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