add day 18

day18/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day18"
+version = "0.1.0"

day18/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "day18"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

day18/input.txt

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day18/src/main.rs

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+//! <https://adventofcode.com/2015/day/18>
+
+fn main() {
+    let mut grid = std::fs::read_to_string("input.txt")
+        .unwrap()
+        .lines()
+        .map(|l| {
+            l.chars()
+                .map(|c| match c {
+                    '#' => 1,
+                    '.' => 0,
+                    _ => panic!(),
+                })
+                .collect::<Vec<_>>()
+        })
+        .collect::<Vec<_>>();
+
+    part1(grid.clone()); // 1061
+
+    grid[0][0] = 1;
+    grid[99][0] = 1;
+    grid[0][99] = 1;
+    grid[99][99] = 1;
+
+    part2(grid)
+}
+
+fn part2(mut grid: Vec<Vec<i32>>) {
+    for _ in 0..100 {
+        let mut new_grid = vec![vec![0; 100]; 100];
+        for x in 0..100 {
+            for y in 0..100 {
+                let neighbor_count = get_neighbor_count(&grid, x, y);
+                if grid[x][y] == 1 && [2, 3].contains(&neighbor_count) {
+                    new_grid[x][y] = 1;
+                }
+                if grid[x][y] == 0 && neighbor_count == 3 {
+                    new_grid[x][y] = 1;
+                }
+            }
+        }
+
+        new_grid[0][0] = 1;
+        new_grid[99][0] = 1;
+        new_grid[0][99] = 1;
+        new_grid[99][99] = 1;
+        std::mem::swap(&mut grid, &mut new_grid);
+    }
+
+    let result = grid.iter().flatten().sum::<i32>();
+    println!("{result}");
+}
+
+fn part1(mut grid: Vec<Vec<i32>>) {
+    for _ in 0..100 {
+        let mut new_grid = vec![vec![0; 100]; 100];
+        for x in 0..100 {
+            for y in 0..100 {
+                let neighbor_count = get_neighbor_count(&grid, x, y);
+                if grid[x][y] == 1 && [2, 3].contains(&neighbor_count) {
+                    new_grid[x][y] = 1;
+                }
+                if grid[x][y] == 0 && neighbor_count == 3 {
+                    new_grid[x][y] = 1;
+                }
+            }
+        }
+        std::mem::swap(&mut grid, &mut new_grid);
+    }
+
+    let result = grid.iter().flatten().sum::<i32>();
+    println!("{result}");
+}
+
+fn get_neighbor_count(grid: &[Vec<i32>], x: usize, y: usize) -> i32 {
+    let mut neighbor_count = 0;
+    for xd in [-1, 0, 1] {
+        for yd in [-1, 0, 1] {
+            if xd == 0 && yd == 0 {
+                continue;
+            }
+
+            let xc = (x as isize) + xd;
+            let yc = (y as isize) + yd;
+
+            if !(0..100).contains(&xc) || !(0..100).contains(&yc) {
+                continue;
+            }
+
+            let xc = xc as usize;
+            let yc = yc as usize;
+
+            if grid[xc][yc] == 1 {
+                neighbor_count += 1;
+            }
+        }
+    }
+
+    neighbor_count
+}