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#include "gmxpre.h"

#include "seed.h"

#include <chrono>

#include "gromacs/utility/basedefinitions.h"
#include "gromacs/utility/fatalerror.h"

namespace gmx
{


/*! \brief Check if the RDRAND random device functioning correctly
 *
 * Due to a bug in AMD Ryzen microcode, RDRAND may always return -1 (0xFFFFFFFF).
 * To avoid that, fall back to using PRNG instead of RDRAND if this happens.
 *
 * \returns The result of the checks.
 */
static bool checkIfRandomDeviceIsFunctional()
{
    std::random_device rd;

    uint64_t randomNumber1 = static_cast<uint64_t>(rd());
    uint64_t randomNumber2 = static_cast<uint64_t>(rd());

    // Due to a bug in AMD Ryzen microcode, RDRAND may always return -1 (0xFFFFFFFF).
    // To avoid that, fall back to using PRNG instead of RDRAND if this happens.
    if (randomNumber1 == 0xFFFFFFFF && randomNumber2 == 0xFFFFFFFF)
    {
        if (debug)
        {
            fprintf(debug,
                    "Hardware random number generator (RDRAND) returned -1 (0xFFFFFFFF) twice in\n"
                    "a row. This may be due to a known bug in AMD Ryzen microcode.");
        }
        return false;
    }
    return true;
}

/*! \brief Get the next pure or pseudo-random number
 *
 * Returns the next random number taken from the hardware generator or from PRNG.
 *
 * \param[in] gen  Pseudo-random/random numbers generator/device to use.
 *
 * \return Random or pseudo-random number.
 */
template<typename GeneratorType>
static uint64_t makeRandomSeedInternal(GeneratorType& gen)
{
    constexpr std::size_t resultBits = std::numeric_limits<uint64_t>::digits;
    constexpr std::size_t numBitsInRandomNumber =
            std::numeric_limits<typename GeneratorType::result_type>::digits;

    uint64_t result = static_cast<uint64_t>(gen());
    // This is needed so that compiler understands that what follows is a dead branch
    // and not complains about shift count larger than number of bits in the result.
    constexpr std::size_t shiftCount = (resultBits < numBitsInRandomNumber) ? numBitsInRandomNumber : 0;
    for (std::size_t bits = numBitsInRandomNumber; bits < resultBits; bits += numBitsInRandomNumber)
    {
        result = (result << shiftCount) | static_cast<uint64_t>(gen());
    }
    return result;
}

uint64_t makeRandomSeed()
{
    bool useRdrand = checkIfRandomDeviceIsFunctional();
    if (useRdrand)
    {
        std::random_device rd;
        return makeRandomSeedInternal(rd);
    }
    else
    {
        int64_t microsecondsSinceEpoch = std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::system_clock::now().time_since_epoch()).count();
        std::mt19937_64 prng(microsecondsSinceEpoch);
        return makeRandomSeedInternal(prng);
    }
}


} // namespace gmx
