loop2 displays "x->(x^2+a) mod n" graph using Graphviz neato command and Gnome image viewer

loop2

#!/bin/bash

#if 0
if [ $0 -nt /tmp/$0 ]; then gcc -xc -Wall -Wextra <(tail -n+2 $0) -o /tmp/$0; fi
/tmp/$0 $1 $2 > $1_$2.dot
neato -Tsvg $1_$2.dot > $1_$2.svg
eog $1_$2.svg
exit
#endif

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

unsigned dif(unsigned a, unsigned b)
{
  return (a<b) ? b-a : a-b;
}

unsigned gcd(unsigned a, unsigned b)
{
  if (a<b)
  {
    if (a==0) return b; else b%=a;
  }
  while (b!=0)
  {
    a%=b;
    if (a==0) return b;
    b%=a;
  }
  return a;
}

char nbsp[3] = { 0xC2, 0xA0, 0x00 };
char buf[10][100];
unsigned int buc=0;
char *lab(unsigned u)
{
  if (++buc==10)  buc=0;
  sprintf(buf[buc], "\"%s%u%s\"", nbsp, u, nbsp);
  return buf[buc];
}

int main(int argc, char *argv[])
{
  unsigned n, a, x;
  assert(argc==3);
  n = atoi(argv[1]);
  a = atoi(argv[2]);

  printf("digraph D {\n");
  printf("size=\"8 8\"\n");

  for(x=0; x<n; ++x)
    if (gcd(dif(x, n-x), n) > 1)
      printf("%s [shape=box]\n", lab(x));

  for(x=0; x<n; ++x)
    printf("%s->%s\n", lab(x), lab((x*x+a)%n));

  printf("}\n");
}

https://www.raspberrypi.org/forums/viewtopic.php?f=33&t=312344&p=1888647#p1888647

Executing this command ...

pi@raspberrypi4B:~/rho $ ./loop2 91 1

... displays this graph (with installed "graphviz" package):