merchant_forgery.rb

class Forgery
  class Merchant
    PRODUCTS = %w(coffee drink gas donut sandwich icecream cigarette)

    PROMOTION_TYPES = [
      lambda {
        ["Buy X P get Y Q free",
          [:X, rand(3)+1],
          [:Y, rand(4)+1],
          [:P, PRODUCTS.rand],
          [:Q, PRODUCTS.rand] ]},
      lambda {
        ["Buy X P get Y% off next Q",
          [:X, rand(3)+1],
          [:Y, (rand(8)+1) * 10],
          [:P, PRODUCTS.rand],
          [:Q, PRODUCTS.rand] ]},
      lambda {
        ["X% off P",
          [:X, (rand(19)+1)*5],
          [:P, PRODUCTS.rand] ]},
      lambda {
        ["Buy P get Q free",
          [:P, PRODUCTS.rand],
          [:Q, PRODUCTS.rand] ]}
    ]

    class << self
      def promotion
        substitutions = PROMOTION_TYPES.rand.call
        text = substitutions.shift
        format_forgery(text, substitutions)
      end

      private

      def format_forgery(text, substitutions)
        substitutions.inject(text) do |output, substitution|
          output.gsub(substitution[0].to_s, substitution[1].to_s)
        end
      end

    end
  end
end