Duoas' Challenge!

import std.array;
import std.conv;
import std.stdio;

void main(string[] args)
{
    if (args.length != 2)
    {
        stderr.writeln("Usage:\n\tprogram.exe number\n");
        return;
    }

    string process(ulong n)
    {
        immutable string[ulong] dictionary = [
            0: "",
            1: "One",
            2: "Two",
            3: "Three",
            4: "Four",
            5: "Five",
            6: "Six",
            7: "Seven",
            8: "Eight",
            9: "Nine",
            10: "Ten",
            11: "Eleven",
            12: "Twelve",
            13: "Thirteen",
            14: "Fourteen",
            15: "Fifteen",
            16: "Sixteen",
            17: "Seventeen",
            18: "Eighteen",
            19: "Nineteen",
            20: "Twenty",
            30: "Thirty",
            40: "Forty",
            50: "Fifty",
            60: "Sixty",
            70: "Seventy",
            80: "Eighty",
            90: "Ninety"
        ];

        immutable string[ulong] levels = [
            1: "",
            1_000: "Thousand",
            1_000_000: "Million",
            1_000_000_000: "Billion",
            1_000_000_000_000: "Trillion",
            1_000_000_000_000_000: "Quadrillion",
            1_000_000_000_000_000_000: "Quintillion"
        ];

        string s;

        while (n != 0)
        {
            static ulong currentLevel = 1;
            string m = to!string(n % 1_000);
            string hundreds;
            string tens;

            if (m.length == 3)
            {
                hundreds = dictionary[parse!ulong(m[0 .. 1])] ~ "Hundred";
                m = m[1 .. $];
            }

            if (m.length == 2)
            {
                if (m[0] == '1')
                    tens ~= dictionary[parse!ulong(m[0 .. $])];
                else
                if (m[0] != '0')
                    tens ~= dictionary[parse!ulong(m[0 .. 1]) * 10];

                if (m[1] != '0' && m[0] != '1')
                    tens ~= dictionary[parse!ulong(m[1 .. 2])];
            }
            else
            if (m.length == 1)
                tens ~= dictionary[parse!ulong(m[0 .. $])];

            if (!tens.empty && n >= 1_000)
                hundreds ~= "And";

            s = hundreds ~ tens ~ (hundreds.empty && tens.empty ? "" : levels[currentLevel]) ~ s;
            currentLevel *= 1_000;
            n /= 1_000;
        }

        return s;
    }

    writeln(process(parse!ulong(args[1])));
}