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day21.py
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day21.py
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import io
import re
from itertools import permutations
# --- Day 21: Scrambled Letters and Hash ---
#
# The computer system you're breaking into uses a weird scrambling
# function to store its passwords. It shouldn't be much trouble to
# create your own scrambled password so you can add it to the system;
# you just have to implement the scrambler.
#
# The scrambling function is a series of operations (the exact list is
# provided in your puzzle input). Starting with the password to be
# scrambled, apply each operation in succession to the string. The
# individual operations behave as follows:
#
# swap position X with position Y means that the letters at indexes
# X and Y (counting from 0) should be swapped.
#
# swap letter X with letter Y means that the letters X and Y should
# be swapped (regardless of where they appear in the string).
#
# rotate left/right X steps means that the whole string should be
# rotated; for example, one right rotation would turn abcd into
# dabc.
#
# rotate based on position of letter X means that the whole string
# should be rotated to the right based on the index of letter X
# (counting from 0) as determined before this instruction does any
# rotations. Once the index is determined, rotate the string to the
# right one time, plus a number of times equal to that index, plus
# one additional time if the index was at least 4.
#
# reverse positions X through Y means that the span of letters at
# indexes X through Y (including the letters at X and Y) should be
# reversed in order.
#
# move position X to position Y means that the letter which is at
# index X should be removed from the string, then inserted such that
# it ends up at index Y.
#
# For example, suppose you start with abcde and perform the following
# operations:
#
# swap position 4 with position 0 swaps the first and last letters,
# producing the input for the next step, ebcda.
#
# swap letter d with letter b swaps the positions of d and b: edcba.
#
# reverse positions 0 through 4 causes the entire string to be
# reversed, producing abcde.
#
# rotate left 1 step shifts all letters left one position, causing
# the first letter to wrap to the end of the string: bcdea.
#
# move position 1 to position 4 removes the letter at position 1
# (c), then inserts it at position 4 (the end of the string): bdeac.
#
# move position 3 to position 0 removes the letter at position 3
# (a), then inserts it at position 0 (the front of the string):
# abdec.
#
# rotate based on position of letter b finds the index of letter b
# (1), then rotates the string right once plus a number of times
# equal to that index (2): ecabd.
#
# rotate based on position of letter d finds the index of letter d
# (4), then rotates the string right once, plus a number of times
# equal to that index, plus an additional time because the index was
# at least 4, for a total of 6 right rotations: decab.
#
# After these steps, the resulting scrambled password is decab.
#
# Now, you just need to generate a new scrambled password and you can
# access the system. Given the list of scrambling operations in your
# puzzle input, what is the result of scrambling abcdefgh?
def swap_position(s, pos1, pos2):
result = ''
for i, c in enumerate(s):
if i == pos1:
result += s[pos2]
elif i == pos2:
result += s[pos1]
else:
result += c
return result
def swap_letter(s, c1, c2):
return s.replace(c1, '_') \
.replace(c2, c1) \
.replace('_', c2)
LEFT = 'left'
RIGHT = 'right'
def rotate_LR(s, way, n):
# #First solution:
#
# result = ''
# for i in range(len(s)):
# if way == LEFT:
# pos = (n + i) % len(s)
# else:
# pos = (i - n) % len(s)
# result += s[pos]
# return result
n = n % len(s)
if way == LEFT:
return s[n:] + s[:n]
else:
return s[-n:] + s[:-n]
def rotate_position(s, letter):
pos = s.index(letter)
if pos >= 4:
rotation = 2 + pos
else:
rotation = 1 + pos
return rotate_LR(s, RIGHT, rotation)
def reverse(s, pos1, pos2):
reverse = s[pos1:pos2+1][::-1]
return s[:pos1] + reverse + s[pos2+1:]
def move(s, pos1, pos2):
result = ''
j = 0
for i in range(len(s)):
if i == pos2: # recopy s[pos1]
result += s[pos1]
else:
if j == pos1: # pass the next j
j+= 1
result += s[j]
j += 1
return result
pattern_swap_position = re.compile('swap position ([0-9]+) with position ([0-9]+)')
pattern_swap_letter = re.compile('swap letter ([a-zA-Z]) with letter ([a-zA-Z])')
pattern_rotate_left_right = re.compile('rotate (left|right) ([0-9]+) step')
pattern_rotate_position = re.compile('rotate based on position of letter ([a-zA-Z])')
pattern_reverse = re.compile('reverse positions ([0-9]+) through ([0-9]+)')
pattern_move = re.compile('move position ([0-9]+) to position ([0-9]+)')
def apply(operation, s):
match = pattern_swap_position.match(operation)
if match is not None:
pos1 = int(match.group(1))
pos2 = int(match.group(2))
return swap_position(s, pos1, pos2)
else:
match = pattern_swap_letter.match(operation)
if match is not None:
c1 = match.group(1)
c2 = match.group(2)
return swap_letter(s, c1, c2)
else:
match = pattern_rotate_left_right.match(operation)
if match is not None:
way = match.group(1)
steps = int(match.group(2))
return rotate_LR(s, way, steps)
else:
match = pattern_rotate_position.match(operation)
if match is not None:
letter = match.group(1)
return rotate_position(s, letter)
else:
match = pattern_reverse.match(operation)
if match is not None:
pos1 = int(match.group(1))
pos2 = int(match.group(2))
return reverse(s, pos1, pos2)
else:
match = pattern_move.match(operation)
if match is not None:
pos1 = int(match.group(1))
pos2 = int(match.group(2))
return move(s, pos1, pos2)
else:
print('Unable to process: {}'.format(operation))
def scramble(operations, s):
for operation in operations:
s = apply(operation, s)
return s
with io.open('inputs/day21.txt', 'r') as f:
start = 'abcdefgh'
operations = [line.strip() for line in f]
s = scramble(operations, start)
print('Scrambling {} gives: {}'.format(start, s))
# --- Part Two ---
#
# You scrambled the password correctly, but you discover that you can't
# actually modify the password file on the system. You'll need to
# un-scramble one of the existing passwords by reversing the scrambling
# process.
#
# What is the un-scrambled version of the scrambled password fbgdceah?
def unscramble(operations, s):
result = None
perms = [''.join(p) for p in permutations(s)]
for perm in perms:
if scramble(operations, perm) == s:
result = perm
break
return result
with io.open('inputs/day21.txt', 'r') as f:
start = 'fbgdceah'
operations = [line.strip() for line in f]
s = unscramble(operations, start)
print('Unscrambling {} gives: {}'.format(start, s))