var Type = {
	"STANDALONE": 0,
	"TEN": 1,
	"MULTIPLE": 2
};

var standalones = {
	"zero": 0,
	"one": 1,
	"two": 2,
	"three": 3,
	"four": 4,
	"five": 5,
	"six": 6,
	"seven": 7,
	"eight": 8,
	"nine": 9,
	"ten": 10,
	"eleven": 11,
	"twelve": 12,
	"thirteen": 13,
	"fourteen": 14,
	"fifteen": 15,
	"sixteen": 16,
	"seventeen": 17,
	"eighteen": 18,
	"nineteen": 19
};

var tens = {
	"twenty": 20,
	"thirty": 30,
	"forty": 40,
	"fifty": 50,
	"sixty": 60,
	"seventy": 70,
	"eighty": 80,
	"ninety": 90
}

var multiples = {
	"hundred": 100,
	"thousand": 1000,
	"million": 1000000,
	"billion": 1000000000
};

var test = 'forty five million eight hundred thirty one thousand';

var textToInt = function(txt) {
	var mult = 1;
	var pieces = txt.split(' ');
	if (pieces.length > 0 && pieces[0] == 'negative')
	{
		mult = -1;
		pieces.splice(0, 1);
	}
	var tokens = []
	for (var i = 0; i < pieces.length; i++)
	{
		var type = -1, p = pieces[i];
		if (standalones[p] != undefined)
		{
			type = Type.STANDALONE;
		} else if (tens[p] != undefined) {
			type = Type.TEN;
		} else if (multiples[p] != undefined) {
			type = Type.MULTIPLE;
		} else {
			throw 'Invalid number';
		}
		tokens.push(type);
	}
	var number = 0;
	var s = 0;
	for (var i = 0; i < pieces.length; i++)
	{
		var p = pieces[i];
		switch (tokens[i])
		{
			case Type.STANDALONE:
				s += standalones[p];
				break;
			case Type.TEN:
				s += tens[p];
				break;
			case Type.MULTIPLE:
				var add = true;
				var m = multiples[p];
				for (var j = i+1; j < pieces.length; j++)
				{
					if (tokens[j] == Type.MULTIPLE && multiples[pieces[j]] > m)
					{
						add = false;
						break;
					}
				}
				if (add)
				{
					number += s * m;
					s = 0;
				} else {
					s *= m;
				}
				break;
		}
	}
	number += s;
	return number * mult;
};

var intToText = function(num)
{
	txt = '';
	if (num < 0)
	{
		txt = 'negative ';
		num = num * -1;
	}
	return txt + intToTextHelper(Math.floor(num));
};

var intToTextHelper = function(num)
{
	if (num < 20)
	{
		for (var s in standalones)
		{
			if (standalones[s] == num)
			{
				return s;
			}
		}
	}

	if (num < 100)
	{
		var r = num % 10, z;
		num -= r;
		for (var t in tens)
		{
			if (tens[t] == num)
			{
				return t + (((z = intToTextHelper(r)) != 'zero') ? ' ' + z : '');
			}
		}
	}

	var prev = 1, prev_txt = '';
	for (var m in multiples)
	{
		if (Math.floor(num / multiples[m]) == 0)
		{
			break;
		}
		prev = multiples[m]; prev_txt = m;
	}
	var divisor = Math.floor(num / prev);
	var remainder = num % prev, z;
	return intToTextHelper(divisor) + ' ' + prev_txt + (((z = intToTextHelper(remainder)) != 'zero') ? ' ' + z : '');
};

var operators = [
	'plus',
	'minus',
	'times',
	'divided by'
];

var parseSentence = function(sentence)
{
	sentence = sentence.toLowerCase();
	var op = 'none', index;
	for (var i = 0; i < operators.length; i++)
	{
		if ((index = sentence.indexOf(operators[i])) != -1)
		{
			op = operators[i];
			break;
		}
	}
	if (op == 'none') { return 'no operation.'; }
	var left = textToInt(sentence.substring(0, index-1)),
		right = textToInt(sentence.substring(index + op.length + 1));
	var res;
	switch (op)
	{
		case 'plus':
			res = left + right;
			break;
		case 'minus':
			res = left - right;
			break;
		case 'times':
			res = left * right;
			break;
		case 'divided by':
			res = left / right;
			break;
		default:
			throw 'what';
	}
	return intToText(res);
};