Cash Register

cash_register.rb

class CashRegister
  US_COINS = {"quarter" => 25, "dime" => 10, "nickel" => 5, "penny" => 1 }
  EU_COINS = {"50_cent" => 50, "20_cent" => 20, "10_cent" => 10, "5_cent" => 5, "2_cent" => 2, "1_cent" => 1 }

  def self.get_change(cash, currency)
    cash_left = cash
    coin_numbers = Hash.new
    coins = self.get_coins(currency)

    # determine the quantity of each coin starting from the largest value
    coins.each {|coin, value|
      num_coins = cash_left / value
      cash_left = cash_left - (num_coins * value)
      coin_numbers[coin] = num_coins
    }
    coin_numbers
  end

  def self.get_coins(currency)
    case currency
    when "EU"
      EU_COINS
    when "US"
      US_COINS
    else
      raise CurrencyError, "Unsupported currency"
    end
  end
end

class CurrencyError < StandardError

end

tc_cash_register.rb

require_relative "cash_register"
require "test/unit"

class TestCashRegister < Test::Unit::TestCase
  def test_get_change_us
    coin_numbers = CashRegister.get_change(63, "US")
    assert_equal(coin_numbers["quarter"], 2)
    assert_equal(coin_numbers["dime"], 1)
    assert_equal(coin_numbers["nickel"], 0)
    assert_equal(coin_numbers["penny"], 3)

    coin_numbers = CashRegister.get_change(117, "US")
    assert_equal(coin_numbers["quarter"], 4)
    assert_equal(coin_numbers["dime"], 1)
    assert_equal(coin_numbers["nickel"], 1)
    assert_equal(coin_numbers["penny"], 2)
  end

  def test_get_change_eu
    coin_numbers = CashRegister.get_change(63, "EU")
    assert_equal(coin_numbers["50_cent"], 1)
    assert_equal(coin_numbers["20_cent"], 0)
    assert_equal(coin_numbers["10_cent"], 1)
    assert_equal(coin_numbers["5_cent"], 0)
    assert_equal(coin_numbers["2_cent"], 1)
    assert_equal(coin_numbers["1_cent"], 1)

    coin_numbers = CashRegister.get_change(88, "EU")
    assert_equal(coin_numbers["50_cent"], 1)
    assert_equal(coin_numbers["20_cent"], 1)
    assert_equal(coin_numbers["10_cent"], 1)
    assert_equal(coin_numbers["5_cent"], 1)
    assert_equal(coin_numbers["2_cent"], 1)
    assert_equal(coin_numbers["1_cent"], 1)

    coin_numbers = CashRegister.get_change(85, "EU")
    assert_equal(coin_numbers["50_cent"], 1)
    assert_equal(coin_numbers["20_cent"], 1)
    assert_equal(coin_numbers["10_cent"], 1)
    assert_equal(coin_numbers["5_cent"], 1)
    assert_equal(coin_numbers["2_cent"], 0)
    assert_equal(coin_numbers["1_cent"], 0)
  end

  def test_unsupported_currency
    assert_raises CurrencyError do
      CashRegister.get_change(100, "CA")
    end
  end
end