Crafty is the "son" (direct descendent) of the chess program Cray Blitz. it is designed totally around the bit-board data structure for reasons of speed of execution, ease of adding new knowledge, and a significantly cleaner overall design. it is written totally in ANSI C with some few UNIX system calls required for I/O, etc.
Source of most of the issues:
http://talkchess.com/forum3/viewtopic.php?f=2&t=71585