Editing Null-move heuristic (section)

== Implementation ==

In employing the null-move heuristic, the computer program first forfeits the turn of the side whose turn it is to move, and then performs an alpha–beta search on the resulting position to a shallower depth than it would have searched the current position had it not used the null move heuristic.  If this shallow search produces a cutoff, it assumes the full-depth search in the absence of a forfeited turn would also have produced a cutoff.  Because a shallow search is faster than deeper search, the cutoff is found faster, accelerating the computer chess program.  If the shallow search fails to produce a cutoff, then the program must make the full-depth search.

This approach makes two assumptions.  First, it assumes that the disadvantage of forfeiting one's turn is greater than the disadvantage of performing a shallower search.  Provided the shallower search is not too much shallower (in practical implementation, the null-move search is usually 2 or 3 [[ply (game theory)|plies]] shallower than the full search would have been), this is usually true.  Second, it assumes that the null-move search will produce a cutoff frequently enough to justify the time spent performing null-move searches instead of full searches.  In practice, this is also usually true.