large number modulus fix

mod

function mod(dividend, divisor) {
    var sDividend = dividend + '';
    var split = sDivisor.length + 1;

    // if dividend < 9007199254740992 (2^53)
    if (sDividend.length < 16) {
        return dividend % divisor;
    }

    // 'abcdefghijklmnopqrstuvwxyz' => ["qrstuvwxyz", "ghijklmnop", "abcdef"]
    var l = sDividend.length, s = [];
    while (l > 0) {
        if (l >= split) {
            s.push(sDividend.slice(-split));
            sDividend = sDividend.substring(0, sDividend.length - split);
        } else {
            s.push(sDividend);
        }
        l -= split;
    }

    // (a + b) % p = (a % p + b % p) % p
    var ret = 0;
    for (var i in s) {
        var b = s[i], t = Math.pow(10, i * split);

        if (!parseInt(b, 10)) continue;

        // (a * b) % p = (a % p * b % p) % p
        ret += mod(
            mod(b, divisor) *
                (t === 1 ? 1 : mod(t, divisor)),
            divisor
        );
    }
    return mod(ret, divisor);
}