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lex.go
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lex.go
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// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Copied from Go's text/template/parse package and modified for yacc.
package yacc
import (
"fmt"
"strings"
"unicode"
"unicode/utf8"
)
// item represents a token or text string returned from the scanner.
type item struct {
typ itemType // The type of this item.
pos Pos // The starting position, in bytes, of this item in the input string.
val string // The value of this item.
}
func (i item) String() string {
switch {
case i.typ == itemEOF:
return "EOF"
case i.typ == itemError:
return i.val
case len(i.val) > 10:
return fmt.Sprintf("%.10q...", i.val)
}
return fmt.Sprintf("%q", i.val)
}
// itemType identifies the type of lex items.
type itemType int
const (
itemError itemType = iota // error occurred; value is text of error
itemEOF
itemComment
itemPct
itemDoublePct
itemIdent
itemColon
itemLiteral
itemExpr
itemPipe
itemNL
)
const eof = -1
// stateFn represents the state of the scanner as a function that returns the next state.
type stateFn func(*lexer) stateFn
// lexer holds the state of the scanner.
type lexer struct {
name string // the name of the input; used only for error reports
input string // the string being scanned
state stateFn // the next lexing function to enter
pos Pos // current position in the input
start Pos // start position of this item
width Pos // width of last rune read from input
lastPos Pos // position of most recent item returned by nextItem
items chan item // channel of scanned items
}
// next returns the next rune in the input.
func (l *lexer) next() rune {
if int(l.pos) >= len(l.input) {
l.width = 0
return eof
}
r, w := utf8.DecodeRuneInString(l.input[l.pos:])
l.width = Pos(w)
l.pos += l.width
return r
}
// peek returns but does not consume the next rune in the input.
func (l *lexer) peek() rune {
r := l.next()
l.backup()
return r
}
// backup steps back one rune. Can only be called once per call of next.
func (l *lexer) backup() {
l.pos -= l.width
}
// emit passes an item back to the client.
func (l *lexer) emit(t itemType) {
l.items <- item{t, l.start, l.input[l.start:l.pos]}
l.start = l.pos
}
// ignore skips over the pending input before this point.
func (l *lexer) ignore() {
l.start = l.pos
}
// lineNumber reports which line we're on, based on the position of
// the previous item returned by nextItem. Doing it this way
// means we don't have to worry about peek double counting.
func (l *lexer) lineNumber() int {
return 1 + strings.Count(l.input[:l.lastPos], "\n")
}
// errorf returns an error token and terminates the scan by passing
// back a nil pointer that will be the next state, terminating l.nextItem.
func (l *lexer) errorf(format string, args ...interface{}) stateFn {
l.items <- item{itemError, l.start, fmt.Sprintf(format, args...)}
return nil
}
// nextItem returns the next item from the input.
func (l *lexer) nextItem() item {
i := <-l.items
l.lastPos = i.pos
return i
}
// lex creates a new scanner for the input string.
func lex(name, input string) *lexer {
l := &lexer{
name: name,
input: input,
items: make(chan item),
}
go l.run()
return l
}
// run runs the state machine for the lexer.
func (l *lexer) run() {
for l.state = lexStart; l.state != nil; {
l.state = l.state(l)
}
}
// state functions
func lexStart(l *lexer) stateFn {
Loop:
for {
switch r := l.next(); {
case r == '/':
return lexComment
case r == '%':
return lexPct
case r == '\n':
l.emit(itemNL)
case r == ':':
l.emit(itemColon)
case r == '|':
l.emit(itemPipe)
case r == '{':
return lexExpr
case isSpace(r):
l.ignore()
case isIdent(r):
return lexIdent
case r == '\'':
return lexLiteral
case r == eof:
l.emit(itemEOF)
break Loop
default:
return l.errorf("invalid character: %v", string(r))
}
}
return nil
}
func lexLiteral(l *lexer) stateFn {
for {
switch l.next() {
case '\'':
l.emit(itemLiteral)
return lexStart
}
}
}
func lexExpr(l *lexer) stateFn {
ct := 1
for {
switch l.next() {
case '{':
ct++
case '}':
ct--
if ct == 0 {
l.emit(itemExpr)
return lexStart
}
}
}
}
func lexComment(l *lexer) stateFn {
switch r := l.next(); r {
case '/':
for {
switch l.next() {
case '\n':
l.backup()
l.emit(itemComment)
return lexStart
}
}
case '*':
for {
switch l.next() {
case '*':
if l.peek() == '/' {
l.next()
l.emit(itemComment)
return lexStart
}
}
}
default:
return l.errorf("expected comment: %c", r)
}
}
func lexPct(l *lexer) stateFn {
switch l.next() {
case '%':
l.emit(itemDoublePct)
return lexStart
case '{':
for {
switch l.next() {
case '%':
if l.peek() == '}' {
l.next()
l.emit(itemPct)
return lexStart
}
}
}
case 'p':
if l.next() != 'r' || l.next() != 'e' || l.next() != 'c' || l.next() != ' ' {
l.errorf("expected %%prec")
}
for {
switch r := l.next(); {
case isIdent(r):
// absorb
default:
l.backup()
l.emit(itemPct)
return lexStart
}
}
default:
ct := 0
for {
switch l.next() {
case ' ':
case '{':
ct++
case '}':
ct--
if ct == 0 {
l.emit(itemPct)
return lexStart
}
case '\n':
if ct == 0 {
l.backup()
l.emit(itemPct)
return lexStart
}
}
}
}
}
func lexIdent(l *lexer) stateFn {
for {
switch r := l.next(); {
case isIdent(r):
// absorb
default:
l.backup()
l.emit(itemIdent)
return lexStart
}
}
}
func isSpace(r rune) bool {
return r == ' ' || r == '\t'
}
func isIdent(r rune) bool {
return r == '_' || unicode.IsLetter(r) || unicode.IsDigit(r)
}