lexer.go

package main

import (
	"fmt"
	"strings"
	"unicode/utf8"
)

type lexer struct {
	name  string    // used only for error reports.
	input string    // the string being scanned.
	start int       // start position of this item.
	pos   int       // current position in the input.
	width int       // width of last rune read.
	items chan item // channel of scanned items.
}

func lex(name, input string) (*lexer, chan item) {
	l := &lexer{
		name:  name,
		input: input,
		items: make(chan item, 2),
	}

	go l.run() // Concurrently run state machine.
	return l, l.items
}

// run lexes the input by executing state functions untill
// the state is nil.
func (l *lexer) run() {
	for state := initState; state != nil; {
		state = state(l)
	}
	close(l.items) // No more tokens will be delivered.
}

// emit passes an item back to the client.
func (l *lexer) emit(t itemType) {
	l.items <- item{t, l.input[l.start:l.pos]}
	l.start = l.pos
}

func (l *lexer) next() rune {
	if l.pos >= len(l.input) {
		l.width = 0
		return eof
	}

	r, width := utf8.DecodeRuneInString(l.input[l.pos:])
	l.width = width
	l.pos += l.width
	return r
}

// ignore skips over the pending input before this point.
func (l *lexer) ignore() {
	l.start = l.pos
}

// backup steps back one rune.
// Can be called only once per call of next.
func (l *lexer) backup() {
	l.pos -= l.width
}

// peek returns but does not consume
// the next rune in the input.
func (l *lexer) peek() rune {
	r := l.next()
	l.backup()
	return r
}

// accept consumes the next rune
// if it's from the valid set
func (l *lexer) accept(valid string) bool {
	if strings.IndexRune(valid, l.next()) >= 0 {
		return true
	}
	l.backup()
	return false
}

// acceptRun consumes a run of runes from the valid set.
func (l *lexer) acceptRun(valid string) {
	for strings.IndexRune(valid, l.next()) >= 0 {
	}
	l.backup()
}

// errorf returns an error token and terminates the scan
// by passing back a nil pointer that will be the next
// state, terminating l.run
func (l *lexer) errorf(format string, args ...interface{}) stateFunc {
	l.items <- item{
		itemError,
		fmt.Sprintf(format, args...),
	}
	return nil
}