gistfile1.rb

# Needs Ruby 1.8.7+ for built in #group_by and #reduce

# Implementation

Product = Struct.new(:code, :name, :price) do
  def line_total(quantity)
    price * quantity
  end
end

class Checkout
  def initialize(products)
    @products = products.group_by{|product| product.code}
    @items = []
  end
  
  def scan(item)
    @items << item
  end
  
  def total
    @items.group_by(&:to_s).map do |code, list| 
      product = @products[code.downcase.to_sym].first
      product.line_total(list.length)
    end.reduce(:+)
  end
end

# Product defintions

fruit_tea    = Product.new(:fr1, 'Fruit tea',     3.11)
strawberries = Product.new(:sr1, 'Strawberries',  5.00)
coffee       = Product.new(:cf1, 'Coffee',        11.23)

products = [fruit_tea, strawberries, coffee]

def fruit_tea.line_total(quantity)
  price * ((quantity.to_i + 1) / 2)
end

def strawberries.line_total(quantity)
  return super if quantity < 3
  quantity * 4.50
end


# Tests

Checkout.new(products).instance_eval do
  scan('FR1')
  scan('SR1')
  scan('FR1')
  scan('FR1')
  scan('CF1')
  puts total
end

Checkout.new(products).instance_eval do
  scan('FR1')
  scan('FR1')
  puts total
end

Checkout.new(products).instance_eval do
  scan('SR1')
  scan('SR1')
  scan('FR1')
  scan('SR1')
  puts total
end