1 files changed, 51 insertions, 17 deletions

diff --git a/2025/04/rust/src/main.rs b/2025/04/rust/src/main.rs
index 5d703c5..3b13140 100644
--- a/2025/04/rust/src/main.rs
+++ b/2025/04/rust/src/main.rs
@@ -1,4 +1,4 @@
-use std::io;
+use std::{collections::VecDeque, io};
 
 fn main() {
     let mut grid: Vec<Vec<Option<usize>>> = io::stdin()
@@ -7,26 +7,60 @@ fn main() {
         .map(|line| line.chars().map(|ch| (ch == '@').then_some(0)).collect())
         .collect();
 
-    for (y, row) in grid.iter().enumerate() {
-        for (x, p) in row.iter().enumerate() {
-            let Some(_) = p else {
-                continue;
-            };
-
-            for nx in x.saturating_sub(1)..=(x + 1).min(row.len() - 1) {
-                for ny in y.saturating_sub(1)..=(y + 1).min(grid.len() - 1) {
-                    // lol quadruple for loop (this makes me sad)
-                    if (x, y) == (nx, ny) {
-                        continue;
-                    }
-                    let mut h = grid.get(ny).and_then(|r| r.get(nx)).unwrap();
-                }
+    // count neighbours
+    points(&grid)
+        .collect::<Vec<_>>()
+        .into_iter()
+        .for_each(|p @ (x, y)| {
+            let ns = neighbours(p, &grid).count();
+            if let Some(n) = grid[y][x].as_mut() {
+                *n = ns;
             }
+        });
+
+    let mut stack = points(&grid)
+        .filter(|&(x, y)| grid[y][x].is_some_and(|n| n < 4))
+        .collect::<VecDeque<_>>();
+    let silver = stack.len();
+    let mut gold = 0;
+
+    while let Some(p @ (x, y)) = stack.pop_back() {
+        if grid[y][x].take().is_none() {
+            continue;
         }
+        gold += 1;
+        // reduce neighbours' neighbour counts by 1
+        neighbours(p, &grid)
+            .collect::<Vec<_>>()
+            .into_iter()
+            .for_each(|(nx, ny)| {
+                if let Some(n) = grid[ny][nx].as_mut() {
+                    *n -= 1;
+                    if *n < 4 {
+                        stack.push_back((nx, ny))
+                    }
+                }
+            });
     }
 
-    let silver: u64 = 0;
-    let gold: u64 = 0;
     println!("silver: {silver}");
     println!("gold: {gold}");
 }
+
+fn neighbours<T>(
+    (x, y): (usize, usize),
+    grid: &Vec<Vec<Option<T>>>,
+) -> impl Iterator<Item = (usize, usize)> {
+    let xs = x.saturating_sub(1)..=(x + 1).min(grid.first().unwrap().len() - 1);
+    let ys = y.saturating_sub(1)..=(y + 1).min(grid.len() - 1);
+
+    xs.flat_map(move |nx| ys.clone().map(move |ny| (nx, ny)))
+        .filter(move |n| *n != (x, y))
+        .filter(|&(nx, ny)| grid.get(ny).and_then(|row| row.get(nx)).unwrap().is_some())
+}
+
+fn points<T>(grid: &Vec<Vec<T>>) -> impl Iterator<Item = (usize, usize)> {
+    let xs = 0..grid.first().unwrap().len();
+    let ys = 0..grid.len();
+    xs.flat_map(move |x| ys.clone().map(move |y| (x, y)))
+}