book-store: Implement exercise

config.json

@@ -923,6 +923,18 @@
         "strings",
         "logic"
       ]
+    },
+    {
+      "uuid": "0ec96460-08be-49a0-973a-4336f21b763c",
+      "slug": "book-store",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 6,
+      "topics": [
+        "loops",
+        "arrays",
+        "logic"
+      ]
     }
   ],
   "foregone": [

exercises/book-store/.meta/generator/book_store_case.rb

@@ -0,0 +1,9 @@
+require 'generator/exercise_case'
+
+class BookStoreCase < Generator::ExerciseCase
+
+  def workload
+    assert_equal { "BookStore.calculate_price(#{basket})" }
+  end
+
+end

exercises/book-store/.meta/solutions/.version

@@ -0,0 +1 @@
+0

exercises/book-store/.meta/solutions/book_store.rb

@@ -0,0 +1,47 @@
+module BookKeeping
+  VERSION = 0
+end
+
+class BookStore
+
+  GROUP_DISCOUNTS = [0, 0.05, 0.1, 0.2, 0.25]
+  INDIVIDUAL_PRICE = 8
+
+  def self.calculate_price(basket)
+    groups = []
+    remaining_books = basket.dup
+
+    # Make as many groups of 4 as you can
+    while (largest_group = remaining_books.uniq).length > 3
+      group_of_four = largest_group[0..3]
+      group_of_four.each {|book| remaining_books.delete_at(remaining_books.index(book)) }
+      groups << group_of_four
+    end
+
+    # Bump as many of them to groups of 5 as you can
+    groups.each do |group|
+      fifth_book = (remaining_books - group).first
+      next unless fifth_book
+
+      group << fifth_book
+      remaining_books.delete_at(remaining_books.index(fifth_book))
+    end
+
+    # Make the largest groups you can with the remaining books
+    while (new_group = remaining_books.uniq).any?
+      new_group.each {|book| remaining_books.delete_at(remaining_books.index(book)) }
+      groups << new_group
+    end
+
+    groups.map {|group| group_price(group.length) }.inject(0,:+)
+  end
+
+  private
+
+  def self.group_price(group_size)
+    discount = GROUP_DISCOUNTS[group_size - 1]
+
+    group_size * INDIVIDUAL_PRICE * (1 - discount)
+  end
+
+end

exercises/book-store/README.md

@@ -0,0 +1,100 @@
+# 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.
+
+* * * *
+
+For installation and learning resources, refer to the
+[exercism help page](http://exercism.io/languages/ruby).
+
+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.
+
+In order to run the test, you can run the test file from the exercise
+directory. For example, if the test suite is called
+`hello_world_test.rb`, you can run the following command:
+
+    ruby hello_world_test.rb
+
+To include color from the command line:
+
+    ruby -r minitest/pride hello_world_test.rb
+
+
+## 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.

exercises/book-store/book_store_test.rb

@@ -0,0 +1,97 @@
+require 'minitest/autorun'
+require_relative 'book_store'
+
+# Common test data version: 1.1.0 a636903
+class BookStoreTest < Minitest::Test
+  def test_only_a_single_book
+    # skip
+    assert_equal 8.0, BookStore.calculate_price([1])
+  end
+
+  def test_two_of_the_same_book
+    skip
+    assert_equal 16.0, BookStore.calculate_price([2, 2])
+  end
+
+  def test_empty_basket
+    skip
+    assert_equal 0.0, BookStore.calculate_price([])
+  end
+
+  def test_two_different_books
+    skip
+    assert_equal 15.2, BookStore.calculate_price([1, 2])
+  end
+
+  def test_three_different_books
+    skip
+    assert_equal 21.6, BookStore.calculate_price([1, 2, 3])
+  end
+
+  def test_four_different_books
+    skip
+    assert_equal 25.6, BookStore.calculate_price([1, 2, 3, 4])
+  end
+
+  def test_five_different_books
+    skip
+    assert_equal 30.0, BookStore.calculate_price([1, 2, 3, 4, 5])
+  end
+
+  def test_two_groups_of_four_is_cheaper_than_group_of_five_plus_group_of_three
+    skip
+    assert_equal 51.2, BookStore.calculate_price([1, 1, 2, 2, 3, 3, 4, 5])
+  end
+
+  def test_group_of_four_plus_group_of_two_is_cheaper_than_two_groups_of_three
+    skip
+    assert_equal 40.8, BookStore.calculate_price([1, 1, 2, 2, 3, 4])
+  end
+
+  def test_two_each_of_first_4_books_and_1_copy_each_of_rest
+    skip
+    assert_equal 55.6, BookStore.calculate_price([1, 1, 2, 2, 3, 3, 4, 4, 5])
+  end
+
+  def test_two_copies_of_each_book
+    skip
+    assert_equal 60.0, BookStore.calculate_price([1, 1, 2, 2, 3, 3, 4, 4, 5, 5])
+  end
+
+  def test_three_copies_of_first_book_and_2_each_of_remaining
+    skip
+    assert_equal 68.0, BookStore.calculate_price([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
+    assert_equal 75.2, BookStore.calculate_price([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
+    assert_equal 102.4, BookStore.calculate_price([1, 1, 2, 2, 3, 3, 4, 5, 1, 1, 2, 2, 3, 3, 4, 5])
+  end
+
+  # Problems in exercism evolve over time, as we find better ways to ask
+  # questions.
+  # The version number refers to the version of the problem you solved,
+  # not your solution.
+  #
+  # Define a constant named VERSION inside of the top level BookKeeping
+  # module, which may be placed near the end of your file.
+  #
+  # In your file, it will look like this:
+  #
+  # module BookKeeping
+  #   VERSION = 1 # Where the version number matches the one in the test.
+  # end
+  #
+  # If you are curious, read more about constants on RubyDoc:
+  # http://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/constants.html
+
+  def test_bookkeeping
+    skip
+    assert_equal 0, BookKeeping::VERSION
+  end
+end