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‎day-6/README.md‎

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# Day 6: Guard Gallivant
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<br>
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## Part 1
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The Historians use their fancy [device](https://adventofcode.com/2024/day/4) again, this time to whisk you all away to the North Pole
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prototype suit manufacturing lab... in the year [1518](https://adventofcode.com/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 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 lab.
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Maybe you can work out where the guard will go ahead of time so that The Historians can search safely?
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You start by making a map (your puzzle input) of the situation. For example:
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```txt
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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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```
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The map shows the current position of the guard with `^` (to indicate the guard is currently facing **up** from the perspective of the map).
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Any **obstructions** - crates, desks, alchemical reactors, etc. - are shown as `#`.
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Lab guards in 1518 follow a very strict patrol protocol which involves repeatedly following these steps:
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- If there is something directly in front of you, turn right 90 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 reaches an obstacle (in this case, a pile of failed suit
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prototypes):
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```txt
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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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```
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Because there is now an obstacle in front of the guard, she turns right before continuing straight in her new facing direction:
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```txt
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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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```
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Reaching another obstacle (a spool of several **very** long polymers), she turns right again and continues downward:
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```txt
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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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```
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This process continues for a while, but the guard eventually leaves the mapped area (after walking past a tank of universal solvent):
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```txt
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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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```
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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
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starting position**, the positions visited by the guard before leaving the area are marked with an `X`:
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```txt
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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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```
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In this example, the guard will visit `41` distinct positions on your map.
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Predict the path of the guard. **How many distinct positions will the guard visit before leaving the mapped area?**
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<br>
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## Part 2
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While The Historians begin working around the guard's patrol route, you borrow their fancy device and step outside the lab. From the safety
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of a supply closet, you time travel through the last few months and [record](https://adventofcode.com/2018/day/4) the nightly status of the
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lab's guard post on the walls of the closet.
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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
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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 **won't** cause a time paradox. They'd like to place the new
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obstruction in such a way that the guard will get **stuck in a loop**, 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 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 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 obstruction would cause the guard to get stuck in a loop. The
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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
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position where the guard moves left/right, and `+` to show a position where the guard 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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```txt
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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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```
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Option two, put a stack of failed suit prototypes in the bottom right quadrant of the mapped area:
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```txt
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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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```
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Option three, put a crate of chimney-squeeze prototype fabric next to the standing desk in the bottom right quadrant:
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```txt
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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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```
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Option four, put an alchemical retroencabulator near the bottom left corner:
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```txt
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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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```
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Option five, put the alchemical retroencabulator a bit to the right instead:
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```txt
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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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```
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Option six, put a tank of sovereign glue right next to the tank of universal solvent:
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```txt
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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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```
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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
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noticing. The important thing is having enough options that you can find one that minimizes time paradoxes, and in this example, there are
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`6` different positions you could choose.
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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
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obstruction?**

‎day-6/day-6.test.ts‎

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import { describe, it } from 'node:test';
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import assert from 'node:assert';
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import path from 'node:path';
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import {
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determineNumberOfDistinctPositionsVisitedByGuard,
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determineNumberOfObstaclePositionsCausingALoop,
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determineNumberOfObstaclePositionsCausingALoopBruteforce,
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} from './day-6';
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describe('Day 6: Guard Gallivant', () => {
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const mapFilePath = path.join(__dirname, 'map.txt');
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it('Part 1: should determine number of distinct positions visited the guard', async () => {
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const expectedNumberOfDistinctPositionsVisitedByGuard = 5153; // Verified for this dataset
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const numberOfDistinctPositionsVisitedByGuard = await determineNumberOfDistinctPositionsVisitedByGuard(mapFilePath);
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assert.strictEqual(numberOfDistinctPositionsVisitedByGuard, expectedNumberOfDistinctPositionsVisitedByGuard);
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});
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it('Part 2: should determine number of obstacle positions causing a loop', async () => {
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const expectedNumberOfObstaclePositionsCausingALoop = 1711; // Verified for this dataset
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const numberOfObstaclePositionsCausingALoop = await determineNumberOfObstaclePositionsCausingALoop(mapFilePath);
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assert.strictEqual(numberOfObstaclePositionsCausingALoop, expectedNumberOfObstaclePositionsCausingALoop);
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});
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it('Part 2 (bruteforce): should determine number of obstacle positions causing a loop', async () => {
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const expectedNumberOfObstaclePositionsCausingALoop = 1711; // Verified for this dataset
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const numberOfObstaclePositionsCausingALoop = await determineNumberOfObstaclePositionsCausingALoopBruteforce(mapFilePath);
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assert.strictEqual(numberOfObstaclePositionsCausingALoop, expectedNumberOfObstaclePositionsCausingALoop);
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});
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});

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