<p>Comma Separated List: <input type="text" pattern="[^,]+((,[^,]+)+)?"></p>
<p><input type="button" value="reverse"></p>
<p><pre id="output"></pre></p>

document.querySelector('input[type="button"]').addEventListener('click', revClick);

function revClick()
{
  let input = document.querySelector('input[type="text"]').value.split(',');
  
  function indexToXY(index, width)
  {
    let x = index % width;
    let y = Math.floor(index / width);
    return [x,y];
  }
  
  function xyToIndex(x, y, width)
  {
    return (x + (y * width));
  }
  
  function reverse(list)
  {
    let width = Math.ceil(Math.sqrt(list.length));
    let rev = list.reduce((oup, item, index) => 
    {
      let [x, y] = indexToXY(index, width);
      let pole = Math.floor(width/2);
      let polex = x - pole;
      let poley = y - pole;
      let r = Math.sqrt((polex*polex)+(poley*poley));
      let fi = Math.atan2(poley, polex);
      fi += Math.PI;
      let newx = Math.round((Math.cos(fi)*r)+pole);
      let newy = Math.round((Math.sin(fi)*r)+pole);
      let newIndex = xyToIndex(newx, newy, width);
      oup[newIndex] = item;
      return oup;
    }, []);
    
    return rev;
  }
  
  document.getElementById('output').innerHTML = reverse(input).filter(x => x).join(',');
}