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scanner.go
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scanner.go
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// Package scanner implements a tokeniser for lua.
// Inspired by https://talks.golang.org/2011/lex.slide#1
package scanner
import (
"fmt"
"strings"
"unicode/utf8"
"github.com/arnodel/golua/token"
)
// Scanner holds the state of the scanner.
type Scanner struct {
name string // used only for error reports.
input []byte // the string being scanned.
start, last, pos token.Pos
items chan *token.Token // channel of scanned items.
state stateFn
errorMsg string
}
type Option func(*Scanner)
// Specializes in scanning a number, used in file:read("n")
func ForNumber() Option {
return func(s *Scanner) {
s.state = scanNumberPrefix
}
}
func WithStartLine(l int) Option {
return func(s *Scanner) {
pos := token.Pos{Line: l, Column: 1}
s.start = pos
s.pos = pos
}
}
// New creates a new scanner for the input string.
func New(name string, input []byte, opts ...Option) *Scanner {
l := &Scanner{
name: name,
input: input,
state: scanToken,
items: make(chan *token.Token, 2), // Two items sufficient.
pos: token.Pos{Line: 1, Column: 1},
start: token.Pos{Line: 1, Column: 1},
}
for _, opt := range opts {
opt(l)
}
return l
}
// stateFn represents the state of the scanner
// as a function that returns the next state.
type stateFn func(*Scanner) stateFn
// emit passes an item back to the client.
func (l *Scanner) emit(tp token.Type) {
lit := l.lit()
if tp == token.INVALID {
fmt.Println("Cannot emit", string(lit))
panic("emit bails out")
}
l.items <- &token.Token{
Type: tp,
Lit: lit,
Pos: l.start,
}
l.start = l.pos
}
func (l *Scanner) lit() []byte {
return l.input[l.start.Offset:l.pos.Offset]
}
// next returns the next rune in the input.
func (l *Scanner) next() rune {
i := l.pos.Offset
if i >= len(l.input) {
l.last = l.pos
// fmt.Println("NEXT EOF")
return -1
}
c, width := utf8.DecodeRune(l.input[i:])
l.last = l.pos
l.pos.Offset += width
i += width
if c == '\n' {
if i < len(l.input) && l.input[i] == '\r' {
l.pos.Offset++
}
l.pos.Line++
l.pos.Column = 1
} else if c == '\r' {
if i < len(l.input) && l.input[i] == '\n' {
l.pos.Offset++
}
l.pos.Line++
l.pos.Column = 1
c = '\n'
} else {
l.pos.Column++
}
// fmt.Println("NEXT", strconv.QuoteRune(c))
return c
}
// ignore skips over the pending input before this point.
func (l *Scanner) ignore() {
l.start = l.pos
l.last = token.Pos{}
}
// backup steps back one rune.
// Can be called only once per call of next.
func (l *Scanner) backup() {
l.pos = l.last
}
// peek returns but does not consume
// the next rune in the input.
func (l *Scanner) peek() rune {
next := l.next()
l.backup()
return next
}
// accept consumes the next rune
// if it's from the valid set.
func (l *Scanner) accept(valid string) bool {
if strings.ContainsRune(valid, l.next()) {
return true
}
l.backup()
return false
}
func (l *Scanner) acceptRune(r rune) bool {
if l.next() == r {
return true
}
l.backup()
return false
}
// 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 *Scanner) errorf(tp token.Type, format string, args ...interface{}) stateFn {
l.errorMsg = fmt.Sprintf(format, args...)
l.items <- &token.Token{
Type: tp,
Lit: l.lit(),
Pos: l.start,
}
return nil
}
// Scan returns the next item from the input (or nil)
func (l *Scanner) Scan() *token.Token {
for {
select {
case item := <-l.items:
return item
default:
if l.state == nil {
return nil
}
l.state = l.state(l)
}
}
}
// ErrorMsg returns the current error message or an empty string if there is none.
func (l *Scanner) ErrorMsg() string {
return l.errorMsg
}