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lexer.go
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lexer.go
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package sexp
import (
"fmt"
"regexp"
"strconv"
)
type Item struct {
Type ItemType
Position int
Value []byte
}
type ItemType int
func (item Item) String() string {
switch item.Type {
case ItemError:
return fmt.Sprintf("Error(%v)", item.Value)
case ItemBracketLeft:
return "("
case ItemBracketRight:
return ")"
case ItemToken:
return fmt.Sprintf("Token(%v)", item.Value)
case ItemQuote:
return fmt.Sprintf("Quote(%v)", item.Value)
case ItemVerbatim:
return fmt.Sprintf("Verbatim(%v)", item.Value)
case ItemEOF:
return "EOF"
default:
return "Unknown(%v)"
}
}
const (
ItemEOF ItemType = iota
ItemError
ItemBracketLeft // (
ItemBracketRight // )
ItemToken // abc Token.
ItemQuote // "abc" Quoted string. May also include length 3"abc"
ItemVerbatim // 3:abc Length prefixed "verbatim" encoding.
// ItemHex // #616263# Hexidecimal string.
// ItemBase64 // {MzphYmM=} Base64 of the verbatim encoding "3:abc"
// ItemBase64Octet // |YWJj| Base64 encoding of the octet-string "abc"
)
var (
reBracketLeft = regexp.MustCompile(`^\(`)
reBracketRight = regexp.MustCompile(`^\)`)
reWhitespace = regexp.MustCompile(`^\s+`)
reVerbatim = regexp.MustCompile(`^(\d+):`)
reQuote = regexp.MustCompile(`^(\d+)?"((?:[^\\"]|\\.)*)"`)
// Strict(er) R.Rivset 1997 draft token + unicode letter support (hello 1997).
// reToken = regexp.MustCompile(`^[\p{L}][\p{L}\p{N}\-./_:*+=]+`)
// Instead a token can be anything including '(', ')' and ' ' so long as you escape them:
reToken = regexp.MustCompile(`^(?:[^\\ ()]|\\.)+`)
)
type stateFn func(*lexer) stateFn
type lexer struct {
input []byte
items chan Item
start int
pos int
state stateFn
matches [][]byte
parens int
}
func (l *lexer) emit(t ItemType) {
switch t {
case ItemBracketLeft:
l.parens++
case ItemBracketRight:
l.parens--
}
if l.parens > 0 && t == ItemEOF {
l.items <- Item{ItemError, l.start, []byte(fmt.Sprintf("Unexpected EOF, %d '(' unmatched", l.parens))}
} else if l.parens < 0 {
l.items <- Item{ItemError, l.start, []byte("Unmatched )")}
} else {
l.items <- Item{t, l.start, l.input[l.start:l.pos]}
}
}
func (l *lexer) Next() Item {
item := <-l.items
return item
}
func (l *lexer) scan(re *regexp.Regexp) bool {
if l.match(re) {
l.start = l.pos
l.pos += len(l.matches[0])
return true
}
return false
}
func (l *lexer) match(re *regexp.Regexp) bool {
if l.matches = re.FindSubmatch(l.input[l.pos:]); l.matches != nil {
return true
}
return false
}
func (l *lexer) run() {
for l.state = lex; l.state != nil; {
l.state = l.state(l)
}
close(l.items)
}
func (l *lexer) errorf(format string, args ...interface{}) stateFn {
l.items <- Item{ItemError, l.start, []byte(fmt.Sprintf(format, args...))}
return nil
}
func lex(l *lexer) stateFn {
// The order is important here, reToken must come last because it'll match reVerbatim and
// reQuote atoms as well.
switch {
case l.pos >= len(l.input):
l.emit(ItemEOF)
return nil
case l.scan(reWhitespace):
return lex
case l.scan(reBracketLeft):
l.emit(ItemBracketLeft)
return lex
case l.scan(reBracketRight):
l.emit(ItemBracketRight)
return lex
case l.scan(reQuote):
// TODO: errorf if length exists and doesn't line up with quote length.
// Don't include quotes in Value.
l.items <- Item{ItemQuote, l.start, []byte(l.matches[2])}
return lex
case l.scan(reVerbatim):
bytes, _ := strconv.ParseInt(string(l.matches[1]), 10, 64)
l.start = l.pos
l.pos += int(bytes)
l.emit(ItemVerbatim)
return lex
case l.scan(reToken):
l.emit(ItemToken)
return lex
}
return l.errorf("Unexpected byte at %d near '%s'.", l.pos, l.near())
}
func (l *lexer) near() []byte {
from := l.pos - 5
if from < 0 {
from = 0
}
near := l.input[from:]
if len(near) < 10 {
return near[:len(near)]
}
return near[:10]
}
/*
Lex S-Expressions.
See http://people.csail.mit.edu/rivest/Sexp.txt
* Unlike the R.Rivest 1997 draft tokens will match any unicode letters.
* Canonical S-Expressions may have spaces between atoms which isn't strictly correct.
*/
func NewLexer(input []byte) *lexer {
l := &lexer{input: input, items: make(chan Item)}
go l.run()
return l
}