Gibbons at Play 1
- 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,
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 1.
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 3. The De Bruijn sequence chosen is even, which implies five leading zeros 4:
/**
* 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
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↩︎