Most striking at the time: symbolic differentiation in some ancient dialect of Lisp. I thought this was an example in McCarthy's original paper, but it turns ou

Most striking at the time: symbolic differentiation in some ancient dialect of Lisp. I thought this was an example in McCarthy's original paper, but it turns out not; I forget where I actually saw it; but anyway, starting from very primitive beginnings and making something that did Real Mathematics is pretty mind-blowing when you're 15 or whatever age I was. TeX: by a large margin the best-documented substantial piece of code I've ever seen, and (by now) with an extremely low bug count. As for the actual code, there's plenty to love and plenty to hate. Lots of things in Norvig's "Paradigms of artificial intelligence programming". Nothing in there is really astonishing, but the standard is consistently extremely high and of course it's accompanied by a wealth of insightful comment, namely the actual text of the book. Various bits of low-level bit-twiddling. I love the population-count trick that begins with x & 0x55555555, but typically it's not actually the best way to do the job (four lookups in a 256-element table will usually be faster), so instead I'll pick the trick for finding whether a word has a zero-byte in it (useful, e.g., for operations like strlen), the idea of which is basically to subtract 0x01010101 and look for evidence of borrows across byte boundaries.