The Will to Rewrite

FSBreak interviewed Austin last week....it's an interesting listen and they cover a lot of ground. A few comments on Laminar's approach to developing software:

That Code Stinks!

Austin is absolutely correct that we (LR) write better software because neither of us are shy about telling the other when a piece of code stinks. But I think Austin deserves the credit for creating this environment. An "ego-free" zone where people can criticize each other honestly and freely is a rare and valuable thing, in many domains, not just music.

When I first came to LR, Austin created this environment by responding positively to feedback, no matter how, um, honest. When I first came to LR 100% of the code was written by him and 0% by me. Thus if I was going to say "this piece of code really needs to be different", it was going to be Austin to either run with it or try to defend his previous work.

To his credit, Austin ran with it, 100% of the time. I can't think of a single time that he didn't come down on the side of "let's make X-Plane better". That set the tone for the environment we have now: one that is data driven, regardless of who the original author is.

I would say this to any programmer who faces a harsh critique of code: good programmers write bad code! I have rewritten the culling code (the code that decides whether an OBJ really needs to be drawn*) perhaps four times now in the last five years. Each time I rewrote the code, it was a big improvement. But that doesn't make the original code a mistake - the previous iterations were still improvements in their day. Programming is an iterative process. It is possible to write code that is both good and valuable to the company and going to need to be torn out a year later.

A Rewrite Is Not A Compatibility Break

Austin also points to the constant rewriting of X-Plane as a source of performance. This is true too - Austin has a zero tolerance policy toward old crufty code. If we know the code has gotten ugly and we know what we would do to make the code clean, we do that, immediately, without delay. Why would you ever put off fixing old code?

Having worked like this for a while, I am now amazed at the extent to which other organizations (including ones I have worked in) are willing to put off cleaning up code organization problems.

Simply put, software companies make money by changing code, and the cost is how long the changes take. If code is organized to make changing it slower, this fundamentally affects the financial viability of the company! (And the longer the code is left messy, the more difficult it will be to clean up later.)

But I must also point out one critical detail: rewriting the code doesn't mean breaking compatibility! Consider the OBJ engine, which has been rewritten more times than I care to remember. It still loads OBJ2 files from X-Plane 620.

When we rework a section of the sim, we make sure that the structure of the code exactly matches what we are working on now (rather than what we were working on two years ago) so that new development fits into the existing code well. But it is not necessary to drop pieces of the code to achieve that. I would describe this "refactoring" as straightening a curvy highway so you can drive faster. If the highway went from LA to San Francisco, it still will - just in less time.

In fact, I think the issue of compatibility in X-Plane's flight model has a lot more to do with whether the goal is to emulate reality or past results. This debate is orthoganal to refactoring code on a regular basis.

* Since OBJs are expensive to draw and there are a huge number of OBJs in a scenery tile, the decision about whether to draw an OBJ is really important to performance. Make bad decisions, you hurt fps. Spend too long debating what to draw, you also hurt fps!