I saw that the source code for Elona, a fun japanese roguelike was available online and decided to take a look. It turns out it is a single, 14mb text file with 444000 lines of code, completely devoid of comments, written in 'Hot Soup Processor', a japanese basic like language.
The code that determines what monsters can spawn is 30000 lines of this (7% of the total game code), which checks for every monster, where if it fails it goto's the next monster, for all 1000 monsters in the game.