2017/advent22.c

   1 #define _GNU_SOURCE 1
   2 #include <stdio.h>
   3 #include <string.h>
   4 #include <stdlib.h>
   5 #include <unistd.h>
   6 #include <stdbool.h>
   7 #include <ctype.h>
   8 #include <assert.h>
   9
  10 #define MAX 3000  // guessing that 10000 iterations won't grow that large
  11
  12 enum dir {
  13   U,
  14   R,
  15   D,
  16   L,
  17 } d;
  18
  19 enum state {
  20   C,
  21   W,
  22   I,
  23   F,
  24 };
  25
  26 static enum state grid[MAX][MAX];
  27
  28 int
  29 main (int argc, char **argv)
  30 {
  31   int cycles = 10000000;
  32   if (argc > 1)
  33     cycles = atoi (argv[1]);
  34   ssize_t nread;
  35   size_t len = 0;
  36   char *line = NULL;
  37   bool first = true;
  38   int x, y;
  39   int count = 0;
  40   while ((nread = getline(&line, &len, stdin)) >= 0) {
  41     if (first) {
  42       assert ((nread & 1) == 0);
  43       first = false;
  44       x = y = MAX / 2 - nread / 2;
  45       printf ("populating starting at %d,%d\n", x, y);
  46     }
  47     char *p = line;
  48     while (*p != '\n') {
  49       if (*p++ == '#')
  50         grid[y][x] = I;
  51       x++;
  52     }
  53     y++;
  54     x -= p - line;
  55     count++;
  56   }
  57   x += count / 2;
  58   y -= count / 2 + 1;
  59   printf ("parsed %dx%d inputs, starting at %d,%d\n", count, count, x, y);
  60   int infected = 0;
  61   for (int i = 0; i < cycles; i++) {
  62     switch (grid[y][x]) {
  63     case C:
  64       d += 3;
  65       break;
  66     case W:
  67       break;
  68     case I:
  69       d++;
  70       break;
  71     case F:
  72       d += 2;
  73       break;
  74     default:
  75       assert (false);
  76     }
  77     infected += (grid[y][x] = (grid[y][x] + 1) % 4) == I;
  78     switch (d % 4) {
  79     case U:
  80       y--;
  81       break;
  82     case R:
  83       x++;
  84       break;
  85     case D:
  86       y++;
  87       break;
  88     case L:
  89       x--;
  90       break;
  91     default:
  92       assert (false);
  93     }
  94     if (!x || !y || x == MAX - 1 || y == MAX - 1) {
  95       printf ("grid is too small at iteration %d\n", i);
  96       return 1;
  97     }
  98   }
  99   printf ("after %d cycles, infected %d nodes\n", cycles, infected);
 100   return 0;
 101 }