg: "input string too short", typ: val.Type()}
        }
        bv, _ := s.Uint()
        val.Index(0).SetUint(bv)
    case String:
        if uint64(vlen) < size {
            return &decodeError{msg: "input string too long", typ: val.Type()}
        }
        if uint64(vlen) > size {
            return &decodeError{msg: "input string too short", typ: val.Type()}
        }
        slice := val.Slice(0, vlen).Interface().([]byte)
        if err := s.readFull(slice); err != nil {
            return err
        }
        // Reject cases where single byte encoding should have been used.
        if size == 1 && slice[0] < 56 {
            return wrapStreamError(ErrCanonSize, val.Type())
        }
    case List:
        return wrapStreamError(ErrExpectedString, val.Type())
    }
    return nil
}

func makeStructDecoder(typ reflect.Type) (decoder, error) {
    fields, err := structFields(typ)
    if err != nil {
        return nil, err
    }
    dec := func(s *Stream, val reflect.Value) (err error) {
        if _, err = s.List(); err != nil {
            return wrapStreamError(err, typ)
        }
        for _, f := range fields {
            err = f.info.decoder(s, val.Field(f.index))
            if err == EOL {
                return &decodeError{msg: "too few elements", typ: typ}
            } else if err != nil {
                return addErrorContext(err, "."+typ.Field(f.index).Name)
            }
        }
        return wrapStreamError(s.ListEnd(), typ)
    }
    return dec, nil
}

// makePtrDecoder creates a decoder that decodes into
// the pointer's element type.
func makePtrDecoder(typ reflect.Type) (decoder, error) {
    etype := typ.Elem()
    etypeinfo, err := cachedTypeInfo1(etype, tags{})
    if err != nil {
        return nil, err
    }
    dec := func(s *Stream, val reflect.Value) (err error) {
        newval := val
        if val.IsNil() {
            newval = reflect.New(etype)
        }
        if err = etypeinfo.decoder(s, newval.Elem()); err == nil {
            val.Set(newval)
        }
        return err
    }
    return dec, nil
}

// makeOptionalPtrDecoder creates a decoder that decodes empty values
// as nil. Non-empty values are decoded into a value of the element type,
// just like makePtrDecoder does.
//
// This decoder is used for pointer-typed struct fields with struct tag "nil".
func makeOptionalPtrDecoder(typ reflect.Type) (decoder, error) {
    etype := typ.Elem()
    etypeinfo, err := cachedTypeInfo1(etype, tags{})
    if err != nil {
        return nil, err
    }
    dec := func(s *Stream, val reflect.Value) (err error) {
        kind, size, err := s.Kind()
        if err != nil || size == 0 && kind != Byte {
            // rearm s.Kind. This is important because the input
            // position must advance to the next value even though
            // we don't read anything.
            s.kind = -1
            // set the pointer to nil.
            val.Set(reflect.Zero(typ))
            return err
        }
        newval := val
        if val.IsNil() {
            newval = reflect.New(etype)
        }
        if err = etypeinfo.decoder(s, newval.Elem()); err == nil {
            val.Set(newval)
        }
        return err
    }
    return dec, nil
}

var ifsliceType = reflect.TypeOf([]interface{}{})

func decodeInterface(s *Stream, val reflect.Value) error {
    if val.Type().NumMethod() != 0 {
        return fmt.Errorf("rlp: type %v is not RLP-serializable", val.Type())
    }
    kind, _, err := s.Kind()
    if err != nil {
        return err
    }
    if kind == List {
        slice := reflect.New(ifsliceType).Elem()
        if err := decodeListSlice(s, slice, decodeInterface); err != nil {
            return err
        }
        val.Set(slice)
    } else {
        b, err := s.Bytes()
        if err != nil {
            return err
        }
        val.Set(reflect.ValueOf(b))
    }
    return nil
}

// This decoder is used for non-pointer values of types
// that implement the Decoder interface using a pointer receiver.
func decodeDecoderNoPtr(s *Stream, val reflect.Value) error {
    return val.Addr().Interface().(Decoder).DecodeRLP(s)
}

func decodeDecoder(s *Stream, val reflect.Value) error {
    // Decoder instances are not handled using the pointer rule if the type
    // implements Decoder with pointer receiver (i.e. always)
    // because it might handle empty values specially.
    // We need to allocate one here in this case, like makePtrDecoder does.
    if val.Kind() == reflect.Ptr && val.IsNil() {
        val.Set(reflect.New(val.Type().Elem()))
    }
    return val.Interface().(Decoder).DecodeRLP(s)
}

// Kind represents the kind of value contained in an RLP stream.
type Kind int

const (
    Byte Kind = iota
    String
    List
)

func (k Kind) String() string {
    switch k {
    case Byte:
        return "Byte"
    case String:
        return "String"
    case List:
        return "List"
    default:
        return fmt.Sprintf("Unknown(%d)", k)
    }
}

var (
    // EOL is returned when the end of the current list
    // has been reached during streaming.
    EOL = errors.New("rlp: end of list")

    // Actual Errors
    ErrExpectedString = errors.New("rlp: expected String or Byte")
    ErrExpectedList   = errors.New("rlp: expected List")
    ErrCanonInt       = errors.New("rlp: non-canonical integer format")
    ErrCanonSize      = errors.New("rlp: non-canonical size information")
    ErrElemTooLarge   = errors.New("rlp: element is larger than containing list")
    ErrValueTooLarge  = errors.New("rlp: value size exceeds available input length")

    // internal errors
    errNotInList    = errors.New("rlp: call of ListEnd outside of any list")
    errNotAtEOL     = errors.New("rlp: call of ListEnd not positioned at EOL")
    errUintOverflow = errors.New("rlp: uint overflow")
)

// ByteReader must be implemented by any input reader for a Stream. It
// is implemented by e.g. bufio.Reader and bytes.Reader.
type ByteReader interface {
    io.Reader
    io.ByteReader
}

// Stream can be used for piecemeal decoding of an input stream. This
// is useful if the input is very large or if the decoding rules for a
// type depend on the input structure. Stream does not keep an
// internal buffer. After decoding a value, the input reader will be
// positioned just before the type information for the next value.
//
// When decoding a list and the input position reaches the declared
// length of the list, all operations will return error EOL.
// The end of the list must be acknowledged using ListEnd to continue
// reading the enclosing list.
//
// Stream is not safe for concurrent use.
type Stream struct {
    r ByteReader

    // number of bytes remaining to be read from r.
    remaining uint64
    limited   bool

    // auxiliary buffer for integer decoding
    uintbuf []byte

    kind    Kind   // kind of value ahead
    size    uint64 // size of value ahead
    byteval byte   // value of single byte in type tag
    kinderr error  // error from last readKind
    stack   []listpos
}

type listpos struct{ pos, size uint64 }

// NewStream creates a new decoding stream reading from r.
//
// If r implements the ByteReader interface, Stream will
// not introduce any buffering.
//
// For non-toplevel values, Stream returns ErrElemTooLarge
// for values that do not fit into the enclosing list.
//
// Stream supports an optional input limit. If a limit is set, the
// size of any toplevel value will be checked against the remaining
// input length. Stream operations that encounter a value exceeding
// the remaining input length will return ErrValueTooLarge. The limit
// can be set by passing a non-zero value for inputLimit.
//
// If r is a bytes.Reader or strings.Reader, the input limit is set to
// the length of r's underlying data unless an explicit limit is
// provided.
func NewStream(r io.Reader, inputLimit uint64) *Stream {
    s := new(Stream)
    s.Reset(r, inputLimit)
    return s
}

// NewListStream creates a new stream that pretends to be positioned
// at an encoded list of the given length.
func NewListStream(r io.Reader, len uint64) *Stream {
    s := new(Stream)
    s.Reset(r, len)
    s.kind = List
    s.size = len
    return s
}

// Bytes reads an RLP string and returns its contents as a byte slice.
// If the input does not contain an RLP string, the returned
// error will be ErrExpectedString.
func (s *Stream) Bytes() ([]byte, error) {
    kind, size, err := s.Kind()
    if err != nil {
        return nil, err
    }
    switch kind {
    case Byte:
        s.kind = -1 // rearm Kind
        return []byte{s.byteval}, nil
    case String:
        b := make([]byte, size)
        if err = s.readFull(b); err != nil {
            return nil, err
        }
        if size == 1 && b[0] < 56 {
            return nil, ErrCanonSize
        }
        return b, nil
    default:
        return nil, ErrExpectedString
    }
}

// Raw reads a raw encoded value including RLP type information.
func (s *Stream) Raw() ([]byte, error) {
    kind, size, err := s.Kind()
    if err != nil {
        return nil, err
    }
    if kind == Byte {
        s.kind = -1 // rearm Kind
        return []byte{s.byteval}, nil
    }
    // the original header has already been read and is no longer
    // available. read content and put a new header in front of it.
    start := headsize(size)
    buf := make([]byte, uint64(start)+size)
    if err := s.readFull(buf[start:]); err != nil {
        return nil, err
    }
    if kind == String {
        puthead(buf, 0x80, 0xB8, size)
    } else {
        puthead(buf, 0xC0, 0xF7, size)
    }
    return buf, nil
}

// Uint reads an RLP string of up to 8 bytes and returns its contents
// as an unsigned integer. If the input does not contain an RLP string, the
// returned error will be ErrExpectedString.
func (s *Stream) Uint() (uint64, error) {
    return s.uint(64)
}

func (s *Stream) uint(maxbits int) (uint64, error) {
    kind, size, err := s.Kind()
    if err != nil {
        return 0, err
    }
    switch kind {
    case Byte:
        if s.byteval == 0 {
            return 0, ErrCanonInt
        }
        s.kind = -1 // rearm Kind
        return uint64(s.byteval), nil
    case String:
        if size > uint64(maxbits/8) {
            return 0, errUintOverflow
        }
        v, err := s.readUint(byte(size))
        switch {
        case err == ErrCanonSize:
            // Adjust error because we're not reading a size right now.
            return 0, ErrCanonInt
        case err != nil:
            return 0, err
        case size > 0 && v < 56:
            return 0, ErrCanonSize
        default:
            return v, nil
        }
    default:
        return 0, ErrExpectedString
    }
}

// List starts decoding an RLP list. If the input does not contain a
// list, the returned error will be ErrExpectedList. When the list's
// end has been reached, any Stream operation will return EOL.
func (s *Stream) List() (size uint64, err error) {
    kind, size, err := s.Kind()
    if err != nil {
        return 0, err
    }
    if kind != List {
        return 0, ErrExpectedList
    }
    s.stack = append(s.stack, listpos{0, size})
    s.kind = -1
    s.size = 0
    return size, nil
}

// ListEnd returns to the enclosing list.
// The input reader must be positioned at the end of a list.
func (s *Stream) ListEnd() error {
    if len(s.stack) == 0 {
        return errNotInList
    }
    tos := s.stack[len(s.stack)-1]
    if tos.pos != tos.size {
        return errNotAtEOL
    }
    s.stack = s.stack[:len(s.stack)-1] // pop
    if len(s.stack) > 0 {
        s.stack[len(s.stack)-1].pos += tos.size
    }
    s.kind = -1
    s.size = 0
    return nil
}

// Decode decodes a value and stores the result in the value pointed
// to by val. Please see the documentation for the Decode function
// to learn about the decoding rules.
func (s *Stream) Decode(val interface{}) error {
    if val == nil {
        return errDecodeIntoNil
    }
    rval := reflect.ValueOf(val)
    rtyp := rval.Type()
    if rtyp.Kind() != reflect.Ptr {
        return errNoPointer
    }
    if rval.IsNil() {
        return errDecodeIntoNil
    }
    info, err := cachedTypeInfo(rtyp.Elem(), tags{})
    if err != nil {
        return err
    }

    err = info.decoder(s, rval.Elem())
    if decErr, ok := err.(*decodeError); ok && len(decErr.ctx) > 0 {
        // add decode target type to error so context has more meaning
        decErr.ctx = append(decErr.ctx, fmt.Sprint("(", rtyp.Elem(), ")"))
    }
    return err
}

// Reset discards any information about the current decoding context
// and starts reading from r. This method is meant to facilitate reuse
// of a preallocated Stream across many decoding operations.
//
// If r does not also implement ByteReader, Stream will do its own
// buffering.
func (s *Stream) Reset(r io.Reader, inputLimit uint64) {
    if inputLimit > 0 {
        s.remaining = inputLimit
        s.limited = true
    } else {
        // Attempt to automatically discover
        // the limit when reading from a byte slice.
        switch br := r.(type) {
        case *bytes.Reader:
            s.remaining = uint64(br.Len())
            s.limited = true
        case *strings.Reader:
            s.remaining = uint64(br.Len())
            s.limited = true
        default:
            s.limited = false
        }
    }
    // Wrap r with a buffer if it doesn't have one.
    bufr, ok := r.(ByteReader)
    if !ok {
        bufr = bufio.NewReader(r)
    }
    s.r = bufr
    // Reset the decoding context.
    s.stack = s.stack[:0]
    s.size = 0
    s.kind = -1
    s.kinderr = nil
    if s.uintbuf == nil {
        s.uintbuf = make([]byte, 8)
    }
}

// Kind returns the kind and size of the next value in the
// input stream.
//
// The returned size is the number of bytes that make up the value.
// For kind == Byte, the size is zero because the value is
// contained in the type tag.
//
// The first call to Kind will read size information from the input
// reader and leave it positioned at the start of the actual bytes of
// the value. Subsequent calls to Kind (until the value is decoded)
// will not advance the input reader and return cached information.
func (s *Stream) Kind() (kind Kind, size uint64, err error) {
    var tos *listpos
    if len(s.stack) > 0 {
        tos = &s.stack[len(s.stack)-1]
    }
    if s.kind < 0 {
        s.kinderr = nil
        // Don't read further if we're at the end of the
        // innermost list.
        if tos != nil && tos.pos == tos.size {
            return 0, 0, EOL
        }
        s.kind, s.size, s.kinderr = s.readKind()
        if s.kinderr == nil {
            if tos == nil {
                // At toplevel, check that the value is smaller
                // than the remaining input length.
                if s.limited && s.size > s.remaining {
                    s.kinderr = ErrValueTooLarge
                }
            } else {
                // Inside a list, check that the value doesn't overflow the list.
                if s.size > tos.size-tos.pos {
                    s.kinderr = ErrElemTooLarge
                }
            }
        }
    }
    // Note: this might return a sticky error generated
    // by an earlier call to readKind.
    return s.kind, s.size, s.kinderr
}

func (s *Stream) readKind() (kind Kind, size uint64, err error) {
    b, err := s.readByte()
    if err != nil {
        return 0, 0, err
    }
    s.byteval = 0
    switch {
    case b < 0x80:
        // For a single byte whose value is in the [0x00, 0x7F] range, that byte
        // is its own RLP encoding.
        s.byteval = b
        return Byte, 0, nil
    case b < 0xB8:
        // Otherwise, if a string is 0-55 bytes long,
        // the RLP encoding consists of a single byte with value 0x80 plus the
        // length of the string followed by the string. The range of the first
        // byte is thus [0x80, 0xB7].
        return String, uint64(b - 0x80), nil
    case b < 0xC0:
        // If a string is more than 55 bytes long, the
        // RLP encoding consists of a single byte with value 0xB7 plus the length
        // of the length of the string in binary form, followed by the length of
        // the string, followed by the string. For example, a length-1024 string
        // would be encoded as 0xB90400 followed by the string. The range of
        // the first byte is thus [0xB8, 0xBF].
        size, err = s.readUint(b - 0xB7)
        if err == nil && size < 56 {
            err = ErrCanonSize
        }
        return String, size, err
    case b < 0xF8:
        // If the total payload of a list
        // (i.e. the combined length of all its items) is 0-55 bytes long, the
        // RLP encoding consists of a single byte with value 0xC0 plus the length
        // of the list followed by the concatenation of the RLP encodings of the
        // items. The range of the first byte is thus [0xC0, 0xF7].
        return List, uint64(b - 0xC0), nil
    default:
        // If the total payload of a list is more than 55 bytes long,
        // the RLP encoding consists of a single byte with value 0xF7
        // plus the length of the length of the payload in binary
        // form, followed by the length of the payload, followed by
        // the concatenation of the RLP encodings of the items. The
        // range of the first byte is thus [0xF8, 0xFF].
        size, err = s.readUint(b - 0xF7)
        if err == nil && size < 56 {
            err = ErrCanonSize
        }
        return List, size, err
    }
}

func (s *Stream) readUint(size byte) (uint64, error) {
    switch size {
    case 0:
        s.kind = -1 // rearm Kind
        return 0, nil
    case 1:
        b, err := s.readByte()
        if err == io.EOF {
            err = io.ErrUnexpectedEOF
        }
        return uint64(b), err
    default:
        start := int(8 - size)
        for i := 0; i < start; i++ {
            s.uintbuf[i] = 0
        }
        if err := s.readFull(s.uintbuf[start:]); err != nil {
            return 0, err
        }
        if s.uintbuf[start] == 0 {
            // Note: readUint is also used to decode integer
            // values. The error needs to be adjusted to become
            // ErrCanonInt in this case.
            return 0, ErrCanonSize
        }
        return binary.BigEndian.Uint64(s.uintbuf), nil
    }
}

func (s *Stream) readFull(buf []byte) (err error) {
    if s.limited && s.remaining < uint64(len(buf)) {
        return ErrValueTooLarge
    }
    s.willRead(uint64(len(buf)))
    var nn, n int
    for n < len(buf) && err == nil {
        nn, err = s.r.Read(buf[n:])
        n += nn
    }
    if err == io.EOF {
        err = io.ErrUnexpectedEOF
    }
    return err
}

func (s *Stream) readByte() (byte, error) {
    if s.limited && s.remaining == 0 {
        return 0, io.EOF
    }
    s.willRead(1)
    b, err := s.r.ReadByte()
    if len(s.stack) > 0 && err == io.EOF {
        err = io.ErrUnexpectedEOF
    }
    return b, err
}

func (s *Stream) willRead(n uint64) {
    s.kind = -1 // rearm Kind
    if s.limited {
        s.remaining -= n
    }
    if len(s.stack) > 0 {
        s.stack[len(s.stack)-1].pos += n
    }
}