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lexer.go
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lexer.go
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package lexer
import (
"bufio"
"bytes"
"io"
"unicode"
)
const (
TokenEnd = iota
TokenAngleLeft
TokenAngleRight
TokenMinus
TokenPlus
TokenClose
TokenComment
TokenEqual
TokenEscape
TokenIdentifier
TokenOpen
TokenQuote
TokenSeparator
TokenStop
TokenVariable
TokenWhitespace
)
// runeClass returns the token class of an input character.
func runeClass(r rune) int {
if unicode.IsSpace(r) {
return TokenWhitespace
}
switch r {
case eof:
return TokenEnd
case '<':
return TokenAngleLeft
case '>':
return TokenAngleRight
case '-':
return TokenMinus
case '+':
return TokenPlus
case '"':
return TokenQuote
case ';':
return TokenStop
case '|':
return TokenSeparator
case '$':
return TokenVariable
case '{':
return TokenOpen
case '}':
return TokenClose
case '#':
return TokenComment
case '=':
return TokenEqual
case '\\':
return TokenEscape
default:
return TokenIdentifier
}
}
// Scanner represents a lexical scanner.
type Scanner struct {
r *bufio.Reader
}
// NewScanner returns a new instance of Scanner.
func NewScanner(r io.Reader) *Scanner {
return &Scanner{r: bufio.NewReader(r)}
}
// Scan returns the next token and literal value.
func (s *Scanner) Scan() (class int, lit string) {
ch := s.read()
class = runeClass(ch)
switch class {
case TokenQuote:
class = TokenIdentifier
lit = s.scanQuoted()
case TokenWhitespace:
s.unread()
lit = s.scanWhitespace()
case TokenComment:
s.unread()
lit = s.scanComment()
case TokenIdentifier:
s.unread()
lit = s.scanIdentifier()
case TokenEscape:
class = TokenIdentifier
lit = s.scanIdentifier()
case TokenEnd:
lit = ``
default:
lit = string(ch)
}
return
}
// ScanIgnoreWhitespace scans the next non-whitespace token.
func (s *Scanner) ScanIgnoreWhitespace() (tok int, lit string) {
tok, lit = s.Scan()
if tok == TokenWhitespace {
tok, lit = s.Scan()
}
return
}
// scanWhitespace consumes the current rune and all contiguous whitespace.
func (s *Scanner) scanWhitespace() string {
var buf bytes.Buffer
buf.WriteRune(s.read())
OUTER:
for {
ch := s.read()
class := runeClass(ch)
switch class {
case TokenWhitespace:
buf.WriteRune(ch)
case TokenEnd:
break OUTER
default:
s.unread()
break OUTER
}
}
return buf.String()
}
func (s *Scanner) scanQuoted() string {
var buf bytes.Buffer
escape := false
OUTER:
for {
ch := s.read()
class := runeClass(ch)
switch {
case class == TokenEscape && !escape:
escape = true
continue
case class == TokenQuote && !escape:
break OUTER
case class == TokenEnd:
break OUTER
default:
buf.WriteRune(ch)
}
escape = false
}
return buf.String()
}
func (s *Scanner) scanComment() string {
var buf bytes.Buffer
buf.WriteRune(s.read())
OUTER:
for {
ch := s.read()
class := runeClass(ch)
switch class {
case TokenEnd:
break OUTER
default:
buf.WriteRune(ch)
}
}
return buf.String()
}
func (s *Scanner) scanIdentifier() string {
var buf bytes.Buffer
buf.WriteRune(s.read())
escape := false
OUTER:
for {
ch := s.read()
class := runeClass(ch)
switch {
case class == TokenEscape && !escape:
escape = true
continue
case class == TokenQuote && !escape:
break OUTER
case class == TokenEnd:
break OUTER
case escape || class == TokenIdentifier:
buf.WriteRune(ch)
default:
s.unread()
break OUTER
}
escape = false
}
return buf.String()
}
// read reads the next rune from the buffered reader.
// Returns the rune(0) if an error occurs (or io.EOF is returned).
func (s *Scanner) read() rune {
ch, _, err := s.r.ReadRune()
if err != nil {
return eof
}
return ch
}
// unread places the previously read rune back on the reader.
func (s *Scanner) unread() { _ = s.r.UnreadRune() }
// eof represents a marker rune for the end of the reader.
var eof = rune(0)