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package lexer
import (
"bytes"
"fmt"
"strings"
"unicode"
"unicode/utf8"
)
// StateFunc captures the movement from one state to the next.
type StateFunc func(*Lexer) StateFunc
// TokenType identifies the tokens emitted.
type TokenType int
const (
// EOFRune is a convenience for EOF
EOFRune rune = -1
// ErrorToken is returned on error
ErrorToken TokenType = -1
// EOFToken is return on EOF
EOFToken TokenType = 0
)
var lineSep = []byte{'\n'}
// Token is returned by the lexer.
type Token struct {
Type TokenType
Value string
Position int
Line int
}
// String implements Stringer
func (t Token) String() string {
return fmt.Sprintf("[%d] %s", t.Type, t.Value)
}
// Lexer represents the lexer machine.
type Lexer struct {
source string
start int
line int
position int
lastWidth int
startState StateFunc
tokens chan Token
history stack
}
// New creates a returns a lexer ready to parse the given source code.
func New(src string, start StateFunc) *Lexer {
return &Lexer{
source: src,
startState: start,
start: 0,
line: 1,
position: 0,
history: newStack(),
}
}
// Start begins executing the Lexer in an asynchronous manner (using a goroutine).
func (l *Lexer) Start() {
go l.StartSync()
}
// StartSync starts the lexer synchronously.
func (l *Lexer) StartSync() {
// Take half the string length as a buffer size.
buffSize := len(l.source) / 2
if buffSize <= 0 {
buffSize = 1
}
l.tokens = make(chan Token, buffSize)
l.run()
}
func (l *Lexer) run() {
state := l.startState
for state != nil {
state = state(l)
}
close(l.tokens)
}
// Current returns the value being being analyzed at this moment.
func (l *Lexer) Current() string {
return l.source[l.start:l.position]
}
// Emit will receive a token type and push a new token with the current analyzed
// value into the tokens channel.
func (l *Lexer) Emit(t TokenType) {
tok := Token{
Type: t,
Value: l.Current(),
Position: l.position,
Line: l.line,
}
l.tokens <- tok
l.checkLines()
l.start = l.position
l.history.clear()
}
func (l *Lexer) checkLines() {
val := l.Current()
l.line += bytes.Count([]byte(val), lineSep)
}
// Next pulls the next rune from the Lexer and returns it, moving the position
// forward in the source.
func (l *Lexer) Next() rune {
var r rune
var s int
str := l.source[l.position:]
if len(str) == 0 {
r, s = EOFRune, 0
} else {
r, s = utf8.DecodeRuneInString(str)
}
l.position += s
l.history.push(r)
return r
}
// Ignore clears the history stack and then sets the current beginning position
// to the current position in the source which effectively ignores the section
// of the source being analyzed.
func (l *Lexer) Ignore() {
l.history.clear()
l.checkLines()
l.start = l.position
}
// Peek performs a Next operation immediately followed by a Backup returning the
// peeked rune.
func (l *Lexer) Peek() rune {
r := l.Next()
l.Backup()
return r
}
// Backup will take the last rune read (if any) and history back. Backups can
// occur more than once per call to Next but you can never history past the
// last point a token was emitted.
func (l *Lexer) Backup() {
r := l.history.pop()
if r > EOFRune {
size := utf8.RuneLen(r)
l.position -= size
if l.position < l.start {
l.position = l.start
}
}
}
// Accept receives a string containing all acceptable strings and will continue
// over each consecutive character in the source until a token not in the given
// string is encountered. This should be used to quickly pull token parts.
func (l *Lexer) Accept(valid string) bool {
if strings.IndexRune(valid, l.Next()) >= 0 {
return true
}
l.Backup() // last next wasn't a match
return false
}
// AcceptRun consumes a run of runes from the valid set.
func (l *Lexer) AcceptRun(valid string) (n int) {
for strings.IndexRune(valid, l.Next()) >= 0 {
n++
}
l.Backup() // last next wasn't a match
return n
}
// SkipWhitespace continues over all unicode whitespace.
func (l *Lexer) SkipWhitespace() {
for {
r := l.Next()
if !unicode.IsSpace(r) {
l.Backup()
break
}
if r == EOFRune {
l.Emit(EOFToken)
break
}
}
}
// Tokens returns the a token channel.
func (l *Lexer) Tokens() <-chan Token {
return l.tokens
}
// NextToken returns the next token from the lexer and done
func (l *Lexer) NextToken() (*Token, bool) {
if tok, ok := <-l.tokens; ok {
return &tok, false
}
return nil, true
}
func (l *Lexer) Error(format string, args ...interface{}) StateFunc {
l.tokens <- Token{
Type: ErrorToken,
Value: fmt.Sprintf(format, args...),
Position: l.position,
Line: l.line,
}
return nil
}
|
