@*Intro. A simple program to make ``random'' squaregraphs, by sort of a
``crocheting'' technique. (Hacked in haste.)

@d maxn 1000

@c
#include <stdio.h>
#include <stdlib.h>
#include "gb_flip.h"
int a[2*maxn+4],d[2*maxn+8];
int move[8*maxn];
int count[maxn];
int seed;
int steps;
main (int argc, char*argv[]) {
  register int j,k,m,t,w;
  @<Process the command line@>;
  a[0]=0,a[1]=1,a[2]=0,a[3]=1;
  d[0]=d[1]=d[2]=d[3]=2;
  w=4;
  for (j=0;j<steps;j++) {
    @<Set |m| to the number of possible moves@>;
    k=gb_unif_rand(m);
    @<Make move |k|@>;
    @<Check for pairs@>;
  }
  @<Output the result@>;
}

@ @<Process the command line@>=
if (argc!=3 || sscanf(argv[1],"%d",&steps)!=1 ||
               sscanf(argv[2],"%d",&seed)!=1) {
  fprintf(stderr,"Usage: %s n seed\n",argv[0]);
  exit(-1);
}
if (steps>=maxn) {
  fprintf(stderr,"Sorry, n should be less than %d!\n",maxn);
  exit(-2);
}
gb_init_rand(seed);

@ @<Set |m| to the number of possible moves@>=
d[w]=d[0], d[w+1]=d[1], a[w]=a[0], a[w+1]=a[1];
for (m=0;m<w;m++) move[m]=m;
for (k=0;k<w;k++) if (d[k+1]>3) move[m++]=maxn+k;
for (k=0;k<w;k++) if (d[k+1]>3 && d[k+2]>3) move[m++]=maxn+maxn+k;

@ @<Make move |k|@>=
if (move[k]<maxn) {
  w+=2, k=move[k];
  for (m=w-1;m>=k+2;m--) d[m+2]=d[m],a[m+2]=a[m];
  d[k+3]=d[k+1]+1, d[k+2]=d[k+1]=2, d[k]=d[k]+1;
  a[k+3]=a[k+1], a[k+2]=j+2, a[k+1]=a[k], a[k]=j+2;
  if (k+3>=w) for (t=0;t+w<=k+3;t++) d[t]=d[w+t],a[t]=a[w+t];
}@+else if (move[k]<maxn+maxn) {
  k=move[k]-maxn;
  d[k+1]=2;
  t=a[k+1], a[k+1]=a[k], a[k]=t;
  if (k+1>=w) for (t=0;t+w<=k+1;t++) d[t]=d[w+t],a[t]=a[w+t];
}@+else {
  k=move[k]-maxn-maxn;
  for (t=0;t<w;t++) if (a[t]==a[k+2] && t!=k+2) a[t]=a[k];
  a[k]=a[k+1], a[k+1]=a[k+3], d[k]=d[k]+1, d[k+1]=d[k+3]+1;
  w-=2;
  for (t=k+2;t<w;t++) a[t]=a[t+2], d[t]=d[t+2];
}

@ @<Check for pairs@>=
for (k=0;k<j+2;k++) count[k]=0;
for (k=0;k<w;k++) count[a[k]]++;
for (k=0;k<j+2;k++)
  if (count[k]!=0 && count[k]!=2)
    fprintf(stderr,"count[%d] is %d!\n",k,count[k]);

@ @<Output the result@>=
for (k=0;k<w;k++) {
  printf(" %d",a[k]);
  if (k%20==19) printf("\n");
}
if (k%20!=0) printf("\n");

@*Index.