Rename SpotReader, remove cruft, and move helpers

cursor/helper.go

@@ -0,0 +1,38 @@
+package cursor
+
+import "io"
+
+// BufferedReaderAt uses a buffer to supplement an io.Reader
+// with limited backward seeking.
+type BufferedReaderAt struct{}
+
+func NewBufferedReaderAt(r io.Reader, minBuffer uint64) *BufferedReaderAt
+
+// ReadAt reads bytes from the underlying reader. If the offset is after
+// the end of the buffer, ReadAt will first read and ignore bytes from the
+// underlying reader until it reaches the offset. If the offset is
+// before the start of the buffer, ReadAt will return an error.
+//
+// If your parser needs unlimited lookahead, you should probably
+// just read the whole input into a slice and use BytesCursor.
+func (b *BufferedReaderAt) ReadAt(dst []byte, offset int64) (int, error)
+
+// RuneReader is an io.RuneReader backed by a Cursor, for compatibility
+// with the regexp package.
+type RuneReader struct {
+	cursor Cursor[byte]
+}
+
+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 (r *RuneReader) Cursor() Cursor[byte] {
+	return r.cursor
+}