diff --git a/Cargo.lock b/Cargo.lock
index ec033d8..c725663 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,10 +2,20 @@
 # It is not intended for manual editing.
 version = 3
 
+[[package]]
+name = "ansi_term"
+version = "0.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d52a9bb7ec0cf484c551830a7ce27bd20d67eac647e1befb56b0be4ee39a55d2"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "aoc21"
 version = "0.1.0"
 dependencies = [
+ "ansi_term",
  "itertools",
 ]
 
@@ -23,3 +33,25 @@ checksum = "a9a9d19fa1e79b6215ff29b9d6880b706147f16e9b1dbb1e4e5947b5b02bc5e3"
 dependencies = [
  "either",
 ]
+
+[[package]]
+name = "winapi"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
+dependencies = [
+ "winapi-i686-pc-windows-gnu",
+ "winapi-x86_64-pc-windows-gnu",
+]
+
+[[package]]
+name = "winapi-i686-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
+
+[[package]]
+name = "winapi-x86_64-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
diff --git a/Cargo.toml b/Cargo.toml
index e2c0e6a..471ab37 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,3 +7,4 @@ edition = "2021"
 
 [dependencies]
 itertools = "0.10.3"
+ansi_term = "0.12.1"
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..905ab40
--- /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 == grid.size() {
+            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() {
+            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];
+
+        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 size(&self) -> usize {
+        N * N
+    }
+}
+
+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
+}
diff --git a/src/main.rs b/src/main.rs
index c90783c..05bb30f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -9,7 +9,8 @@ mod day07;
 mod day08;
 mod day09;
 mod day10;
+mod day11;
 
 fn main() {
-    day10::day10();
+    day11::day11();
 }