Table Auto Scroll do not scroll to the end of the table! Scroll table from top to Bottom, then Back Up (and repeat)

[macano333]

1. I have a code that on button press runs autoscroll on a single table and when I use the autoscroll speed of 2500, the autoscroll works as it should to the end of the table, but when I change the speed to let’s say 11500 autoscroll works but not to the end of the table, 70% of the board just does autoscroll, so what should I need to change in the code to fix this error?
2. The same code and same table but I need an option to Scroll Page from Top to Bottom, then Back Up (and Repeat), please check the code of my autoscroll option and what should I change or insert to enable this option?

ini_set('MAX_EXECUTION_TIME', -1);

     define('SRT_STATE_SUBNUMBER', 0);
     define('SRT_STATE_TIME',      1);
     define('SRT_STATE_TEXT',      2);
     define('SRT_STATE_BLANK',     3);

     $lines   = file("uploads/".$file_name);

     $subs    = array();
     $state   = SRT_STATE_SUBNUMBER;
     $subNum  = 0;
     $subText = '';
     $subTime = '';

     foreach($lines as $line) {
         switch($state) {
             case SRT_STATE_SUBNUMBER:
                 $subNum = trim($line);
                 $state  = SRT_STATE_TIME;
                 break;

             case SRT_STATE_TIME:
                 $subTime = trim($line);
                 $state   = SRT_STATE_TEXT;
                 break;

             case SRT_STATE_TEXT:
                 if (trim($line) == '') {
                     $sub = new stdClass;
                     $sub->number = $subNum;
                     list($sub->startTime, $sub->stopTime) = explode(' --> ', $subTime);
                     $sub->text   = $subText;
                     $subText     = '';
                     $state       = SRT_STATE_SUBNUMBER;

                     $subs[]      = $sub;
                 } else {
                     $subText .= $line;
                 }
                 break;
         }
     }

     if ($state == SRT_STATE_TEXT) {
         // if file was missing the trailing newlines, we'll be in this
         // state here.  Append the last read text and add the last sub.
         $sub->text = $subText;
         $subs[] = $sub;
     }
     ?>

      document.getElementById('table').style.display = 'block';
      }
      function autoScroll(){
         window.scrollBy(0,400)
 var objDiv = document.getElementById("table");
smooth_scroll_to(objDiv, objDiv.scrollHeight, 15900);
      }

      var smooth_scroll_to = function(element, target, duration) {
target = Math.round(target);
duration = Math.round(duration);
if (duration < 0) {
    return Promise.reject("bad duration");
}
if (duration === 0) {
    element.scrollTop = target;
    return Promise.resolve();
}

var start_time = Date.now();
var end_time = start_time + duration;

var start_top = element.scrollTop;
var distance = target - start_top;

var smooth_step = function(start, end, point) {
    if(point <= start) { return 0; }
    if(point >= end) { return 1; }
    var x = (point - start) / (end - start); 
    return x*x*(3 - 2*x);
}

return new Promise(function(resolve, reject) {
    var previous_top = element.scrollTop;

    var scroll_frame = function() {
        if(element.scrollTop != previous_top) {
            reject("interrupted");
            return;
        }

        var now = Date.now();
        var point = smooth_step(start_time, end_time, now);
        var frameTop = Math.round(start_top + (distance * point));
        element.scrollTop = frameTop;

        if(now >= end_time) {
            resolve();
            return;
        }

        if(element.scrollTop === previous_top
            && element.scrollTop !== frameTop) {
            resolve();
            return;
        }
        previous_top = element.scrollTop;
setTimeout(scroll_frame, 0); } setTimeout(scroll_frame, 0); }); }