Sean Eron Anderson
Compiler and author of bit-twiddling-hacks (graphics.stanford.edu/~seander/bithacks.html), the canonical Stanford-hosted collection of bit-manipulation snippets assembled with contributions from many named developers (~1997–2005). The reference most other bit-trick sources — including count-set-bits-so — cite as the authority.
Related
bit-twiddling-hacks · bit-manipulation · population-count