crc.cc

// Copyright (c) 2015-2019, FlightAware LLC.
// Copyright (c) 2015, Oliver Jowett <oliver@mutability.co.uk>
// All rights reserved.

// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are met:

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// * Redistributions in binary form must reproduce the above copyright notice,
//   this list of conditions and the following disclaimer in the documentation
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#include <iomanip>
#include <iostream>

#include "crc.h"

#include <boost/preprocessor/repetition/enum.hpp>

namespace crc {
    namespace detail {
        // generates the CRC table at compile time (!)
        const std::uint32_t crc_polynomial = 0xfff409U;

        template <std::uint32_t c, int k = 8> struct crcgen : crcgen<((c & 0x00800000) ? crc_polynomial : 0) ^ (c << 1), k - 1> {};

        template <std::uint32_t c> struct crcgen<c, 0> {
            enum { value = (c & 0x00FFFFFF) };
        };

#define CRCGEN(Z, N, _) detail::crcgen<N << 16>::value
        std::uint32_t crc_table[256] = {BOOST_PP_ENUM(256, CRCGEN, _)};
#undef CRCGEN

        std::map<std::uint32_t, unsigned> syndromes_short;
        std::map<std::uint32_t, unsigned> syndromes_long;

        void init_syndromes() {
            syndromes_short.clear();
            std::vector<std::uint8_t> message_short(56 / 8);
            for (unsigned i = 5; i < 56; ++i) {
                message_short[i / 8] ^= 1 << (7 - (i & 7));
                syndromes_short[message_residual(message_short)] = i;
                message_short[i / 8] ^= 1 << (7 - (i & 7));
            }

            syndromes_long.clear();
            std::vector<std::uint8_t> message_long(112 / 8);
            for (unsigned i = 5; i < 112; ++i) {
                message_long[i / 8] ^= 1 << (7 - (i & 7));
                syndromes_long[message_residual(message_long)] = i;
                message_long[i / 8] ^= 1 << (7 - (i & 7));
            }
        }
    } // namespace detail
};    // namespace crc