2016/advent18.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
   8 #define MAX 100 // cheat by pre-inspecting input
   9
  10 int
  11 main (int argc, char **argv)
  12 {
  13   int rows = 40;
  14   if (argc > 1)
  15     rows = atoi (argv[1]);
  16   int safe = 0;
  17   bool grid[2][MAX + 2] = { 0 }; // true for trap
  18   int c;
  19   int width = 0;
  20   while ((c = getchar ()) != '\n') {
  21     if (c == '^')
  22       grid[0][width + 1] = true;
  23     width++;
  24   }
  25   for (int i = 0; i < rows; i++)
  26     for (int j = 1; j <= width; j++) {
  27       if (!grid[i & 1][j])
  28         safe++;
  29       grid[!(i & 1)][j] = grid[i & 1][j - 1] ^ grid[i & 1][j + 1];
  30     }
  31   printf ("found %d safe tiles in %d rows\n", safe, rows);
  32   return 0;
  33 }