Discounts on multiple items free lowest price.

[Tanker]

I did several scenarios using coupons, making a discount of 2x1 was easy I even did it in a way that it applies by it self when criteria is met ...

But then we jump to the 4 x 2, now here is a different story..

Scenario 1

• Item 1 [ Qty 1 ] $ 100 | $100
• Item 2 [ Qty 3 ] $ 90  | $270
  • Discount: $ 180
  • Shipment: $0.0 (Free)
• Total to be pay: $190

Here everything seems just fine it works, and we can play with two items and it works for a discount of 4 x 2

Scenario 2

• Item 1 [ Qty 1 ] $ 100 | $100
• Item 2 [ Qty 1 ] $ 90  | $90
• Item 3 ] Qty 2 ] $60 | $120
  • Discount: $ 120
  • Shipment: $0.0 (Free)
• Total to be pay: $190

But what if Scenario 3

• Item 1 [ Qty 1 ] $ 100 | $100
• Item 2 [ Qty 2 ] $ 90  | $180
• Item 3 ] Qty 1 ] $60 | $60
  • Discount: $ 150
  • Shipment: $0.0 (Free)
• Total to be pay: $220

And there is the problem!

Coupons, 2x1, 4x2 are almost the same it applies to the whole store for selected items, on 2x1 the user must have 2 items, for 4x2 the user must have 4 items, it will discount 100% to the item with the lowest price.

The problem with scenario 3 is that there is only 1 item with the lowest price (item 3 $60), so it should make a discount on that item, but then because the user qualifies for a 4 x 2 the system need to make another discount, so since the item 3 has been discounted then the next lowest price should item 2 $90, but instead it makes another discount of item 3 when that item has been calculated and it should not make another discount of that item because there is only 1 of it... it should look for the next item with the lowest price...

Does anybody else has this type of discounts or problem?

How did you solved it...?
Is there a module that can do this type of discounts? 2x1, 4x2, 6x3, 8x4, 10x5...etc

If there isn't I'm willing to do a core or an overwrite class, I just don't know which class to do my own calculations...

I need to get items, Qty,  prices and categories... and with that make my own calculations and return the discounts made and the total to be pay...

Thank you in advance!