rational_and_bigdecimal_multiplication_and_division.rb

# Out of the box, Ruby's Rational class doesn't know how to multiply or divide a BigDecimal;
# You get: "TypeError: Rational can't be coerced into BigDecimal" when you try.

class Rational
  alias :"old_/" :"/"
  def /(a)
    begin
      # Keep existing behavior if we can
      send(:"old_/", a)
    rescue TypeError
      # Note that Rational() will handle decimal denominators; it finds the appropriate integral denominator
      Rational(self.numerator, self.denominator * a )
    end
  end

  alias :"old_*" :"*"
  def *(a)
    begin
      send(:"old_*", a)
    rescue TypeError
      # Numerator must have gcd() (ie: be an Integer), so modify the denominator instead
      Rational( self.numerator, self.denominator / a )
    end
  end
end


class TestMathExtensions < Test::Unit::TestCase
  def test_rational_and_bigdecimal_division
    r = 1 / 4
    d = BigDecimal.new("2")
    assert_equal (r / d), 0.125
  end

  def test_rational_and_bigdecimal_multiplication
    r = 1 / 4
    d = BigDecimal.new("2")
    assert_equal (r * d), 0.5
  end
end