day 24

data/day24/input

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src/bin/day24.rs

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+use std::collections::HashSet;
+
+const INPUT: &str = include_str!("../../data/day24/input");
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
+enum Direction {
+    East,
+    SouthEast,
+    SouthWest,
+    West,
+    NorthWest,
+    NorthEast,
+}
+
+impl Direction {
+    const ALL: [Direction; 6] = [
+        Self::East,
+        Self::SouthEast,
+        Self::SouthWest,
+        Self::West,
+        Self::NorthWest,
+        Self::NorthEast,
+    ];
+
+    fn symbol(self) -> &'static str {
+        match self {
+            Self::East => "e",
+            Self::SouthEast => "se",
+            Self::SouthWest => "sw",
+            Self::West => "w",
+            Self::NorthWest => "nw",
+            Self::NorthEast => "ne",
+        }
+    }
+
+    fn parse_list(line: &str) -> Vec<Self> {
+        let mut line = line.trim();
+        let mut result = Vec::new();
+        'outer: while !line.is_empty() {
+            for &d in Self::ALL.iter() {
+                if let Some(rem) = line.strip_prefix(d.symbol()) {
+                    result.push(d);
+                    line = rem;
+                    continue 'outer;
+                }
+            }
+            panic!("Invalid directions: {:?}", line);
+        }
+        result
+    }
+
+    // -> (row, col)
+    fn as_offset(self) -> (i32, i32) {
+        match self {
+            Self::East => (0, 1),
+            Self::SouthEast => (1, 1),
+            Self::SouthWest => (1, 0),
+            Self::West => (0, -1),
+            Self::NorthWest => (-1, -1),
+            Self::NorthEast => (-1, 0),
+        }
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash, Default, Debug)]
+struct Position {
+    row: i32,
+    // the "0"-column goes from south-west to north-east through (0/0)
+    col: i32,
+}
+
+impl Position {
+    fn adjacent(self) -> impl Iterator<Item=Self> {
+        Direction::ALL.iter().map(move |&d| self + d)
+    }
+}
+
+impl std::ops::Add<Direction> for Position {
+    type Output = Position;
+
+    fn add(self, rhs: Direction) -> Self::Output {
+        let (row, col) = rhs.as_offset();
+        Self {
+            row: self.row + row,
+            col: self.col + col,
+        }
+    }
+}
+
+fn daily_flip(black_tiles: HashSet<Position>) -> HashSet<Position> {
+    let mut new_black_tiles: HashSet<Position> = HashSet::new();
+    let mut dont_flip_to_black: HashSet<Position> = HashSet::new();
+    for &tile in &black_tiles {
+        let mut count_adj_black = 0;
+        for adj_tile in tile.adjacent() {
+            if black_tiles.contains(&adj_tile) {
+                count_adj_black += 1;
+            } else if !new_black_tiles.contains(&adj_tile) && !dont_flip_to_black.contains(&adj_tile) {
+                // white adjacent: maybe needs to be flipped to black
+                let mut white_count_adj_black = 0;
+                for adj_tile_2 in adj_tile.adjacent() {
+                    if black_tiles.contains(&adj_tile_2) {
+                        white_count_adj_black += 1;
+                        if white_count_adj_black > 2 { break; }
+                    }
+                }
+                if white_count_adj_black == 2 {
+                    new_black_tiles.insert(adj_tile);
+                } else {
+                    // don't check again
+                    dont_flip_to_black.insert(adj_tile);
+                }
+            }
+        }
+        if count_adj_black == 1 || count_adj_black == 2 {
+            new_black_tiles.insert(tile);
+        }
+    }
+    new_black_tiles
+}
+
+fn main() {
+    let directions = INPUT.lines().map(Direction::parse_list).collect::<Vec<_>>();
+    let flip_tiles = directions.iter().map(|tile| {
+        tile.iter().fold(Position::default(), |tile, &dir| tile + dir)
+    }).collect::<Vec<_>>();
+    let mut black_tiles = HashSet::new();
+    for &tile in &flip_tiles {
+        if black_tiles.contains(&tile) {
+            black_tiles.remove(&tile);
+        } else {
+            black_tiles.insert(tile);
+        }
+    }
+    println!("Black tiles: {}", black_tiles.len());
+    for _ in 0..100 {
+        black_tiles = daily_flip(black_tiles);
+    }
+    println!("Black tiles: {}", black_tiles.len());
+}