CRC32.cpp

#include "CRC32.h"

#include <array>

constexpr auto create_crc_table() noexcept
{
    std::array<unsigned int, 256> ret{};

    for (auto i = 0u; i < 256u; ++i)
    {
        auto value = i;

        for (auto j = 8u; j; --j)
        {
            // bit-reverse 0x04C11DB7U;
            auto const polynomial = (value & 1u) ? 0xEDB88320u : 0u;
            value = (value >> 1u) ^ polynomial;
        }

        ret[i] = value;
    }

    return ret;
}

unsigned int CRC32::compute(
    void const* const buffer, long long const length) noexcept
{
    static constexpr auto const crc_table = create_crc_table();

    auto const data = static_cast<unsigned char const*>(buffer);

    for (auto i = data; i < &data[length]; ++i)
    {
        auto const index = static_cast<unsigned char>((_value & 0xFFu) ^ *i);
        _value = (_value >> 8u) ^ crc_table[index];
    }

    return ~_value;
}