2016/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
   9 int regs[4];
  10 int cache[4];
  11 char *program[50]; // sized based on pre-inspecting file
  12
  13 int
  14 get (const char *value)
  15 {
  16   if (isalpha(*value))
  17     return regs[*value - 'a'];
  18   return atoi (value);
  19 }
  20
  21 void
  22 set (const char *reg, int value)
  23 {
  24   if (getenv ("DEBUG"))
  25     printf (" setting %s to %d\n", reg, value);
  26   regs[*reg - 'a'] = value;
  27 }
  28
  29 int main(int argc, char **argv)
  30 {
  31   int instr = 0;
  32   char buf[250]; // sized based on pre-inspecting file
  33   int nread = fread (buf, 1, sizeof buf, stdin);
  34   char *p = buf;
  35   while (p < buf + nread) {
  36     program[instr++] = p;
  37     p = strchr (p, '\n');
  38     *p++ = '\0';
  39   }
  40   printf ("program consists of %d instructions\n", instr);
  41   int initial = -1;
  42  retry:
  43   regs[0] = ++initial;
  44   memset (cache, 0, sizeof cache);
  45   if (getenv ("DEBUG"))
  46     printf ("attempting execution with a=%d\n", initial);
  47   unsigned int pc = 0;
  48   long long count = 0;
  49   int expected = 0;
  50   while (pc < instr && expected < 50) {
  51     char arg1[10], arg2[10];
  52     unsigned int line;
  53     count++;
  54     if (getenv ("DEBUG"))
  55       printf ("count=%lld pc=%d a=%d b=%d c=%d d=%d, executing %s\n", count,
  56               pc, regs[0], regs[1], regs[2], regs[3], program[pc]);
  57     sscanf (program[pc], "%*s %9s %9s", arg1, arg2);
  58     switch (program[pc][0]) {
  59     case 'c': // cpy x y
  60       if (isalpha (*arg2))
  61         set (arg2, get (arg1));
  62       break;
  63     case 'i': // inc x
  64       set (arg1, get (arg1) + 1);
  65       break;
  66     case 'd': // dec x
  67       set (arg1, get (arg1) - 1);
  68       break;
  69     case 'j': // jnz x y
  70       if (get (arg1))
  71         pc += get (arg2) - 1;
  72       break;
  73     case 't': // tgl x
  74       line = get (arg1) + pc;
  75       if (line < instr) {
  76         if (getenv ("DEBUG"))
  77           printf (" rewriting instruction %d, was %s\n", line, program[line]);
  78         switch (*program[line]) {
  79         case 'i':
  80           memcpy (program[line], "dec", 3);
  81           break;
  82         case 'd':
  83         case 't':
  84           memcpy (program[line], "inc", 3);
  85           break;
  86         case 'c':
  87           memcpy (program[line], "jnz", 3);
  88           break;
  89         case 'j':
  90           memcpy (program[line], "cpy", 3);
  91           break;
  92         default:
  93           return 1;
  94         }
  95       }
  96       break;
  97     case 'o': // out x
  98       line = get (arg1);
  99       if (line != (expected & 1)) {
 100         //if (getenv ("DEBUG"))
 101           printf ("after %lld ops and %d out, got %d, with initial a=%d\n",
 102                   count, expected, line, initial);
 103         goto retry;
 104       }
 105       if (initial == 175)
 106         printf (" %d %d %d %d: %d\n", regs[0], regs[1], regs[2], regs[3], line);
 107       if (!line) {
 108         if (!memcmp (regs, cache, sizeof regs))
 109           printf ("same state seen, looping sequence found?\n");
 110         memcpy (cache, regs, sizeof regs);
 111       }
 112       expected++;
 113       break;
 114     default:
 115       return 1;
 116     }
 117     pc++;
 118   }
 119   int potential = 0;
 120   if (pc < instr) {
 121     printf ("potential candidate for start %d, with %d out\n", initial,
 122             expected);
 123     if (potential++ < 10)
 124       goto retry;
 125   }
 126   printf ("after %lld operations, final content of register a: %d\n",
 127           count, regs[0]);
 128   return 0;
 129 }