Gibbon
Gibbons at Play [1]
Gibbon,
an UCI compliant open source chess engine by Eric Marathée written in C++. Gibbon originated from Small-C in 2005, playing Massy 2006 and improving since then. While featuring some bitboards, it has no annoying magic bitboard stuff, and is a counter approach of a Fruit like programming style, full of gotos, and difficult to follow control flow due to indent style and preprocessor switches for conditional compilation. Gibbon computes a few nodes, but is an attempt to compute the right ones [2].
Contents
Features
Search
- Principal Variation Search (PVS) with Iterative Deepening and Aspiration Windows
- Two-tier Transposition Table, Pawn Hash Table
- Move Ordering with Hash Move and Killer Moves
- Internal Iterative Deepening
- Enhanced Transposition Cutoff (conditional compiled)
Selectivity
- Null Move Pruning with R = 4
- Non-PV nodes pruning (Late Move Reductions Fail-Low/Fail-High History)
- Futility Pruning
- Extended Futility Pruning
- Extensions (Check Extensions, Recapture Extensions, Passed Pawn Extensions)
Quiescence Search
- One check allowed at the horizon
- Quiescence heuristic for checks winning hanging pieces
- Static Exchange Evaluation (SEE)
- One Reply Extensions in quiescence
Mate at a Glance
- Mate in one recognition for queen moves
- Heuristic to identify moves threatening mate with queen and piece
Evaluation
- Lazy Evaluation
- Mobility
- Pawn Structure considering king placement
- Passed Pawns
- Trade down bonus that encourages the winning side to trade pieces but no pawns
- Incentive to keep the queen in case of material down
- Incentive to keep at least one pawn
- Bishop Pair
Opening
Middlegame
- King Safety considering castling rights
- Outposts
Endgame
- Bishops of Opposite Colors
- Rule of the Square
- King Pawn Tropism
- King Centralization
- Draw Evaluation
- Pawn Endgame
Interior Node Recognizer
- KPK
- KRPKR
- KQKQ
- Wrong Color Bishop and Rook Pawn
- KNKP
- KBKP
- KRKP
Misc
- => Gibbon knows all legal moves, material, mobility, and check-giving squares
BitScan with Reset
In serializing bitboards, Gibbon applies a bitscan with reset based on the De Bruijn multiplication approach published by Charles Leiserson, Harald Prokop and Keith H. Randall in 1998 [4]. The De Bruijn sequence chosen is even, which implies five leading zeros [5]:
/**
* bitScanForward
* @author Charles E. Leiserson
* Harald Prokop
* Keith H. Randall
* "Using de Bruijn Sequences to Index a 1 in a Computer Word"
* @param bb bitboard to scan
* @precondition bb != 0
* @return index (0..63) of least significant one bit
*/
const char xindex64[64] = {
63, 0, 58, 1, 59, 47, 53, 2,
60, 39, 48, 27, 54, 33, 42, 3,
61, 51, 37, 40, 49, 18, 28, 20,
55, 30, 34, 11, 43, 14, 22, 4,
62, 57, 46, 52, 38, 26, 32, 41,
50, 36, 17, 19, 29, 10, 13, 21,
56, 45, 25, 31, 35, 16, 9, 12,
44, 24, 15, 8, 23, 7, 6, 5
};
#define set_bit_to_0(x64,xr) (x64) &= ~((unsigned __int64)1<<(xr))
char SEARCH::bitScanForward_Clear(unsigned __int64 &bb)
{
char index;
unsigned __int64 debruijn64 =(unsigned __int64)(0x07EDD5E59A4E28C2);
ASSERT (bb != 0);
// Ignorer le warning de compilation
index=xindex64[((bb & -bb) * debruijn64) >> 58];
set_bit_to_0(bb, index);
return index;
}
See also
Forum Posts
- Gibbon 2.32a : 2219 by Patrick Buchmann, CCC, April 16, 2007
- Gibbon 2.41a : 2258 by Patrick Buchmann, CCC, June 19, 2007
- Gibbon 2.42c : 2200 by Patrick Buchmann, CCC, October 01, 2007
- Gibbon 2.51a : 2249 by Patrick Buchmann, CCC, February 12, 2009
- Gibbon 2.52a : 2208 by Patrick Buchmann, CCC, March 03, 2009
External Links
Chess Engine
Misc
References
- ↑ Gibbons at Play - Chinese painting 15th century, Hanging scroll, ink and colors on paper, Current location: National Palace Museum, Taipei, Taiwan, Gibbons Wikipedia.de (German)
- ↑ gibbon home page
- ↑ Features based on the gibbon home page
- ↑ Charles E. Leiserson, Harald Prokop, Keith H. Randall (1998). Using de Bruijn Sequences to Index a 1 in a Computer Word. pdf
- ↑ gibbon home page, Gibbon 2.57.a - uci - Cchess_2.h, Cchess_4.h, maj.cpp
