2019/day3.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 #include <limits.h>
   8
   9 #define LIMIT 20000
  10 static int a[LIMIT*2][LIMIT*2];
  11
  12 static int do_debug = -1;
  13 void debug(const char *fmt, ...) {
  14   va_list ap;
  15   if (do_debug < 0)
  16     do_debug = !!getenv("DEBUG");
  17   if (do_debug > 0) {
  18     va_start(ap, fmt);
  19     vfprintf(stderr, fmt, ap);
  20     va_end(ap);
  21   }
  22 }
  23
  24 enum dir { U, R, D, L };
  25
  26 int main(int argc, char **argv) {
  27   int i, d, count = 0, line = 0, len = 0;
  28   int x = LIMIT, y = LIMIT;
  29   int best1 = LIMIT * 2, best2 = INT_MAX;
  30
  31   if (argc > 1)
  32     if (!(stdin = freopen(argv[1], "r", stdin))) {
  33       perror("failure");
  34       exit(2);
  35     }
  36
  37   while (line < 2) {
  38     count++;
  39     switch (getchar()) {
  40     case 'U': d = U; break;
  41     case 'R': d = R; break;
  42     case 'D': d = D; break;
  43     case 'L': d = L; break;
  44     default:
  45       printf("unexpected input at %d\n", count);
  46       exit(1);
  47     }
  48     if (scanf("%d", &i) != 1) {
  49       printf("unexpected input at %d\n", count);
  50       exit(1);
  51     }
  52     while (i--) {
  53       len++;
  54       switch (d) {
  55       case U: x++; break;
  56       case R: y++; break;
  57       case D: x--; break;
  58       case L: y--; break;
  59       }
  60       if (x < 0 || x >= LIMIT*2 || y < 0 || y >= LIMIT*2 || len > INT_MAX) {
  61         printf("recompile with larger LIMIT\n");
  62         exit(2);
  63       }
  64       if (line) {
  65         if (a[x][y]) {
  66           debug("intersect at %d,%d time %d+%d=%d\n", x-LIMIT, y-LIMIT,
  67                 a[x][y], len, a[x][y] + len);
  68           if (abs(x-LIMIT) + abs(y-LIMIT) < best1)
  69             best1 = abs(x-LIMIT) + abs(y-LIMIT);
  70           if (a[x][y] + len < best2)
  71             best2 = a[x][y] + len;
  72         }
  73       } else if (!a[x][y]) {
  74         a[x][y] = len;
  75       }
  76     }
  77     switch (getchar()) {
  78     case ',': break;
  79     case '\n':
  80       debug("parsed a line len=%d\n", len);
  81       ++line;
  82       len = 0;
  83       x = y = LIMIT;
  84       break;
  85     default:
  86       printf("unexpected input at %d\n", count);
  87       exit(1);
  88     }
  89   }
  90   printf("after %d segments, best distance %d, best time %d\n", count,
  91          best1, best2);
  92   return 0;
  93 }