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# Book Store

To try and encourage more sales of different books from a popular 5 book
series, a bookshop has decided to offer discounts on multiple book purchases.

One copy of any of the five books costs $8.

If, however, you buy two different books, you get a 5%
discount on those two books.

If you buy 3 different books, you get a 10% discount.

If you buy 4 different books, you get a 20% discount.

If you buy all 5, you get a 25% discount.

Note: that if you buy four books, of which 3 are
different titles, you get a 10% discount on the 3 that
form part of a set, but the fourth book still costs $8.

Your mission is to write a piece of code to calculate the
price of any conceivable shopping basket (containing only
books of the same series), giving as big a discount as
possible.

For example, how much does this basket of books cost?

- 2 copies of the first book
- 2 copies of the second book
- 2 copies of the third book
- 1 copy of the fourth book
- 1 copy of the fifth book

One way of grouping these 8 books is:

- 1 group of 5 --> 25% discount (1st,2nd,3rd,4th,5th)
- +1 group of 3 --> 10% discount (1st,2nd,3rd)

This would give a total of:

- 5 books at a 25% discount
- +3 books at a 10% discount

Resulting in:

- 5 x (8 - 2.00) == 5 x 6.00 == $30.00
- +3 x (8 - 0.80) == 3 x 7.20 == $21.60

For a total of $51.60

However, a different way to group these 8 books is:

- 1 group of 4 books --> 20% discount  (1st,2nd,3rd,4th)
- +1 group of 4 books --> 20% discount  (1st,2nd,3rd,5th)

This would give a total of:

- 4 books at a 20% discount
- +4 books at a 20% discount

Resulting in:

- 4 x (8 - 1.60) == 4 x 6.40 == $25.60
- +4 x (8 - 1.60) == 4 x 6.40 == $25.60

For a total of $51.20

And $51.20 is the price with the biggest discount.

## Implementation

Define a single Go func, Cost, which calculates the cost
for a given list of books based on the defined discounts.

Use the following signature for func Cost:

```
func Cost(books []int) int
```
Cost will return the total cost (after discounts) in cents.
For example, for a single book, the cost is 800 cents, which equals $8.00.
Only integer calculations are necessary for this exercise.



## Running the tests

To run the tests run the command `go test` from within the exercise directory.

If the test suite contains benchmarks, you can run these with the `--bench` and `--benchmem`
flags:

    go test -v --bench . --benchmem

Keep in mind that each reviewer will run benchmarks on a different machine, with
different specs, so the results from these benchmark tests may vary.

## Further information

For more detailed information about the Go track, including how to get help if
you're having trouble, please visit the exercism.io [Go language page](http://exercism.io/languages/go/resources).

## Source

Inspired by the harry potter kata from Cyber-Dojo. [http://cyber-dojo.org](http://cyber-dojo.org)

## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.