var shellSort = function(arr) {
  var n = arr.length;

  for (var gap = n / 2 | 0; gap > 0; gap = gap / 2 | 0) {
    for (var groupIndex = 0; groupIndex < gap; groupIndex++) {
      for (var currIndex = groupIndex + gap; currIndex < n; currIndex += gap) {
        var cur = arr[currIndex];
        var pre = currIndex - gap;

        while (pre >= groupIndex && cur < arr[pre]) {
            arr[pre + gap] = arr[pre];
            pre -= gap;
        }

        arr[pre + gap] = cur;
      }
    }
  }
};