Super Scanner
Back to the Supermarket. We'll implement the code for a checkout system that handles pricing schemes such as "apples cost 50 cents, three apples cost $1.30".
Step 1
Let's model the various options for supermarket pricing.
Some things in supermarkets have simple prices: this can of beans costs $0.65. Other things have more complex prices. For example:
- three for a dollar (so what's the price if I buy 4, or 5?)
- $1.99/pound (so what does 4 ounces cost?)
- buy two, get one free (so does the third item have a price?)
Bonus
To make it better, you can consider things like:
- Start Date / End Date of an event
- How to keep an audit trail of pricing decisions?
Step 2
We'll have to implement the code for a supermarket checkout that calculates the
total price of a number of items. In a normal supermarket, things are identified
using Stock Keeping Units, or SKUs. In our store, we'll use individual letters
of the alphabet ( A , B , C , ...). Our goods are priced individually.
In addition, some items are multipriced: buy n of them, and they'll cost you y
cents. For example, item A might cost 50 cents individually, but this week
we have a special offer: buy three As and they'll cost you $1.30.
| Item | Unit Price | Special Price |
|---|---|---|
| A | 50 | 3 for 130 |
| B | 30 | 2 for 45 |
| C | 20 | |
| D | 15 |
Our checkout accepts items in any order, so that if we scan a B , an A, and
another B ,we'll recognize the two Bs and price them at 45 (for a total
price so far of 95). Because the pricing changes frequently, we need to be
able to pass in a set of pricing rules each time we start handling a checkout
transaction.
The interface to the checkout should look like:
co = CheckOut.new(pricing_rules)
co.scan(item)
co.scan(item)
...
price = co.total