2016/advent13.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
   9 #define MAX 75
  10 int grid[MAX][MAX];
  11
  12 int main(int argc, char **argv)
  13 {
  14   int seed = 1350;
  15   if (argc == 2)
  16     seed = atoi (argv[1]);
  17   int i, j;
  18   for (i = 0; i < MAX; i++)
  19     for (j = 0; j < MAX; j++)
  20       grid[i][j] = (~__builtin_popcount (j*j + 3*j + 2*i*j + i + i*i
  21                                          + seed) & 1) - 1;
  22   int gen = 1;
  23   grid[1][1] = gen;
  24   int set = 1;
  25   bool change = true;
  26   while (change) {
  27     change = false;
  28     for (i = 0; i < MAX; i++)
  29       for (j = 0; j < MAX; j++)
  30         if (grid[i][j] == gen) {
  31           if (i && !grid[i - 1][j]) {
  32             change = true;
  33             if (gen < 51)
  34               set++;
  35             grid[i - 1][j] = gen + 1;
  36           }
  37           if (i + 1 < MAX && !grid[i + 1][j]) {
  38             change = true;
  39             if (gen < 51)
  40               set++;
  41             grid[i + 1][j] = gen + 1;
  42           }
  43           if (j && !grid[i][j - 1]) {
  44             change = true;
  45             if (gen < 51)
  46               set++;
  47             grid[i][j - 1] = gen + 1;
  48           }
  49           if (j + 1 < MAX && !grid[i][j + 1]) {
  50             change = true;
  51             if (gen < 51)
  52               set++;
  53             grid[i][j + 1] = gen + 1;
  54           }
  55         }
  56     gen++;
  57   }
  58   if (getenv ("DEBUG"))
  59     for (i = 0; i < MAX; i++) {
  60       for (j = 0; j < MAX; j++)
  61         putchar (grid[i][j] > 0 ? 'O' : grid[i][j] ? '#' : '.');
  62       putchar ('\n');
  63     }
  64   printf ("steps to x=31,y=39 is %d\n", grid[39][31] - 1);
  65   printf ("%d places reachable within 50 steps\n", set);
  66   return 0;
  67 }