RomanNumerals.java

//initial

import org.junit.Test;

import static org.junit.Assert.assertEquals;

public class RomanNumerals {

    @Test
    public void test1() {
        assertEquals(1, toNumber("I"));
        assertEquals(2, toNumber("II"));
        assertEquals(3, toNumber("III"));
        assertEquals(4, toNumber("IV"));
        assertEquals(5, toNumber("V"));
        assertEquals(6, toNumber("VI"));
        assertEquals(7, toNumber("VII"));
        assertEquals(8, toNumber("VIII"));
        assertEquals(9, toNumber("IX"));
        assertEquals("X", toRoman(10));
        assertEquals(10, toNumber("X"));
        assertEquals("XXI", toRoman(21));
        assertEquals(29, toNumber("XXIX"));
        assertEquals(36, toNumber("XXXVI"));
        assertEquals(49, toNumber("XLIX"));
        assertEquals("XLIX", toRoman(49));
        assertEquals("LXXI", toRoman(71));
        assertEquals("MCMLIV", toRoman(1954));
        assertEquals("CDLXXI", toRoman(471));
        assertEquals("DCLXXI", toRoman(671));
        assertEquals("MDCLXXI", toRoman(1671));
        assertEquals("MCMLIV", toRoman(1954));
        assertEquals(1954, toNumber("MCMLIV"));
        assertEquals("MCMXC", toRoman(1990));
        assertEquals(1990, toNumber("MCMXC"));
        assertEquals(2014, toNumber("MMXIV"));
        assertEquals("MMXIV", toRoman(2014));
        assertEquals(1967, toNumber("MCMLXVII"));
        assertEquals("MCMLXVII", toRoman(1967));
        assertEquals("MMDCLXXI", toRoman(2671));
    }

    private int toNumber(String input) {
        if(input.isEmpty()) return 0;
        final String inputString = input;
        int length = inputString.length();
        int val;
        while((val = fromRoman(inputString.substring(0, length))) == -1) {
            length -= 1;
        }
        String tail = input.substring(length);
        return val + toNumber(tail);
    }

    private String toRoman(int i) {
        if(i <= 10) {
            switch (i) {
                case 1:
                    return "I";
                case 2:
                    return "II";
                case 3:
                    return "III";
                case 4:
                    return "IV";
                case 5:
                    return "V";
                case 6:
                    return "VI";
                case 7:
                    return "VII";
                case 8:
                    return "VIII";
                case 9:
                    return "IX";
                case 10:
                    return "X";
            }
        } else if (i == 40) {
            return "XL";
        } else if (i <= 50) {
            if(i == 50 ) return "L";
            if(i > 40) return toRoman(40) + toRoman(i - 40);
            return leftOrRight(i, "X", 10);
        }else if (i == 90){
            return "XC";
        } else if (i <= 100) {
            if (i == 100) return "C";
            if(i > 90) return toRoman(90) + toRoman(i - 90);
            return toRoman(50) + toRoman(i - 50);
        } else if (i == 400) {
            return "CD";
        } else if (i <= 500) {
            if (i == 500) return "D";
            if(i > 400) return toRoman(400) + toRoman(i - 400);
            return leftOrRight(i, "C", 100);
        } else if (i == 900) {
            return "CM";
        } else if (i <= 1000) {
            if(i == 1000) return "M";
            if(i > 900) return "CM" + toRoman(i - 900);
            return toRoman(500) + toRoman(i - 500);
        }
        return leftOrRight(i, "M", 1000);
    }

    private String leftOrRight(int num, String rom, int eur) {
        int b = num / eur;
        String head = "";
        for(int j = 0; j < b; j++) {
            head += rom;
        }
        int rest = b * eur;
        return head + toRoman(num - rest);
    }

    private int fromRoman(String i) {
        if(i.length() > 4) return -1;
        switch (i) {
            case "I": return 1;
            case "II": return 2;
            case "III": return 3;
            case "IV": return 4;
            case "V": return 5;
            case "VI": return 6;
            case "VII": return 7;
            case "VIII": return 8;
            case "IX": return 9;
            case "X": return 10;
            case "XX": return 20;
            case "XXX": return 30;
            case "XL": return 40;
            case "L": return 50;
            case "XC": return 90;
            case "C": return 100;
            case "CC": return 200;
            case "CCC": return 300;
            case "CD": return 400;
            case "D": return 500;
            case "CM": return 900;
            case "M": return 1000;
            default:
                return -1;
        }
    }
}