- toString() uses a stream for conversion while genes() returns a direct reference to the string, without copies in between. The speed up is very small compared to the improvement from switching to storing a string of abbreviations instead of a vector of step names inside chromosomes but there's basically no downside to this change so it's still worth it.
- Before this change the order of chromosomes with the same fitness in a population depended on the initial order set when the population was first created. Now it only depends on the individual.
- The length comparison is not strictly necessary (lexicographical order covers that) but it makes the intention clear and the comparison slightly faster when chromosomes have different lengths.
- Mostly for readability and convenience. This significantly shortens calls to sort().
- I could define it as Individual::operator< instead but it would be inconsistent with operator== because it does not compare the chromosomes, only fitness. It could result in an unintuitive situation where (a <= b <= a) does not necessarily imply (a == b).
- Until now the source code was being parsed during every fitness computation. Now the parsed program is reused and only the optimisation steps are applied each time.
- I need some sample .yul files for testing but I see that existing tests generally have source code hard-coded in them rather than in standalone .yul files. There are lots of .yul files but they seem to be automatically processed by a special test case rather loaded ad-hoc by manually created tests.
- Program and Population required a file name until now. I'm making them accept loaded source code to be able to give them data hard-coded in a test.
