From: H.G. Muller <h.g.muller@hccnet.nl>
Date: Thu, 3 Nov 2011 16:37:34 +0000 (+0100)
Subject: Fix under-promotion hash mixup
X-Git-Tag: 4.8S~23
X-Git-Url: http://winboard.nl/cgi-bin?p=fairymax.git;a=commitdiff_plain;h=a93cdacb82bd916e110d71611754bb9ef9be1e67;hp=6cd25f6e4f2a524bdd85f34583e47afbae746488

Fix under-promotion hash mixup

The hash table was invalidated on under-promotion by adding the promotion
'deficit' to the J hash key. But for some fairy-pieces (in particular
Spartan G vs W) this difference apparently caused false hits in the
null-move search, leading to misqualification of positions as stalemate.
Presumably because the J key is also modified by stm, and different
stm and promo-deficit could cancel each other. In stead we now modify
the Z hash key with the promo-deficit.
---

diff --git a/fairymax.c b/fairymax.c
index e43bd46..c68ca20 100644
--- a/fairymax.c
+++ b/fairymax.c
@@ -222,7 +222,7 @@ int k,q,l,e,E,z,n;      /* (q,l)=window, e=current eval. score, E=e.p. sqr.*/
         {if(v+I&&x==K&y==L&gt==GT)             /*   if move found          */
          {Q=-e-i;O=F;LL=L;prom=gt;
           if(b[y]-u&15)prom=b[y]-=PromPiece,   /* under-promotion, correct */
-                       J+=PromPiece;           /*  piece & invalidate hash */
+                       Z+=PromPiece;           /*  piece & invalidate hash */
           a->D=99;a->V=0;                      /* lock game in hash as draw*/
           R-=i/FAC;                            /*** total captd material ***/
           Fifty = t|p<3?0:Fifty+1;