day24 part1

src/day24.rs

@@ -0,0 +1,72 @@
+use aoc_runner_derive::aoc;
+use std::collections::HashMap;
+
+#[derive(Debug, Copy, Clone)]
+enum Direction {
+    E,
+    SE,
+    SW,
+    W,
+    NW,
+    NE,
+}
+
+impl Direction {
+    fn parse(input: &[u8]) -> Option<(&[u8], Direction)> {
+        match input {
+            &[b's', b'e', ..] => Some((&input[2..], Direction::SE)),
+            &[b's', b'w', ..] => Some((&input[2..], Direction::SW)),
+            &[b'n', b'e', ..] => Some((&input[2..], Direction::NE)),
+            &[b'n', b'w', ..] => Some((&input[2..], Direction::NW)),
+            &[b'e', ..] => Some((&input[1..], Direction::E)),
+            &[b'w', ..] => Some((&input[1..], Direction::W)),
+            _ => None,
+        }
+    }
+
+    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> {
+            type Item = Direction;
+            fn next(&mut self) -> Option<Self::Item> {
+                match Direction::parse(self.0) {
+                    None => None,
+                    Some((rem, dir)) => {
+                        self.0 = rem;
+                        Some(dir)
+                    }
+                }
+            }
+        }
+        Iter(input)
+    }
+}
+
+#[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)
+        })
+    }) {
+        *tiles.entry(pos).or_default() += 1;
+    }
+
+    tiles
+        .values()
+        .filter(|&times_flipped| times_flipped % 2 == 1)
+        .count()
+}

src/lib.rs

@@ -14,6 +14,7 @@ pub mod day20;
 pub mod day21;
 pub mod day22;
 pub mod day23;
+pub mod day24;
 pub mod day3;
 pub mod day4;
 pub mod day5;