decode_reader.go
func (w *window) writeByte(c byte) {
	w.buf[w.w] = c
	w.w = (w.w + 1) & w.mask
}
func (w *window) writeByte(c byte) {
	w.buf[w.w] = c
	w.w = (w.w + 1) & w.mask
}
// writeByte writes c to the end of the window