Cabify Mobile Challenge

Besides providing exceptional transportation services, Cabify also runs a physical store which sells (only) 3 products:

Code         | Name                |  Price
-------------------------------------------------
VOUCHER      | Cabify Voucher      |   5.00€
TSHIRT       | Cabify T-Shirt      |  20.00€
MUG          | Cafify Coffee Mug   |   7.50€

Various departments have insisted on the following discounts:

• The marketing department believes in 2-for-1 promotions (buy two of the same product, get one free), and would like for there to be a 2-for-1 special on VOUCHER items.

• The CFO insists that the best way to increase sales is with discounts on bulk purchases (buying x or more of a product, the price of that product is reduced), and demands that if you buy 3 or more TSHIRT items, the price per unit should be 19.00€.

Cabify's checkout process allows for items to be scanned in any order, and should return the total amount to be paid. The interface for the checkout process looks like this (ruby):

co = Checkout.new(pricing_rules)
co.scan("VOUCHER")
co.scan("VOUCHER")
co.scan("TSHIRT")
price = co.total

Using ruby (>= 2.0), implement a checkout process that fulfills the requirements.

Examples:

Items: VOUCHER, TSHIRT, MUG
Total: 32.50€

Items: VOUCHER, TSHIRT, VOUCHER
Total: 25.00€

Items: TSHIRT, TSHIRT, TSHIRT, VOUCHER, TSHIRT
Total: 81.00€

Items: VOUCHER, TSHIRT, VOUCHER, VOUCHER, MUG, TSHIRT, TSHIRT
Total: 74.50€

The code should:

• Be written as production-ready code. You will write production code.
• Be easy to grow and easy to add new functionality.
• Have notes attached, explaining the solution and why certain things are included and others are left out.