Solution to http://adventofcode.com/2016/day/4

d4p1.py

#!/usr/bin/env python3

# Solution to http://adventofcode.com/2016/day/4 part 1
# Mark Jenkins <mark@markjenkins.ca>

from sys import stdin
from collections import Counter
from itertools import islice

CHECKSUM_SIZE = 5

def parse_room(line):
    parts = line.split("-")
    sector_id_and_checksum = parts[-1].split("[")
    return (
        ''.join(c for c in parts[:-1] if c!="-"), # name letters without "-"
        int(sector_id_and_checksum[0]), # sector id
        sector_id_and_checksum[1][0:CHECKSUM_SIZE] # checksum
        )

def checksum(name):
    counts = Counter(name)
    # make a sorted iterable of the counts and letters,
    # use reverse=True to get the counts in descending order
    #
    # to take full advantage of sorted, we reverse the ordering of the
    # alphabet by converting to integer and subtracting from 'z'
    # so that the use of reverse=True doesn't give us z-a, but instead a-z
    sorted_count_let_num = sorted(
        ( (count, ord('z') - ord(let) )
          for let, count in counts.items()),
        reverse=True)

    # construct a string using the first CHECKSUM_SIZE entries
    return ''.join(
        chr(ord('z') - da_let) # fix the letter reversing and convert back
        for da_count, da_let in islice(sorted_count_let_num, 
                                       0, CHECKSUM_SIZE) # islice
    ) # join

def valid_room(name, checksum_in):
    return checksum(name)==checksum_in

print( sum( sector_id
            for name, sector_id, checksum in (parse_room(line)
                                              for line in stdin )
            if valid_room(name, checksum)
        ) # sum
       ) # print

d4p2.py

#!/usr/bin/env python3

# Solution to http://adventofcode.com/2016/day/4 part 2
# Mark Jenkins <mark@markjenkins.ca>

from sys import stdin
from collections import Counter
from itertools import islice

CHECKSUM_SIZE = 5

def parse_room(line):
    parts = line.split("-")
    sector_id_and_checksum = parts[-1].split("[")
    return (
        parts[:-1], # name in parts
        int(sector_id_and_checksum[0]), # sector id
        sector_id_and_checksum[1][0:CHECKSUM_SIZE] # checksum
        )

def checksum(name):
    counts = Counter(name)
    # make a sorted iterable of the counts and letters,
    # use reverse=True to get the counts in descending order
    #
    # to take full advantage of sorted, we reverse the ordering of the
    # alphabet by converting to integer and subtracting 'z'
    # so that the use of reverse=True doesn't give us z-a, but instead a-z
    sorted_count_let_num = sorted(
        ( (count, ord('z') - ord(let) )
          for let, count in counts.items()),
        reverse=True)

    # construct a string using the first CHECKSUM_SIZE entries
    return ''.join(
        chr(ord('z') - da_let) # fix the letter reversing and convert back
        for da_count, da_let in islice(sorted_count_let_num, 
                                       0, CHECKSUM_SIZE) # islice
    ) # join

def valid_room(name, checksum_in):
    return checksum(''.join(name)) ==checksum_in

def decrypt_letter(c, key):
    return chr(
        ( (ord(c) - ord('a') + key) % 26 ) +
        ord('a')
    ) # chr

def decrypt_room(name, sector_id):
    return ' '.join( ''.join( decrypt_letter(c, sector_id) for c in name_part )
                     for name_part in name )
print( '\n'.join( "%s %s" % (decrypt_room(name, sector_id), sector_id)
                  for name, sector_id, checksum in (parse_room(line)
                                                    for line in stdin )
                  if valid_room(name, checksum)
              ) # join
   ) # print