calculate_money_percentage.rb

module Cm
  module Shared
    # Calculates a percentage of the amount passed in and returns it rounded to 2 decimals (i.e. cents),
    # as a BigDecimal object.
    #
    # Some takeaways from "Invoices: How to properly round and calculate totals" (https://goo.gl/cHvuPa)
    # - it is very easy to create invoices where numbers don't add up and a few cents are missing.
    # - don't pass around unrounded values. Once you round, use the rounded value for all further totals.
    # - the rounding of money amounts is *not* a matter of presentation, and should never happen in a view.
    # - don't ever use the float data type in Ruby or MySQL for money
    #
    module CalculateMoneyPercentage
      def calculate_money_percentage(amount, percentage)
        (number_to_decimal(amount) * (number_to_decimal(percentage) / number_to_decimal(100))).round(2)
      end

      private

      def number_to_decimal(amount)
        precision = 10
        BigDecimal.new(amount, precision)
      end
    end
  end
end