[Issue 6244] Add powmod / modpow function to std.math

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=6244

--- Comment #1 from josvanuden at gmail.com <josvanuden at gmail.com> ---
ulong powmod(ulong b, ulong e, ulong m)  {
    ulong r = 1;
    for (; e > 0; e >>= 1) {
        if (e & 1) {
            r = (r * b) % m;
        }
        b = (b * b) % m;
    }
    return r;
}

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