Magento – SKU Auto Generator with Prefix and sufix

I am trying to create a code that generated:

1) a random prefix sku with four digits(3 integers and 1 char) that depends on the category (on the last sub category) so for all product that are in the same sub-category the prefix should be the same!

2) a random suffix three digits (it could be only integers or 2 integers 1 char but no only char's)

3) between the prefix and suffix the should be a – (minus)

I already have a code that creates a the next sku, that Increment the SKU of the last edit product

<?php
$dbread = Mage::getSingleton('core/resource')->getConnection('core_read');
$sql = $dbread->query("SELECT * FROM catalog_product_entity ORDER BY created_at DESC LIMIT 1");
$res = $sql->fetch();
  ?>
 <script type="text/javascript">
   if(document.getElementById('sku').value == ""){
 document.getElementById('sku').value = <?php echo (int)$res["sku"] + 1; ?>;
}
 </script>

Hope you could help me

EDIT

SO that is the category and if I add a product and Select X more then one category, then it should take only the prefix from the last selected category, the last one is Men

Example
root
    Clothing & Shoes   x                      Prefix: 2088-
                    Clothing    x             Prefix: 2058-
                            Women
                            Men    x          Prefix: 2002-
                    Shoes

maybe we could take th category id number + 2 digits more if there is only one or two digits only