cargo fmt

src/day17.rs

@@ -1,6 +1,101 @@
 use aoc_runner_derive::aoc;
+use std::collections::HashSet;
 
 #[aoc(day17, part1)]
 fn solve_d17_p1(input: &str) -> usize {
-    42
+    let mut world = HashSet::new();
+    for (y, line) in input.split('\n').enumerate() {
+        for (x, _) in line
+            .as_bytes()
+            .iter()
+            .copied()
+            .enumerate()
+            .filter(|&(_, byte)| byte == b'#')
+        {
+            world.insert((x as isize, y as isize, 0isize, 0isize));
+        }
+    }
+    for _ in 0..6 {
+        world = step(world);
+    }
+    world.len()
+}
+
+fn neighbors(
+    world: &HashSet<(isize, isize, isize, isize)>,
+    (x, y, z, w): (isize, isize, isize, isize),
+) -> usize {
+    let mut count = 0;
+    for xa in x - 1..=x + 1 {
+        for ya in y - 1..=y + 1 {
+            for za in z - 1..=z + 1 {
+                for wa in w - 1..=w + 1 {
+                    count += world.contains(&(xa, ya, za, wa)) as usize;
+                }
+            }
+        }
+    }
+    count - world.contains(&(x, y, z, w)) as usize
+}
+
+fn step(world: HashSet<(isize, isize, isize, isize)>) -> HashSet<(isize, isize, isize, isize)> {
+    struct MinMax {
+        x_min: isize,
+        x_max: isize,
+        y_min: isize,
+        y_max: isize,
+        z_min: isize,
+        z_max: isize,
+        w_min: isize,
+        w_max: isize,
+    }
+    let MinMax {
+        x_min,
+        x_max,
+        y_min,
+        y_max,
+        z_min,
+        z_max,
+        w_min,
+        w_max,
+    } = world.iter().fold(
+        MinMax {
+            x_min: isize::MAX,
+            x_max: isize::MIN,
+            y_min: isize::MAX,
+            y_max: isize::MIN,
+            z_min: isize::MAX,
+            z_max: isize::MIN,
+            w_min: isize::MAX,
+            w_max: isize::MIN,
+        },
+        |minmax, &(x, y, z, w)| MinMax {
+            x_min: std::cmp::min(minmax.x_min, x),
+            x_max: std::cmp::max(minmax.x_max, x),
+            y_min: std::cmp::min(minmax.y_min, y),
+            y_max: std::cmp::max(minmax.y_max, y),
+            z_min: std::cmp::min(minmax.z_min, z),
+            z_max: std::cmp::max(minmax.z_max, z),
+            w_min: std::cmp::min(minmax.w_min, w),
+            w_max: std::cmp::max(minmax.w_max, w),
+        },
+    );
+    let mut new_world = HashSet::new();
+    for x in x_min - 1..=x_max + 1 {
+        for y in y_min - 1..=y_max + 1 {
+            for z in z_min - 1..=z_max + 1 {
+                for w in w_min - 1..=w_max + 1 {
+                    let active = world.contains(&(x, y, z, w));
+                    let n = neighbors(&world, (x, y, z, w));
+                    if active && (n == 2 || n == 3) {
+                        new_world.insert((x, y, z, w));
+                    }
+                    if !active && n == 3 {
+                        new_world.insert((x, y, z, w));
+                    }
+                }
+            }
+        }
+    }
+    new_world
 }