Prison After N Days

let  prisonAfterNDays = function(cells, N) {

    let cache = {},
        counter = 1,
        cycle = false,
        len = 8;
    let dup = [...cells];

    function getNextCycle(data) {
        let lastCycle = data[1];
        let cycleDays = counter - lastCycle;
        console.log(lastCycle, cycleDays);
        let nextCycle = N - ((N - lastCycle) % cycleDays); //?.
        // let nextCycle = counter; //?
        // while (nextCycle + counter < N) {
        //     nextCycle = nextCycle += cycleDays;
        // }

        return nextCycle; //?
    }

    while(counter <= N) {
        let key = dup.join(''),
            data = cache[key],
            row = [];

        if (data && !cycle) {
            cycle = true;
            counter = getNextCycle(data);
        }

        if (data) {
            dup = data[0];
            counter++; //?
        }
        else {
            for (let i = 0; i < len; i++) {
                row[i] = dup[i - 1] === dup[i + 1] ? 1 : 0;
            }
            cache[key] = [[...row], counter];
            dup = cache[key][0];
            counter++; //?
        }
    }

    return dup;
};