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Upon entering D() for scoring a new position,
we first look in the hash table to see what we know about this position.
A retrieved table entry that was calculated at sufficient depth that contains an appropriate type result
(i.e. exact within the window, or the proper bound type when outside the current window)
is immediately used by returning the value without any further search.
If the position was found in the table, it can also be of insifficient depth, or it can be incompatibility with the current window (e.g. an upper bound above beta, where we need lower bounds). In both cases we use the best move from the table as a first try in the search that we now have to do. We then start the iterative deepening at the depth of the table result, or the requested depth if that was lower (and we could not accept the table value because of bad window). After all, the only thing used from previous iterations is the best move, and we already have one from the table. (This might be unjustified if the table value was a fail low, where all moves considered but all were deemed so bad that they were not even worth figuring out how bad. We then might have no idea at all what the best move is. A move from fail high at least must have some merit... The behavior when the the hash value is incompatible with the window is clearly a weak spot in micro-Max, much more advanced algorithms are concievable, deciding with which depth to start the iteration based on the positioning of the table value compared to the current window, and the likelyhood this implies for the 'best move' to be a useful guess.)
If the depth of the table result was that of QS,
we don't even use the best move, because there might be none
(in which case the table score represents the static evaluation).
If the position was not in the table at all,
(the entry is empty, as seen from a->K being zero),
or if the table is 'full'
(i.e. in the 8 accessible places,
in which case the dummy entry A[0] was retrieved, also empty)
iteration starts at a depth of zero, with no best move.
Just as in normal iterative deepening without a hash table.
Below the code that implements the hash-table retrieval is highlighted:
/***************************************************************************/
/* micro-Max, */
/* A chess program smaller than 2KB (of non-blank source), by H.G. Muller */
/***************************************************************************/
/* version 3.2 (2000 characters) features: */
/* - recursive negamax search */
/* - quiescence search with recaptures */
/* - recapture extensions */
/* - (internal) iterative deepening */
/* - best-move-first 'sorting' */
/* - a hash table storing score and best move */
/* - full FIDE rules (expt minor ptomotion) and move-legality checking */
#define F(I,S,N) for(I=S;I<N;I++)
#define W(A) while(A)
#define K(A,B) *(int*)(T+A+(B&8)+S*(B&7))
#define J(A) K(y+A,b[y])-K(x+A,u)-K(H+A,t)
#define U 16777224
struct _ {int K,V;char X,Y,D;} A[U]; /* hash table, 16M+8 entries*/
int V=112,M=136,S=128,I=8e3,C=799,Q,N,i; /* V=0x70=rank mask, M=0x88 */
char O,K,L,
w[]={0,1,1,3,-1,3,5,9}, /* relative piece values */
o[]={-16,-15,-17,0,1,16,0,1,16,15,17,0,14,18,31,33,0, /* step-vector lists */
7,-1,11,6,8,3,6, /* 1st dir. in o[] per piece*/
6,3,5,7,4,5,3,6}, /* initial piece setup */
b[129], /* board: half of 16x8+dummy*/
T[1035], /* hash translation table */
n[]=".?+nkbrq?*?NKBRQ"; /* piece symbols on printout*/
D(k,q,l,e,J,Z,E,z,n) /* recursive minimax search, k=moving side, n=depth*/
int k,q,l,e,J,Z,E,z,n; /* (q,l)=window, e=current eval. score, E=e.p. sqr.*/
{ /* e=score, z=prev.dest; J,Z=hashkeys; return score*/
int j,r,m,v,d,h,i=8,F,G;
char t,p,u,x,y,X,Y,H,B;
struct _*a=A;
/* lookup pos. in hash table*/
j=(k*E^J)&U-9; /* try 8 consec. locations */
while((h=A[++j].K)&&h-Z&&--i); /* first empty or match */
a+=i?j:0; /* dummy A[0] if miss & full*/
if(a->K) /* hit: pos. is in hash tab */
{d=a->D;v=a->V;X=a->X; /* examine stored data */
if(d>=n) /* if depth sufficient: */
{if(v>=l|X&S&&v<=q|X&8)return v; /* use if window compatible */
d=n-1; /* or use as iter. start */
}X&=~M;Y=a->Y; /* with best-move hint */
Y=d?Y:0; /* don't try best at d=0 */
}else d=X=Y=0; /* start iter., no best yet */
N++; /* node count (for timing) */
W(d++<n|z==8&N<1e7&d<98) /* iterative deepening loop */
{x=B=X; /* start scan at prev. best */
Y|=8&Y>>4; /* request try noncastl. 1st*/
m=d>1?-I:e; /* unconsidered:static eval */
do{u=b[x]; /* scan board looking for */
if(u&k) /* own piece (inefficient!)*/
{r=p=u&7; /* p = piece type (set r>0) */
j=o[p+16]; /* first step vector f.piece*/
W(r=p>2&r<0?-r:-o[++j]) /* loop over directions o[] */
{A: /* resume normal after best */
y=x;F=G=S; /* (x,y)=move, (F,G)=castl.R*/
do{H=y+=r; /* y traverses ray */
if(Y&8)H=y=Y&~M; /* sneak in prev. best move */
if(y&M)break; /* board edge hit */
if(p<3&y==E)H=y^16; /* shift capt.sqr. H if e.p.*/
t=b[H];if(t&k|p<3&!(r&7)!=!t)break; /* capt. own, bad pawn mode */
i=99*w[t&7]; /* value of capt. piece t */
if(i<0||E-S&&b[E]&&y-E<2&E-y<2)m=I; /* K capt. or bad castling */
if(m>=l)goto C; /* abort on fail high */
if(h=d-(y!=z)) /* remaining depth(-recapt.)*/
{v=p<6?b[x+8]-b[y+8]:0; /* center positional pts. */
b[G]=b[H]=b[x]=0;b[y]=u&31; /* do move, strip virgin-bit*/
if(!(G&M)){b[F]=k+6;v+=30;} /* castling: put R & score */
if(p<3) /* pawns: */
{v-=9*(((x-2)&M||b[x-2]!=u)+ /* structure, undefended */
((x+2)&M||b[x+2]!=u)-1); /* squares plus bias */
if(y+r+1&S){b[y]|=7;i+=C;} /* promote p to Q, add score*/
}
v=-D(24-k,-l-(l>e),m>q?-m:-q,-e-v-i, /* recursive eval. of reply */
J+J(0),Z+J(8)+G-S,F,y,h); /* J,Z: hash keys */
v-=v>e; /* delayed-gain penalty */
if(z==9) /* called as move-legality */
{if(v!=-I&x==K&y==L) /* checker: if move found */
{Q=-e-i;O=F;return l;} /* & not in check, signal */
v=m; /* (prevent fail-lows on */
} /* K-capt. replies) */
b[G]=k+38;b[F]=b[y]=0;b[x]=u;b[H]=t; /* undo move,G can be dummy */
if(Y&8){m=v;Y&=~8;goto A;} /* best=1st done,redo normal*/
if(v>m){m=v;X=x;Y=y|S&G;} /* update max, mark with S */
} /* if non castling */
t+=p<5; /* fake capt. for nonsliding*/
if(p<3&6*k+(y&V)==S /* pawn on 3rd/6th, or */
||(u&~24)==36&j==7&& /* virgin K moving sideways,*/
G&M&&b[G=(x|7)-(r>>1&7)]&32 /* 1st, virgin R in corner G*/
&&!(b[G^1]|b[G^2]) /* 2 empty sqrs. next to R */
){F=y;t--;} /* unfake capt., enable e.p.*/
}W(!t); /* if not capt. continue ray*/
}}}W((x=x+9&~M)-B); /* next sqr. of board, wrap */
C:if(m>I/4|m<-I/4)d=99; /* mate is indep. of depth */
m=m+I?m:-D(24-k,-I,I,0,J,Z,S,z,1)/2; /* best loses K: (stale)mate*/
if(!a->K|(a->X&M)!=M|a->D<=d) /* if new/better type/depth:*/
{a->K=Z;a->V=m;a->D=d;A->K=0; /* store in hash,dummy stays*/
a->X=X|8*(m>q)|S*(m<l);a->Y=Y; /* empty, type (limit/exact)*/
} /* encoded in X S,8 bits */
/*if(z==8)printf("%2d ply, %9d searched, %6d by (%2x,%2x)\n",d-1,N,m,X,Y&0x77);*/
}
if(z&8){K=X;L=Y&~M;}
return m;
}
main()
{
int j,k=8,*p,c[9];
F(i,0,8)
{b[i]=(b[i+V]=o[i+24]+40)+8;b[i+16]=18;b[i+96]=9; /* initial board setup*/
F(j,0,8)b[16*j+i+8]=(i-4)*(i-4)+(j-3.5)*(j-3.5); /* center-pts table */
} /*(in unused half b[])*/
F(i,M,1035)T[i]=random()>>9;
W(1) /* play loop */
{F(i,0,121)printf(" %c",i&8&&(i+=7)?10:n[b[i]&15]); /* print board */
p=c;W((*p++=getchar())>10); /* read input line */
N=0;
if(*c-10){K=c[0]-16*c[1]+C;L=c[2]-16*c[3]+C;}else /* parse entered move */
D(k,-I,I,Q,1,1,O,8,0); /* or think up one */
F(i,0,U)A[i].K=0; /* clear hash table */
if(D(k,-I,I,Q,1,1,O,9,2)==I)k^=24; /* check legality & do*/
}
}
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