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Move Generation: En Passant

The basic move generator only plays chess as it used to be several hundred years ago, before e.p. capture and castling where invented. It does not even allow pawns to move two squares from their initial position. These 'advanced moves' unfortunately add a relatively large amount of complexity to the move generator, because they violate the basic philosophy of most chess moves. In castling two pieces move, in e.p. capture one does not capture the piece by stepping in its place. In addition, these moves cannot be performed at just any time, and we have to test all the conditions that have to be obeyed.

Pawn's Double Move

The discussion of the pawn's double move was deferred to this page, because it is intimately tied to the e.p. capture. The basic move generator terminates the inner loop for crawling pieces after the first iteration, by faking a capture through incrementing the captured piece t. Pawns on the 2nd/7th row have to be given one extra iteration, though. This is simply done by decrementing t back to its original value if the target square y of the current iteration is on the 3rd/6th row. To test this for both colors simultaneaously the expressions `p<3&6*k+(y&V)==S` is used. It effectively compares the rank number of the pawn, `(y&V)` with the rank from which they have to make the extra step. Which rank this is, is derived from the color k of the pawn: by multiplying this by 6 we obtain either 0x30 or 0x60, which are subtracted from `S=0x80` to optain the sought rank numbers 0x50 (2nd) and 0x20 (6th). In addition the field from which the move was extended is remembered in F, which normally holds the invalid square code S, for passing it to the next ply so that the move generator there knows where it can try an e.p. capture.

E.p. Capture

The e.p. square is thus passed as a parameter E to the next level of the search tree. If a pawn tries to move to this square, (`p<3 & y==E`) it must be through a diagonal step (it cannot come from the square to which the opponent just moved!). For e.p. capture this move does have to be executed, and we have to be able to make a piece disappear that is not in the target square y when the move is later performed. To this end the square of the captured piece is held in a separete variable H, which is normally set to y, but for e.p. capture is set to the 'companion square' `E^16`, by flipping the lowest bit of the rank number.

Below the code lines involved in Pawn double move and e.p. capture are 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=9,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  */
W((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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