Static NULL Move

[thevinenator]

Please explain what this code (from CPW) is doing.

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    if (depth < 3
        && (!is_pv)
        && (!flagInCheck)
        && (abs(beta - 1) > -INF+100))
    {
        int static_eval = eval(alpha, beta, 1);
 
        int eval_margin = 120 * depth;
        if (static_eval - eval_margin >= beta)
            return static_eval - eval_margin;
    }

if not in PV and not in check...but what is

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(abs(beta - 1) > -INF+100))

doing?
won't beta almost always be greater than the low mate limit?

[H.G.Muller]

Indeed,it will. So you will only refrain from doing this if you have mate scores. Which makes sense, because the margin is in centiPawn, and mate scores are not be measured in centiPawns.

This is just an inefficient implementation of futility pruning / razoring. Normally you do the test (in reverse, because of negamax) in the parent node, to not waste any time on MakeMove.