diff --git a/input/day11.txt b/input/day11.txt
new file mode 100644
index 0000000..db2a113
--- /dev/null
+++ b/input/day11.txt
@@ -0,0 +1,10 @@
+1553421288
+5255384882
+1224315732
+4258242274
+1658564216
+6872651182
+5775552238
+5622545172
+8766672318
+2178374835
diff --git a/src/day11.rs b/src/day11.rs
new file mode 100644
index 0000000..06477c4
--- /dev/null
+++ b/src/day11.rs
@@ -0,0 +1,148 @@
+use ansi_term::{Color, Style};
+use std::collections::{HashSet, VecDeque};
+use std::fmt::{Debug, Formatter};
+use std::ops::RangeInclusive;
+
+pub fn day11() {
+    // let input = r#"
+    // 11111
+    // 19991
+    // 19191
+    // 19991
+    // 11111
+    //     "#;
+
+    // let grid: Grid<5> = input.into();
+    // dbg!(&grid);
+    // dbg!(&grid.step());
+    // dbg!(&grid.step().step());
+
+    // let input = r#"
+    // 5483143223
+    // 2745854711
+    // 5264556173
+    // 6141336146
+    // 6357385478
+    // 4167524645
+    // 2176841721
+    // 6882881134
+    // 4846848554
+    // 5283751526
+    //     "#;
+    let input = include_str!("../input/day11.txt");
+
+    let mut grid: Grid<10> = input.into();
+    let mut total_flashes = 0;
+
+    println!("{:?}", grid);
+
+    for step in 0..100 {
+        let (new, flashed) = grid.step();
+        // println!("after step: {}, new grid: {:?}", step + 1, new);
+        grid = new;
+        total_flashes += flashed;
+    }
+
+    println!("grid: {:?}, total flashes: {}", grid, total_flashes);
+
+    let mut grid2: Grid<10> = input.into();
+    let mut step = 0;
+    loop {
+        step += 1;
+        let (new, flashed) = grid2.step();
+        grid2 = new;
+        if flashed == 100 {
+            break;
+        }
+    }
+    println!("grid: {:?}, after step: {}", grid2, step);
+}
+
+struct Grid<const N: usize> {
+    grid: [[u32; N]; N],
+}
+
+impl<const N: usize> Debug for Grid<N> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "\n").expect("write");
+        for row in 0..N {
+            for col in 0..N {
+                let mut style = Style::new();
+                if self.grid[row][col] == 0 {
+                    style = Color::White.bold();
+                }
+                write!(
+                    f,
+                    "{}{}{}",
+                    style.prefix(),
+                    self.grid[row][col],
+                    style.suffix()
+                )
+                .expect("write");
+            }
+            write!(f, "\n").expect("write");
+        }
+        Ok(())
+    }
+}
+
+impl<const N: usize> From<&str> for Grid<N> {
+    fn from(s: &str) -> Self {
+        let mut grid = [[0; N]; N];
+        for (row, line) in s.trim().lines().enumerate() {
+            if row >= N {
+                panic!("invalid grid size");
+            }
+            for (col, c) in line.trim().chars().enumerate() {
+                grid[row][col] = c.to_digit(10).unwrap();
+            }
+        }
+        Self { grid }
+    }
+}
+
+impl<const N: usize> Grid<N> {
+    fn step(&self) -> (Grid<N>, usize) {
+        let mut new_grid = [[0; N]; N];
+
+        // increment all
+        for row in 0..N {
+            for col in 0..N {
+                new_grid[row][col] = self.grid[row][col] + 1;
+            }
+        }
+
+        let mut to_flash = (0..N)
+            .flat_map(|row| (0..N).map(move |col| (row, col)))
+            .filter(|&(row, col)| new_grid[row][col] > 9)
+            .collect::<VecDeque<(usize, usize)>>();
+        let mut already_flashed: HashSet<(usize, usize)> = HashSet::new();
+        while let Some((row, col)) = to_flash.pop_front() {
+            if already_flashed.contains(&(row, col)) {
+                continue;
+            }
+
+            already_flashed.insert((row, col));
+            for r in adjacent(row, N) {
+                for c in adjacent(col, N) {
+                    new_grid[r][c] += 1;
+                    if new_grid[r][c] > 9 && !already_flashed.contains(&(r, c)) {
+                        to_flash.push_back((r, c));
+                    }
+                }
+            }
+        }
+
+        for &(row, col) in already_flashed.iter() {
+            new_grid[row][col] = 0;
+        }
+
+        (Self { grid: new_grid }, already_flashed.len())
+    }
+}
+
+fn adjacent(n: usize, max: usize) -> RangeInclusive<usize> {
+    let min = if n > 0 { n - 1 } else { n };
+    let max = if n < max - 1 { n + 1 } else { n };
+    min..=max
+}