diff --git a/2021/inputs/11.txt b/2021/inputs/11.txt
new file mode 100644
index 0000000..efb0e67
--- /dev/null
+++ b/2021/inputs/11.txt
@@ -0,0 +1,10 @@
+5651341452
+1381541252
+1878435224
+6814831535
+3883547383
+6473548464
+1885833658
+3732584752
+1881546128
+5121717776
diff --git a/2021/src/day11.rs b/2021/src/day11.rs
index 113ba49..e1e4062 100644
--- a/2021/src/day11.rs
+++ b/2021/src/day11.rs
@@ -1,9 +1,114 @@
 use std::io::Read;
 
-pub fn part1(_input: &mut dyn Read) -> String {
-    todo!()
+fn read_grid<'a>(input: &'_ mut dyn Read, buffer: &'a mut Vec<u8>) -> Vec<&'a mut [u8]> {
+    input.read_to_end(buffer).unwrap();
+
+    let mut grid: Vec<&mut [u8]> = buffer.split_mut(|&b| b == b'\n').collect();
+
+    grid.iter_mut()
+        .flat_map(|line| line.iter_mut())
+        .for_each(|b| *b -= b'0');
+
+    grid
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+fn advance(grid: &mut [&mut [u8]], todo: &mut Vec<(i8, i8)>) -> usize {
+    let mut flashes = 0;
+
+    grid.iter_mut()
+        .enumerate()
+        .flat_map(|(y, line)| {
+            line.iter_mut()
+                .enumerate()
+                .map(move |(x, value)| (x, y, value))
+        })
+        .for_each(|(x, y, value)| {
+            *value += 1;
+            if *value > 9 {
+                todo.push((x as i8, y as i8));
+            }
+        });
+
+    while let Some((x, y)) = todo.pop() {
+        flashes += 1;
+
+        for dy in -1..=1 {
+            for dx in -1..=1 {
+                if dx == 0 && dy == 0 {
+                    continue;
+                }
+
+                let nx = usize::try_from(x + dx);
+                let ny = usize::try_from(y + dy);
+
+                if let (Ok(nx), Ok(ny)) = (nx, ny) {
+                    if let Some(value) = grid.get_mut(ny).and_then(|line| line.get_mut(nx)) {
+                        *value += 1;
+                        if *value == 10 {
+                            todo.push((nx as i8, ny as i8));
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    grid.iter_mut()
+        .flat_map(|line| line.iter_mut())
+        .filter(|b| **b > 9)
+        .for_each(|b| *b = 0);
+
+    flashes
+}
+
+pub fn part1(input: &mut dyn Read) -> String {
+    let mut buffer = Vec::new();
+
+    let mut grid = read_grid(input, &mut buffer);
+    let mut todo = Vec::new();
+
+    (0..100)
+        .map(|_| advance(&mut grid, &mut todo))
+        .sum::<usize>()
+        .to_string()
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    let mut buffer = Vec::new();
+
+    let mut grid = read_grid(input, &mut buffer);
+    let mut todo = Vec::new();
+
+    let target = grid.iter().map(|line| line.len()).sum();
+
+    (1..)
+        .find_map(|step| {
+            let flashes = advance(&mut grid, &mut todo);
+            if flashes == target {
+                Some(step)
+            } else {
+                None
+            }
+        })
+        .unwrap()
+        .to_string()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use crate::test_implementation;
+
+    const SAMPLE: &[u8] = include_bytes!("samples/11.txt");
+
+    #[test]
+    fn sample_part1() {
+        test_implementation(part1, SAMPLE, 1656);
+    }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE, 195);
+    }
 }
diff --git a/2021/src/samples/11.txt b/2021/src/samples/11.txt
new file mode 100644
index 0000000..03743f6
--- /dev/null
+++ b/2021/src/samples/11.txt
@@ -0,0 +1,10 @@
+5483143223
+2745854711
+5264556173
+6141336146
+6357385478
+4167524645
+2176841721
+6882881134
+4846848554
+5283751526