import std.bigint : BigInt;

​

BigInt factorial_recursive(ulong n = 10000)

{

    return n <= 1 ? BigInt(1) : n * factorial_recursive(n - 1);

}

​

BigInt factorial_loop(ulong n = 10000)

{

    if (n <= 1)

        return BigInt(1);

​

    BigInt res = 2 * n;

​

    foreach (i; 3 .. n)

        res *= i;

    return res;

}

​

BigInt factorial_parallel(ulong n = 10000)

{

    import std.range : iota;

    import std.parallelism : taskPool;

​

    return taskPool.reduce!"a * b"(BigInt(1), iota(1, n + 1));

}

​

void main()

{

    import std.stdio : writefln;

    import std.datetime.stopwatch : benchmark;

​

    import std.parallelism;

    import std.stdio;

    defaultPoolThreads.writeln;

    auto r = benchmark!(factorial_recursive, factorial_loop, factorial_parallel)(100);

    writefln!"recursive:\t%s\nloop:\t\t%s\nparallel:\t%s"(r[0], r[1], r[2]);

}