module.exports = parse;

//following http://www.w3.org/TR/css3-selectors/#nth-child-pseudo

//[ ['-'|'+']? INTEGER? {N} [ S* ['-'|'+'] S* INTEGER ]?
var re_nthElement = /^([+\-]?\d*n)?\s*(?:([+\-]?)\s*(\d+))?$/;

/*
	parses a nth-check formula, returns an array of two numbers
*/
function parse(formula) {
    formula = formula.trim().toLowerCase();

    if (formula === "even") {
        return [2, 0];
    } else if (formula === "odd") {
        return [2, 1];
    } else {
        var parsed = formula.match(re_nthElement);

        if (!parsed) {
            throw new SyntaxError(
                "n-th rule couldn't be parsed ('" + formula + "')"
            );
        }

        var a;

        if (parsed[1]) {
            a = parseInt(parsed[1], 10);
            if (isNaN(a)) {
                if (parsed[1].charAt(0) === "-") a = -1;
                else a = 1;
            }
        } else a = 0;

        return [a, parsed[3] ? parseInt((parsed[2] || "") + parsed[3], 10) : 0];
    }
}