Implemented (but not tested) cursor.RuneReader

cursor/cursor.go

@@ -25,6 +25,9 @@ type Cursor[Datum any] interface {
 
 	// Pos returns the Cursor's position within the source.
 	Pos() uint64
+
+	// At returns a new cursor at the position pos.
+	At(pos uint64) Cursor[Datum]
 }
 
 type SliceCursor[Datum any] struct {
@@ -56,6 +59,11 @@ func (sc SliceCursor[Datum]) Pos() uint64 {
 	return sc.offset
 }
 
+func (sc SliceCursor[Datum]) At(pos uint64) Cursor[Datum] {
+	sc.offset = pos
+	return sc
+}
+
 type ReaderAtCursor struct {
 	r   io.ReaderAt
 	pos uint64
@@ -77,6 +85,11 @@ func (rac ReaderAtCursor) Pos() uint64 {
 	return rac.pos
 }
 
+func (rac ReaderAtCursor) At(pos uint64) Cursor[byte] {
+	rac.pos = pos
+	return rac
+}
+
 // StringCursor is identical to SliceCursor[byte], but uses a string as its data source.
 // The advantage is that creating a StringCursor does not require copying the source
 // string into a []byte.
@@ -105,3 +118,8 @@ func (sc StringCursor) Read(dst []byte) (n uint64, next Cursor[byte], err error)
 func (sc StringCursor) Pos() uint64 {
 	return sc.offset
 }
+
+func (sc StringCursor) At(pos uint64) Cursor[byte] {
+	sc.offset = pos
+	return sc
+}

cursor/cursor_test.go

@@ -73,6 +73,7 @@ func testCursor[C cursor.Cursor[byte]](t *testing.T, makeCursor func([]byte) C)
 		test.ErrorIs(t, err, io.EOF)
 		test.EqOp(t, len(data), int(n))
 	}))
+	t.Run("At", Todo)
 }
 
 func TestSliceCursor(t *testing.T) {

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
 }