Solve day23

day23/input

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day23/input.test

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+....#..
+..###.#
+#...#.#
+.#...##
+#.###..
+##.#.##
+.#..#..

day23/src/main.rs

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-fn main() {
-    println!("Hello, world!");
+use std::{
+    collections::{HashMap, HashSet},
+    fs::File,
+    io::{BufRead, BufReader},
+    ops::Index,
+};
+
+type Elf = (isize, isize);
+
+const ACTIONS: [Dir; 4] = [Dir::North, Dir::South, Dir::West, Dir::East];
+
+#[derive(Debug, Default, PartialEq, Eq)]
+enum SolvePuzzle {
+    First,
+    #[default]
+    Second,
+}
+
+struct Playground {
+    elves: HashSet<Elf>,
+    first_action_index: usize,
+}
+
+impl Playground {
+    fn parse_input<R: BufRead>(reader: R) -> Self {
+        let elves = reader
+            .lines()
+            .map(Result::unwrap)
+            .enumerate()
+            .flat_map(|(y, line)| {
+                line.into_bytes()
+                    .into_iter()
+                    .enumerate()
+                    .filter_map(move |(x, byte)| {
+                        if byte == b'#' {
+                            Some((x as isize, y as isize))
+                        } else {
+                            None
+                        }
+                    })
+            })
+            .collect();
+        Self {
+            elves,
+            first_action_index: 0,
+        }
+    }
+
+    fn simulate_round(&mut self) -> HasMoved {
+        let mut any_moved = false;
+        let proposed_moves: HashMap<Elf, Elf> = self
+            .elves
+            .iter()
+            .copied()
+            .filter_map(|elf| {
+                let surr = Surrounding::for_elf(&self.elves, &elf);
+                // Do nothing if there's noone around
+                if surr.is_empty() {
+                    None
+                } else {
+                    let mut proposed_move = None;
+                    for action_idx in self.first_action_index..self.first_action_index + 4 {
+                        let action = ACTIONS[action_idx % ACTIONS.len()];
+                        if surr.is_dir_empty(action) {
+                            proposed_move = Some(action);
+                            break;
+                        }
+                    }
+                    proposed_move.map(|dir| (elf, dir.apply_to(elf)))
+                }
+            })
+            .collect();
+        let mut illegal_moves: HashSet<Elf> = HashSet::new();
+        proposed_moves
+            .iter()
+            .fold(HashSet::new(), |mut set, (_from, to)| {
+                if set.contains(&to) {
+                    illegal_moves.insert(*to);
+                } else {
+                    set.insert(to);
+                }
+                set
+            });
+        proposed_moves
+            .into_iter()
+            .filter(|(_from, to)| !illegal_moves.contains(to))
+            .for_each(|(from, to)| {
+                self.elves.remove(&from);
+                self.elves.insert(to);
+                any_moved = true;
+            });
+        self.first_action_index = (self.first_action_index + 1) % ACTIONS.len();
+        match any_moved {
+            true => HasMoved::Yes,
+            false => HasMoved::No,
+        }
+    }
+
+    fn count_empty_tiles(self) -> usize {
+        // Calculate empty tiles
+        let (min_x, max_x, min_y, max_y, total_elf_count) = self.elves.into_iter().fold(
+            (isize::MAX, isize::MIN, isize::MAX, isize::MIN, 0),
+            |(min_x, max_x, min_y, max_y, count), (x, y)| {
+                (
+                    min_x.min(x),
+                    max_x.max(x),
+                    min_y.min(y),
+                    max_y.max(y),
+                    count + 1,
+                )
+            },
+        );
+        ((max_x - min_x + 1) * (max_y - min_y + 1) - total_elf_count) as usize
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+enum Dir {
+    North,
+    East,
+    South,
+    West,
+}
+
+impl Dir {
+    fn apply_to(&self, (x, y): Elf) -> Elf {
+        match self {
+            Dir::North => (x, y - 1),
+            Dir::East => (x + 1, y),
+            Dir::South => (x, y + 1),
+            Dir::West => (x - 1, y),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+enum DirDiag {
+    North = 0,
+    NorthEast,
+    East,
+    SouthEast,
+    South,
+    SouthWest,
+    West,
+    NorthWest,
+}
+
+#[derive(Debug)]
+struct Surrounding {
+    raw: [bool; 8],
+}
+
+impl Surrounding {
+    fn for_elf(elves: &HashSet<Elf>, &(x, y): &Elf) -> Self {
+        let raw = [
+            (x, y - 1),
+            (x + 1, y - 1),
+            (x + 1, y),
+            (x + 1, y + 1),
+            (x, y + 1),
+            (x - 1, y + 1),
+            (x - 1, y),
+            (x - 1, y - 1),
+        ]
+        .into_iter()
+        .map(|(x, y)| elves.get(&(x, y)).is_some())
+        .collect::<Vec<_>>()
+        .try_into()
+        .unwrap();
+        Self { raw }
+    }
+
+    fn is_empty(&self) -> bool {
+        !self.raw.iter().any(|&b| b)
+    }
+
+    fn is_dir_empty(&self, action: Dir) -> bool {
+        use DirDiag::*;
+        let any = match action {
+            Dir::North => self[North] || self[NorthWest] || self[NorthEast],
+            Dir::East => self[East] || self[NorthEast] || self[SouthEast],
+            Dir::South => self[South] || self[SouthWest] || self[SouthEast],
+            Dir::West => self[West] || self[NorthWest] || self[SouthWest],
+        };
+        !any
+    }
+}
+
+enum HasMoved {
+    Yes,
+    No,
+}
+
+fn solve_first_puzzle<R: BufRead>(reader: R) -> usize {
+    let mut playground = Playground::parse_input(reader);
+    for _ in 1..11 {
+        playground.simulate_round();
+    }
+    playground.count_empty_tiles()
+}
+
+fn solve_second_puzzle<R: BufRead>(reader: R) -> usize {
+    let mut playground = Playground::parse_input(reader);
+    let mut curr_round = 1;
+    loop {
+        let has_moved = playground.simulate_round();
+        if let HasMoved::No = has_moved {
+            break curr_round;
+        }
+        curr_round += 1;
+    }
+}
+
+fn main() {
+    let mut args = ::std::env::args().skip(1);
+    let file = args.next().unwrap_or_else(|| String::from("./input"));
+    let solve = args
+        .next()
+        .map(|arg| match arg.as_str() {
+            "first" => SolvePuzzle::First,
+            "second" => SolvePuzzle::Second,
+            _ => unreachable!(),
+        })
+        .unwrap_or_default();
+    let file = BufReader::new(File::open(file).expect("Opening file"));
+    match solve {
+        SolvePuzzle::First => println!("{}", solve_first_puzzle(file)),
+        SolvePuzzle::Second => println!("{}", solve_second_puzzle(file)),
+    };
+}
+
+impl Index<DirDiag> for Surrounding {
+    type Output = bool;
+
+    fn index(&self, index: DirDiag) -> &Self::Output {
+        &self.raw[index as usize]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::io::Cursor;
+
+    use super::*;
+
+    #[test]
+    fn example_on_first() {
+        let reader = Cursor::new(include_str!("../input.test"));
+        assert_eq!(solve_first_puzzle(reader), 110);
+    }
+
+    #[test]
+    fn example_on_second() {
+        let reader = Cursor::new(include_str!("../input.test"));
+        assert_eq!(solve_second_puzzle(reader), 20);
+    }
 }