Day 17 Complete

2016/day17/problem

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