day24 part 2

src/day24.rs

@@ -1,6 +1,21 @@
 use aoc_runner_derive::aoc;
 use std::collections::HashMap;
 
+#[derive(Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq, Hash)]
+struct Pos(isize, isize);
+impl Pos {
+    fn step(self, dir: Direction) -> Self {
+        match dir {
+            Direction::E => Pos(self.0 + 2, self.1),
+            Direction::SE => Pos(self.0 + 1, self.1 -1),
+            Direction::SW => Pos(self.0 -1, self.1 -1),
+            Direction::W => Pos(self.0 -2, self.1),
+            Direction::NE => Pos(self.0 + 1, self.1 + 1),
+            Direction::NW => Pos(self.0 -1, self.1 + 1),
+        }
+    }
+}
+
 #[derive(Debug, Copy, Clone)]
 enum Direction {
     E,
@@ -24,17 +39,6 @@ impl Direction {
         }
     }
 
-    fn step(self) -> (isize, isize) {
-        match self {
-            Direction::E => (2, 0),
-            Direction::SE => (1, -1),
-            Direction::SW => (-1, -1),
-            Direction::W => (-2, 0),
-            Direction::NE => (1, 1),
-            Direction::NW => (-1, 1),
-        }
-    }
-
     fn iter(input: &[u8]) -> impl Iterator<Item = Direction> + '_ {
         struct Iter<'a>(&'a [u8]);
         impl<'a> Iterator for Iter<'a> {
@@ -53,13 +57,26 @@ impl Direction {
     }
 }
 
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+enum Tile {
+    Black,
+    White,
+}
+impl Tile {
+    fn flip(&mut self) {
+        *self = match *self {
+            Tile::Black => Tile::White,
+            Tile::White => Tile::Black,
+        };
+    }
+}
+
 #[aoc(day24, part1)]
 fn solve_d24_p1(input: &str) -> usize {
     let mut tiles: HashMap<_, usize> = HashMap::new();
     for pos in input.split('\n').map(|line| {
-        Direction::iter(line.as_bytes()).fold((0, 0), |pos, dir| {
-            let (x_step, y_step) = dir.step();
-            (pos.0 + x_step, pos.1 + y_step)
+        Direction::iter(line.as_bytes()).fold(Pos(0, 0), |pos, dir| {
+            pos.step(dir)
         })
     }) {
         *tiles.entry(pos).or_default() += 1;
@@ -70,3 +87,73 @@ fn solve_d24_p1(input: &str) -> usize {
         .filter(|&times_flipped| times_flipped % 2 == 1)
         .count()
 }
+
+#[aoc(day24, part2)]
+fn solve_d24_p2(input: &str) -> usize {
+    let mut floor: HashMap<_, Tile> = HashMap::new();
+    for pos in input.split('\n').map(|line| {
+        Direction::iter(line.as_bytes()).fold(Pos(0, 0), |pos, dir| {
+            pos.step(dir)
+        })
+    }) {
+        floor.entry(pos).or_insert(Tile::White).flip()
+    }
+
+    for _ in 0..100 {
+        let mut tiles_visited = HashMap::new();
+        for &pos in floor.keys() {
+            run(&floor, &mut tiles_visited, pos);
+        }
+        for pos in tiles_visited
+            .into_iter()
+            .filter_map(|(k, should_flip)| if should_flip { Some(k) } else { None })
+        {
+            floor.entry(pos).or_insert(Tile::White).flip();
+        }
+    }
+
+    floor.values().filter(|&&tile| tile == Tile::Black).count()
+}
+
+fn run(
+    floor: &HashMap<Pos, Tile>,
+    tiles_visited: &mut HashMap<Pos, bool>,
+    pos: Pos,
+) {
+    if tiles_visited.contains_key(&pos) {
+        return;
+    }
+    let n = neighbors(floor, pos);
+    let black_tiles = n
+        .iter()
+        .copied()
+        .filter(|&tile| tile == Tile::Black)
+        .count();
+    let tile = floor.get(&pos).copied().unwrap_or(Tile::White);
+    if (tile == Tile::Black && (black_tiles == 0 || black_tiles > 2))
+        || (tile == Tile::White && black_tiles == 2)
+    {
+        tiles_visited.insert(pos, true);
+    } else {
+        tiles_visited.insert(pos, false);
+    }
+    if black_tiles > 0 {
+        run(floor, tiles_visited, pos.step(Direction::E));
+        run(floor, tiles_visited, pos.step(Direction::SE));
+        run(floor, tiles_visited, pos.step(Direction::SW));
+        run(floor, tiles_visited, pos.step(Direction::W));
+        run(floor, tiles_visited, pos.step(Direction::NE));
+        run(floor, tiles_visited, pos.step(Direction::NW));
+    }
+}
+
+fn neighbors(floor: &HashMap<Pos, Tile>, pos: Pos) -> [Tile; 6] {
+    [
+        floor.get(&pos.step(Direction::E)).copied().unwrap_or(Tile::White),
+        floor.get(&pos.step(Direction::SE)).copied().unwrap_or(Tile::White),
+        floor.get(&pos.step(Direction::SW)).copied().unwrap_or(Tile::White),
+        floor.get(&pos.step(Direction::W)).copied().unwrap_or(Tile::White),
+        floor.get(&pos.step(Direction::NE)).copied().unwrap_or(Tile::White),
+        floor.get(&pos.step(Direction::NW)).copied().unwrap_or(Tile::White),
+    ]
+}