Conditional Synapse Embed

maybe_on.js

<script type="text/javascript">
var onswipeSometimes = function() {
  var usr = "f";
  var fractionToRedirect = '3/4';
  var parts = fractionToRedirect.split('/');
  var total = parseInt(parts[1]);
  var redirect = parseInt(parts[0]);
  var randomNumber = Math.floor(Math.random()*total + 1); // Number between 1 and total inclusive
  if (randomNumber <= redirect)  {
    var synapse = 'http://assets.onswipe.com/synapse/on.js?usr=' + usr;
    var embed = document.createElement('script');
    embed.setAttribute('type', 'text/javascript');
    embed.setAttribute('id', 'onswipe_synapse');
    embed.setAttribute('src', synapse);
    var parent = document.getElementsByTagName('head')[0];
    var first = parent.firstChild;
    parent.appendChild(embed, first);
  }
}();
</script>