2017/advent25.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 30000 // educated guess
  11 typedef enum State State;
  12 enum State {
  13   A, B, C, D, E, F,
  14 };
  15 typedef struct state state;
  16 struct state {
  17   bool write;
  18   bool right;
  19   State next;
  20 };
  21 bool tape[MAX];
  22
  23 // open-coding based on the input, instead of parsing it...
  24 #if 0 // sample
  25 #define LIMIT 6
  26 state states[2][2] = {
  27   { { true, true, B }, { false, false, B }, },
  28   { { true, false, A }, { true, true, A }, },
  29 };
  30
  31 #else // day25.input
  32 #define LIMIT 12861455
  33 state states[6][2] = {
  34   { { true, true, B }, { false, false, B }, },
  35   { { true, false, C }, { false, true, E }, },
  36   { { true, true, E }, { false, false, D }, },
  37   { { true, false, A }, { true, false, A }, },
  38   { { false, true, A }, { false, true, F }, },
  39   { { true, true, E }, { true, true, A }, },
  40 };
  41 #endif
  42
  43 int main(int argc, char **argv)
  44 {
  45   int limit = LIMIT;
  46   if (argc > 1)
  47     limit = atoi (argv[1]);
  48   int sum = 0;
  49   State S = A;
  50   int pos = MAX/2;
  51   int min = MAX;
  52   int max = 0;
  53   while (limit--) {
  54     state *s = &states[S][tape[pos]];
  55     if (!tape[pos] && s->write)
  56       sum++;
  57     else if (tape[pos] && !s->write)
  58       sum--;
  59     tape[pos] = s->write;
  60     S = s->next;
  61     pos = pos + (s->right ? 1 : -1);
  62     if (pos < 0 || pos == MAX) {
  63       printf ("still %d ops to perform, out of tape, used %d-%d\n",
  64               limit, min, max);
  65       return 1;
  66     }
  67     if (pos < min)
  68       min = pos;
  69     else if (pos > max)
  70       max = pos;
  71   }
  72   printf ("checksum is %d\n", sum);
  73   return 0;
  74 }