I'm not sure how much this applies to open source. Closed source, quite possibly, since it probably won't survive long enough / need to change enough / need enough people to understand it.
Open source, meanwhile, requires being understandable to new people. It needs to explain itself, or it needs to be idiomatic (within its specialty). If neither, you're condemning people to rewrite it or waste time trying to figure out the original intent.
The second half of what I said explained how the code becomes less readable. If someone wants to hack on that code, they better understand the algorithm it implements, which means at a minimum they've probably absorbed the same 2000 page spec the original developers absorbed (and know exactly what 15 means). No amount of #define can fix that, although in the right circumstance some comments might help.
Open source, meanwhile, requires being understandable to new people. It needs to explain itself, or it needs to be idiomatic (within its specialty). If neither, you're condemning people to rewrite it or waste time trying to figure out the original intent.