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Null Move Reductions work similar to Feldmann's Fail-High Reductions, but use a Null Move Search rather than static evaluation and reduces by a amount of four plies rather than one. Omid David and Nathan S. Netanyahu introduced so called Extended Null-Move Reductions ^[1] with an adaptive R based on depth.

If a null move fail-high occurs - opposed to Null Move Pruning - the search is reduced by four plies, rather than pruned. Thus, Null Move Reductions are therefor less vulnerable to Zugzwang and might even applied in (late) endings.

if ( nullMoveAllowed && ...) {
   R = depth > 6 ? 4 : 3;
   makeNullMove()
   score = -zwSearch(1-beta, depth-R-1) // -AlphaBeta (0-beta, 1-beta, depth-R-1)
   unmakeNullMove();
   if (score >= beta ) {
      depth -= 4; // reduce search
      if ( depth <= 0 )
          return Evaluate(); // Quiescence
   }
}
// continue search

For zwSearch, see Zero Window Search inside the Principal Variation Search.

See also

• Double Null Move
• Fail-High Reductions
• Null Move Observation
• Null Move Pruning
• Standing Pat in Quiescence Search

Forum Posts

• Extended Null-Move Reductions by Alvaro Cardoso, CCC, August 20, 2010
• Null move alterative in endgames by Aleks Peshkov, CCC, November 16, 2011
• thoughts on null-move reduction by Youri Matiounine, CCC, June 14, 2015 » Null Move Pruning

References

1. ^ Omid David, Nathan S. Netanyahu (2008). Extended Null-Move Reductions. CG 2008, pdf
   

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