Incremental Updates

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Incremental updates keep global or shared structures up to date after making and unmaking moves on the internal board, likely inside a depth-first, alpha-beta like search-algorithm. Incremental update on the same global or shared data ever and ever, will likely keep it in fast cache memory, and saves memory bandwidth compared to a Copy-Make approach, which needs a make-update on the copy as well, but no unmake-update rather than decrementing a ply-index or fetching a pointer.

=Chess Position= Incremental updates by make/unmake are obligatory for square centric board arrays like mailbox and 0x88 along with their piece-lists, as well for the classical bitboard board-definition. A move only affects a small fraction of the complete board structure and the update cost is small compared to a copy of the whole board. Also one usually doesn't need an array of boards for random access, but only the current one.

However, there are aspects of a chess position, which can not generally restored from a child-position by unmaking a move, and which therefor need to be restored by either re-playing the whole game record from the root position, or to be memorized inside an array indexed by ply or on a stack (LIFO), most notably ep state, castling rights and the halfmove clock enforcing the fify-move rule. Also, if one doesn't keep a captured piece (if any) inside the move object, one has to memorize those pieces elsewhere as well, to restore the target square of the move about to unmake.

=Material and Hash keys= Subject of incremental updates are further data which only change conditionally on certain move types, like material signature, only affected by captures or promotions, or for instance zobrist keys for a pawn hash table, only affected by moving/capturing pawns. Standard zobrist keys for TT purpose and dependent on ep state and castling rights, are slightly more complicated to restore. If stored inside a record anyway to detect repetitions, one has the option to restore the key after unmake from that record.

It is a good idea to compare incremental updated stuff like zobrist keys with its from scratch generated counterparts for debug purposes.

=Evaluation= Other global structures or variables as candidates of an incremental update are related to Evaluation, beside the mentioned material (balance), lazy evaluation terms, like the sum of all piece-square values.

=Attack table= Some programs keep Attack and Defend Maps up to date incrementally, which is runtime dependent on the number of squares whose attacks-to are influenced by the move. Due to its size and utilization, copy-make is no issue, but on the fly generation if actually needed.

=See also=
 * Attack and Defend Maps
 * BCH Hashing
 * Bitboard Update By Move
 * Color Flipping
 * Copy-Make
 * Encoding Moves
 * Lazy Evaluation
 * Make Move
 * Material
 * Unmake Move
 * Zobrist Hashing

=Publications=
 * John J. Scott (1969). A chess-playing program. Machine Intelligence Vol. 4
 * Peter W. Frey (1983). The Alpha-Beta Algorithm: Incremental Updating, Well-Behaved Evaluation Functions, and Non-Speculative Forward Pruning. Computer Game-Playing (ed. Max Bramer), pp. 285-289. Ellis Horwood Limited

=Forum Posts=

2005 ...

 * Incremental updating for positional evaluation by Steven Edwards, CCC, March 27, 2008
 * Incremental Zobrist - slow? by Vlad Stamate, CCC, June 20, 2009 » Zobrist Hashing
 * undo move vs. Position Cloning by BoldReceiver, CCC, September 16, 2009
 * Re: undo move vs. Position Cloning by Don Dailey, CCC, September 16, 2009 » Doch

2010 ...

 * Incremental or non-incremental PST evaluation calcs by Mark Pearce, CCC, January 26, 2012 » Piece-Square Tables
 * copy/make vs make/unmake by Robert Hyatt, CCC, January 07, 2014
 * Incrementally-updated attack map by Harm Geert Muller, April 21, 2014 » Attack and Defend Maps
 * Memory usage in make/unmake vs logic complexity by Matthew Lai, CCC, August 30, 2014

2015 ...

 * Incremental update by Matthew R. Brades, CCC, December 19, 2015

=External Links=
 * Search Techniques in REBEL (Lazy Eval) from How Rebel Plays Chess by Ed Schröder » Lazy Evaluation, Rebel

=References=

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