erambler/content/post/adventofcode/2017/08-i-heard-you-like-registers.md

title	description	slug	date	tags	series
I Heard You Like Registers — Python — #adventofcode Day 8	In which I implement a very simple assembly-type language.	day-08	2017-12-10T19:33:46+00:00	Technology Learning Advent of Code Python	aoc2017

Today's challenge describes a simple instruction set for a CPU, incrementing and decrementing values in registers according to simple conditions. We have to interpret a stream of these instructions, and to prove that we've done so, give the highest value of any register, both at the end of the program and throughout the whole program.

→ Full code on GitHub

!!! commentary This turned out to be a nice straightforward one to implement, as the instruction format was easily parsed by regular expression, and Python provides the eval function which made evaluating the conditions a doddle.

Import various standard library bits that we'll use later.

import re
import fileinput as fi
from math import inf
from collections import defaultdict

We could just parse the instructions by splitting the string, but using a regular expression is a little bit more robust because it won't match at all if given an invalid instruction.

INSTRUCTION_RE = re.compile(r'(\w+) (inc|dec) (-?\d+) if (.+)\s*')

def parse_instruction(instruction):
    match = INSTRUCTION_RE.match(instruction)
    return match.group(1, 2, 3, 4)

Executing an instruction simply checks the condition and if it evaluates to True updates the relevant register.

def exec_instruction(registers, instruction):
    name, op, value, cond = instruction

    value = int(value)
    if op == 'dec':
        value = -value

    if eval(cond, globals(), registers):
        registers[name] += value

highest_value returns the maximum value found in any register.

def highest_value(registers):
    return sorted(registers.items(), key=lambda x: x[1], reverse=True)[0][1]

Finally, loop through all the instructions and carry them out, updating global_max as we go. We need to be able to deal with registers that haven't been accessed before. Keeping the registers in a dictionary means that we can evaluate the conditions directly using eval above, passing it as the locals argument. The standard dict will raise an exception if we try to access a key that doesn't exist, so instead we use collections.defaultdict, which allows us to specify what the default value for a non-existent key will be. New registers start at 0, so we use a simple lambda to define a function that always returns 0.

global_max = -inf
registers = defaultdict(lambda: 0)
for i in map(parse_instruction, fi.input()):
    exec_instruction(registers, i)
    global_max = max(global_max, highest_value(registers))

print('Max value:', highest_value(registers))
print('All-time max:', global_max)