2017/advent5.c

   1 #define _GNU_SOURCE 1
   2 #include <stdio.h>
   3 #include <string.h>
   4 #include <stdlib.h>
   5 #include <unistd.h>
   6
   7 int main(void) {
   8   /* slurp in all of stdin. Cheat - we know our input file size, so
   9      pick a buffer larger than that */
  10   static char buf[5000];
  11   int len = 0;
  12   while (len < sizeof buf) {
  13     int ret = read(STDIN_FILENO, buf + len, sizeof buf - len);
  14     if (ret < 0)
  15       return 1;
  16     if (!ret)
  17       break;
  18     len += ret;
  19   }
  20   if (len == sizeof buf)
  21     return 2;
  22   /* count lines */
  23   int count = 0;
  24   char *p = buf;
  25   while ((p = memchr (p, '\n', buf + len - p)) && p++)
  26     count++;
  27   printf("found %d lines\n", count);
  28   /* populate array */
  29   int *array = calloc(sizeof(int), count);
  30   p = buf;
  31   char *q;
  32   int i = 0;
  33   while ((q = memchr (p, '\n', buf + len - p))) {
  34     *q = '\0';
  35     array[i++] = atoi(p);
  36     p = q + 1;
  37   }
  38   if (getenv("DEBUG")) {
  39     for (i = 0; i < count; i++)
  40       printf("%d ", array[i]);
  41     puts("\n");
  42   }
  43   /* run through maze */
  44   int pc = 0;
  45   int steps = 0;
  46   while (pc >= 0 && pc < count) {
  47     steps++;
  48 #if 0 // part 1
  49     pc += array[pc]++;
  50 #else // part 2
  51     int jump = array[pc];
  52     if (jump >= 3)
  53       array[pc]--;
  54     else
  55       array[pc]++;
  56     pc += jump;
  57 #endif
  58     if (getenv("DEBUG"))
  59       printf("moving to pc %d\n", pc);
  60   }
  61   printf("exited in %d steps\n", steps);
  62   return 0;
  63 }