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// Copyright 2009 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.
package ebnf
import (
"io"
"strconv"
"text/scanner"
)
type parser struct {
errors errorList
scanner scanner.Scanner
pos scanner.Position // token position
tok rune // one token look-ahead
lit string // token literal
}
func (p *parser) next() {
p.tok = p.scanner.Scan()
p.pos = p.scanner.Position
p.lit = p.scanner.TokenText()
}
func (p *parser) error(pos scanner.Position, msg string) {
p.errors = append(p.errors, newError(pos, msg))
}
func (p *parser) errorExpected(pos scanner.Position, msg string) {
msg = `expected "` + msg + `"`
if pos.Offset == p.pos.Offset {
// the error happened at the current position;
// make the error message more specific
msg += ", found " + scanner.TokenString(p.tok)
if p.tok < 0 {
msg += " " + p.lit
}
}
p.error(pos, msg)
}
func (p *parser) expect(tok rune) scanner.Position {
pos := p.pos
if p.tok != tok {
p.errorExpected(pos, scanner.TokenString(tok))
}
p.next() // make progress in any case
return pos
}
func (p *parser) parseIdentifier() *Name {
pos := p.pos
name := p.lit
p.expect(scanner.Ident)
return &Name{pos, name}
}
func (p *parser) parseToken() *Token {
pos := p.pos
value := ""
if p.tok == scanner.String {
value, _ = strconv.Unquote(p.lit)
// Unquote may fail with an error, but only if the scanner found
// an illegal string in the first place. In this case the error
// has already been reported.
p.next()
} else {
p.expect(scanner.String)
}
return &Token{pos, value}
}
// ParseTerm returns nil if no term was found.
func (p *parser) parseTerm() (x Expression) {
pos := p.pos
switch p.tok {
case scanner.Ident:
x = p.parseIdentifier()
case scanner.String:
tok := p.parseToken()
x = tok
const ellipsis = '…' // U+2026, the horizontal ellipsis character
if p.tok == ellipsis {
p.next()
x = &Range{tok, p.parseToken()}
}
case '(':
p.next()
x = &Group{pos, p.parseExpression()}
p.expect(')')
case '[':
p.next()
x = &Option{pos, p.parseExpression()}
p.expect(']')
case '{':
p.next()
x = &Repetition{pos, p.parseExpression()}
p.expect('}')
}
return x
}
func (p *parser) parseSequence() Expression {
var list Sequence
for x := p.parseTerm(); x != nil; x = p.parseTerm() {
list = append(list, x)
}
// no need for a sequence if list.Len() < 2
switch len(list) {
case 0:
p.errorExpected(p.pos, "term")
return &Bad{p.pos, "term expected"}
case 1:
return list[0]
}
return list
}
func (p *parser) parseExpression() Expression {
var list Alternative
for {
list = append(list, p.parseSequence())
if p.tok != '|' {
break
}
p.next()
}
// len(list) > 0
// no need for an Alternative node if list.Len() < 2
if len(list) == 1 {
return list[0]
}
return list
}
func (p *parser) parseProduction() *Production {
name := p.parseIdentifier()
p.expect('=')
var expr Expression
if p.tok != '.' {
expr = p.parseExpression()
}
p.expect('.')
return &Production{name, expr}
}
func (p *parser) parse(filename string, src io.Reader) Grammar {
p.scanner.Init(src)
p.scanner.Filename = filename
p.next() // initializes pos, tok, lit
grammar := make(Grammar)
for p.tok != scanner.EOF {
prod := p.parseProduction()
name := prod.Name.String
if _, found := grammar[name]; !found {
grammar[name] = prod
} else {
p.error(prod.Pos(), name+" declared already")
}
}
return grammar
}
// Parse parses a set of EBNF productions from source src.
// It returns a set of productions. Errors are reported
// for incorrect syntax and if a production is declared
// more than once; the filename is used only for error
// positions.
//
func Parse(filename string, src io.Reader) (Grammar, error) {
var p parser
grammar := p.parse(filename, src)
return grammar, p.errors.Err()
}
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