adventofcode/2018/day20/problem

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2019-11-08 21:01:07 +00:00
Advent of Code
--- Day 20: A Regular Map ---
While you were learning about instruction pointers, the Elves made
considerable progress. When you look up, you discover that the North Pole
base construction project has completely surrounded you.
The area you are in is made up entirely of rooms and doors. The rooms are
arranged in a grid, and rooms only connect to adjacent rooms when a door is
present between them.
For example, drawing rooms as ., walls as #, doors as | or -, your current
position as X, and where north is up, the area you're in might look like
this:
##### #.|.# #-### #.|X# #####
You get the attention of a passing construction Elf and ask for a map. "I
don't have time to draw out a map of this place - it's huge. Instead, I can
give you directions to every room in the facility!" He writes down some
directions on a piece of parchment and runs off. In the example above, the
instructions might have been ^WNE$, a regular expression or "regex" (your
puzzle input).
The regex matches routes (like WNE for "west, north, east") that will take
you from your current room through various doors in the facility. In
aggregate, the routes will take you through every door in the facility at
least once; mapping out all of these routes will let you build a proper map
and find your way around.
^ and $ are at the beginning and end of your regex; these just mean that the
regex doesn't match anything outside the routes it describes. (Specifically,
^ matches the start of the route, and $ matches the end of it.) These
characters will not appear elsewhere in the regex.
The rest of the regex matches various sequences of the characters N (north),
S (south), E (east), and W (west). In the example above, ^WNE$ matches only
one route, WNE, which means you can move west, then north, then east from
your current position. Sequences of letters like this always match that
exact route in the same order.
Sometimes, the route can branch. A branch is given by a list of options
separated by pipes (|) and wrapped in parentheses. So, ^N(E|W)N$ contains a
branch: after going north, you must choose to go either east or west before
finishing your route by going north again. By tracing out the possible
routes after branching, you can determine where the doors are and,
therefore, where the rooms are in the facility.
For example, consider this regex: ^ENWWW(NEEE|SSE(EE|N))$
This regex begins with ENWWW, which means that from your current position,
all routes must begin by moving east, north, and then west three times, in
that order. After this, there is a branch. Before you consider the branch,
this is what you know about the map so far, with doors you aren't sure about
marked with a ?:
#?#?#?#?# ?.|.|.|.? #?#?#?#-# ?X|.? #?#?#
After this point, there is (NEEE|SSE(EE|N)). This gives you exactly two
options: NEEE and SSE(EE|N). By following NEEE, the map now looks like this:
#?#?#?#?# ?.|.|.|.? #-#?#?#?# ?.|.|.|.? #?#?#?#-# ?X|.? #?#?#
Now, only SSE(EE|N) remains. Because it is in the same parenthesized group
as NEEE, it starts from the same room NEEE started in. It states that
starting from that point, there exist doors which will allow you to move
south twice, then east; this ends up at another branch. After that, you can
either move east twice or north once. This information fills in the rest of
the doors:
#?#?#?#?# ?.|.|.|.? #-#?#?#?# ?.|.|.|.? #-#?#?#-# ?.?.?X|.? #-#-#?#?#
?.|.|.|.? #?#?#?#?#
Once you've followed all possible routes, you know the remaining unknown
parts are all walls, producing a finished map of the facility:
######### #.|.|.|.# #-####### #.|.|.|.# #-#####-# #.#.#X|.# #-#-#####
#.|.|.|.# #########
Sometimes, a list of options can have an empty option, like (NEWS|WNSE|).
This means that routes at this point could effectively skip the options in
parentheses and move on immediately. For example, consider this regex and
the corresponding map:
^ENNWSWW(NEWS|)SSSEEN(WNSE|)EE(SWEN|)NNN$
########### #.|.#.|.#.# #-###-#-#-# #.|.|.#.#.# #-#####-#-# #.#.#X|.#.#
#-#-#####-# #.#.|.|.|.# #-###-###-# #.|.|.#.|.# ###########
This regex has one main route which, at three locations, can optionally
include additional detours and be valid: (NEWS|), (WNSE|), and (SWEN|).
Regardless of which option is taken, the route continues from the position
it is left at after taking those steps. So, for example, this regex matches
all of the following routes (and more that aren't listed here):
 ENNWSWWSSSEENEENNN • ENNWSWWNEWSSSSEENEENNN • ENNWSWWNEWSSSSEENEESWENNNN
 ENNWSWWSSSEENWNSEEENNN
By following the various routes the regex matches, a full map of all of the
doors and rooms in the facility can be assembled.
To get a sense for the size of this facility, you'd like to determine which
room is furthest from you: specifically, you would like to find the room for
which the shortest path to that room would require passing through the most
doors.
 In the first example (^WNE$), this would be the north-east corner 3
doors away. • In the second example (^ENWWW(NEEE|SSE(EE|N))$), this would
be the south-east corner 10 doors away. • In the third example
(^ENNWSWW(NEWS|)SSSEEN(WNSE|)EE(SWEN|)NNN$), this would be the north-east
corner 18 doors away.
Here are a few more examples:
Regex: ^ESSWWN(E|NNENN(EESS(WNSE|)SSS|WWWSSSSE(SW|NNNE)))$ Furthest room
requires passing 23 doors
############# #.|.|.|.|.|.# #-#####-###-# #.#.|.#.#.#.# #-#-###-#-#-#
#.#.#.|.#.|.# #-#-#-#####-# #.#.#.#X|.#.# #-#-#-###-#-# #.|.#.|.#.#.#
###-#-###-#-# #.|.#.|.|.#.# #############
Regex: ^WSSEESWWWNW(S|NENNEEEENN(ESSSSW(NWSW|SSEN)|WSWWN(E|WWS(E|SS))))$
Furthest room requires passing 31 doors
############### #.|.|.|.#.|.|.# #-###-###-#-#-# #.|.#.|.|.#.#.#
#-#########-#-# #.#.|.|.|.|.#.# #-#-#########-# #.#.#.|X#.|.#.#
###-#-###-#-#-# #.|.#.#.|.#.|.# #-###-#####-### #.|.#.|.|.#.#.#
#-#-#####-#-#-# #.#.|.|.|.#.|.# ###############
What is the largest number of doors you would be required to pass through to
reach a room? That is, find the room for which the shortest path from your
starting location to that room would require passing through the most doors;
what is the fewest doors you can pass through to reach it?
Your puzzle answer was 4155.
--- Part Two ---
Okay, so the facility is big.
How many rooms have a shortest path from your current location that pass
through at least 1000 doors?
Your puzzle answer was 8434.
Both parts of this puzzle are complete! They provide two gold stars: **
At this point, you should return to your Advent calendar and try another
puzzle.
If you still want to see it, you can get your puzzle input.
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