Implemented (but not tested) cursor.RuneReader

cursor/helper.go

@@ -1,6 +1,11 @@
 package cursor
 
-import "io"
+import (
+	"errors"
+	"fmt"
+	"io"
+	"unicode/utf8"
+)
 
 // BufferedReaderAt uses a buffer to supplement an io.Reader
 // with limited backward seeking.
@@ -31,12 +36,20 @@ func NewRuneReader(c Cursor[byte]) *RuneReader {
 	return &RuneReader{cursor: c}
 }
 
-func (r *RuneReader) Read(dst []byte) (int, error) {
-	n, c, err := r.cursor.Read(dst)
-	r.cursor = c
-	return int(n), err
+func (rr *RuneReader) ReadRune() (r rune, size int, err error) {
+	var b [4]byte
+	s := b[:]
+	n, next, err := rr.cursor.Read(s)
+	if err != nil && !errors.Is(err, io.EOF) {
+		rr.cursor = next
+		return 0, 0, fmt.Errorf("ReadRune: %w", err)
+	}
+	s = s[:n]
+	r, size = utf8.DecodeRune(s)
+	rr.cursor = rr.cursor.At(rr.cursor.Pos() + uint64(size))
+	return r, size, err
 }
 
-func (r *RuneReader) Cursor() Cursor[byte] {
-	return r.cursor
+func (rr *RuneReader) Cursor() Cursor[byte] {
+	return rr.cursor
 }