Day 18, part 2

src/bin/d18p2.rs

@@ -0,0 +1,10 @@
+use aoc22::{day18, util};
+
+pub fn main() {
+    let droplet = day18::parse_droplet(&util::parse_input());
+
+    println!(
+        "Outer surface area: {}",
+        day18::outer_surface_area(&droplet)
+    );
+}

src/day18.rs

@@ -1,6 +1,15 @@
+use std::collections::HashSet;
+
 use itertools::Itertools;
 
-pub fn parse_droplet(input: &String) -> Vec<Vec<Vec<bool>>> {
+#[derive(Clone, Copy, Debug)]
+pub enum Material {
+    Air,
+    Lava,
+    Steam,
+}
+
+pub fn parse_droplet(input: &String) -> Vec<Vec<Vec<Material>>> {
     let mut cubes: Vec<(usize, usize, usize)> = Vec::new();
 
     for line in input.lines() {
@@ -19,19 +28,19 @@ pub fn parse_droplet(input: &String) -> Vec<Vec<Vec<bool>>> {
     let (x_min, x_max) = cubes.iter().map(|c| c.0).minmax().into_option().unwrap();
     let (y_min, y_max) = cubes.iter().map(|c| c.1).minmax().into_option().unwrap();
     let (z_min, z_max) = cubes.iter().map(|c| c.2).minmax().into_option().unwrap();
-    let x_len = x_max - x_min + 1;
-    let y_len = y_max - y_min + 1;
-    let z_len = z_max - z_min + 1;
+    let x_len = x_max - x_min + 3;
+    let y_len = y_max - y_min + 3;
+    let z_len = z_max - z_min + 3;
 
-    let mut result = vec![vec![vec![false; z_len]; y_len]; x_len];
+    let mut result = vec![vec![vec![Material::Air; z_len]; y_len]; x_len];
     for (x, y, z) in cubes {
-        result[x - x_min][y - y_min][z - z_min] = true;
+        result[x - x_min + 1][y - y_min + 1][z - z_min + 1] = Material::Lava;
     }
 
     result
 }
 
-pub fn surface_area(droplet: &Vec<Vec<Vec<bool>>>) -> usize {
+pub fn surface_area(droplet: &Vec<Vec<Vec<Material>>>) -> usize {
     let x_len = droplet.len();
     assert!(x_len > 0);
     let y_len = droplet[0].len();
@@ -100,7 +109,7 @@ enum ArgOrder {
 }
 
 fn surface_area_dir(
-    droplet: &Vec<Vec<Vec<bool>>>,
+    droplet: &Vec<Vec<Vec<Material>>>,
     lines: &Vec<(usize, usize, Vec<usize>)>,
     order: ArgOrder,
 ) -> usize {
@@ -113,11 +122,119 @@ fn surface_area_dir(
                 ArgOrder::XZY => droplet[*a][*c][*b],
                 ArgOrder::YZX => droplet[*c][*a][*b],
             };
-            if last_empty && occupied {
+            if last_empty && matches!(occupied, Material::Lava) {
                 result += 1;
             }
-            last_empty = !occupied;
+            last_empty = matches!(occupied, Material::Air);
         }
     }
     result
 }
+
+pub fn outer_surface_area(droplet: &Vec<Vec<Vec<Material>>>) -> usize {
+    let x_len = droplet.len();
+    assert!(x_len > 0);
+    let y_len = droplet[0].len();
+    assert!(y_len > 0);
+    let z_len = droplet[0][0].len();
+    assert!(z_len > 0);
+
+    let mut next = Vec::new();
+
+    // top/bottom
+    for x in 0..x_len {
+        for y in 0..y_len {
+            next.push((x, y, 0));
+            next.push((x, y, z_len - 1));
+        }
+    }
+    // left/right
+    for x in 0..x_len {
+        for z in 0..z_len {
+            next.push((x, 0, z));
+            next.push((x, y_len - 1, z));
+        }
+    }
+    // fore/back
+    for y in 0..y_len {
+        for z in 0..z_len {
+            next.push((0, y, z));
+            next.push((x_len - 1, y, z));
+        }
+    }
+
+    let mut visited: HashSet<_> = next.iter().map(|c| c.clone()).collect();
+    let generators: [Box<dyn Fn(&Coord) -> Option<Coord>>; 6] = [
+        Box::new(|c| go_fore(c)),
+        Box::new(|c| go_back(c, x_len)),
+        Box::new(|c| go_left(c)),
+        Box::new(|c| go_right(c, y_len)),
+        Box::new(|c| go_up(c)),
+        Box::new(|c| go_down(c, z_len)),
+    ];
+
+    let mut result = 0;
+    while let Some(n) = next.pop().clone() {
+        for gen in &generators {
+            if let Some(c) = gen(&n) {
+                let occupied = droplet[c.0][c.1][c.2];
+                if matches!(occupied, Material::Air) && visited.insert(c) {
+                    next.push(c);
+                } else if matches!(occupied, Material::Lava) {
+                    result += 1;
+                }
+            }
+        }
+    }
+    result
+}
+
+type Coord = (usize, usize, usize);
+
+fn go_fore(c: &Coord) -> Option<Coord> {
+    if c.0 > 0 {
+        Some((c.0 - 1, c.1, c.2))
+    } else {
+        None
+    }
+}
+
+fn go_back(c: &Coord, x_len: usize) -> Option<Coord> {
+    if c.0 < x_len - 1 {
+        Some((c.0 + 1, c.1, c.2))
+    } else {
+        None
+    }
+}
+
+fn go_left(c: &Coord) -> Option<Coord> {
+    if c.1 > 0 {
+        Some((c.0, c.1 - 1, c.2))
+    } else {
+        None
+    }
+}
+
+fn go_right(c: &Coord, y_len: usize) -> Option<Coord> {
+    if c.1 < y_len - 1 {
+        Some((c.0, c.1 + 1, c.2))
+    } else {
+        None
+    }
+}
+
+fn go_up(c: &Coord) -> Option<Coord> {
+    if c.2 > 0 {
+        Some((c.0, c.1, c.2 - 1))
+    } else {
+        None
+    }
+}
+
+fn go_down(c: &Coord, z_len: usize) -> Option<Coord> {
+    if c.2 < z_len - 1 {
+        Some((c.0, c.1, c.2 + 1))
+    } else {
+        None
+    }
+}