module NumberNames {
    void run() {
        @Inject Console console;

        Int[] tests = [0, 1, -1, 11, -17, 42, 99, 100, 101, -111, 1000, 1234, 10000, 100000,
                       123456789000, 0x123456789ABCDEF];
        for (Int test : tests) {
            console.print($"{test} = {toEnglish(test)}");
        }
    }

    static String[] digits = ["zero", "one", "two", "three", "four",
                              "five", "six", "seven", "eight", "nine"];
    static String[] teens  = ["ten", "eleven", "twelve", "thirteen", "fourteen",
                              "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"];
    static String[] tens   = ["zero", "ten", "twenty", "thirty", "forty",
                              "fifty", "sixty", "seventy", "eighty", "ninety"];
    static String[] ten3rd = ["?", "thousand", "million", "billion", "trillion",
                              "quadrillion", "quintillion"];

    static String toEnglish(Int n) {
        StringBuffer buf = new StringBuffer();
        if (n < 0) {
            "negative ".appendTo(buf);
            n = -n;
        }

        format3digits(n, buf);
        return buf.toString();
    }

    static void format3digits(Int n, StringBuffer buf, Int nested=0) {
        (Int left, Int right) = n /% 1000;
        if (left != 0) {
            format3digits(left, buf, nested+1);
        }

        if (right != 0 || (left == 0 && nested==0)) {
            if (right >= 100) {
                (left, right) = (right /% 100);
                digits[left].appendTo(buf);
                " hundred ".appendTo(buf);
                if (right != 0) {
                    format2digits(right, buf);
                }
            } else {
                format2digits(right, buf);
            }

            if (nested > 0) {
                ten3rd[nested].appendTo(buf).add(' ');
            }
        }
    }

    static void format2digits(Int n, StringBuffer buf) {
        switch (n) {
        case 0..9:
            digits[n].appendTo(buf).add(' ');
            break;

        case 10..19:
            teens[n-10].appendTo(buf).add(' ');
            break;

        default:
            (Int left, Int right) = n /% 10;
            tens[left].appendTo(buf);
            if (right == 0) {
                buf.add(' ');
            } else {
                buf.add('-');
                digits[right].appendTo(buf).add(' ');
            }
            break;
        }
    }
}