text.mod/graphviz.mod/graphviz/cmd/gvpr/lib/cycle

/* Detect directed cycle and print one if found */
BEG_G{
  node_t tp, hp;
  node_t stk[node_t];
  $tvtype = TV_prepostfwd;
  $tvroot = fstnode($);
}

N {
  if (stk[$]) {
      stk[$] = NULL;
  }
  else if ($tvedge == NULL) {  /* current root */
    stk[$] = $;
  }
  else {
    stk[$] = $tvedge.tail;
  }
}
E {
  if (stk[$.head]) {
     tp = $.tail;
     hp = $.head;
     while (tp != $.head) {
        printf ("%s -> %s\n", tp.name, hp.name);
        hp = tp;
        tp = stk[tp];
      }
     printf ("%s -> %s\n", tp.name, hp.name);
     exit(0);
  }
}