function encode(input) {
    var encoding = [];
    var prev, count, i;
    for (count = 1, prev = input[0], i = 1; i < input.length; i++) {
        if (input[i] != prev) {
            encoding.push([count, prev]);
            count = 1;
            prev = input[i];
        }
        else 
            count ++;
    }
    encoding.push([count, prev]);
    return encoding;
}