static int crc(byte[] buf, int offset, int len)
    {
        return ~updateCrc(buf, offset, len);
    }
@Test
    void testCRCImpl()
    {
        byte[] b1 = "Hello World!".getBytes();
        assertEquals(472456355, PNGConverter.crc(b1, 0, b1.length));
        assertEquals(-632335482, PNGConverter.crc(b1, 2, b1.length - 4));
    }