Hunting Zebras in Outer Space

I once took a graduate course: ”Physiology of Excitable Cells I” run by an eccentric British professor who wanted to teach in a far-reaching classical sense. (In the first class on hearing my surname he commanded ”Next week you will tell the class the connection between your name and Sherlock Holmes”; perhaps a bit off-topic given the course title). One thing he said that has stayed with me is that common problems have common solutions. He was discussing how all the medical students he taught were quick to assume a patient had a rare condition, instead of something more mundane. The tag line they teach medical students is:

“When you hear the sound of hooves, think of horses not zebras”.

I use this phrase a lot when it comes to debugging issues in software. It’s far too easy to believe I am the victim of some fiendishly clever interaction in my system or (so help me) a compiler bug. The answer is (to first order) always that I have done something very dumb or just failed to see an edge case. After a long time writing code I am a lot less likely to get caught by this. However, it did happen to me with N-Body and I was so amazed at how dumb I had been I had to close the laptop and go for walk.

The issue related to the use of sprites for a collision. They seemed to work pretty well except on a level where I had a number of other masses moving around. The latent trap was that I felt guilty about the quick way I had pushed sprites into the code. I decided not to worry too much about optimizing them and allocated a sprite group on the fly. I know it’s better to pre-allocate a sprite pool and all that – but at the time I added them my main focus was elsewhere and I knew I would have to come back and clean them up.

So when I hit an issue where collisions on a busy level resulted in a serious pause in the game loop – I decided the time had come to finally put in sprite pools. That HAD to be the source of the issue. Off I went, with the added fun of feature creep in the refactoring. All in all I spent about ten hours. It made no difference.

The real issue showed itself when I was doing the feature creep part of the refactoring. The loop nesting was wrong and I was running the sprite evolution inside a loop that was running over each body in a level. When there were five masses, I evolved the sprites five times per clock tick and things got really slow. Instead of looking hard for a simple error I had jumped to the conclusion that it must be due memory inefficiency and my use of on-the-fly allocations. Doh! I was amazed I had gone on such a long Zebra hunt.

I have used the Zebra hunting metaphor for such a long time that sometimes I now use it without setting the context properly. I was working with our very bright overseas team in India but thought that this time they were on the wrong track, chasing a theory that seemed too intricate. When my manager asked me how they were making out I responded ”Oh, I think they’re off Zebra hunting”. He replied ”Really?! Do they have a holiday or something?”. I then explained my metaphor as well as the fact that India does not have Zebras…

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