checkout_spec.rb

require 'matchers.rb'

describe CheckOut do
  let(:checkout) { CheckOut.new(RULES) }

  context "calculating totals" do
    subject { checkout }

    it { should price("").at(0) }
    it { should price("A").at(50) }
    it { should price("AB").at(80) }
    it { should price("CDBA").at(115) }

    it { should price("AA").at(100) }
    it { should price("AAA").at(130) }
    it { should price("AAAA").at(180) }
    it { should price("AAAAA").at(230) }
    it { should price("AAAAAA").at(260) }

    it { should price("AAAB").at(160) }
    it { should price("AAABB").at(175) }
    it { should price("AAABBD").at(190) }
    it { should price("DABABA").at(190) }
  end

  it "calculates incremental totals" do
    checkout.total.should eq(0)
    checkout.scan("A");  checkout.total.should eq(50)
    checkout.scan("B");  checkout.total.should eq(80)
    checkout.scan("A");  checkout.total.should eq(130)
    checkout.scan("A");  checkout.total.should eq(160)
    checkout.scan("B");  checkout.total.should eq(175)
  end
end

matchers.rb

RSpec::Matchers.define :price do |goods|
  match do |checkout|
    goods.split(//).each { |item| checkout.scan(item) }
    (@actual_total = checkout.total) == @price
  end

  chain :at do |price|
    @price = price
  end

  failure_message_for_should do |checkout|
    "expected checkout to price #{goods.inspect} at #{@price}, got #{@actual_total}"
  end

  description do
    "price #{goods.inspect} at #{@price}"
  end
end