buy 1 get 1 from same category half price off

Need the ability so that if user buys 1 item another item from the same category only (of equal or lesser value) will be 50% off automatically in the shopping cart. it would work the same with 2 items, they could add 2 more items and get 50% off each. and 3,4,5, etc etc. if the user has 4 products in the cart the 2 highest priced items should be used as the benchmark for the 50% off Example: item 1: $600 item 2: $150 item 3: $200 item 4: $300 item 2 and 3 will be the ones given the discount. which category it applies to should be able to be picked in the back end. as well as the percentage amount. The discount must be clearly shown in the shopping cart pages. I will give you access to a sandbox webpage upon being chosen. Please ask as many questions before hand. The questions you ask are the best way for me to see that you understand. Thank you.