how do you test algorithms?

gistfile1.rb

# how do you test algorithms?

class Pythagoream < Struct.new(:a, :b)
  def result
    a**2 + b**2
  end
end

# I prefer to "invert" the algorithm somehow
# or calculate the expected result another way.
# The idea is to give me a "double-entry book-keeping" benefit :-
# f(x) should equal g(x), where g(x) is an alternative implementation of f(x)

def test_pythagoream
  opposite = 3.to_f
  adjacent = 4.to_f
  theta = Math.atan(opposite/adjacent)
  expected = adjacent / Math.cos(theta)
  assert_in_delta expected, Pythagoream.new(opposite, adjacent).result, 0.01
end

# Otherwise I think using explicit expected results
# is better than duplicating the algorithm in the test :-

def test_pythagoream
  assert_equal 25, Pythagoream.new(3, 4).result
end

# Here's a nicer example of where "inverting" the algorithm is useful :-
# f`(f(x)) should equal x, where f`(x) is the inverse of f(x)

class SquareRooter < Struct.new(:x)
  def result
    Math.sqrt(x)
  end
end

def test_square_rooter
  root = 5.0
  square = 5.0 ** 2
  assert_in_delta root, SquareRooter.new(square).result, 0.01
end