an open source chess engine under the GNU General Public License v2.0  that plays Bughouse and Crazyhouse - a separate version also orthodox chess. Sunsetter is written by Georg von Zimmermann in C++, recent versions co-authored by Ben Dean-Kawamura , available at GitHub, compliant to the Chess Engine Communication Protocol .
Sunsetter is a bitboard engine which uses rotated bitboards to calculate sliding piece attacks . The board structure further keeps an 8x8 board, and Crazyhouse specific, a bitboard of promoted pawns, and piece-lists of each player's reserve (in hand), also keeping some incremental updated attack tables. Pseudo-legal moves are generated in four stages.
Sunsetter performs NegaScout with transposition table within its iterative deepening framework using a zero aspiration window to decide whether the forced re-search of the first move half-opens the window in plus or minus infinity direction . Null move pruning, extended futility pruning, razoring and fractional extensions are used to shape the tree, which has considerable larger branching factor in Crazyhouse. Sunsetter features book learning based on the final result, where root moves receive a bonus if persistent from advantageous positions in previously won games.
- Sunsetter(C) source released by Georg von Zimmermann, CCC, June 04, 2001
- Very interesting crazyhouse position-to Georg Zimmerman (sunsetter test) by Lieven Clarisse, CCC, November 11, 2002
- Sunsetter C10 Anomaly by Matthias Gemuh, CCC, December 04, 2003
- Reborn of sunsetter? by velocidrom, Winboard Forum, August 31, 2013
- Sunsetter (crazyhouse engine) issue with ponder on, linux by Robert Tournevisse, CCC, December 09, 2013
- Sunsetter 9 under Arena GUI by Arnaud lohéac, CCC, May 17, 2017
- GitHub - georgvonzimmermann/Sunsetter: A crazyhouse chess playing program
- Sunsetter from SourceForge
- Sunsetter - at SourceForge.net
- Sunsetter compiled by Jim Ablett, hosted by Kirill Kryukov
- Sunsetter 10.3 64-bit in CCRL 40/4
- Sunset from Fiji over Pacific ocean with coconut palm tree, image by Andrew Mandemaker, April 17, 2006, CC BY-SA 2.5, Wikimedia Commons
- Sunsetter/COPYING at master · georgvonzimmermann/Sunsetter · GitHub
- Sunsetter/README.txt at master · georgvonzimmermann/Sunsetter · GitHub
- Sunsetter/interface.cpp at master · georgvonzimmermann/Sunsetter · GitHub
- based on GitHub - georgvonzimmermann/Sunsetter: A crazyhouse chess playing program
- Sunsetter/bitboard.cpp at master · georgvonzimmermann/Sunsetter · GitHub
- Sunsetter/search.cpp at master · georgvonzimmermann/Sunsetter · GitHub
- Sunsetter/evaluate.cpp at master · georgvonzimmermann/Sunsetter · GitHub