TDD and random numbers in ruby

I’m about to TDD a Ruby class whose behaviour will involve the use of random numbers. I expect the algorithms within the class to evolve as I implement new stories, so I don’t want to design and build a testing mechanism that will be brittle when those changes occur. But before I can write the next example, I need to figure out how to control the random numbers needed by the code under test. Off the top of my head I can think of four options:

  1. One way would be to set a fixed seed just before each test and then simply let the random algorithm do its thing. But for each new behaviour I would need to guess the appropriate seed, which is likely to be time-consuming. Furthermore, the relationship between each magic seed and the specific behaviour tested is likely to be obscure, possibly requiring comments to document it for the future reader. And finally, if the algorithm later evolves in such a way as to consume random numbers in a different order or quantity, the seed may turn out to be inappropriate, leading to broken tests or, worse, tests that pass but which no longer test the planned behaviour.
  2. Alternatively I could redefine Kernel::rand — but that could potentially interfere with stuff I don’t know about elsewhere in the object space.
  3. Within the class under test I could self-encapsulate the call to Kernel::rand, and then override the encapsulating method in a subclass for the tests. But then I’m not testing the class itself.
  4. Finally, I could parameterize the class, passing to it an object that generates random numbers for it. This option appears to give me complete control, without being too brittle or trampling on anyone else in the object space.

So I’ll go with option 4. Right now, though, I’m not sure what interface the randomizer object should provide to the calling class. Looking ahead, I expect I’ll most likely want to select a random item from an array, which means selecting a random integer in the range 0...(array.length). And for this next example all I’ll need is a couple of different randomizers that return 0 or 1 on every request, so I’ll simply pass in a proc:

obj.randomize_using { |ceil| 0 }

And if ever I need to provide a specific sequence of varying random numbers, I can do it like this:

rands = [1, 0, 2]
obj.randomize_using { |ceil| rands.shift || 0 }

Later that same day…

The class I’m developing has evolved quite a lot and split into three. And suddenly, with the most recent change, three of the tests have begun failing. A little investigation reveals that the code is now consuming a random number when it didn’t need to in the past, and so some of my randomizer procs now provide inappropriate values. It turns out that two of the failing examples actually boil down to being a single test of a piece that has now been refactored out into another class; by refactoring the tests to match I can remove the dependency on random numbers altogether. And the last broken test is fixed by providing a randomizer that respects the ceiling passed to it (not an unreasonable request):

obj.randomize_using { |ceil| [2, ceil-1].min }

This works, and I get no more surprises during the session.


2 thoughts on “TDD and random numbers in ruby

  1. I came across this recently, and a quick and dirty way of doing this is just to repeat the assertion a set amount of times. ie.

    500.times do |i|
    rand.should_not == 2

    Simplistic example. It’s not flawless, if you are testing the actual values you have a small chance of getting a test failure, but without going too crazy it works alright.

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