Re: [boost] crc // wrong poly for crc_xmodem_type

16 May 2017

      Hi Pavel,

2017-05-15 19:40 GMT+03:00 Pavel Kretov :
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Hi Sergei.
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I've discovered that current typedef crc_xmodem_type uses wrong poly.
I'm not an expert in CRCs and I have no idea whether Boost's implementation is
wrong, but it's interesting that:
bin(0x8408) = reverse(bin(0x1021))
Maybe this isn't a real problem, but simply someone decided to intentionally
flip the poly in the CRC implementation, and the computation still leads to
correct results?
——— Pavel K.
I believe this is real problem

CRC16 calculator from here
https://www.lammertbies.nl/comm/info/crc-calculation.html

XMODEM for string "123456789" gives 0x31C3

Test case
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>>>>>>>> cut here <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
#define CATCH_CONFIG_MAIN
// This tells Catch to provide a main() - only do this in one cpp file
#include "catch.hpp"
#include 

TEST_CASE("boost_checksum_crc16", "[checksum]")
{

    std::string xmt{"123456789"};

    boost::crc_xmodem_type bxmt;
    bxmt.process_bytes(xmt.c_str(), xmt.length());
    unsigned xmv = bxmt.checksum();
    CHECK(xmv == 0x31C3);

    boost::crc_optimal<16, 0x1021, 0, 0, false, false> crc_xmt;
    crc_xmt.process_bytes(xmt.c_str(), xmt.length());
    unsigned xmvr = crc_xmt.checksum();
    CHECK(xmvr == 0x31C3);
}
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>>>>>>>> cut here <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
First one check fails with

../test/test_3.cpp:136: FAILED:
  CHECK( xmv == 0x31C3 )
with expansion:
  3187 (0xc73) == 12739 (0x31c3)

The second one is passed.

-- 
Best Regards,
Sergei Nikulov