checkout.rb

def assert_equal(expected, actual)
  print expected == actual ? "." : "Expected #{expected.inspect} but got #{actual.inspect}\n"
end

class SummingRule
  def initialize(item, price)
    @item = item
    @price = price
  end

  def apply(basket)
    basket.count(@item) * @price
  end
end

class DiscountRule
  def initialize(item, count, discount)
    @item = item
    @count = count
    @discount = discount
  end

  def apply(basket)
    (basket.count(@item) / @count) * -@discount
  end
end

class Checkout
  def initialize
    @basket = []
    @rules = [
      SummingRule.new("A", 50),
      SummingRule.new("B", 30),
      DiscountRule.new("A", 3, 20),
      DiscountRule.new("B", 2, 15)
    ]
  end

  def total
    total = 0
    @rules.each do |rule|
      total += rule.apply(@basket)
    end
    total
  end

  def scan(item)
    @basket.push(item)
  end
end

checkout = Checkout.new
assert_equal(0, checkout.total)
checkout.scan("A")
assert_equal(50, checkout.total)
checkout.scan("B")
assert_equal(80, checkout.total)

checkout = Checkout.new
checkout.scan("A")
checkout.scan("A")
checkout.scan("A")
assert_equal(130, checkout.total)

checkout = Checkout.new
checkout.scan("B")
checkout.scan("B")
assert_equal(45, checkout.total)


puts