// considered read operations.
func (b *Reader) UnreadByte() error {
	if b.lastByte < 0 || b.r == 0 && b.w > 0 {
		return ErrInvalidUnreadByte
	}
	// b.r > 0 || b.w == 0
	if b.r > 0 {
		b.r--
	} else {
		// b.r == 0 && b.w == 0
		b.w = 1
	}
	b.buf[b.r] = byte(b.lastByte)
	b.lastByte = -1
	b.lastRuneSize = -1
	return nil
}

	var rp *repeatedByte
	if len(r.buf) > 0 {
		rp = &r.buf[len(r.buf)-1]
		// Fast path, if p is entirely the same byte repeated.
		if lastByte := rp.b; len(p) > 0 && p[0] == lastByte {
			if bytes.Count(p, []byte{lastByte}) == len(p) {
				rp.n += int64(len(p))
				return len(p), nil
			}
		}
	}

	for _, b := range p {
		if rp == nil || rp.b != b {