whatcanGOwrong

go/pkg/mod/golang.org/x/exp@v0.0.0-20230224173230-c95f2b4c22f2/ebnf/parser.go

@@ -0,0 +1,189 @@
+// 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 || p.tok == scanner.RawString {
+		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, scanner.RawString:
+		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()
+}