[ Ruby ] Simple minitest + object-oriented recursion example "With interest"

with_interest.rb

require 'minitest/autorun'
require 'minitest/pride'

class WithInterest

  def initialize amount, interest, years = 1
    @amount   = amount
    @interest = interest
    @years    = years
  end

  def calculate
    last_year? ? product : next_round_product 
  end

  private

  def product
    @amount * @interest
  end

  def next_round_product
    product + (next_round * @interest)
  end

  def next_round
    WithInterest.new(@amount, @interest, @years - 1).calculate
  end

  def last_year?
    @years == 1
  end

end

describe WithInterest do

  it "calculates the interest on one year" do
    WithInterest.new(100, 1.06).calculate.must_equal 106
  end

  it "calculates the interest with multiple years" do
    sum = (((100 * 1.06) + 100) * 1.06 + 100) * 1.06
    WithInterest.new(100, 1.06, 3).calculate.must_equal sum
  end

end