Rust

Only part 1 because I’m meant to be leaving for a holiday in a few hours and haven’t packed yet. Part two looks simple enough to add:

Edit: I did the change on my phone (which was painful).

use std::{collections::HashSet, fs, str::FromStr};

use color_eyre::eyre::{Report, Result};

type GridPosition = usize;

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
enum Direction {
    N,
    E,
    S,
    W,
}

impl Direction {
    fn clockwise(&self) -> Self {
        match self {
            Direction::N => Direction::E,
            Direction::E => Direction::S,
            Direction::S => Direction::W,
            Direction::W => Direction::N,
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum Thing {
    Guard(Direction),
    Obstruction,
    Space,
}

#[derive(Debug, PartialEq, Eq)]
struct LabMap {
    grid: Vec<Thing>,
    width: usize,
    height: usize,
}

impl FromStr for LabMap {
    type Err = Report;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let grid = s
            .chars()
            .filter_map(|ch| {
                use Thing::*;
                match ch {
                    '^' => Some(Guard(Direction::N)),
                    '>' => Some(Guard(Direction::E)),
                    'v' => Some(Guard(Direction::S)),
                    '<' => Some(Guard(Direction::W)),
                    '#' => Some(Obstruction),
                    '.' => Some(Space),
                    '\n' => None,
                    _ => unreachable!(),
                }
            })
            .collect::<Vec<_>>();
        let width = s
            .chars()
            .position(|ch| ch == '\n')
            .ok_or_else(|| Report::msg("grid width cannot be zero, or one line"))?;
        let height = grid.len() / width;
        Ok(Self {
            grid,
            width,
            height,
        })
    }
}

impl LabMap {
    fn neighbour(&self, i: GridPosition, dir: Direction) -> Option<GridPosition> {
        let width = self.width;
        let length = self.grid.len();
        use Direction::*;
        match dir {
            N if i >= width => Some(i - width),
            E if i % width != width - 1 => Some(i + 1),
            S if i + width < length => Some(i + width),
            W if i % width != 0 => Some(i - 1),
            _ => None,
        }
    }

    fn guard_pos(&self) -> Option<(GridPosition, Direction)> {
        self.grid
            .iter()
            .enumerate()
            .filter_map(|(pos, &thing)| match thing {
                Thing::Guard(dir) => Some((pos, dir)),
                _ => None,
            })
            .next()
    }

    fn path_len(&self) -> usize {
        let mut positions = HashSet::new();
        let mut next = self.guard_pos();
        while let Some((pos, dir)) = next {
            positions.insert(pos);

            next = self.neighbour(pos, dir).map(|npos| match self.grid[npos] {
                Thing::Space | Thing::Guard(_) => (npos, dir),
                Thing::Obstruction => (pos, dir.clockwise()),
            });
        }
        positions.len()
    }

    fn num_loops(&self) -> usize {
        (0..self.grid.len())
            .filter(|&pos| matches!(self.grid[pos], Thing::Space))
            .map(|pos| {
                let mut grid = self.grid.clone();
                grid[pos] = Thing::Obstruction;
                LabMap {
                    grid,
                    width: self.width,
                    height: self.height,
                }
            })
            .filter(LabMap::is_loop)
            .count()
    }

    fn is_loop(&self) -> bool {
        let mut positions = HashSet::new();
        let mut next = self.guard_pos();
        while let Some((pos, dir)) = next {
            let is_new = positions.insert((pos, dir));
            if !is_new {
                return true;
            }

            next = self.neighbour(pos, dir).map(|npos| match self.grid[npos] {
                Thing::Space | Thing::Guard(_) => (npos, dir),
                Thing::Obstruction => (pos, dir.clockwise()),
            });
        }
        false
    }
}

fn part1(filepath: &str) -> Result<usize> {
    let input = fs::read_to_string(filepath)?;
    let map = LabMap::from_str(&input)?;
    Ok(map.path_len())
}

fn part2(filepath: &str) -> Result<usize> {
    let input = fs::read_to_string(filepath)?;
    let map = LabMap::from_str(&input)?;
    Ok(map.num_loops())
}

fn main() -> Result<()> {
    color_eyre::install()?;

    println!("Part 1: {}", part1("input.txt")?);
    println!("Part 2: {}", part2("input.txt")?);
    Ok(())
}