TalkChess.com

[Pawel Koziol]

We have implemented the stuff Edmund described in Glass and it works reasonably well, except that it doesn't scale with time control. Thus, an engine that performs as about 2000 Elo at blitz feels like 1800 in a longer game.

Edmund is right that it's better to get pseudorandom values from position hash ( randomPercentage = hashKey % 100 rather than randomPercentage = rand() % 100 )

later You can code something like

for all moves
{
if ( randomPercentage < errorPercentage
&& ply > canErrFromPly ) newScore = alpha;
else newScore = Search(...)
}

be sure, though, that engine actually tries at least one legal move, otherwhise You might get a false mate score.

Another useful thing is slowing the engine down - humans tend to move faster when an engine replies instantly, and thus they play much weaker chess. Also, the algorithm described here results in engine reporting intimidating (if fake) depth, which looks plain ugly without a slowdown.

You might also want to look at the Phalanx source code - it also has a routine that allows forgetting about certain moves during search, depending on the move properties (for example longer moves and backward moves tend to be omitted more often)
_________________
Pawel Koziol
http://www.pkoziol.cal24.pl/rodent/rodent.htm