parser: add initial Parser emitting non-whitespace tokens

Signed-off-by: Alejandro Mery <amery@jpi.io>

parser/parser.go

@@ -0,0 +1,105 @@
+// Package parser parses dosini-style files
+package parser
+
+import (
+	"io"
+	"log"
+
+	"asciigoat.org/core/lexer"
+)
+
+// Parser parses a dosini-style document
+type Parser struct {
+	src *lexer.Reader
+	pos lexer.Position
+
+	// OnToken is called for each identified token. if it returns an error
+	// parsing is interrupted.
+	OnToken func(pos lexer.Position, typ TokenType, value string) error
+
+	// OnError is called in case of a parsing error, and it's allowed
+	// to replace the error returned by [Parser.Run].
+	// OnError is called for io.EOF, but [Parser.Run] will consider it
+	// normal termination.
+	OnError func(pos lexer.Position, content string, err error) error
+}
+
+func defaultOnToken(pos lexer.Position, typ TokenType, value string) error {
+	log.Printf("%s:%v:%v: %q", typ, pos.Line, pos.Column, value)
+	return nil
+}
+
+func defaultOnError(pos lexer.Position, content string, err error) error {
+	log.Printf("%s:%v:%v: %q: %s", "error", pos.Line, pos.Column, content, err)
+
+	return lexer.Error{
+		Line:   pos.Line,
+		Column: pos.Column,
+
+		Content: content,
+		Err:     err,
+	}
+}
+
+func (p *Parser) setDefaults() {
+	if p.OnToken == nil {
+		p.OnToken = defaultOnToken
+	}
+	if p.OnError == nil {
+		p.OnError = defaultOnError
+	}
+}
+
+func (p *Parser) emitString(typ TokenType) error {
+	s := p.src.Emit()
+	err := p.OnToken(p.pos, typ, s)
+	p.pos.StepN(len(s))
+
+	return err
+}
+
+func (p *Parser) emitError(content string, err error) (lexer.StateFn, error) {
+	err2 := p.OnError(p.pos, content, err)
+	switch {
+	case err2 != nil:
+		// return wrapped error
+		return nil, err2
+	default:
+		// return original error
+		return nil, err
+	}
+}
+
+// stepLine discards the data and moves the position
+// to the next line.
+func (p *Parser) stepLine() {
+	p.src.Discard()
+	p.pos.StepLine()
+}
+
+// stepRune discards the data and moves the position
+// one rune forward on the same line.
+func (p *Parser) stepRune() {
+	p.src.Discard()
+	p.pos.Step()
+}
+
+// stepString discards the data and moves the position
+// forward on the same line the length of the discarded
+// content.
+func (p *Parser) stepString() {
+	s := p.src.Emit()
+	p.pos.StepN(len(s))
+}
+
+// NewParser creates a dosini-style parser using
+// an [io.Reader] as source
+func NewParser(r io.Reader) *Parser {
+	if r == nil {
+		return nil
+	}
+
+	return &Parser{
+		src: lexer.NewReader(r),
+	}
+}