Funny your example is rc5, I wrote exactly what you describe to generate 32-bit cookies in a random prototype a few years ago: https://github.com/jcalvinowens/sdvr/blob/main/rc5.c

It is cute, but surely there's a more efficient way than RC5? There are bijective hash functions which are much cheaper (murmur, at least).

In my case, performance was utterly unimportant.

But is Murmur actually bijective?