crc7.cpp

#include <iostream>

constexpr uint8_t m_len = 40;
constexpr uint8_t crc_len = 7;
constexpr uint8_t msb_shift = m_len + crc_len - 1;
constexpr uint8_t g_len = 8;
constexpr uint8_t g_shift = m_len + crc_len - g_len;
constexpr uint8_t g = (1 << 7) | (1 << 3) | (1 << 0); // G(x) = x^7 + x^3 + 1

constexpr uint8_t crc7_impl(uint64_t crc, uint64_t msb, uint64_t msk, uint8_t iter) {
    return iter == 0 ? (uint8_t) crc : crc7_impl(crc & msb ? crc ^ msk : crc, msb >> 1, msk >> 1, iter - 1);
}

constexpr uint8_t crc7(uint64_t m) {
    return crc7_impl(m << crc_len, (uint64_t) 1 << msb_shift, (uint64_t) g << g_shift, g_shift + 1);
}

int main() {
    // 0100000000000000000000000000000000000000 -> 1001010
    std::cout << ((int) crc7(274877906944)) << " = " << 74 << std::endl;

    // 0101000100000000000000000000000000000000 -> 0101010
    std::cout << ((int) crc7(347892350976)) << " = " << 42 << std::endl;

    // 0001000100000000000000000000100100000000 -> 0110011
    std::cout << ((int) crc7(73014446336)) << " = " << 51 << std::endl;

    // 0000000000000000000000000000000000000000 -> 0000000
    std::cout << ((int) crc7(0)) << " = " << 0 << std::endl;

    return 0;
}