2016/advent23.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 char *program[50]; // sized based on pre-inspecting file
  11
  12 int
  13 get (const char *value)
  14 {
  15   if (isalpha(*value))
  16     return regs[*value - 'a'];
  17   return atoi (value);
  18 }
  19
  20 void
  21 set (const char *reg, int value)
  22 {
  23   if (getenv ("DEBUG"))
  24     printf (" setting %s to %d\n", reg, value);
  25   regs[*reg - 'a'] = value;
  26 }
  27
  28 int main(int argc, char **argv)
  29 {
  30   int instr = 0;
  31   char buf[200]; // sized based on pre-inspecting file
  32   int nread = fread (buf, 1, sizeof buf, stdin);
  33   char *p = buf;
  34   while (p < buf + nread) {
  35     program[instr++] = p;
  36     p = strchr (p, '\n');
  37     *p++ = '\0';
  38   }
  39   printf ("program consists of %d instructions\n", instr);
  40   unsigned int pc = 0;
  41   regs[0] = 12;
  42   long long count = 0;
  43   while (pc < instr) {
  44     char arg1[10], arg2[10];
  45     unsigned int line;
  46     count++;
  47     if (getenv ("DEBUG"))
  48       printf ("count=%lld pc=%d a=%d b=%d c=%d d=%d, executing %s\n", count,
  49               pc, regs[0], regs[1], regs[2], regs[3], program[pc]);
  50     sscanf (program[pc], "%*s %9s %9s", arg1, arg2);
  51     switch (program[pc][0]) {
  52     case 'c': // cpy x y
  53       if (isalpha (*arg2))
  54         set (arg2, get (arg1));
  55       break;
  56     case 'i': // inc x
  57       set (arg1, get (arg1) + 1);
  58       break;
  59     case 'd': // dec x
  60       set (arg1, get (arg1) - 1);
  61       break;
  62     case 'j': // jnz x y
  63       if (get (arg1))
  64         pc += get (arg2) - 1;
  65       break;
  66     case 't': // tgl x
  67       line = get (arg1) + pc;
  68       if (line < instr) {
  69         if (getenv ("DEBUG"))
  70           printf (" rewriting instruction %d, was %s\n", line, program[line]);
  71         switch (*program[line]) {
  72         case 'i':
  73           memcpy (program[line], "dec", 3);
  74           break;
  75         case 'd':
  76         case 't':
  77           memcpy (program[line], "inc", 3);
  78           break;
  79         case 'c':
  80           memcpy (program[line], "jnz", 3);
  81           break;
  82         case 'j':
  83           memcpy (program[line], "cpy", 3);
  84           break;
  85         default:
  86           return 1;
  87         }
  88       }
  89       break;
  90     default:
  91       return 1;
  92     }
  93     pc++;
  94   }
  95   printf ("after %lld operations, final content of register a: %d\n",
  96           count, regs[0]);
  97   return 0;
  98 }