In the original design, each byte of input is treated with a round of the FNV hashing.
https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function

In Ethash input data is hashed in 32-bit chunks following the FNV-1 formula for single round.
This is not changed in ProgPoW except for using FNV-1a formula.

This is "more correct" implementation: https://godbolt.org/z/tO3Lqt.