2016/advent16.c

   1 #define _GNU_SOURCE 1
   2 #include <stdio.h>
   3 #include <string.h>
   4 #include <stdlib.h>
   5 #include <unistd.h>
   6
   7 #define GOAL 35651584
   8 char buf[GOAL * 2];
   9
  10 int
  11 main (int argc, char **argv)
  12 {
  13   int goal = GOAL;
  14   char *seed = "01110110101001000";
  15   if (argc > 1)
  16     seed = argv[1];
  17   if (argc == 3)
  18     goal = atoi (argv[2]);
  19   int len = stpcpy (buf, seed) - buf;
  20   while (len < goal) {
  21     buf[len] = '0';
  22     for (int i = 0; i < len; i++)
  23       buf[2 * len - i] = buf[i] ^ 1;
  24     len = 2 * len + 1;
  25   }
  26   len = goal;
  27   if (len < 50)
  28     printf ("buf contents: %.*s\n", len, buf);
  29   do {
  30     for (char *p = buf, *q = buf; p - buf < len; p += 2, q++)
  31       *q = '0' + (p[0] == p[1]);
  32     buf[len >>= 1] = '\0';
  33     printf ("checksum length %d\n", len);
  34   } while (!(len & 1));
  35   printf ("final checksum: %s\n", buf);
  36   return 0;
  37 }