Moments in Coding

You know what is scary? I will tell you a thing that is scary.

When you perform a somewhat spanning refactor —

Well let me back up a moment. I have a general approach to coding, and other things, which goes like this:

  • Charge ahead, make some progress without worrying too much about long-term consequences.
  • Step back and look at the results.
  • Now I am an expert. An expert on one very very tiny field of knowledge, but one that is immediately applicable to my needs…
  • Double back to step one, and revisit it. It may feel like “not making progress”… but with experience you know that certain things really do lead to long-term benefits. Eat your vegetables and whole grains, kid.

The scope of these revisitations varies. Sometimes I’ll bang out a solution, say, Hooray, and revert my files to do it again in the same hour. Other times I’ll go back a month later and refactor while keeping tests passing.

Where were we. Yes, earlier this week I was revisiting the base class for all Metareal objects (some would call them “game objects”). After spinning up the editor, and implementing four or five of them, I had a pretty good idea of what the life cycle should be, and a better idea of the distribution of responsibilities between World, Level, Editor, and Object.

When I started banging out the code, I didn’t really even have those names. The Level object was handling some world duties, and some Editor duties. And the Editor was handling SDL events directly, and so on. What they now reboiled down to is:

  • Editor. One of several possible top level apps. Instantiates a Level and a World. Gets first crack at UI events, passes them to the World sometimes.
  • World. Owns the list of objects. Owns the Level (after Editor loads it and gives it to the World). Responds to ticks, and owns the renderer.
  • Level. Turns files into object lists, and vice versa.
  • Object. Receives in-world events like player-touches and ticks and messages. May have renderable portions, or volume or collision presence. Has some “Editor-only” affordances like debug-info messages, getTriangleCount().

Nothing too radical, but took a little while to settle into this orderly form. So this refactor I was doing, I had a pretty clear idea, and some notes, and I dug into it. Lots of temporary ifdef-ing, and new unit tests along the way. Which brings us to the posting topic.

The thing that is Scary

Sometimes a thing is scary. It makes me question the plausibility of this whole endeavor. A person can’t write a big computer program… it can’t be done!

Mccoy

After laying out all the parts on the workbench, and 15 hours of coding, iterating, testing, everything was working again, looking good. Except the CPU and frame rate were all wrong. Previously, I could run a million triangles and fifty thousand collision zones at 12% and 60 FPS. But now it was slogging. The CPU was erratic. 15%. 45%. Debugger break, go, and back to 20%. And the frame rate would zoom to 60 (Hooray!) and then stumble to 38 (Wafna! Wafna!).

I ran the old version on a laptop side by side. The performance counters all matched reasonably: the same amount of “work” was being done, as expected. The memory footprint was comparable. Still no memory leaks.

But I had rejiggered quite a bit. What had changed? I kept profiling and iterating. Nothing. How could this be??

Illusion

Finally I rolled back to the previous version on the same computer… and saw that the old version also was behaving all wrong. Then I ran some other games, and they were terrible.

A reboot fixed everything. The refactor had worked just fine.

I’ll close with a small observation, to keep handy in special moment of confusion. (For example, the day before shipping when half the subsystems suddenly fail, and the test server breaks, and and and…)

The inexplicable usually involves a coincidence.

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