From 178e2e50117b6c5f05c5af1696dcad1376663d1a Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Sat, 17 Dec 2022 22:02:11 -0800
Subject: [PATCH] 2022 Day 18 Part 2

---
 2022/src/bin/day18.rs | 174 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 154 insertions(+), 20 deletions(-)

diff --git a/2022/src/bin/day18.rs b/2022/src/bin/day18.rs
index 1a34fee..3c8139e 100644
--- a/2022/src/bin/day18.rs
+++ b/2022/src/bin/day18.rs
@@ -1,20 +1,15 @@
+use std::collections::{HashMap, VecDeque};
+
 const INPUT: &str = include_str!("../../input/day18.txt");
 
 fn main() {
-    println!("{}", part1(INPUT));
+    let cubes = parse(INPUT);
+    let open_faces = part1(cubes.clone());
+    println!("Part 1: {}", open_faces);
+    println!("Part 2: {}", part2(cubes, open_faces));
 }
 
-fn part1(input: &str) -> usize {
-    let cubes: Vec<Cube> = aoc::lines(input)
-        .map(|line| {
-            let coords: Vec<&str> = line.split(',').collect();
-            let x = coords[0].parse().unwrap();
-            let y = coords[1].parse().unwrap();
-            let z = coords[2].parse().unwrap();
-            Cube::new(x, y, z)
-        })
-        .collect();
-
+fn part1(cubes: Vec<Cube>) -> usize {
     let mut shared_faces = 0;
     for (c1, c2) in itertools::iproduct!(&cubes, &cubes) {
         if c1 == c2 {
@@ -29,22 +24,161 @@ fn part1(input: &str) -> usize {
     6 * cubes.len() - shared_faces
 }
 
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+fn part2(cubes: Vec<Cube>, open_faces: usize) -> usize {
+    let min = cubes.iter().map(Cube::min_coord).min().unwrap();
+    let max = cubes.iter().map(Cube::max_coord).max().unwrap();
+
+    let mut grid: HashMap<Cube, Material> = cubes
+        .iter()
+        .cloned()
+        .map(|c| (c, Material::Water))
+        .collect();
+
+    let lo = Cube {
+        x: min - 1,
+        y: min - 1,
+        z: min - 1,
+    };
+
+    let hi = Cube {
+        x: max + 1,
+        y: max + 1,
+        z: max + 1,
+    };
+
+    grid.insert(lo, Material::Steam);
+    grid.insert(hi, Material::Steam);
+
+    steam(&mut grid, lo, hi);
+    air(&mut grid, lo, hi);
+
+    let mut insulated_faces = 0;
+    for (&cube, &mat) in &grid {
+        if mat == Material::Air {
+            for neighbor in cube.neighbors() {
+                if let Some(Material::Water) = grid.get(&neighbor) {
+                    insulated_faces += 1;
+                }
+            }
+        }
+    }
+
+    open_faces - insulated_faces
+}
+
+fn steam(grid: &mut HashMap<Cube, Material>, lo: Cube, hi: Cube) {
+    let mut queue: VecDeque<Cube> = grid
+        .iter()
+        .filter_map(|(&cube, &mat)| {
+            if mat == Material::Steam {
+                Some(cube)
+            } else {
+                None
+            }
+        })
+        .collect();
+
+    while let Some(src) = queue.pop_front() {
+        for dst in src.neighbors() {
+            if dst.out_of_bounds(lo, hi) {
+                continue;
+            }
+
+            if let Some(_mat) = grid.get(&dst) {
+                continue;
+            }
+
+            let old = grid.insert(dst, Material::Steam);
+            assert!(old.is_none());
+
+            queue.push_back(dst);
+        }
+    }
+}
+
+fn air(grid: &mut HashMap<Cube, Material>, lo: Cube, hi: Cube) {
+    for (x, y, z) in itertools::iproduct!(lo.x..=hi.x, lo.y..=hi.y, lo.z..=hi.z) {
+        let cube = Cube::new(x, y, z);
+        grid.entry(cube).or_insert(Material::Air);
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+enum Material {
+    Water,
+    Steam,
+    Air,
+}
+
+fn parse(input: &str) -> Vec<Cube> {
+    aoc::lines(input)
+        .map(|line| {
+            let coords: Vec<&str> = line.split(',').collect();
+            let x = coords[0].parse().unwrap();
+            let y = coords[1].parse().unwrap();
+            let z = coords[2].parse().unwrap();
+            Cube::new(x, y, z)
+        })
+        .collect()
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 struct Cube {
-    x: u32,
-    y: u32,
-    z: u32,
+    x: i32,
+    y: i32,
+    z: i32,
 }
 
 impl Cube {
-    fn new(x: u32, y: u32, z: u32) -> Self {
+    fn new(x: i32, y: i32, z: i32) -> Self {
         Self { x, y, z }
     }
 
     fn touches(&self, other: &Self) -> bool {
-        let dx = (self.x as i32 - other.x as i32).abs();
-        let dy = (self.y as i32 - other.y as i32).abs();
-        let dz = (self.z as i32 - other.z as i32).abs();
+        let dx = (self.x - other.x).abs();
+        let dy = (self.y - other.y).abs();
+        let dz = (self.z - other.z).abs();
         (dx + dy + dz) == 1
     }
+
+    fn max_coord(&self) -> i32 {
+        (self.x).max(self.y).max(self.z)
+    }
+
+    fn min_coord(&self) -> i32 {
+        (self.x).min(self.y).min(self.z)
+    }
+
+    fn neighbors(&self) -> Vec<Self> {
+        let deltas = vec![
+            (-1, 0, 0),
+            (0, -1, 0),
+            (0, 0, -1),
+            (1, 0, 0),
+            (0, 1, 0),
+            (0, 0, 1),
+        ];
+
+        deltas
+            .iter()
+            .map(|(dx, dy, dz)| Self {
+                x: self.x + dx,
+                y: self.y + dy,
+                z: self.z + dz,
+            })
+            .collect()
+    }
+
+    fn out_of_bounds(&self, lo: Self, hi: Self) -> bool {
+        vec![
+            self.x < lo.x,
+            self.y < lo.y,
+            self.z < lo.z,
+            self.x > hi.x,
+            self.y > hi.y,
+            self.z > hi.z,
+        ]
+        .iter()
+        .any(|b| *b)
+    }
 }