256 lines
6.6 KiB
Markdown
256 lines
6.6 KiB
Markdown
|
## \-\-- Day 6: Guard Gallivant \-\--
|
||
|
|
|
||
|
|
The Historians use their fancy [device](4) again, this time to whisk you
|
||
|
|
all away to the North Pole prototype suit manufacturing lab\... in the
|
||
|
|
year [1518](/2018/day/5)! It turns out that having direct access to
|
||
|
|
history is very convenient for a group of historians.
|
||
|
|
|
||
|
|
You still have to be careful of time paradoxes, and so it will be
|
||
|
|
important to avoid anyone from 1518 while The Historians search for the
|
||
|
|
Chief. Unfortunately, a single *guard* is patrolling this part of the
|
||
|
|
lab.
|
||
|
|
|
||
|
|
Maybe you can work out where the guard will go ahead of time so that The
|
||
|
|
Historians can search safely?
|
||
|
|
|
||
|
|
You start by making a map (your puzzle input) of the situation. For
|
||
|
|
example:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
.........#
|
||
|
|
..........
|
||
|
|
..#.......
|
||
|
|
.......#..
|
||
|
|
..........
|
||
|
|
.#..^.....
|
||
|
|
........#.
|
||
|
|
#.........
|
||
|
|
......#...
|
||
|
|
|
||
|
|
The map shows the current position of the guard with `^` (to indicate
|
||
|
|
the guard is currently facing *up* from the perspective of the map). Any
|
||
|
|
*obstructions* - crates, desks, alchemical reactors, etc. - are shown as
|
||
|
|
`#`.
|
||
|
|
|
||
|
|
Lab guards in 1518 follow a very strict patrol protocol which involves
|
||
|
|
repeatedly following these steps:
|
||
|
|
|
||
|
|
- If there is something directly in front of you, turn right 90
|
||
|
|
degrees.
|
||
|
|
- Otherwise, take a step forward.
|
||
|
|
|
||
|
|
Following the above protocol, the guard moves up several times until she
|
||
|
|
reaches an obstacle (in this case, a pile of failed suit prototypes):
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....^....#
|
||
|
|
..........
|
||
|
|
..#.......
|
||
|
|
.......#..
|
||
|
|
..........
|
||
|
|
.#........
|
||
|
|
........#.
|
||
|
|
#.........
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Because there is now an obstacle in front of the guard, she turns right
|
||
|
|
before continuing straight in her new facing direction:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
........>#
|
||
|
|
..........
|
||
|
|
..#.......
|
||
|
|
.......#..
|
||
|
|
..........
|
||
|
|
.#........
|
||
|
|
........#.
|
||
|
|
#.........
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Reaching another obstacle (a spool of several *very* long polymers), she
|
||
|
|
turns right again and continues downward:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
.........#
|
||
|
|
..........
|
||
|
|
..#.......
|
||
|
|
.......#..
|
||
|
|
..........
|
||
|
|
.#......v.
|
||
|
|
........#.
|
||
|
|
#.........
|
||
|
|
......#...
|
||
|
|
|
||
|
|
This process continues for a while, but the guard eventually leaves the
|
||
|
|
mapped area (after walking past a tank of universal solvent):
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
.........#
|
||
|
|
..........
|
||
|
|
..#.......
|
||
|
|
.......#..
|
||
|
|
..........
|
||
|
|
.#........
|
||
|
|
........#.
|
||
|
|
#.........
|
||
|
|
......#v..
|
||
|
|
|
||
|
|
By predicting the guard\'s route, you can determine which specific
|
||
|
|
positions in the lab will be in the patrol path. *Including the guard\'s
|
||
|
|
starting position*, the positions visited by the guard before leaving
|
||
|
|
the area are marked with an `X`:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....XXXXX#
|
||
|
|
....X...X.
|
||
|
|
..#.X...X.
|
||
|
|
..XXXXX#X.
|
||
|
|
..X.X.X.X.
|
||
|
|
.#XXXXXXX.
|
||
|
|
.XXXXXXX#.
|
||
|
|
#XXXXXXX..
|
||
|
|
......#X..
|
||
|
|
|
||
|
|
In this example, the guard will visit `41` distinct positions on your
|
||
|
|
map.
|
||
|
|
|
||
|
|
Predict the path of the guard. *How many distinct positions will the
|
||
|
|
guard visit before leaving the mapped area?*
|
||
|
|
|
||
|
|
Your puzzle answer was `5212`.
|
||
|
|
|
||
|
|
## \-\-- Part Two \-\-- {#part2}
|
||
|
|
|
||
|
|
While The Historians begin working around the guard\'s patrol route, you
|
||
|
|
borrow their fancy device and step outside the lab. From the safety of a
|
||
|
|
supply closet, you time travel through the last few months and
|
||
|
|
[record](/2018/day/4) the nightly status of the lab\'s guard post on the
|
||
|
|
walls of the closet.
|
||
|
|
|
||
|
|
Returning after what seems like only a few seconds to The Historians,
|
||
|
|
they explain that the guard\'s patrol area is simply too large for them
|
||
|
|
to safely search the lab without getting caught.
|
||
|
|
|
||
|
|
Fortunately, they are *pretty sure* that adding a single new obstruction
|
||
|
|
*won\'t* cause a time paradox. They\'d like to place the new obstruction
|
||
|
|
in such a way that the guard will get [*stuck in a
|
||
|
|
loop*]{title="This vulnerability was later fixed by having the guard always turn left instead."},
|
||
|
|
making the rest of the lab safe to search.
|
||
|
|
|
||
|
|
To have the lowest chance of creating a time paradox, The Historians
|
||
|
|
would like to know *all* of the possible positions for such an
|
||
|
|
obstruction. The new obstruction can\'t be placed at the guard\'s
|
||
|
|
starting position - the guard is there right now and would notice.
|
||
|
|
|
||
|
|
In the above example, there are only `6` different positions where a new
|
||
|
|
obstruction would cause the guard to get stuck in a loop. The diagrams
|
||
|
|
of these six situations use `O` to mark the new obstruction, `|` to show
|
||
|
|
a position where the guard moves up/down, `-` to show a position where
|
||
|
|
the guard moves left/right, and `+` to show a position where the guard
|
||
|
|
moves both up/down and left/right.
|
||
|
|
|
||
|
|
Option one, put a printing press next to the guard\'s starting position:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....+---+#
|
||
|
|
....|...|.
|
||
|
|
..#.|...|.
|
||
|
|
....|..#|.
|
||
|
|
....|...|.
|
||
|
|
.#.O^---+.
|
||
|
|
........#.
|
||
|
|
#.........
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Option two, put a stack of failed suit prototypes in the bottom right
|
||
|
|
quadrant of the mapped area:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....+---+#
|
||
|
|
....|...|.
|
||
|
|
..#.|...|.
|
||
|
|
..+-+-+#|.
|
||
|
|
..|.|.|.|.
|
||
|
|
.#+-^-+-+.
|
||
|
|
......O.#.
|
||
|
|
#.........
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Option three, put a crate of chimney-squeeze prototype fabric next to
|
||
|
|
the standing desk in the bottom right quadrant:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....+---+#
|
||
|
|
....|...|.
|
||
|
|
..#.|...|.
|
||
|
|
..+-+-+#|.
|
||
|
|
..|.|.|.|.
|
||
|
|
.#+-^-+-+.
|
||
|
|
.+----+O#.
|
||
|
|
#+----+...
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Option four, put an alchemical retroencabulator near the bottom left
|
||
|
|
corner:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....+---+#
|
||
|
|
....|...|.
|
||
|
|
..#.|...|.
|
||
|
|
..+-+-+#|.
|
||
|
|
..|.|.|.|.
|
||
|
|
.#+-^-+-+.
|
||
|
|
..|...|.#.
|
||
|
|
#O+---+...
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Option five, put the alchemical retroencabulator a bit to the right
|
||
|
|
instead:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....+---+#
|
||
|
|
....|...|.
|
||
|
|
..#.|...|.
|
||
|
|
..+-+-+#|.
|
||
|
|
..|.|.|.|.
|
||
|
|
.#+-^-+-+.
|
||
|
|
....|.|.#.
|
||
|
|
#..O+-+...
|
||
|
|
......#...
|
||
|
|
|
||
|
|
Option six, put a tank of sovereign glue right next to the tank of
|
||
|
|
universal solvent:
|
||
|
|
|
||
|
|
....#.....
|
||
|
|
....+---+#
|
||
|
|
....|...|.
|
||
|
|
..#.|...|.
|
||
|
|
..+-+-+#|.
|
||
|
|
..|.|.|.|.
|
||
|
|
.#+-^-+-+.
|
||
|
|
.+----++#.
|
||
|
|
#+----++..
|
||
|
|
......#O..
|
||
|
|
|
||
|
|
It doesn\'t really matter what you choose to use as an obstacle so long
|
||
|
|
as you and The Historians can put it into position without the guard
|
||
|
|
noticing. The important thing is having enough options that you can find
|
||
|
|
one that minimizes time paradoxes, and in this example, there are `6`
|
||
|
|
different positions you could choose.
|
||
|
|
|
||
|
|
You need to get the guard stuck in a loop by adding a single new
|
||
|
|
obstruction. *How many different positions could you choose for this
|
||
|
|
obstruction?*
|
||
|
|
|
||
|
|
Your puzzle answer was `1767`.
|
||
|
|
|
||
|
|
Both parts of this puzzle are complete! They provide two gold stars:
|
||
|
|
\*\*
|
||
|
|
|
||
|
|
At this point, you should [return to your Advent calendar](/2024) and
|
||
|
|
try another puzzle.
|
||
|
|
|
||
|
|
If you still want to see it, you can [get your puzzle
|
||
|
|
input](6/input).
|
||
|
|
|