Last Updated: September 18, 2020
·
1.989K
· graup

Infinite sliding windows in Javascript

I needed to code a slideshow that should infinitely repeat items when sliding to the left and right. The slide show would always show n slides at the same time. To me that sounded a lot like a sliding window algorithm, only with an added feature of infinitely repeating.

As I needed to have this in Angular.js, I created a simple Javascript function to achieve this "infinite sliding window" algorithm as I call it.

/**
 * Returns a sliding window over data
 * with lenth len, starting from cur
 * repeating data infinitely to fit the window
 */
function infiniteSlidingWindow(data, cur, len) {
    var win = [],
        num = data.length,
        numVisible = len;
    // A negative cur is the same as num - abs(cur)
    if (cur < 0) {
        cur = num + cur;
    }
    // Now keep adding items until we have enough
    while (win.length < numVisible) {
        var first = (win.length?0:Math.abs(cur)%num),
            missing = numVisible - win.length,
            last = Math.min(first + missing, num);
        win = win.concat(data.slice(first, last));
    }
    return win;
}

I actually used a slight modification as you don't want to have duplicates when using ng-repeat.

/**
 * Use this slight modification if you need to avoid duplicate items.
 * This keeps the window fixed when there are not enough items to fill it.
 */
function infiniteSlidingWindowNoRepeats(data, cur, len) {
    if (data.length < len) {
        len = data.length;
        cur = 0;
    }
    return infiniteSlidingWindow(data, cur, len);
}

Here are some tests to throw in your console or Node.js interpreter, showing the expected behavior.

/* Tests */
var data = [0,1,2,3,4,5,6,7,8,9];
var result;

result = infiniteSlidingWindow(data, 0, 3);
console.log([0,1,2].join() == result.join());

result = infiniteSlidingWindow(data, 1, 3);
console.log([1,2,3].join() == result.join());

result = infiniteSlidingWindow(data, -1, 3);
console.log([9,0,1].join() == result.join());

result = infiniteSlidingWindow(data, -11, 5);
console.log([1,2,3,4,5].join() == result.join());

result = infiniteSlidingWindow(data, 2, 10);
console.log([2,3,4,5,6,7,8,9,0,1].join() == result.join());

data = [0,1,2,3];

result = infiniteSlidingWindow(data, 0, 5);
console.log([0,1,2,3,0].join() == result.join());

result = infiniteSlidingWindow(data, 2, 5);
console.log([2,3,0,1,2].join() == result.join());

result = infiniteSlidingWindowNoRepeats(data, 2, 5);
console.log(data.join() == result.join());

Have fun with that!

1 Response
Add your response

Nice approach!
How would you support a finite sliding window as well?

BTW, i think you got a bug when you check the negative curr, i think it should be:
if (curr < 0) { curr = num + (curr % num); }

over 1 year ago ·