exercise.rb

Product = Struct.new(:name, :price)
Offer = Struct.new(:product_name, :min_quantity, :offer_type, :discount) do
  def calculate(quantity)
    return quantity if min_quantity > quantity

    case offer_type
    when '2x1' then quantity / 2 + quantity % 2
    when 'discount' then (1 - discount.to_f) * quantity
    else quantity
    end
  end
end

class Store
  attr_accessor :products, :offers

  def initialize
    @products = []
    @offers = []
  end

  def find_product(product_name)
    @products.detect { |product| product.name == product_name }
  end

  def find_offer(product_name)
    @offers.detect { |offer| offer.product_name == product_name }
  end
end

class Checkout
  attr_reader :products, :store

  def initialize(store)
    @products = {}
    @store = store
  end

  def add_product(array)
    if @products[array[0]]
      @products[array[0]] += array[1]
    else
      @products[array[0]] = array[1]
    end
  end

  def calculate
    products.inject(0) do |total, (name, quantity)|
      product = store.find_product(name)
      offer = store.find_offer(name)
      total_quantity = offer ? offer.calculate(quantity) : quantity
      total + total_quantity * product.price
    end
  end
end

store = Store.new
store.products << Product.new('grapes', 5)
store.products << Product.new('apples', 3)
store.products << Product.new('peaches', 7)

store.offers << Offer.new('grapes', 2, '2x1')
store.offers << Offer.new('apples', 2, 'discount', 0.2)

products = [['grapes', 1], ['apples', 0], ['peaches',  1]]
# products = [['grapes', 1], ['apples', 1], ['peaches', 1]]
# products = [['grapes', 2], ['apples', 2], ['peaches', 1]]
# products = [['grapes', 3], ['apples', 5], ['peaches', 2]]
# products = [['peaches', 7], ['grapes', 7], ['apples', 7]]

checkout = Checkout.new(store)
products.map { |product| checkout.add_product(product) }
p checkout.calculate