lexer: Implement Reader.UnreadRune() and Reader.PeekRune()

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

lexer/reader.go

@@ -2,6 +2,7 @@ package lexer
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"strings"
 	"unicode/utf8"
@@ -18,7 +19,14 @@ const (
 
 // implemented interfaces
 var (
-	_ io.RuneReader = (*Reader)(nil)
+	_ io.RuneReader  = (*Reader)(nil)
+	_ io.RuneScanner = (*Reader)(nil)
+)
+
+var (
+	// ErrInvalidUnreadRune indicates UnreadRune() was calls after an
+	// action other than a successful ReadRune()
+	ErrInvalidUnreadRune = errors.New("invalid UnreadRune() call")
 )
 
 // Reader is a RuneReader aimed at implementing text parsers
@@ -28,6 +36,8 @@ type Reader struct {
 	buf    []byte
 	off    int
 	cursor int
+
+	lastRuneSize int
 }
 
 // String returns what's already Read but not yet emitted or discarded
@@ -54,6 +64,9 @@ func (b *Reader) Discard() {
 		// step
 		b.off = b.cursor
 	}
+
+	// and prevent UnreadRune()
+	b.lastRuneSize = -1
 }
 
 // ready tells how many bytes are ready to decode
@@ -139,6 +152,8 @@ func (b *Reader) ReadRune() (rune, int, error) {
 	for {
 		err := b.needsBytes(count)
 		if err != nil {
+			b.lastRuneSize = -1
+
 			return 0, 0, err
 		}
 
@@ -155,10 +170,34 @@ func (b *Reader) ReadRune() (rune, int, error) {
 	r, l := utf8.DecodeRune(b.buf[b.cursor:])
 	// step over
 	b.cursor += l
+	// and remember for UnreadRune()
+	b.lastRuneSize = l
 
 	return r, l, nil
 }
 
+// UnreadRune moves the cursor where it was before the last call to ReadRune
+func (b *Reader) UnreadRune() error {
+	if b.lastRuneSize > 0 {
+		b.cursor -= b.lastRuneSize
+		b.lastRuneSize = -1
+		return nil
+	}
+
+	return ErrInvalidUnreadRune
+}
+
+// PeekRune returns information about the next rune without moving the
+// cursor
+func (b *Reader) PeekRune() (rune, int, error) {
+	r, l, err := b.ReadRune()
+	if err != nil {
+		return r, l, err
+	}
+	err = b.UnreadRune()
+	return r, l, err
+}
+
 // NewReader creates a new runes [Reader] using the given [io.Reader]
 func NewReader(r io.Reader) *Reader {
 	if r == nil {