squareup_spec.rb

require 'bigdecimal'

class Payment
  FEE_PERCENTAGE = BigDecimal.new('2.9')
  FEE_SET_AMOUNT = BigDecimal.new('0.15')

  attr_reader :amount

  def initialize(amount)
    @amount = BigDecimal.new(amount.to_s)
  end

  def fee_amount
    (rounded_fee_percentage + FEE_SET_AMOUNT).to_f
  end

  private

  def rounded_fee_percentage
    fee_percentage = FEE_PERCENTAGE / 100
    (@amount * fee_percentage).round(2)
  end
end


RSpec::Matchers.define :have_a_fee_amount_of do |amount|
  match do |payment|
    payment.fee_amount == amount
  end

  failure_message_for_should do |payment|
    "expected a payment of $#{payment.amount.to_f} to have a fee amount of $#{amount}"
  end

  failure_message_for_should_not do |actual|
    "expected a payment of $#{payment.amount.to_f} to not have a fee amount of $#{amount}"
  end

  description do
    "have a fee_amount of 2.9% + $0.15 ($#{amount})"
  end
end


module PaymentExampleHelpers
  def when_amount_is(amount, &block)
    context "with an amount of $#{amount}" do
      subject { Payment.new(amount) }
      instance_eval(&block)
    end
  end
end


RSpec.configure do |config|
  config.extend(PaymentExampleHelpers)
end


# although the combination of when_amount_is and have_a_fee_amount_of matcher
# increases the test size by a fair few lines, the idea was to remove duplication
# within the test descriptions and code. Also, this spec layout is more flexible 
# to adding further tests within each when_amount_is block.
#
# as a side note, `rspec -f squareup_spec.rb` produces nice test documentation
#
describe Payment do
  when_amount_is(10.00) do
    it { should have_a_fee_amount_of(0.44) }
  end
  
  when_amount_is(42.00) do
    it { should have_a_fee_amount_of(1.37) }
  end
end