99 lines
3.9 KiB
Plaintext
99 lines
3.9 KiB
Plaintext
Advent of Code
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--- Day 17: Two Steps Forward ---
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You're trying to access a secure vault protected by a 4x4 grid of small
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rooms connected by doors. You start in the top-left room (marked S), and you
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can access the vault (marked V) once you reach the bottom-right room:
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#########
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#S| | | #
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#-#-#-#-#
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# | | | #
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#-#-#-#-#
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# | | | #
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#-#-#-#-#
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# | | |
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####### V
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Fixed walls are marked with #, and doors are marked with - or |.
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The doors in your current room are either open or closed (and locked) based
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on the hexadecimal MD5 hash of a passcode (your puzzle input) followed by a
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sequence of uppercase characters representing the path you have taken so far
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(U for up, D for down, L for left, and R for right).
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Only the first four characters of the hash are used; they represent,
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respectively, the doors up, down, left, and right from your current
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position. Any b, c, d, e, or f means that the corresponding door is open;
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any other character (any number or a) means that the corresponding door is
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closed and locked.
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To access the vault, all you need to do is reach the bottom-right room;
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reaching this room opens the vault and all doors in the maze.
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For example, suppose the passcode is hijkl. Initially, you have taken no
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steps, and so your path is empty: you simply find the MD5 hash of hijkl
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alone. The first four characters of this hash are ced9, which indicate that
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up is open (c), down is open (e), left is open (d), and right is closed and
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locked (9). Because you start in the top-left corner, there are no "up" or
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"left" doors to be open, so your only choice is down.
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Next, having gone only one step (down, or D), you find the hash of hijklD.
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This produces f2bc, which indicates that you can go back up, left (but
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that's a wall), or right. Going right means hashing hijklDR to get 5745 -
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all doors closed and locked. However, going up instead is worthwhile: even
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though it returns you to the room you started in, your path would then be
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DU, opening a different set of doors.
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After going DU (and then hashing hijklDU to get 528e), only the right door
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is open; after going DUR, all doors lock. (Fortunately, your actual passcode
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is not hijkl).
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Passcodes actually used by Easter Bunny Vault Security do allow access to
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the vault if you know the right path. For example:
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• If your passcode were ihgpwlah, the shortest path would be DDRRRD.
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• With kglvqrro, the shortest path would be DDUDRLRRUDRD.
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• With ulqzkmiv, the shortest would be DRURDRUDDLLDLUURRDULRLDUUDDDRR.
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Given your vault's passcode, what is the shortest path (the actual path, not
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just the length) to reach the vault?
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Your puzzle answer was DRRDRLDURD.
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--- Part Two ---
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You're curious how robust this security solution really is, and so you
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decide to find longer and longer paths which still provide access to the
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vault. You remember that paths always end the first time they reach the
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bottom-right room (that is, they can never pass through it, only end in it).
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For example:
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• If your passcode were ihgpwlah, the longest path would take 370 steps.
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• With kglvqrro, the longest path would be 492 steps long.
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• With ulqzkmiv, the longest path would be 830 steps long.
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What is the length of the longest path that reaches the vault?
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Your puzzle answer was 618.
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References
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Visible links
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. http://adventofcode.com/
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. http://adventofcode.com/2016/about
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. http://adventofcode.com/2016/sponsors
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. https://en.wikipedia.org/wiki/MD5
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. http://adventofcode.com/2016
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