2019/day2.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
   8 #define LIMIT 400
   9 static int a[LIMIT];
  10 static int len;
  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 void dump(void) {
  25   if (!do_debug)
  26     return;
  27   for (int i = 0; i < len; i++)
  28     debug("%d,", a[i]);
  29   debug("\n");
  30 }
  31
  32 int main(int argc, char **argv) {
  33   int i, *pc, count = 0;
  34   bool done = false;
  35
  36   if (argc > 1)
  37     if (!(stdin = freopen(argv[1], "r", stdin))) {
  38       perror("failure");
  39       exit(2);
  40     }
  41
  42   while (scanf("%d%*[,\n]", &i) == 1) {
  43     a[len++] = i;
  44     if (len > LIMIT) {
  45       printf("recompile with larger LIMIT\n");
  46       exit(2);
  47     }
  48   }
  49   printf("Read %u slots\n", len);
  50   dump();
  51
  52   if (argc > 3) {
  53     a[1] = atoi(argv[2]);
  54     a[2] = atoi(argv[3]);
  55   }
  56
  57   pc = a;
  58   while (!done) {
  59     count++;
  60     if (pc[0] == 99) {
  61       debug("halting\n");
  62     } else {
  63       debug("executing %d,%d(%d),%d(%d),%d\n", pc[0], pc[1], a[pc[1]],
  64             pc[2], a[pc[2]], pc[3]);
  65       if (pc - a >= len || pc[1] >= len || pc[2] >= len || pc[3] >= len) {
  66         printf("out-of-bounds reference at instruction %d\n", (int)(pc - a));
  67         exit(1);
  68       }
  69     }
  70     switch (*pc) {
  71     case 1:
  72       a[pc[3]] = a[pc[1]] + a[pc[2]];
  73       break;
  74     case 2:
  75       a[pc[3]] = a[pc[1]] * a[pc[2]];
  76       break;
  77     case 99:
  78       done = true;
  79       break;
  80     }
  81     pc += 4;
  82     dump();
  83   }
  84   printf("after %d opcodes, slot 0 holds %d\n", count, a[0]);
  85   dump();
  86   return 0;
  87 }