@*Intro. I'm (hastily) writing this program in order to get some basic
experience with a generalization of the game of Reversi (which is popularly
called ``Othello'' in its principal variant).
I consider an $m\times n$ board, whose rows are numbered \.1, \.2, \.3, \dots,
and whose columns are named \.a, \.b, \.c, \dots.
A cell is identified by letter and digit. (I don't forbid $m>9$ or
$n>26$; but bugs will show up if those parameters get too big.)
When prompted, the user should provide (on |stdin|) the name of the cell where
the next move is to be made, followed by a newline (aka `return').
The newline can optionally be preceded by `\.!', in which case the
contents of the current board will be printed (on |stdout|).
The first four moves simply place pieces on the board; they are made without
captures. (They are considered to be moves $-3,ドル $-2,ドル $-1,ドル and 0ドル,ドル
so that move~1 will be the normal first move.) In standard Othello we
have $m=n=9,ドル and the preliminary moves are \.{d5}, \.{e5}, \.{e4}, \.{d4};
the first move might then be \.{d3}, in which case the color of \.{d4}
will be reversed.
This program actually allows more than two players. With $p$ players,
we start with move 1ドル-p^2,ドル and use the first $p^2$ moves to set up the
initial position.
I don't attempt to do anything tricky. Cells of the board contain $-1$
if they're unoccupied, $c$ if they are occupied and the top color is~$c$.
The colors are 0, 1, \dots,~$p-1$. The value of |board[i][j]| is
maintained for 0ドル\le i\le m+1$ and 0ドル\le j\le n+1,ドル but the
boundary cells remain unoccupied.
@d m 8 /* this many rows */
@d n 8 /* this many columns */
@d p 2 /* this many players */
@c
#include <stdio.h>
#include <stdlib.h>
char board[m+2][n+2]; /* cells of the current board position */
int move; /* the current move number */
char deli[8]={-1,-1,-1,0,0,1,1,1},
 delj[8]={-1,0,1,-1,1,-1,0,1}; /* the eight directions */
char buffer[8]; /* used for input */
int total[p]; /* how many pieces show this color? */
@<subroutines@>;
void main(void) {
 register int i,j,k,l,pass,player;
 for (i=0;i<=m+1;i++) for (j=0;j<=n+1;j++) board[i][j]=-1; for (move=1-p*p,player=0;;move++,player=(player+1)% p) { for (pass=0;pass<p;pass++) { for (i=1;i<=m;i++) for (j=1;j<=n;j++) if (islegal(i,j,player)) goto nextmove; printf("(player %c cannot move)\n", '0'+player@t)@>);
 player=(player+1)%p;
 }
 break; /* the game is over: &#124;p&#124; passes in a row */
nextmove: printf("Move %d, player %c: ",
 move,'0'+player);
 fflush(stdout); /* make sure the user sees the prompt */
 @<set &#124;i&#124; and &#124;j&#124; to the coordinates of the next move@>;
 makemove(i,j,player);
 if (buffer[2]=='!') print_board();
 }
 print_board();
}
@ @<set &#124;i&#124; and &#124;j&#124; to the coordinates of the next move@>=
if (!fgets(buffer,8,stdin)) {
 fprintf(stderr,"Unexpected end of input!\n");
 exit(-1);
}
j=buffer[0]-'a'+1,i=buffer[1]-'0';
if (i<1 &#124;&#124; i>m &#124;&#124; j<1 &#124;&#124; j>n) {
 fprintf(stderr,"Cell `%c%c' doesn't exist!\n",
 buffer[0],buffer[1]);
 print_board();
 goto nextmove;
}
if (!islegal(i,j,player)) {
 fprintf(stderr,"No! `%c%c' isn't a legal move for %c.\n",
 buffer[0],buffer[1],'0'+player);
 print_board();
 goto nextmove;
}
@ @<sub...@>=
void print_board(void) {
 register i,j,k;
 for (k=0;k<p;k++) total[k]=0; for (i=1;i<=m;i++) { for (j=1;j<=n;j++) { k=board[i][j]; if (k>=0) total[k]++;
 printf("%c",
 k<0?'.': '0'+k); } if (i==m) for (k=0;k<p;k++) printf(" %d", total[k]); printf("\n"); } } @ This subroutine decides whether or not it's OK to place a piece of color~&#124;c&#124; in cell $(i,j)$ of the board. We assume (without checking) that 1ドル\le i\le m,ドル 1ドル\le j\le n,ドル and 0ドル\le c<p$. @<sub...@>=
int islegal(int i,int j,int c) {
 register int ii,jj,k,l;
 if (board[i][j]>=0) return 0; /* already occupied */
 if (move<=0) return 1; /* we're just gettin' started */ for (k=0;k<8;k++) @<if direction &#124;k&#124; allows a move, &#124;return 1&#124;; otherwise &#124;continue&#124;@>;
 return 0;
}
@ @<if direction &#124;k&#124; allows a move, &#124;return 1&#124;; otherwise &#124;continue&#124;@>=
{
 for (ii=i+deli[k],jj=j+delj[k],l=0;board[ii][jj]>=0;
 ii+=deli[k],jj+=delj[k],l++)
 if (board[ii][jj]==c) goto maybe;
 continue; /* no occurrences of &#124;c&#124; in direction &#124;k&#124; */
maybe:@+if (l) return 1; /* yes, that move reverses at least &#124;l&#124; cells */
 continue; /* two adjacent &#124;c&#124;'s */
}
@ @<sub...@>=
void makemove(int i,int j,int c) {
 register int ii,jj,k;
 board[i][j]=c;
 if (move<=0) return; /* just gettin' started */ for (k=0;k<8;k++) @<do all reversals in direction &#124;k&#124;@>;
}
@ @<do all reversals in direction &#124;k&#124;@>=
{
 for (ii=i+deli[k],jj=j+delj[k]; board[ii][jj]>=0;
 ii+=deli[k],jj+=delj[k])
 if (board[ii][jj]==c) goto reverse;
 continue; /* no occurrences of &#124;c&#124; in direction &#124;k&#124; */
reverse:@+for (ii-=deli[k],jj-=delj[k];ii!=i &#124;&#124; jj!=j;
 ii-=deli[k],jj-=delj[k]) board[ii][jj]=c;
}
@*Index.
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