From d21a6a6d97d17a8bddc6c949b871cb3f65682823 Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Fri, 6 Dec 2024 21:04:27 +0100
Subject: [PATCH] Solve 06

---
 src/bin/06.rs | 181 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 181 insertions(+)
 create mode 100644 src/bin/06.rs

diff --git a/src/bin/06.rs b/src/bin/06.rs
new file mode 100644
index 0000000..9b183b0
--- /dev/null
+++ b/src/bin/06.rs
@@ -0,0 +1,181 @@
+use aoc::*;
+use glam::IVec2;
+use std::collections::HashMap;
+use std::iter;
+
+const INPUT: &str = include_str!("../../input/06");
+
+fn main() {
+    assert_example!(part1, "06-test", 41);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "06-test", 6);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    Lab::parse(input).patrol().count_visited()
+}
+
+fn part2(input: &str) -> usize {
+    let lab = Lab::parse(input);
+
+    lab.clone()
+        .patrol()
+        .tiles
+        .into_iter()
+        .filter_map(|(pos, tile)| match tile {
+            Tile::Visited => Some(pos),
+            _ => None,
+        })
+        .filter(|&modification| modification != lab.start)
+        .filter(|&modification| {
+            let mut lab = lab.clone();
+            lab.place_wall(modification);
+            lab.loops()
+        })
+        .count()
+}
+
+#[derive(Clone, PartialEq, Eq, Debug)]
+enum Tile {
+    Start,
+    Empty,
+    Wall,
+    Visited,
+}
+
+impl Tile {
+    fn parse(input: char) -> Self {
+        match input {
+            '^' => Tile::Start,
+            '.' => Tile::Empty,
+            '#' => Tile::Wall,
+            other => panic!("unexpected tile '{other}'"),
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+struct Lab {
+    start: IVec2,
+    tiles: HashMap<IVec2, Tile>,
+    size: IVec2,
+}
+
+impl Lab {
+    fn patrol(mut self) -> Self {
+        let mut pos = self.start;
+        let mut dir = Direction::Up;
+
+        while self.contains(pos) {
+            self.mark_visited(pos);
+            (pos, dir) = self.advance(pos, dir);
+        }
+
+        self
+    }
+
+    /// <https://en.wikipedia.org/wiki/Cycle_detection#Floyd's_tortoise_and_hare>
+    fn loops(&self) -> bool {
+        let (mut tortoise_pos, mut tortoise_dir) = (self.start, Direction::Up);
+        let (mut hare_pos, mut hare_dir) = (self.start, Direction::Up);
+
+        loop {
+            (tortoise_pos, tortoise_dir) = self.advance(tortoise_pos, tortoise_dir);
+            (hare_pos, hare_dir) = self.advance(hare_pos, hare_dir);
+            (hare_pos, hare_dir) = self.advance(hare_pos, hare_dir);
+
+            if !self.contains(hare_pos) {
+                return false;
+            }
+
+            if hare_pos == tortoise_pos && hare_dir == tortoise_dir {
+                return true;
+            }
+        }
+    }
+
+    fn advance(&self, pos: IVec2, mut dir: Direction) -> (IVec2, Direction) {
+        while self.looking_at_wall(pos, dir) {
+            dir = dir.rotate();
+        }
+        (pos + dir.vec(), dir)
+    }
+
+    fn place_wall(&mut self, pos: IVec2) {
+        self.tiles.insert(pos, Tile::Wall);
+    }
+
+    fn mark_visited(&mut self, pos: IVec2) {
+        self.tiles.insert(pos, Tile::Visited);
+    }
+
+    fn contains(&self, position: IVec2) -> bool {
+        position.x <= self.size.x && position.y <= self.size.y && position.x >= 0 && position.y >= 0
+    }
+
+    fn looking_at_wall(&self, position: IVec2, d: Direction) -> bool {
+        let check = position + d.vec();
+        self.tiles.get(&check) == Some(&Tile::Wall)
+    }
+
+    fn count_visited(&self) -> usize {
+        self.tiles
+            .values()
+            .filter(|x| matches!(x, Tile::Visited))
+            .count()
+    }
+
+    fn parse(input: &str) -> Self {
+        let tiles: HashMap<_, _> = input
+            .lines()
+            .enumerate()
+            .flat_map(|(y, line)| line.chars().enumerate().zip(iter::repeat(y)))
+            .map(|((x, c), y)| {
+                let pos = IVec2::new(x as i32, y as i32);
+                let tile = Tile::parse(c);
+                (pos, tile)
+            })
+            .collect();
+
+        let start = *tiles
+            .iter()
+            .find(|(_, c)| matches!(c, Tile::Start))
+            .unwrap()
+            .0;
+
+        let size = tiles.keys().fold(IVec2::new(0, 0), |a, b| {
+            IVec2::new(a.x.max(b.x), a.y.max(b.y))
+        });
+
+        Self { tiles, start, size }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+enum Direction {
+    Up,
+    Right,
+    Down,
+    Left,
+}
+
+impl Direction {
+    fn rotate(self) -> Direction {
+        match self {
+            Direction::Up => Direction::Right,
+            Direction::Right => Direction::Down,
+            Direction::Down => Direction::Left,
+            Direction::Left => Direction::Up,
+        }
+    }
+
+    fn vec(self) -> IVec2 {
+        match self {
+            Direction::Up => IVec2::new(0, -1),
+            Direction::Right => IVec2::new(1, 0),
+            Direction::Down => IVec2::new(0, 1),
+            Direction::Left => IVec2::new(-1, 0),
+        }
+    }
+}