Two tasks. Part 1 was about calculating a checksum using an unusual algorithm. Two slightly
interesting things. Counter object, which greatly simplified the code. And a way to sort dict
descending by value, then ascending by key: sorted(lettercounter, key=lambda x: (-x[1],x[0])).
Get it? Let's start from the inside. We define a lambda function for key, it returns a tuple.
x[1] is the value, - for reversing the sort order. Then x[0] is the key.
Part 2 was a simple cypher - addition, subtraction and modulo is all that's needed.
#!/usr/bin/python3
import re
from collections import Counter
rx1=re.compile(r"([a-z-]+)([0-9]+)\[([a-z]+)\]")
INPUTFILE="04-input.txt"
def calculate_sum(name):
name=name.replace("-","")
lettercounter=Counter(name).most_common()
sortedcounter=sorted(lettercounter, key=lambda x: (-x[1],x[0])) # sort descending by value, then ascending by key
checksum=""
for l,_ in sortedcounter[:5]:
checksum+=l
return checksum
def decrypt_name(name, sector):
newname=""
for letter in name:
if letter=="-":
newletter=" "
else:
lettercode=ord(letter)
lettercode+=sector%26
if lettercode>ord("z"):
lettercode-=26
newletter=chr(lettercode)
newname+=newletter
#print(f"{letter} becomes {newletter}")
return newname
sum_sector=0
with open(INPUTFILE) as f:
for line in f:
if roomsearch := rx1.search(line):
roomname, roomsector, roomsum=roomsearch.groups()
newsum=calculate_sum(roomname)
if roomsum == newsum:
newname=decrypt_name(roomname,int(roomsector))
sum_sector+=int(roomsector)
#print(f"Correct room: {roomname}, sector {roomsector}, checksum {roomsum}, new name {newname}")
if "northpole" in newname:
print("Part 2, ID of northpole storage: ", roomsector)
else:
print("Wrong room format (or wrong regexp): ", line)
print("Part 1, sum of sectors: ", sum_sector)