15 puzzle in javascript

http://www.geocities.jp/teruakigemma/nikoniko/puzzle.html
used prototype.js.

var puzzle;
var zeropos;

function swap_puzzle(x,flag) {
  var tmp = puzzle[x];
  puzzle[x] = puzzle[zeropos];
  puzzle[zeropos] = tmp;
  if (flag) {
    $('cell' + zeropos).src = puzzle[zeropos] + '.png';
    $('cell' + x).src = puzzle[x] + '.png';
    if ($A($R(0,15)).all(function(x) {return x == puzzle[x]}))
      $('main').appendChild(new Element('span').update('nicely done!'));
  };
  zeropos = x;
}

function shuffle() {
  for (var i = 0; i < 5; i++) {
    var a = [];
    if (Math.floor(zeropos / 4) != 0) a.push(zeropos - 4);
    if (Math.floor(zeropos / 4) != 3) a.push(zeropos + 4);
    if (zeropos % 4 != 0) a.push(zeropos - 1);
    if (zeropos % 4 != 3) a.push(zeropos + 1);
    swap_puzzle(a[Math.floor(Math.random() * a.length)],false);
  };
}

function click_cell(x) {
  if(x == zeropos - 4) swap_puzzle(zeropos - 4, true);
  if(x == zeropos + 4) swap_puzzle(zeropos + 4, true);
  if(x == zeropos - 1) swap_puzzle(zeropos - 1, true);
  if(x == zeropos + 1) swap_puzzle(zeropos + 1, true);
}

function main() {
  puzzle = $A($R(0,15));
  zeropos = 0;
  shuffle();
  var e = $('main');
  while (e.firstChild) e.removeChild(e.firstChild);
  $A($R(0,15)).inGroupsOf(4).each(function(x) {
      x.each(function(y) {
   var img = new Element('img',{id:'cell' + y,src:puzzle[y] + '.png',width:64,height:64});
   img.observe('click',function () {click_cell(y)});
   e.appendChild(img)});
      e.appendChild(new Element('br'))});
}

window.onload = main;