2017/advent19.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 202 // cheat by pre-inspecting input with wc
  10 char grid[MAX][MAX + 1];
  11
  12 enum dir {
  13   U,
  14   D,
  15   L,
  16   R,
  17 };
  18
  19 int
  20 main (int argc, char **argv)
  21 {
  22   int rows = 0;
  23   char out[11]; // cheat by pre-inspecting 'tr -d -c A-Z < day19.input'
  24   char *p = out;
  25   while (fgets (grid[rows], MAX + 1, stdin) && grid[rows][0])
  26     rows++;
  27   int width = strchr (grid[0], '\n') - grid[0];
  28   printf ("read %d rows, width %d\n", rows, width);
  29   int r = 0, c;
  30   enum dir d = D;
  31   c = strchr (grid[0], '|') - grid[0];
  32   bool done = false;
  33   int steps = 0;
  34   while (!done) {
  35     if (getenv ("DEBUG"))
  36       printf ("at grid[%d][%d]='%c', direction %d\n", r, c, grid[r][c], d);
  37     steps++;
  38     if (isalpha (grid[r][c]))
  39       *p++ = grid[r][c];
  40     switch (d) {
  41     case U:
  42       if (r && grid[r - 1][c] != ' ')
  43         r--;
  44       else if (c < width - 1 && grid[r][c + 1] != ' ') {
  45         d = R;
  46         c++;
  47       } else if (c && grid[r][c - 1] != ' ') {
  48         d = L;
  49         c--;
  50       } else
  51         done = true;
  52       break;
  53     case D:
  54       if (r < rows - 1 && grid[r + 1][c] != ' ')
  55         r++;
  56       else if (c < width - 1 && grid[r][c + 1] != ' ') {
  57         d = R;
  58         c++;
  59       } else if (c && grid[r][c - 1] != ' ') {
  60         d = L;
  61         c--;
  62       } else
  63         done = true;
  64       break;
  65     case L:
  66       if (c && grid[r][c - 1] != ' ')
  67         c--;
  68       else if (r < rows - 1 && grid[r + 1][c] != ' ') {
  69         d = D;
  70         r++;
  71       } else if (r && grid[r - 1][c] != ' ') {
  72         d = U;
  73         r--;
  74       } else
  75         done = true;
  76       break;
  77     case R:
  78       if (c < width - 1 && grid[r][c + 1] != ' ')
  79         c++;
  80       else if (r < rows - 1 && grid[r + 1][c] != ' ') {
  81         d = D;
  82         r++;
  83       } else if (r && grid[r - 1][c] != ' ') {
  84         d = U;
  85         r--;
  86       } else
  87         done = true;
  88       break;
  89     }
  90   }
  91   printf ("encountered %s after %d steps\n", out, steps);
  92   return 0;
  93 }