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lexer.go
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lexer.go
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package mapaccess
import (
"fmt"
"strings"
"unicode"
"unicode/utf8"
)
type item struct {
typ itemType // The type of this item.
val string // The key value of this item.
}
// itemType identifies the type of lex items.
type itemType int
const (
itemError itemType = iota
itemEOF
itemIdentifier
itemArrayIndex
itemDot
)
const eof = -1
// lexStateFn represents the state of the scanner as a function that returns the next state.
type lexStateFn func(*lexer) lexStateFn
// lexer holds the state of the scanner.
type lexer struct {
input string // the string being scanned
pos int // current position in the input
start int // start position of this item
width int // width of last rune read from input
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 = 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.input[l.start:l.pos]}
l.start = l.pos
}
// accept consumes the next rune if it's from the valid set.
func (l *lexer) accept(valid string) bool {
if strings.ContainsRune(valid, l.next()) {
return true
}
l.backup()
return false
}
// acceptRun consumes a run of runes from the valid set.
func (l *lexer) acceptRun(valid string) {
for strings.ContainsRune(valid, l.next()) {
}
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.nextItem.
func (l *lexer) errorf(format string, args ...interface{}) lexStateFn {
l.items <- item{itemError, fmt.Sprintf(format, args...)}
return nil
}
// nextItem returns the next item from the input.
// Called by the parser, not in the lexing goroutine.
func (l *lexer) nextItem() item {
return <-l.items
}
// lex creates a new scanner for the input string.
func lex(input string) *lexer {
l := &lexer{
input: input,
items: make(chan item),
}
go l.run()
return l
}
// run runs the state machine for the lexer.
func (l *lexer) run() {
defer close(l.items)
for state := lexStart; state != nil; {
state = state(l)
}
}
func lexStart(l *lexer) lexStateFn {
next := l.peek()
switch next {
case '[':
return lexArrayIndexAction
default:
return lexIdentifier
}
}
// lexIdentifier scans an alphanumeric.
func lexIdentifier(l *lexer) lexStateFn {
for {
switch r := l.next(); {
case isValidIdentifierRune(r):
// absorb.
default:
l.backup()
next := l.peek()
if l.pos == l.start {
if next != -1 {
return l.errorf("expected identifier")
}
l.emit(itemEOF)
return nil
}
l.emit(itemIdentifier)
switch next {
case '.':
return lexDotAction
case '[':
return lexArrayIndexAction
case -1:
l.emit(itemEOF)
return nil
default:
return l.errorf("expected <.> or <array index>")
}
}
}
}
// lexDotAction expects a dot.
func lexDotAction(l *lexer) lexStateFn {
r := l.next()
switch r {
case '.':
l.emit(itemDot)
next := l.peek()
switch next {
case '[':
return lexArrayIndexAction
default:
return lexIdentifier
}
default:
return l.errorf("bad character")
}
}
// lexArrayIndexAction scans for [\d+].
func lexArrayIndexAction(l *lexer) lexStateFn {
if !l.accept("[") {
return l.errorf("missing closing bracket [ at array index <[]>")
}
if !l.accept("0123456789") {
return l.errorf("missing digits in array index <[]>")
}
digits := "0123456789"
l.acceptRun(digits)
if !l.accept("]") {
return l.errorf("missing closing bracket ] at array index <[]>")
}
l.emit(itemArrayIndex)
next := l.peek()
switch next {
case '.':
return lexDotAction
case '[':
return lexArrayIndexAction
default:
return lexEOF
}
}
// lexEOF scans for EOF.
func lexEOF(l *lexer) lexStateFn {
if l.next() == -1 {
l.emit(itemEOF)
return nil
}
return lexIdentifier
}
// isAlphaNumeric reports whether r is an alphabetic, digit, or underscore.
func isValidIdentifierRune(r rune) bool {
return strings.ContainsRune("_-", r) || unicode.IsLetter(r) || unicode.IsDigit(r)
}