A lot of software spends its time mapping strings to values. A web server must identify the HTTP method (GET, POST, …). It must recognize header names. A URL parser must recognize the protocol (http, https, ftp, …). A compiler must recognize keywords.
It works. But notice something: the keys are known when you write the code. They never change. And yet we pay at runtime as if they could change at any time: we hash the whole string, we pick a bucket, we follow a pointer, we compare, maybe we walk a list.
A hash function maps keys to slots. When two keys land in the same slot, we have a collision, and we must deal with it. Collisions are what make hash tables complicated and slow.
If you know all of your keys ahead of time, you can construct a hash function designed for your keys, a function that never produces a collision. It is called a perfect hash function. The idea is old: the GNU gperf tool has been doing it since 1989. You give it a list of keywords, and it generates C code.
