Difference between revisions of "Move Generation"

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=Special Generators=  
 
=Special Generators=  
Most programs use special move generators for the [[Quiescence Search|quiescence search]], sometimes supplemented by one for getting out of [[Check|check]]. These special cases can be made more efficient than generating and testing each possible move to fit specific criteria. For example, if the king is in check, the only possible legal moves are to capture the attacking piece, block the attacker if it is a "ray" piece, or move the king to safety. Special generators for the quiescence search might want to generate checks in addition to [[Captures|captures]] and [[Promotions|promotions]]. They can use the fact that a knight or bishop must start off on the [[Color of a Square#SameColor|same color square]] as the opponent king if they are to attack it. And rooks can only generate at most 2 checking moves...to the [[Intersection Squares|squares]] with the rook's column and the king's row or the king's column and the rooks row.  
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Most programs use special move generators for the [[Quiescence Search|quiescence search]], sometimes supplemented by one for getting out of [[Check|check]]. These special cases can be made more efficient than generating and testing each possible move to fit specific criteria. For example, if the king is in check, the only possible legal moves are to capture the attacking piece, block the attacker if it is a "ray" piece, or move the king to safety. Special generators for the quiescence search might want to generate checks in addition to [[Captures|captures]] and [[Promotions|promotions]]. They can use the fact that a knight or bishop must start off on the [[Color of a Square#SameColor|same color square]] as the opponent king if they are to attack it. And rooks can only generate at most 2 checking moves...to the [[Intersection Squares|squares]] with the rook's column and the king's row or the king's column and the rooks row.  
  
 
Similar tricks can be used for generating possible moves out of check, which must be by the king, capturing the opponent's checking piece, or blocking its attack if it is a ray piece. When in [[Double Check|double check]], only king moves are permitted.
 
Similar tricks can be used for generating possible moves out of check, which must be by the king, capturing the opponent's checking piece, or blocking its attack if it is a ray piece. When in [[Double Check|double check]], only king moves are permitted.
  
=Chunk move generation=
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=Chunk Move Generation=
 
With [[Move Ordering|move ordering]] in mind, chess programs, while traversing pieces and their move-target sets once, store and buffer generated moves inside one or two [[Move List|move lists]] (i.e. for [[Tactical Moves|tactical]] and [[Quiet Moves|quiet moves]]), which is convenient for book-keeping and assigning scores based on [[MVV-LVA]], [[Static Exchange Evaluation|SEE]], [[History Heuristic|history]], [[Piece-Square Tables|piece square table]] etc., to later perform a [https://en.wikipedia.org/wiki/Selection_sort selection sort] before actually [[Make Move|making the move]].
 
With [[Move Ordering|move ordering]] in mind, chess programs, while traversing pieces and their move-target sets once, store and buffer generated moves inside one or two [[Move List|move lists]] (i.e. for [[Tactical Moves|tactical]] and [[Quiet Moves|quiet moves]]), which is convenient for book-keeping and assigning scores based on [[MVV-LVA]], [[Static Exchange Evaluation|SEE]], [[History Heuristic|history]], [[Piece-Square Tables|piece square table]] etc., to later perform a [https://en.wikipedia.org/wiki/Selection_sort selection sort] before actually [[Make Move|making the move]].
 
<span id="Staged"></span>
 
<span id="Staged"></span>
=Staged move generation=  
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=Staged Move Generation=  
 
Some programs do not generate all moves at once, but do it in several stages (i.e. [[Hash Move|hash move]] first, then [[Captures|captures]], then [[Killer Move|killer moves]], then all the rest in a chunk) on the premise that if one of the early moves causes a cutoff, then we may save on the effort of generating the rest of the moves <ref>[http://www.talkchess.com/forum/viewtopic.php?t=27657 Move generation: staged vs all-at-once] by [[Steven Edwards]], [[CCC]], April 30, 2009</ref>.
 
Some programs do not generate all moves at once, but do it in several stages (i.e. [[Hash Move|hash move]] first, then [[Captures|captures]], then [[Killer Move|killer moves]], then all the rest in a chunk) on the premise that if one of the early moves causes a cutoff, then we may save on the effort of generating the rest of the moves <ref>[http://www.talkchess.com/forum/viewtopic.php?t=27657 Move generation: staged vs all-at-once] by [[Steven Edwards]], [[CCC]], April 30, 2009</ref>.
  
 
=Debugging=  
 
=Debugging=  
 
It is important to ensure that the move generator works properly. Although this could be tested by playing many games, a better approach is to write a [[Perft]] function. This function [[Recursion|recursively]] generates moves for the current position and all children up to a certain depth, and by counting all the leaf nodes, it can be compared to a [[Perft Results|table of values]] to test its accuracy.
 
It is important to ensure that the move generator works properly. Although this could be tested by playing many games, a better approach is to write a [[Perft]] function. This function [[Recursion|recursively]] generates moves for the current position and all children up to a certain depth, and by counting all the leaf nodes, it can be compared to a [[Perft Results|table of values]] to test its accuracy.
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<span id="Reverse"></span>
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=Un-Move Generation=
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'''Un-Move Generation''' produces a list of all possible moves (including [[Captures|captures]] and [[Promotions|promotions]]) which could be [[Unmake Move|un-made]] from a target position to reach all possible legal predecessor or parent positions for [[Retrograde Analysis|retrograde analysis]] tasks.
  
 
=See also=
 
=See also=
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* [http://www.talkchess.com/forum/viewtopic.php?t=54465 Black/White symmetry in move generation] by Jeffery A Esposito, [[CCC]], November 25, 2014 » [[Color Flipping]]
 
* [http://www.talkchess.com/forum/viewtopic.php?t=54465 Black/White symmetry in move generation] by Jeffery A Esposito, [[CCC]], November 25, 2014 » [[Color Flipping]]
 
* [http://www.talkchess.com/forum/viewtopic.php?t=54704 Symmetric move generation using bitboards] by [[Lasse Hansen]], [[CCC]], December 20, 2014
 
* [http://www.talkchess.com/forum/viewtopic.php?t=54704 Symmetric move generation using bitboards] by [[Lasse Hansen]], [[CCC]], December 20, 2014
* [http://www.talkchess.com/forum/viewtopic.php?t=54796 Reverse move generation] by Kostas Oreopoulos, December 30, 2014 » [[Retrograde Analysis]]
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* [http://www.talkchess.com/forum/viewtopic.php?t=54796 Reverse move generation] by Kostas Oreopoulos, [[CCC]], December 30, 2014 » [[Move Generation#Reverse|Un-Move Generation]], [[Retrograde Analysis]]
 
==2015 ...==
 
==2015 ...==
 
* [http://www.talkchess.com/forum/viewtopic.php?t=55275 On bitboard legal move generation] by [[Lasse Hansen]], [[CCC]], February 09, 2015
 
* [http://www.talkchess.com/forum/viewtopic.php?t=55275 On bitboard legal move generation] by [[Lasse Hansen]], [[CCC]], February 09, 2015
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==2020 ...==
 
==2020 ...==
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=2&t=74917 Comparing 4 move generators: 0x88 vs 10x12 vs 10x12 + bitboards HYBRID vs Pure MAGIC BITBOARDS] by [[Maksim Korzh]], [[CCC]], August 28, 2020
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=2&t=74917 Comparing 4 move generators: 0x88 vs 10x12 vs 10x12 + bitboards HYBRID vs Pure MAGIC BITBOARDS] by [[Maksim Korzh]], [[CCC]], August 28, 2020
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'''2021'''
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=76835 Staged move generation?] by [[Niels Abildskov]], [[CCC]], March 10, 2021 » [[Move Generation#Staged|Staged Move Generation]]
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=76835 Staged move generation?] by [[Niels Abildskov]], [[CCC]], March 10, 2021 » [[Move Generation#Staged|Staged Move Generation]]
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77350 Being silly with perft and legal move generation] by [[Jakob Progsch]], [[CCC]], May 19, 2021 » [[Perft]], [[#Legal|Legal Move Generation]], [[En passant]]
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77350 Being silly with perft and legal move generation] by [[Jakob Progsch]], [[CCC]], May 19, 2021 » [[Perft]], [[#Legal|Legal Move Generation]], [[En passant]]
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* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77671 Efficiently Generated Legal moves question] by Pedro Duran, [[CCC]], July 08, 2021 » [[Legal Move]]
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77671 Efficiently Generated Legal moves question] by Pedro Duran, [[CCC]], July 08, 2021 » [[Legal Move]]
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77906 Move generator advice] by Ellie Moore, [[CCC]], August 08, 2021
 
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=77906 Move generator advice] by Ellie Moore, [[CCC]], August 08, 2021
* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=78084 legal or pseudolegal move generator?] by Pier Carlo, [[CCC]], September 03, 2021
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* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=78084 Legal or pseudolegal move generator?] by Pier Carlo, [[CCC]], September 03, 2021
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* [http://www.talkchess.com/forum3/viewtopic.php?f=7&t=78164 Distinguish rook and bishop move efficiently] by [[Guido Flohr]], [[CCC]], September 14, 2021
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* [https://www.talkchess.com/forum3/viewtopic.php?f=7&t=78913 Fast reverse move generation] by koedem, [[CCC]], December 18, 2021 » [[Move Generation#Reverse|Un-Move Generation]], [[Retrograde Analysis]]
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: [https://www.talkchess.com/forum3/viewtopic.php?f=7&t=78913&start=1 Re: Fast reverse move generation] by [[Peter Österlund]], [[CCC]], December 18, 2021 » [[Texel]]
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'''2022'''
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* [https://www.talkchess.com/forum3/viewtopic.php?f=7&t=79365 Move generation for bitboards] by [[Elias Nilsson]], [[CCC]], February 16, 2022
  
 
=External Links=  
 
=External Links=  

Latest revision as of 17:39, 12 March 2022

Home * Board Representation * Move Generation

Blast Generation [1]

Generation of moves is a basic part of a chess engine with many variations concerning a generator or an iterator to loop over moves inside the search routine. The implementation heavily depends on the board representation, and it can be generalized into two types, pseudo-legal and legal move generation.

Legality

Pseudo-legal

In Pseudo-legal move generation pieces obey their normal rules of movement, but they're not checked beforehand to see if they'll leave the king in check. It is left up to the move-making function to test the move, or it is even possible to let the king remain in check and only test for the capture of the king on the next move.

Legal

In Legal move generation, as the name implies, only legal moves are generated, which means extra time must be spent to make sure the king isn't going to be left or placed in check after each move. Pins are the main difficulty, particularly when en passant is involved.

Special Generators

Most programs use special move generators for the quiescence search, sometimes supplemented by one for getting out of check. These special cases can be made more efficient than generating and testing each possible move to fit specific criteria. For example, if the king is in check, the only possible legal moves are to capture the attacking piece, block the attacker if it is a "ray" piece, or move the king to safety. Special generators for the quiescence search might want to generate checks in addition to captures and promotions. They can use the fact that a knight or bishop must start off on the same color square as the opponent king if they are to attack it. And rooks can only generate at most 2 checking moves...to the squares with the rook's column and the king's row or the king's column and the rooks row.

Similar tricks can be used for generating possible moves out of check, which must be by the king, capturing the opponent's checking piece, or blocking its attack if it is a ray piece. When in double check, only king moves are permitted.

Chunk Move Generation

With move ordering in mind, chess programs, while traversing pieces and their move-target sets once, store and buffer generated moves inside one or two move lists (i.e. for tactical and quiet moves), which is convenient for book-keeping and assigning scores based on MVV-LVA, SEE, history, piece square table etc., to later perform a selection sort before actually making the move.

Staged Move Generation

Some programs do not generate all moves at once, but do it in several stages (i.e. hash move first, then captures, then killer moves, then all the rest in a chunk) on the premise that if one of the early moves causes a cutoff, then we may save on the effort of generating the rest of the moves [2].

Debugging

It is important to ensure that the move generator works properly. Although this could be tested by playing many games, a better approach is to write a Perft function. This function recursively generates moves for the current position and all children up to a certain depth, and by counting all the leaf nodes, it can be compared to a table of values to test its accuracy.

Un-Move Generation

Un-Move Generation produces a list of all possible moves (including captures and promotions) which could be un-made from a target position to reach all possible legal predecessor or parent positions for retrograde analysis tasks.

See also

General

Board Array

0x88
Vector Attacks

Bitboards

Hardware

Chess Programs

Publications

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Forum Posts

1990 ...

Re: move generators in computer chess, Tricky bit tricks by Marcel van Kervinck, rgcc, October 20, 1994 » Traversing Subsets of a Set

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2005 ...

Re: Yet another new bitboard move generation method by Harm Geert Muller, Winboard Forum, October 01, 2006 [4]
Re: Move generation: staged vs all-at-once by Lance Perkins, CCC, April 30, 2009

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2012

2014

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2016

2017

2019

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2021

Re: Fast reverse move generation by Peter Österlund, CCC, December 18, 2021 » Texel

2022

External Links

References

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