Performance issue in Bigint::pow function

[JK000000]

In function Bigint::pow there are two indentical recruisive calls, that could be replaced by just one call
Consider changing

Bigint Bigint::pow(int const &power, std::map<int, Bigint> &lookup)
{
    if (power == 1) return *this;
    if (lookup.count(power)) return lookup[power];

    int closestPower = 1;
    while (closestPower < power) closestPower <<= 1;
    closestPower >>= 1;

    //here pow(power / 2, lookup) is called twice
    if (power == closestPower) lookup[power] = pow(power / 2, lookup) * pow(power / 2, lookup);
    else lookup[power] = pow(closestPower, lookup) * pow(power - closestPower, lookup);

    return lookup[power];
}

to

Bigint Bigint::pow(int const &power, std::map<int, Bigint> &lookup)
{
    if (power == 1) return *this;
    if (lookup.count(power)) return lookup[power];

    int closestPower = 1;
    while (closestPower < power) closestPower <<= 1;
    closestPower >>= 1;

    if (power == closestPower)
    {
        auto tmp = pow(power / 2, lookup);
        lookup[power] = tmp * tmp;
    }
    else
    {
        lookup[power] = pow(closestPower, lookup) * pow(power - closestPower, lookup);
    }

    return lookup[power];
}