 # cjavdev's solution

## to Book Store in the Ruby Track

Published at Mar 10 2019 · 0 comments
Instructions
Test suite
Solution

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.

For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

``````gem install minitest
``````

If you would like color output, you can `require 'minitest/pride'` in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

``````ruby book_store_test.rb
``````

To include color from the command line:

``````ruby -r minitest/pride book_store_test.rb
``````

## Source

Inspired by the harry potter kata from Cyber-Dojo. 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.

### book_store_test.rb

``````require 'minitest/autorun'
require_relative 'book_store'

# Common test data version: 1.4.0 33c6b60
class BookStoreTest < Minitest::Test
def test_only_a_single_book
# skip
end

def test_two_of_the_same_book
skip
end

skip
end

def test_two_different_books
skip
end

def test_three_different_books
skip
end

def test_four_different_books
skip
basket = [1, 2, 3, 4]
end

def test_five_different_books
skip
basket = [1, 2, 3, 4, 5]
end

def test_two_groups_of_four_is_cheaper_than_group_of_five_plus_group_of_three
skip
basket = [1, 1, 2, 2, 3, 3, 4, 5]
end

def test_two_groups_of_four_is_cheaper_than_groups_of_five_and_three
skip
basket = [1, 1, 2, 3, 4, 4, 5, 5]
end

def test_group_of_four_plus_group_of_two_is_cheaper_than_two_groups_of_three
skip
basket = [1, 1, 2, 2, 3, 4]
end

def test_two_each_of_first_4_books_and_1_copy_each_of_rest
skip
basket = [1, 1, 2, 2, 3, 3, 4, 4, 5]
end

def test_two_copies_of_each_book
skip
basket = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5]
end

def test_three_copies_of_first_book_and_2_each_of_remaining
skip
basket = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1]
end

def test_three_each_of_first_2_books_and_2_each_of_remaining_books
skip
basket = [1, 1, 2, 2, 3, 3, 4, 4, 5, 5, 1, 2]
end

def test_four_groups_of_four_are_cheaper_than_two_groups_each_of_five_and_three
skip
basket = [1, 1, 2, 2, 3, 3, 4, 5, 1, 1, 2, 2, 3, 3, 4, 5]
end
end``````
``````class BookStore
DISCOUNT = {
0 => 0,
1 => 0,
2 => 5,
3 => 10,
4 => 20,
5 => 25
}
@groups = {}

best_price = Float::INFINITY
current_price = price(group)
if current_price < best_price
best_price = current_price
end
end
best_price
end

end

groups = []
results.each do |result|
groups << result + [[item]]
result.each do |sub_result|
# Prune tree by ignoring sub groups
# with duplicate items.
if !sub_result.include?(item)
other_results = result.select{|r| r.object_id != sub_result.object_id}
this_result = sub_result + [item]
groups << other_results + [this_result]
end
end
end
groups
end

def self.price(groups)
# memoize prices so we only calculate once per group.
@prices = {}
groups.sum do |g|
if @prices.key?(g)
@prices[g]
else
@prices[g] = BookStore.new(g).calculate
end
end
end

def initialize(items)
@items = items
end

def calculate
group_count = @items.uniq.count
full_price = (@items.length - group_count) * 8.0
discount = DISCOUNT[group_count] || 0
full_price + (8.00 * group_count) * (100 - discount) / 100
end
end``````