Fletcher algorithm of controll summ

gistfile1.txt

function calcFletcherSum(data) {
        var length = data.length, i = 0, sum1 = 0xFF, sum2 = 0xFF;
        while (length) {
            var tlen = length > 21 ? 21 : length;
            length -= tlen;
            do {
                var ch = data.charCodeAt(i++);
                if (ch > 255) {
                    var ch2 = ch >> 8;
                    ch &= 0xFF;
                    ch ^= ch2;
                }
                sum1 += ch;
                sum2 += sum1;
            } while (--tlen);
            sum1 = (sum1 & 0xFF) + (sum1 >> 8);
            sum2 = (sum2 & 0xFF) + (sum2 >> 8);
        }
        var result = (((sum1 & 0xFF) + (sum1 >> 8)) << 8) | ((sum2 & 0xFF) + (sum2 >> 8));
        return result == 0xFFFF ? 0 : result;
    }