y2025d4

y2025/src/days/d4.rs

@@ -0,0 +1,109 @@
+use itertools::Itertools;
+
+fn neighbors(grid: &Vec<Vec<char>>, position: (isize, isize)) -> i32 {
+    let width = grid[0].len() as isize;
+    let height = grid.len() as isize;
+    let mut num_tp = 0;
+    for y_offset in -1..=1 {
+        for x_offset in -1..=1 {
+            let search_position = (position.0 + x_offset, position.1 + y_offset);
+            if search_position == position
+                || search_position.0 < 0
+                || search_position.0 >= width
+                || search_position.1 < 0
+                || search_position.1 >= height
+            {
+                continue;
+            }
+            let cell = grid[search_position.1 as usize][search_position.0 as usize];
+            if cell == '@' {
+                num_tp += 1;
+            }
+        }
+    }
+    num_tp
+}
+
+pub fn process_part1(input: &str) -> i32 {
+    let grid = input
+        .lines()
+        .map(|line| line.chars().collect_vec())
+        .collect_vec();
+    let width = grid[0].len() as isize;
+    let height = grid.len() as isize;
+    let mut accessible_tp = 0;
+    for y in 0..height {
+        for x in 0..width {
+            if grid[y as usize][x as usize] != '@' {
+                continue;
+            }
+            let neighbors = neighbors(&grid, (x, y));
+            if neighbors < 4 {
+                accessible_tp += 1;
+            }
+        }
+    }
+    accessible_tp
+}
+
+pub fn process_part2(input: &str) -> i32 {
+    let mut grid = input
+        .lines()
+        .map(|line| line.chars().collect_vec())
+        .collect_vec();
+    let width = grid[0].len() as isize;
+    let height = grid.len() as isize;
+    let mut accessible_tp_total = 0;
+    loop {
+        let mut accessible_tp = 0;
+        let mut new_grid = grid.clone();
+        for y in 0..height {
+            for x in 0..width {
+                if grid[y as usize][x as usize] != '@' {
+                    continue;
+                }
+                let neighbors = neighbors(&grid, (x, y));
+                if neighbors < 4 {
+                    accessible_tp += 1;
+                    new_grid[y as usize][x as usize] = '.';
+                } else {
+                    new_grid[y as usize][x as usize] = '@';
+                }
+            }
+        }
+        if accessible_tp == 0 {
+            break;
+        }
+        grid = new_grid;
+        accessible_tp_total += accessible_tp;
+    }
+    accessible_tp_total
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const INPUT: &str = "..@@.@@@@.
+@@@.@.@.@@
+@@@@@.@.@@
+@.@@@@..@.
+@@.@@@@.@@
+.@@@@@@@.@
+.@.@.@.@@@
+@.@@@.@@@@
+.@@@@@@@@.
+@.@.@@@.@.";
+
+    #[test]
+    fn part1() {
+        let result = process_part1(INPUT);
+        assert_eq!(result, 13);
+    }
+
+    #[test]
+    fn part2() {
+        let result = process_part2(INPUT);
+        assert_eq!(result, 43);
+    }
+}