Cosmic Number

Main.java

import java.util.*;

/**
 * Cosmic Number
 * License: Unlicensed
 **/
public class Main {
    private static final int BILLION  = 1000000000;
    private static final int MILLION  =    1000000;
    private static final int THOUSAND =       1000;
    private static final int HUNDRED  =        100;
    private static final int TEN      =         10;
    private static final int ONE      =          1;

    public static void main(String[] args) {
        final Scanner scanner = new Scanner(System.in);
        int input = scanner.nextInt();
        scanner.nextLine();    
        while (true) {
            int last = 0;
            for (final String term : numberToStr(input))
                last += term.length();
            if (input == last) break;
            System.out.format("%d is %d, ", input, last);
            input = last;
        }
        System.out.format("%d is cosmic\n", input);
    }

    private static List<String> numberToStr(final int input) {
        int value = input;
        List<String> terms = new ArrayList(5);
        if (value >= BILLION) {
            final int billions = value / BILLION;
            value -= billions * BILLION;
            terms.addAll(numberToStr(billions));
            terms.add("billion");
        }
        if (value >= MILLION) {
            final int millions = value / MILLION;
            value -= millions * MILLION;
            terms.addAll(numberToStr(millions));
            terms.add("million");
        }
        if (value >= THOUSAND) {
            final int thousands = value / THOUSAND;
            value -= thousands * THOUSAND;
            terms.addAll(numberToStr(thousands));
            terms.add("thousand");
        }
        if (value >= HUNDRED) {
            final int hundreds = value / HUNDRED;
            value -= hundreds * HUNDRED;
            terms.addAll(numberToStr(hundreds));
            terms.add("hundred");
        }
        if (value >= TEN) {
            final int tens = value / TEN;
            value -= tens * TEN;
            switch (tens) {
                case 0:
                    break;
                case 1:
                    // Extract right now.
                    final Pair<Integer, Integer> oneExtract = extractOnes(value);
                    value = oneExtract.first;
                    final int v = Integer.parseInt(new StringBuilder().append(tens /* 1 */).append(oneExtract.second).toString());
                    switch (v) {
                        case 10:
                            terms.add("ten");
                            break;
                        case 11:
                            terms.add("eleven");
                            break;
                        case 12:
                            terms.add("twelve");
                            break;
                        case 13:
                            terms.add("thirteen");
                            break;
                        case 14:
                            terms.add("fourteen");
                            break;
                        case 15:
                            terms.add("fifteen");
                            break;
                        case 16:
                            terms.add("sixteen");
                            break;
                        case 17:
                            terms.add("seventeen");
                            break;
                        case 18:
                            terms.add("eighteen");
                            break;
                        case 19:
                            terms.add("nineteen");
                            break;
                        default:
                            throw new IllegalStateException("Unknown 10 ~ 19 value " + v);
                    }
                    break;
                case 2:
                    terms.add("twenty");
                    break;
                case 3:
                    terms.add("thirty");
                    break;
                case 4:
                    terms.add("forty");
                    break;
                case 5:
                    terms.add("fifty");
                    break;
                case 6:
                    terms.add("sixty");
                    break;
                case 7:
                    terms.add("seventy");
                    break;
                case 8:
                    terms.add("eighty");
                    break;
                case 9:
                    terms.add("ninety");
                    break;
                default:
                    throw new IllegalStateException("Unknown tens " + tens);
            }
        }

        
        final Pair<Integer, Integer> oneExtract = extractOnes(value);
        value = oneExtract.first;
        switch (oneExtract.second) {
            case 0:
                break;
            case 1:
                terms.add("one");
                break;
            case 2:
                terms.add("two");
                break;
            case 3:
                terms.add("three");
                break;
            case 4:
                terms.add("four");
                break;
            case 5:
                terms.add("five");
                break;
            case 6:
                terms.add("six");
                break;
            case 7:
                terms.add("seven");
                break;
            case 8:
                terms.add("eight");
                break;
            case 9:
                terms.add("nine");
                break;
            default:
                throw new IllegalStateException("Unknown digit " + oneExtract.second);
        }
        if (value != 0) throw new IllegalStateException("Unexpected left out");
        
        return terms;
    }

    private static Pair<Integer, Integer> extractOnes(int value) {
        int ones = 0;
        if (value >= ONE) {
            ones = value / 1;
            value -= ones * 1;
        }
        return new Pair(value, ones); 
    }

    private static class Pair<A, B> {
        public final A first;
        public final B second;
        public Pair(A first, B second) {
            this.first = first;
            this.second = second;
        }
    }
}