2018/day7a.c

   1 #define _GNU_SOURCE 1
   2 #include <stdio.h>
   3 #include <string.h>
   4 #include <stdlib.h>
   5 #include <stdarg.h>
   6 #include <stdbool.h>
   7
   8 void debug(const char *fmt, ...) {
   9   va_list ap;
  10   if (getenv("DEBUG")) {
  11     va_start(ap, fmt);
  12     vfprintf(stderr, fmt, ap);
  13     va_end(ap);
  14   }
  15 }
  16
  17 static struct data {
  18   char state;
  19   char wait[26];
  20 } list[26];
  21
  22 int main(int argc, char **argv) {
  23   size_t len = 0, count = 0;
  24   char *line;
  25   int i, j;
  26   char a, b;
  27   char *p;
  28
  29   /* Part 1 - read in lines */
  30   if (argc > 1)
  31     if (!(stdin = freopen(argv[1], "r", stdin))) {
  32       perror("failure");
  33       exit(2);
  34     }
  35
  36   while (getline(&line, &len, stdin) >= 0) {
  37     ++count;
  38     if (sscanf(line, "Step %c must be finished before step %c can begin. ", &a,
  39                &b) != 2 || (unsigned)(a-'A') > 25 || (unsigned)(b-'A') > 25) {
  40       fprintf(stderr, "bad input\n");
  41       exit(1);
  42     }
  43     list[a - 'A'].state = 1;
  44     list[b - 'A'].state = 1;
  45     strchrnul(list[b - 'A'].wait, '\0')[0] = a;
  46   }
  47   printf("Read %zu lines\n", count);
  48
  49   /* Part 2 - determine output */
  50   for (i = 0; i < 26; i++)
  51     for (j = 0; j < 26; j++) {
  52       if (list[j].state != 1)
  53         continue;
  54       p = list[j].wait;
  55       while (*p) {
  56         if (list[*p - 'A'].state < 2)
  57           break;
  58         p++;
  59       }
  60       if (*p)
  61         continue;
  62       list[j].state = 2;
  63       printf("%c", j + 'A');
  64       break;
  65     }
  66   printf("\n");
  67   return 0;
  68 }