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lexer.go
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lexer.go
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package jsonlexer
import (
"bufio"
"errors"
"fmt"
"io"
)
type Token byte
const (
TokenErr Token = iota
TokenBeginArray
TokenBeginObject
TokenEndArray
TokenEndObject
TokenNameSeparator
TokenValueSeparator
TokenString
TokenNumber
TokenTrue
TokenFalse
TokenNull
)
type Lexer struct {
br *bufio.Reader
buf []byte
err error
}
func New(r io.Reader) *Lexer {
return &Lexer{
br: bufio.NewReader(r),
buf: make([]byte, 0, 128),
}
}
func (l *Lexer) Buf() []byte {
return l.buf
}
func (l *Lexer) Err() error {
return l.err
}
func (l *Lexer) Token() Token {
l.buf = l.buf[:1]
for {
c, err := l.br.ReadByte()
if err != nil {
l.err = err
return TokenErr
}
l.buf[0] = c
// Cases are ordered by decreasing expected frequency.
switch c {
case '"':
return l.readString()
case ',':
return TokenValueSeparator
case ':':
return TokenNameSeparator
case '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-':
return l.readNumber(c)
case 'f':
return l.readLiteral("alse", TokenFalse)
case 't':
return l.readLiteral("rue", TokenTrue)
case 'n':
return l.readLiteral("ull", TokenNull)
case '{':
return TokenBeginObject
case '}':
return TokenEndObject
case '[':
return TokenBeginArray
case ']':
return TokenEndArray
case ' ', '\n', '\r', '\t':
continue
default:
l.err = fmt.Errorf("invalid character %q looking for beginning of value", c)
return TokenErr
}
}
}
func (l *Lexer) readLiteral(s string, t Token) Token {
for i := range s {
c, err := l.br.ReadByte()
if err != nil {
if err == io.EOF {
err = io.ErrUnexpectedEOF
}
l.err = err
return TokenErr
}
if s[i] != c {
l.err = errors.New("bad literal name")
return TokenErr
}
}
c, err := l.br.ReadByte()
if err != nil {
if err == io.EOF {
return t
}
}
if err := l.br.UnreadByte(); err != nil {
l.err = err
return TokenErr
}
if !isValueBoundaryChar(c) {
l.err = errors.New("bad literal name")
return TokenErr
}
return t
}
func isValueBoundaryChar(c byte) bool {
switch c {
case '[', '{', ']', '}', ',', '"':
return true
case 0x20, 0x0a, 0x0d, 0x09:
return true
}
return false
}
func (l *Lexer) readNumber(c byte) Token {
l.buf = append(l.buf[:0], c)
for {
c, err := l.br.ReadByte()
if err != nil {
if err == io.EOF {
return TokenNumber
}
l.err = err
return TokenErr
}
if !isNumberChar(c) {
if err := l.br.UnreadByte(); err != nil {
l.err = err
return TokenErr
}
return TokenNumber
}
l.buf = append(l.buf, c)
}
}
func isNumberChar(c byte) bool {
return c-'0' < 10 || c == '.' || c == 'e' || c == 'E' || c == '-' || c == '+'
}
func (l *Lexer) readString() Token {
var escape bool
for {
c, err := l.br.ReadByte()
if err != nil {
l.err = err
if err == io.EOF {
l.err = io.ErrUnexpectedEOF
}
return TokenErr
}
l.buf = append(l.buf, c)
if c == '"' && !escape {
return TokenString
}
escape = c == '\\' && !escape
}
}