Because it happens in the lexer, and depending on how you design your symbol table lookups, you may want to use the same hash function for all table lookups, in

Because it happens in the lexer, and depending on how you design your symbol table lookups, you may want to use the same hash function for all table lookups, instead of calculating different hashes for each table. Symbol table lookups can compare hash value first (store the full hash code per entry) to skip the expensive string comparison too. Perfect hashing isn't always what you want in any case. For many languages, especially symbolically flavoured ones like C, most strings found by the lexer aren't keywords. A hash table which is mostly empty and is statically verified not to contain keyword collisions is ideal. Keep the hash code calculation separate, and do it only once. Little point in repeatedly calculating it.