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lexer.go
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lexer.go
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package tdat
import (
"fmt"
"io"
"strconv"
"strings"
)
type tokenType byte
const (
textToken tokenType = iota + 1
separatorToken
newlineToken
eofToken
)
func (tt tokenType) String() string {
switch tt {
case textToken:
return "text"
case separatorToken:
return "separator"
case newlineToken:
return "newline"
case eofToken:
return "eof"
}
panic("unknown token type")
}
// A token is a lexeme.
type token struct {
line int
pos int
ttype tokenType
text string
}
func (t *token) String() string {
return fmt.Sprintf("%d:%d %s(%s)", t.line, t.pos, t.ttype, t.text)
}
// The lexer scans input for tokens. It iterates over the
// runes in the reader processes each rune one-by-one.
// Initially, the lexer points to the first rune.
// At the end of the reader (if the reader has no more runes to read),
// the lexer stops.
type lexer struct {
reader io.RuneReader
r rune
err error
line int
pos int
}
func newLexer(reader io.RuneReader) *lexer {
l := &lexer{
reader,
-1,
nil,
1,
0,
}
l.read()
return l
}
// Next parses and returns the next token or returns an error.
// If the lexer has stopped (after reading the last rune from the reader)
// next will always return a eof token.
// Since eof is considered normal, eof is never returned as error.
// The returned token must not be retained by the caller.
// The returned token is valid until the next invocation of next.
func (l *lexer) next() (*token, error) {
// eat whitespace
for {
if l.err != nil {
return nil, l.err
}
if l.r > ' ' || l.r == 0 || l.r == '\n' {
break
}
l.read()
}
// scan next token
switch l.r {
case 0:
return &token{l.line, l.pos, eofToken, ""}, nil
case '|':
line, pos := l.line, l.pos
l.read()
return &token{line, pos, separatorToken, ""}, nil
case '\n':
line, pos := l.line, l.pos
l.read()
return &token{line, pos, newlineToken, ""}, nil
case '"':
line, pos := l.line, l.pos
text, err := l.readQuotedText()
if err != nil {
return nil, err
}
return &token{line, pos, textToken, text}, nil
default:
line, pos := l.line, l.pos
text, err := l.readText()
if err != nil {
return nil, err
}
return &token{line, pos, textToken, text}, nil
}
}
// readText will collect the next runes, up to (and not including) the next
// separator or newline or EOF, whichever comes first.
// It trims trailing whitespace.
func (l *lexer) readText() (string, error) {
runes := make([]rune, 0, 40)
for {
if l.err != nil {
return "", l.err
}
if l.r == 0 || l.r == '|' || l.r == '\n' {
return strings.TrimSpace(string(runes)), nil
}
runes = append(runes, l.r)
l.read()
}
}
// readQuotedText will collect the next runes, up to the
// first unescaped double quote '"', which will close a quoted string.
// Escaping applies, unicode escaping also.
func (l *lexer) readQuotedText() (string, error) {
runes := make([]rune, 0, 40)
for {
l.read()
if l.err != nil {
return "", l.err
}
switch l.r {
case 0:
return "", l.errorf("unterminated string")
case '\\':
r, err := l.readEscapeSequence()
if err != nil {
return "", err
}
runes = append(runes, r)
case '"':
l.read()
return strings.TrimSpace(string(runes)), nil
default:
runes = append(runes, l.r)
}
}
}
func (l *lexer) readEscapeSequence() (rune, error) {
l.read()
switch {
case l.err != nil:
return 0, l.err
case l.r == 0:
return 0, l.errorf("unterminated escape sequence")
case l.r == 'b':
return '\b', nil
case l.r == 't':
return '\t', nil
case l.r == 'n':
return '\n', nil
case l.r == 'f':
return '\f', nil
case l.r == 'r':
return '\r', nil
case l.r == 'u':
return l.readUniodeEscapeSequence()
case l.r == '"':
return '"', nil
case l.r == '\\':
return '\\', nil
}
return 0, l.errorf("illegal escape sequence")
}
func (l *lexer) readUniodeEscapeSequence() (rune, error) {
runes := []rune{'\\', 'u', 0, 0, 0, 0}
for i := 2; i < 6; i++ {
l.read()
if l.err != nil {
return 0, l.err
}
if l.r == 0 {
return 0, l.errorf("unterminated escape sequence")
}
runes[i] = l.r
}
value, _, _, err := strconv.UnquoteChar(string(runes), 0)
return value, err
}
// read advances the lexer by reading the next rune from the reader.
// If the reader has no more runes (eof), special rune 0 is set as
// the current rune and no error is set.
// read will also update line and pos.
// Rune codepoint U+0000 ocurring in the input is considered an error:
// Valid input must not contain '\0' characters.
func (l *lexer) read() {
if l.r == 0 || l.err != nil {
return
}
if l.r == '\n' {
l.line++
l.pos = 1
} else {
l.pos++
}
r, _, err := l.reader.ReadRune()
if err != nil {
if err == io.EOF {
l.r = 0
l.err = nil
} else {
l.err = err
}
return
}
if r < 0x20 && (r != 0x09 && r != 0x0A && r != 0x0D) {
err = l.errorf("invalid char 0x%x", r)
}
l.r = r
l.err = err
}
func (l *lexer) errorf(format string, args ...interface{}) error {
msg := fmt.Sprintf(format, args...)
return fmt.Errorf("line %d, pos %d: %s", l.line, l.pos, msg)
}