Document Year 2022 Day 18

Author: maneatingape

src/year2022/day18.rs

@@ -1,23 +1,36 @@
+//! # Boiling Boulders
+//!
+//! The lava droplet is a fixed size so we can use a one dimensional fixed size array to store the
+//! cube data for speed.
+//!
+//! For part two we use the [flood fill](https://en.wikipedia.org/wiki/Flood_fill) algorithm
+//! starting from any corner to fill the outside space with water. We then use the same exposed
+//! edge counting approach as part one, but only considering faces that touch a water drop.
 use crate::util::iter::*;
 use crate::util::parse::*;
 
 const SIZE: usize = 22;
 
 pub fn parse(input: &str) -> Vec<u32> {
     let mut cube = vec![0; SIZE * SIZE * SIZE];
+    // Leave a 1 layer boundary around the outside for the part two flood fill
+    // and also so that we don't have to use boundary checks when checking neighbors.
     input.iter_unsigned().chunk::<3>().for_each(|[x, y, z]: [usize; 3]| {
         cube[(x + 1) * SIZE * SIZE + (y + 1) * SIZE + (z + 1)] = 1;
     });
     cube
 }
 
 pub fn part1(input: &[u32]) -> u32 {
+    // The exposed surface area is the 6 faces of the cubes minus any neighbors.
     count(input, |x| 6 - x)
 }
 
 pub fn part2(input: &[u32]) -> u32 {
     let mut cube = input.to_vec();
+    // "Paint" the outside of the cube with water drops.
     flood_fill(&mut cube, 0);
+    // Divide by 8 so that we only count water cubes.
     count(&cube, |x| x >> 3)
 }
 
@@ -26,6 +39,7 @@ fn count(cube: &[u32], adjust: fn(u32) -> u32) -> u32 {
 
     for i in 0..cube.len() {
         if cube[i] == 1 {
+            // No need for boundary checks as all cubes are at least 1 away from the edge.
             total += adjust(
                 cube[i - 1]
                     + cube[i + 1]
@@ -42,7 +56,9 @@ fn count(cube: &[u32], adjust: fn(u32) -> u32) -> u32 {
 
 fn flood_fill(cube: &mut [u32], i: usize) {
     if cube.get(i) == Some(&0) {
+        // Use 8 as the nearest power of two greater than 6.
         cube[i] = 8;
+        // We may wrap around to an opposite edge but that will also be water.
         flood_fill(cube, i.saturating_sub(1));
         flood_fill(cube, i + 1);
         flood_fill(cube, i.saturating_sub(SIZE));