Stories about Software


TDD For Breaking Problems Apart

If I have to write some code that’s going to do something rather complex, it’s easy for my mind to start swimming with possibilities, as mentioned in the magic boxes post. In that post, I talked about thinking in abstractions and breaking problems apart and alluded to TDD being a natural fit for this paradigm without going into much detail. Today and in my next post about this I’m going to go into that detail.

Let’s say I’m tasked with coding some class or classes that keep track of bowling scores. I don’t think “okay, what is my score at the start of a bowling game?” I think things like “I’ll need to have some iteration logic that remembers back at least two frames, since that’s how far back we can go, and I’ll probably need something to look ahead to handle the 10th frame, and maybe there’s a design pattern for that, and…” Like I said, swimming. These are the insane ramblings of an over-caffeinated maniac more than they’re a calm and methodical solution to a somewhat, but not terribly complex problem. In a way, this is like premature optimization. I’m solving problems that I anticipate having rather than problems that I actually have right at this moment.

TDD is thus like a deep breath, a relaxing cup of some hot beverage, and perhaps a bit of meditation or zoning out. All of this noise fades out of my head and I can actually start figuring out how things work. The reason for this is that the essence of TDD is breaking the task into small, manageable problems and solving those while simultaneously ensuring that they stay solved. It’s like a check-list. Forget all of this crap about iterators and design patterns — let’s establish a base case: “Score at the beginning of a game is zero.” That’s pretty easy to solve and pretty hard to get frantic over.

With TDD, the first that I’m going to do is write a test and I’m going to write only enough of a test to fail (which includes writing something that doesn’t compile. So, let’s do that:

Alright, since there is no such type as “BowlingScoreCalculator”, this test fails by virtue of non-compiling. I then define the type and the test goes green (I’m using NCrunch, so I’m never actually building or running anything — I just see dots change colors). Next, I want to assert that the score property is equal to zero after the constructor executes:

This also fails, since that property doesn’t exist, and I solve this problem by declaring an auto-implemented property, which actually makes the whole test pass. And just like that, I’ve notched my first passing test and step toward a functional bowling calculator. The game starts with a score of zero.

What’s next? Well, I suppose the simplest thing that we should say is that if I bowl a frame with a score of zero for the first throw and one on the second throw, my score is now 1. Okay, so what is a frame, and what kind of data structure should I use to represent it, and what would be the ideal API for that, and is score really best represented as a property, and — BZZT!! Stop it. We’re solving small, simple problems one at a time. And the only problem I have at the moment is “lack of failing test”. I’m going to quickly decide that the mechanism for bowling a frame is going to be BowlFrame(int, int) so that I know the name of my next sub-test class for naming purposes. Writing code until something fails, I get:

This fails because there is no “bowl frame” method, so I add it. CodeRush’s “declare method” refactoring does the trick nicely, populating the new method with a thrown NotImplementedException, which gives me a red test instead of a non-compile. I have to make the test pass before moving on, so I delete it by deleting the throw, and then I add an assert that score is equal to 1. This fails, and I make it pass:

Now, with a green test, I have the option to solve problems in existing code (i.e. “refactor”). For instance, I don’t like that I have a useless, empty constructor, so I delete it and note that my tests still pass. This is another easy problem that I can solve with confidence.

From here, I think I’d like a non-obtuse implementation of scoring that first frame. I think I’ll test that if I bowl a 2 and then a 3, the score is equal to five. For the first time, I’m not going to have any non-compiling failings, so I write some code and see red, obviously, which I get rid of by making the whole thing look like this:

All right, the calculator is now doing a good job of handling a low scoring first frame. So, let’s correct a little thing or two with the refactor cycle, making sure that whatever we do is a problem that’s easily solvable, isolated, and small in scope. I’m thinking I don’t like the magic numbers 2, 3 and 5 in that last test. Let’s try making it look like this:

Yes, it’s perfectly acceptable to refactor your unit tests during the “refactor” phase. Treat these guys as first class code and keep them clean or nobody will want them around. I have another bone to pick with this code, which is the duplication of the constructor logic in each test. Let’s fix that too:

There, that looks better to me now. Now, what’s wrong with the production code? What problem can we solve? I can think of a few things: we can bowl more than 10 per frame, we can bowl negative numbers, and this thing will only keep track of the most recent frame. There are many other issues as well, but we’re already trending toward overload here, so let’s get back on Easy Street and figure out what to do if the user gives us goofy input.

I write a test that fails:

And then make it pass (first by declaring the nested exception and then by throwing it when the first frame is negative). I also take a shortcut in terms of obtuseness of my TDD here and make it throw the exception if either parameter is negative. I consider this acceptable, personally, since I believe that within the red-green-refactor discipline it’s a matter of some discretion how simple “the simplest thing” is. I’m not going to be accepting negative parameters for one and not the other and I know it. At any rate, after getting this to pass, I now refactor my method parameter names to be more descriptive and set the Score setter to private (which I should have done at first anyway) and the production class looks like this:

Now I’m going to tackle the similar problem of allowing too much scoring for a frame. I want to solve the problem that scores of greater than 10 are currently allowed and I’m going to do this with the test case for 11. TDD is not a smoke testing approach nor is it an exhaustive approach, so I’m not going to write tests for 11, 12, 13… 200 or anything like that. As such, I try to hit edge cases whenever possible and that’s what I’ll do here:

I make this pass, and then I go back and then decide I’m not a huge fan of that “10” in both my test and in the production class. Zero I can let slide because of it’s sort of universal value in scoring of competitions, but 10 deserves a descriptive name. I’m going to call it “Mark” since that’s what a frame of 10 is known as in bowling.

Now that we’ve sufficiently guarded against bad inputs, I’d say it’s time to start thinking about aggregation. The easiest way to do that is to have a frame of 1 and then another frame of one, and make sure that we have a score of 2:

How cool is it that I fix this by changing “Score = firstThrow + secondThrow;” to “Score += firstThrow + secondThrow;” Talk about the simplest solution possible!

Now, I don’t like the way that test is written with 5 int literals in there. This is telling me that it might be time to rethink the way I’m handling frame. I don’t really want to think too much about it now, but I know that there’s going to be a hard-fail when I get to the 10th frame, which can have three throws so I’m already not married to this implementation. And now it’s already starting to feel awkward.

So what if I created a simple frame object? How would that look? Well, I’ll cover that next time as I flesh out this exercise. I’m hoping I’ve at least sold you somewhat on the notion that TDD forces you to chunk problems into manageable pieces and solve them that way without getting carried away. Next time I’ll circle back a bit to the idea of “magic boxes” as we decide how to divide up the concept of “frame” and “game” while adhering to the practice of TDD.

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[…] pluralsight course, I use the example of a method that identifies numbers as prime or not, and in a series of posts I did last fall on TDD, I use the example of something that calculates a bowling score. I’ve […]


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