04.py

# --- Day 4: Security Through Obscurity ---

import re

with open("input/04.txt", "r") as rooms:
  sectors = northpole_objects = 0
  alphabet = "abcdefghijklmnopqrstuvwxyz"

  for room in rooms:
    regex = re.search(r"([a-z\-]*)-([0-9]+)\[([a-z]*)\]", room)

    roomname = regex.group(1)
    sector = int(regex.group(2))
    checksum = regex.group(3)

    letters = "".join(roomname.split("-"))

    uniq = {}
    # calculated checksum
    cs = ""

    for l in letters:
      if not l in uniq.keys():
        uniq[l] = letters.count(l)

    i = max(uniq.values()) # work backwards down to 1

    while i > 0:
      chars = [k for k, v in uniq.items() if v == i]
      chars.sort()
      cs += "".join(chars)
      i -= 1

    if(cs[:5] == checksum):
      sectors += sector

    decrypted = ""

    for r in roomname:
      if r == "-":
        decrypted += " "
      else:
        loc = alphabet.find(r)
        decrypted += alphabet[(loc + sector) % len(alphabet)]

    if decrypted == "northpole object storage":
      northpole_objects = sector

  print(sectors)
  print(northpole_objects)