AoC 2016 Day 12: Assembunny

Source: Leonardo’s Monorail

Part 1: Create a virtual machine that has four registers (a, b, c, and d) and can process the following instructions:

  • cpy x y - copies x into y (x can be an integer or a register)
  • inc x - increases register x by one
  • dec x - decreases register x by one
  • jnz x y - jumps over y instructions if x is not zero (x can be an integer or a register)

What is the final value in register a?

Let’s make a virtual machine!

class APC(object):
    def __init__(self, code):
        self._code = code
        self._pc = 0
        self._registers = {k: 0 for k in 'abcd'}

    def __repr__(self):
        return 'APC<{}, {}, {}>'.format(

    def run(self):
        def val(x):
                return int(x)
                return self._registers[x]

            while True:
                cmd, *args = self._code[self._pc]
      '{} running {}({})'.format(self, cmd, args))

                if cmd == 'cpy':
                    self._registers[args[1]] = val(args[0])

                elif cmd == 'inc':
                    self._registers[args[0]] += 1

                elif cmd == 'dec':
                    self._registers[args[0]] -= 1

                elif cmd == 'jnz':
                    if val(args[0]) != 0:
                        self._pc += val(args[1]) - 1

                self._pc += 1

        except Exception as ex:
  'Exception: {}'.format(ex))
            self.exception = ex

In true EAFP style, I’m using the out of bounds exception to halt execution. Other than that, the translations are pretty direct.

instructions = [
    for line in fileinput.input(args.files)
    if line.strip()

apc = APC(instructions)

print('Initial:', apc)
print('Final:', apc)

Part 2: If register c starts with the value 1, what is the final value left in a?

Let’s modify the __init__ function to take in initial values for any registers and then read those in with argparse :

class APC(object):
    def __init__(self, code, registers):
        self._code = code
        self._pc = 0
        self._registers = {k: registers.get(k, 0) for k in 'abcd'}

registers = {
    arg.split('=')[0].strip(): int(arg.split('=')[1].strip())
    for arg in args.registers

apc = APC(instructions, registers)

Nice when part 1 is flexible enough to solve part 2 with minimal changes.