modern_roman_numerals.js

var assert = require("assert")

var arabic_for_roman = function(roman) {
    var arabic_value_for = {
	"I" : 1,
	"V" : 5,
	"X" : 10,
	"L" : 50,
	"C" : 100,
	"D" : 500,
	"M" : 1000,
    };

    var arabic = 0;

    for (var i = 0; i < roman.length; i++) {
        var current = arabic_value_for[roman[i]];
        var next    = arabic_value_for[roman[i+1]];
        if (next > current) {
            arabic -= current;
        } else {
            arabic += current;
        }
    }

    return arabic;
}

var expected_value = {
    "I"      : 1,
    "II"     : 2,
    "III"    : 3,
    "IV"     : 4,
    "V"      : 5,
    "VI"     : 6,
    "IX"     : 9,
    "X"      : 10,
    "XV"     : 15,
    "XX"     : 20,
    "XXX"    : 30,
    "L"      : 50,
    "LXVI"   : 66,
    "C"      : 100,
    "D"      : 500,
    "CMXLVI" : 946,
    "M"      : 1000,
}

describe('arabic_for_roman converter', function() {
    Object.keys(expected_value).forEach(function (key) {
	it('should return ' + expected_value[key] + ' for ' + key, function() {
	    assert.equal(expected_value[key], arabic_for_roman(key))
	})
    })
})

// Now depending on the rules you go by, these may be treated as errors.
// This implementation will however work out reasonable values for them.
// {
//     "VV"        : 10,
//     "LL"        : 100,
//     "DDD"       : 1500,
//     "CMDCCXLVI" : 1646,
// }