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|
package compiler
import (
"bufio"
"fmt"
"io"
)
type tokenKind string
const (
tokenKindChar = tokenKind("char")
tokenKindAnyChar = tokenKind(".")
tokenKindRepeat = tokenKind("*")
tokenKindAlt = tokenKind("|")
tokenKindGroupOpen = tokenKind("(")
tokenKindGroupClose = tokenKind(")")
tokenKindEOF = tokenKind("eof")
)
type token struct {
kind tokenKind
char rune
}
const nullChar = '\u0000'
func newToken(kind tokenKind, char rune) *token {
return &token{
kind: kind,
char: char,
}
}
type lexer struct {
src *bufio.Reader
lastChar rune
prevChar rune
reachedEOF bool
}
func newLexer(src io.Reader) *lexer {
return &lexer{
src: bufio.NewReader(src),
lastChar: nullChar,
prevChar: nullChar,
reachedEOF: false,
}
}
func (l *lexer) next() (*token, error) {
c, eof, err := l.read()
if err != nil {
return nil, err
}
if eof {
return newToken(tokenKindEOF, nullChar), nil
}
switch c {
case '*':
return newToken(tokenKindRepeat, nullChar), nil
case '.':
return newToken(tokenKindAnyChar, nullChar), nil
case '|':
return newToken(tokenKindAlt, nullChar), nil
case '(':
return newToken(tokenKindGroupOpen, nullChar), nil
case ')':
return newToken(tokenKindGroupClose, nullChar), nil
case '\\':
c, eof, err := l.read()
if err != nil {
return nil, err
}
if eof {
return nil, &SyntaxError{
message: "incompleted escape sequence; unexpected EOF follows \\ character",
}
}
switch {
case c == '\\' || c == '.' || c == '*' || c == '|' || c == '(' || c == ')':
return newToken(tokenKindChar, c), nil
default:
return nil, &SyntaxError{
message: fmt.Sprintf("invalid escape sequence '\\%s'", string(c)),
}
}
default:
return newToken(tokenKindChar, c), nil
}
}
func (l *lexer) read() (rune, bool, error) {
c, _, err := l.src.ReadRune()
if err != nil {
if err == io.EOF {
l.prevChar = l.lastChar
l.lastChar = nullChar
l.reachedEOF = true
return nullChar, true, nil
}
return nullChar, false, err
}
l.prevChar = l.lastChar
l.lastChar = c
return c, false, nil
}
func (l *lexer) restore() error {
if l.reachedEOF {
l.lastChar = l.prevChar
l.prevChar = nullChar
l.reachedEOF = false
return l.src.UnreadRune()
}
if l.lastChar == nullChar {
return fmt.Errorf("the lexer failed to call restore() because the last character is null")
}
l.lastChar = l.prevChar
l.prevChar = nullChar
return l.src.UnreadRune()
}
|
