Implementation day 24.

2020/inputs/24.txt

@@ -0,0 +1,313 @@
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2020/samples/24.txt

@@ -0,0 +1,20 @@
+sesenwnenenewseeswwswswwnenewsewsw
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+seswneswswsenwwnwse
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2020/src/day24.rs

@@ -1,12 +1,139 @@
+use std::collections::HashMap;
+use std::collections::HashSet;
 use std::io::Read;
+use std::mem::swap;
 
+use crate::common::Lines;
 use crate::Solution;
 
+type Pos = (i32, i32);
+
+const DIRECTIONS: [Pos; 6] = [(-2, 0), (2, 0), (-1, -1), (-1, 1), (1, -1), (1, 1)];
+
+fn convert_pos(s: &str) -> Pos {
+    let mut s = s.chars();
+
+    let mut x = 0;
+    let mut y = 0;
+
+    while let Some(c) = s.next() {
+        match c {
+            'e' => x += 2,
+            'w' => x -= 2,
+            'n' | 's' => {
+                if c == 'n' {
+                    y += 1;
+                } else {
+                    y -= 1;
+                }
+
+                match s.next() {
+                    Some('e') => x += 1,
+                    Some('w') => x -= 1,
+                    _ => panic!(),
+                }
+            }
+            _ => panic!(),
+        }
+    }
+
+    (x, y)
+}
+
+fn step(current: &HashSet<Pos>, target: &mut HashSet<Pos>) {
+    let mut black_count: HashMap<Pos, u32> = current.iter().map(|&pos| (pos, 0)).collect();
+
+    for &(x, y) in current {
+        for &(dx, dy) in &DIRECTIONS {
+            let pos = (x + dx, y + dy);
+
+            *black_count.entry(pos).or_default() += 1;
+        }
+    }
+
+    target.clear();
+
+    for (pos, neighbours) in black_count {
+        let is_black = current.contains(&pos);
+
+        let going_black = match neighbours {
+            1 | 2 if is_black => true,
+            2 if !is_black => true,
+            _ => false,
+        };
+
+        if going_black {
+            target.insert(pos);
+        }
+    }
+}
+
+fn get_black_tiles(input: &mut dyn Read) -> HashSet<Pos> {
+    let mut black = HashSet::new();
+
+    for line in Lines::new(input) {
+        let pos = convert_pos(&line);
+
+        if !black.insert(pos) {
+            black.remove(&pos);
+        }
+    }
+
+    black
+}
+
 #[derive(Default)]
 pub struct Day24;
 
 impl Solution for Day24 {
-    fn part1(&mut self, _input: &mut dyn Read) -> String {
-        todo!()
+    fn part1(&mut self, input: &mut dyn Read) -> String {
+        get_black_tiles(input).len().to_string()
+    }
+
+    fn part2(&mut self, input: &mut dyn Read) -> String {
+        let mut state = get_black_tiles(input);
+        let mut scratch_pad = state.clone();
+
+        for _ in 0..100 {
+            step(&state, &mut scratch_pad);
+            swap(&mut state, &mut scratch_pad);
+        }
+
+        state.len().to_string()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::test_implementation;
+
+    use super::*;
+
+    const SAMPLE: &[u8] = include_bytes!("../samples/24.txt");
+
+    #[test]
+    fn test_convert_pos() {
+        assert_eq!((1, -1), convert_pos("esew"));
+        assert_eq!((0, 0), convert_pos("nwwswee"));
+    }
+
+    #[test]
+    fn test_step() {
+        let state = get_black_tiles(&mut SAMPLE.clone());
+        let mut target = state.clone();
+
+        step(&state, &mut target);
+
+        assert_eq!(15, target.len());
+    }
+
+    #[test]
+    fn sample_part1() {
+        test_implementation!(Day24, 1, SAMPLE, 10);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation!(Day24, 2, SAMPLE, 2208);
     }
 }