105 lines
4.3 KiB
Plaintext
105 lines
4.3 KiB
Plaintext
Advent of Code
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--- Day 21: Scrambled Letters and Hash ---
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The computer system you're breaking into uses a weird scrambling function to
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store its passwords. It shouldn't be much trouble to create your own
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scrambled password so you can add it to the system; you just have to
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implement the scrambler.
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The scrambling function is a series of operations (the exact list is
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provided in your puzzle input). Starting with the password to be scrambled,
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apply each operation in succession to the string. The individual operations
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behave as follows:
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• swap position X with position Y means that the letters at indexes X and
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Y (counting from 0) should be swapped.
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• swap letter X with letter Y means that the letters X and Y should be
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swapped (regardless of where they appear in the string).
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• rotate left/right X steps means that the whole string should be rotated;
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for example, one right rotation would turn abcd into dabc.
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• rotate based on position of letter X means that the whole string should
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be rotated to the right based on the index of letter X (counting from 0)
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as determined before this instruction does any rotations. Once the index
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is determined, rotate the string to the right one time, plus a number of
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times equal to that index, plus one additional time if the index was at
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least 4.
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• reverse positions X through Y means that the span of letters at indexes
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X through Y (including the letters at X and Y) should be reversed in
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order.
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• move position X to position Y means that the letter which is at index X
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should be removed from the string, then inserted such that it ends up at
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index Y.
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For example, suppose you start with abcde and perform the following
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operations:
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• swap position 4 with position 0 swaps the first and last letters,
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producing the input for the next step, ebcda.
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• swap letter d with letter b swaps the positions of d and b: edcba.
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• reverse positions 0 through 4 causes the entire string to be reversed,
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producing abcde.
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• rotate left 1 step shifts all letters left one position, causing the
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first letter to wrap to the end of the string: bcdea.
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• move position 1 to position 4 removes the letter at position 1 (c), then
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inserts it at position 4 (the end of the string): bdeac.
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• move position 3 to position 0 removes the letter at position 3 (a), then
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inserts it at position 0 (the front of the string): abdec.
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• rotate based on position of letter b finds the index of letter b (1),
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then rotates the string right once plus a number of times equal to that
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index (2): ecabd.
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• rotate based on position of letter d finds the index of letter d (4),
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then rotates the string right once, plus a number of times equal to that
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index, plus an additional time because the index was at least 4, for a
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total of 6 right rotations: decab.
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After these steps, the resulting scrambled password is decab.
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Now, you just need to generate a new scrambled password and you can access
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the system. Given the list of scrambling operations in your puzzle input,
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what is the result of scrambling abcdefgh?
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Your puzzle answer was cbeghdaf.
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--- Part Two ---
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You scrambled the password correctly, but you discover that you can't
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actually modify the password file on the system. You'll need to un-scramble
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one of the existing passwords by reversing the scrambling process.
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What is the un-scrambled version of the scrambled password fbgdceah?
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Your puzzle answer was bacdefgh.
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References
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Visible links
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. http://adventofcode.com/
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. https://en.wikipedia.org/wiki/File_system_permissions
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. https://en.wikipedia.org/wiki/Passwd
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. http://adventofcode.com/2016
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. http://adventofcode.com/2016/day/21/input
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