1This file contains a description of GNU's heuristics. 2 Copyright (C) 1986, 1987 Free Software Foundation, Inc. 3 4This file is part of CHESS. 5 6CHESS is distributed in the hope that it will be useful, 7but WITHOUT ANY WARRANTY. No author or distributor 8accepts responsibility to anyone for the consequences of using it 9or for whether it serves any particular purpose or works at all, 10unless he says so in writing. Refer to the CHESS General Public 11License for full details. 12 13Everyone is granted permission to copy, modify and redistribute 14CHESS, but only under the conditions described in the 15CHESS General Public License. A copy of this license is 16supposed to have been given to you along with CHESS so you 17can know your rights and responsibilities. It should be in a 18file named COPYING. Among other things, the copyright notice 19and this notice must be preserved on all copies. */ 20 21 -- requested by main author 22Heuristic descriptions for CHESS. 23 24Revision: 12-16-87 25 26Copyright (c) 1987 by John Stanback 27 28 Here is a brief description of the heuristics used in the positional 29 evaluator of the GNU Chess program. Many heuristics are functions of the 30 stage of the game which is based on the total non-pawn material remaining 31 for both sides. 32 33 34PAWNS 35 The material value of a pawn is 100 points. Isolated pawns get a 36 penalty depending on which file they occupy: 37 (12,14,16,20,20,16,14,12) for files (a..h). 38 Doubled pawns (which are not also isolated) get a penalty of 12 39 points. Backward pawns (defined simply as not being defended by a 40 pawn with the square in front also not defended by a a pawn) are 41 penalized 6 points. A 4 point penalty is also invoked for each attack 42 by the opponent to a backward pawn and for a backward pawn on a 43 half-open file. Pawn Advancement in the centre is given a bonus of 44 about 4 points per rank in the opening increasing to about 8 points 45 per rank in the ending. Advancement on the edges is given a lower 46 bonus. Pawns on the e and d files and on the 2nd rank are given a 10 47 point penalty. An additional penalty of 15 points is invoked if these 48 pawns are also blocked. Pawns within 2 squares of the king are given 49 a 10 point bonus. Passed pawns are given a bonus for increasing rank 50 which is a function of stage of the game and of whether the opponent 51 blocks or attacks one or more squares in front of the pawn or if the 52 opponents king is in the square of the pawn. This bonus ranges from 53 about 15 points for a pawn on the second rank up to about 300 points 54 for a passed pawn on the 7th rank which can't be stopped from 55 queening. 56 57 58KNIGHTS 59 The material value of a knight is 330 points. The main heuristic for 60 knights is a bonus for proximity to the centre. This varies from 0 61 points in the corners to 30 points in the centre. Knights are also 62 given a bonus for being within 2 squares of each enemy piece. This 63 bonus is a function of the stage of the game, equalling 4 points in 64 the end game. A penalty of 1 point per square is given for distance 65 from either king. A bonus of up to 8 points (depends on stage) is 66 given for knights which can't be driven away by enemy pawns. 67 68 69BISHOPS 70 The material value of a bishop is 330 points. Bishops are given a 71 bonus as material falls off the board equalling 10 points in the end 72 game. Bishops get a bonus for mobility and Xray mobility thru pieces 73 but not pawns. This bonus ranges from -4 points for a totally blocked 74 bishop up to 18 points for a bishop attacking 12 or more squares. 75 Xray attacks on an enemy R,Q,K or any undefended piece are given an 8 76 point bonus. Bishops are given a bonus of 14 points if they lie on 77 the edge of the board up to 22 points if the lie in the centre. A 78 bishop is given a bonus of up to 5 points for each attack to a square 79 adjacent to the enemy king. 80 81 82ROOKS 83 The material value of a rook is 520 points. Rook mobility is handled 84 similiarly to bishops with a bonus of 0 points if blocked up to 20 85 points if attacking 12 squares or more. A bonus of 8 points for Xray 86 attacks is handled as it is for bishops. Rooks are given a bonus of 87 10 points for occupying a file with no friendly pawns and a bonus of 88 4 points if no enemy pawns lie on that file. After the opening Rooks 89 are penalized slightly depending on "taxicab" distance to the enemy 90 king. 91 92 93QUEENS 94 The material value of a queen is 980 points. The only heuristic for a 95 queen is that after the opening it is penalized slightly for 96 "taxicab" distance to the enemy king. 97 98 99KINGS 100 Kings are given a penalty for proximity to the centre in the opening 101 and a bonus for proximity to the centre in the endgame. The penalty 102 is about 24 points for being in the centre in the opening with a 103 bonus of about 36 points for being in the centre in the endgame. 104 Kings are penalized for lying on an open or half-open file or if the 105 adjacent file closest to the corner is open or half-open. This 106 penalty is up to 23 points in the opening and goes to zero in the end 107 game. The King is penalized up to 8 points if there are no pawns 108 immediately adjacent. A penalty is invoked depending on the number of 109 "safe" checks available by the opponent. This penalty ranges from 6 110 points for one such check to 50 points for 4 or more. Depending on 111 game stage, Kings are given up to 10 points for castling and a 112 penalty of up to 40 points for moving before castling. 113 114 115SPECIAL 116 If more than one piece is "hung" (attacked and not defended or 117 attacked by an enemy piece of lower value) an extra penalty of 10 118 points is invoked for that side and the search may be extended one 119 ply. Pinned or trapped pieces are treated similarly. A special mating 120 routine is used if one side has only a king and the other has mating 121 material. 122 123