pick_banner.js

ads = [
  {width: 1000, height:500},
  {width: 200, height:200},
  {width: 600, height:750},
  {width: 600, height:600}
];

screen_ratio = window.document.width / window.document.height;
minimum_area = window.document.width * window.document.height * 0.8;

ads.sort(function(a, b) {
  var a_ratio_to_screen = Math.round(Math.abs(a.width / a.height - screen_ratio) * 100)/100,
    b_ratio_to_screen = Math.round(Math.abs(b.width / b.height - screen_ratio) * 100)/100,
    a_area = a.width * a.height,
    b_area = b.width * b.height;

  // determine the best fit and size

  // if they are both bigger than minimum area
  if(a_area > minimum_area && b_area > minimum_area) {
    // pick the one with best proportions
    return a_ratio_to_screen < b_ratio_to_screen ? 1 : -1;
  } else {
    // pick the bigger ad first
    return a_area < b_area ? 1 : -1;
  }
});