WANTED: Qty discount by category by table

I'm looking for a contribution which will allow a shopper to buy multiple products in various quantities and receive a quantity discount with several levels based on the total number of items purchased from that category.

For example:

Category "Shirts"

Products - "Design 1" "Design 2" "Design 3" etc.

Rule: 20% discount when purchasing more than 10 total shirts (any quantity of any design)
25% if purchasing more than 15 total shirts (any quantity of any design)

I have already seen:

http://www.zen-cart.com/modules/mydownloads/singlefile.php?lid=296

"A drop in quantity discount order_total module. It allows you to specify the discount percentage or flat rate based on the number of items bought. This is not products_id based and is site-wide. Any combination of items will work as it runs of the total number of items in the cart."

Which doesn't meet my needs because the discounts in this contrib are calculated based on the total number of items in the cart. Because I have several categories (which need their own discount tables) the total number of items in the cart could be from more than one category.

Is there anyone who can help me develop this contribution?

Well, I've finished the coding and testing for this extension.

It involves creating two files, adding a require() statement and two function calls into /includes/classes/shopping_cart.php, and creating a table in the database to hold the discount data.

I haven't created an administration GUI for it at this time.