diff --git a/2021/src/common.rs b/2021/src/common.rs
index 751c8e4..162f2c1 100644
--- a/2021/src/common.rs
+++ b/2021/src/common.rs
@@ -102,25 +102,32 @@ where
     }
 }
 
+#[derive(Default)]
 pub struct BitSet {
     buffer: Vec<u32>,
 }
 
 impl BitSet {
     pub fn new() -> Self {
-        Self::with_capacity(0)
+        Self::default()
     }
 
     pub fn with_capacity(capacity: usize) -> Self {
-        let buffer = Vec::with_capacity(capacity);
+        let buffer = Vec::with_capacity(capacity / 32);
 
         Self { buffer }
     }
 
-    pub fn insert(&mut self, value: usize) -> bool {
+    fn convert_value(value: usize) -> (usize, u32) {
         let chunk = value / 32;
         let bit = 1 << (31 - (value % 32));
 
+        (chunk, bit)
+    }
+
+    pub fn insert(&mut self, value: usize) -> bool {
+        let (chunk, bit) = Self::convert_value(value);
+
         if self.buffer.len() <= chunk + 1 {
             self.buffer.resize(chunk + 1, 0);
         }
@@ -135,4 +142,13 @@ impl BitSet {
     pub fn len(&self) -> usize {
         self.buffer.iter().map(|c| c.count_ones() as usize).sum()
     }
+
+    pub fn contains(&self, value: usize) -> bool {
+        let (chunk, bit) = Self::convert_value(value);
+
+        self.buffer
+            .get(chunk)
+            .map(|&c| c & bit != 0)
+            .unwrap_or(false)
+    }
 }
diff --git a/2021/src/day20.rs b/2021/src/day20.rs
index 9f113bf..fa67954 100644
--- a/2021/src/day20.rs
+++ b/2021/src/day20.rs
@@ -1,10 +1,125 @@
-use std::collections::HashSet;
+use std::fmt::Display;
 use std::io::Read;
-use std::mem::swap;
+use std::ops::Index;
+
+use crate::common::BitSet;
 
 type Translation = [bool; 512];
-type Point = (i32, i32);
-type Field = HashSet<Point>;
+
+struct Field {
+    width: usize,
+    height: usize,
+    infinity: bool,
+    finite: BitSet,
+}
+
+impl Field {
+    pub fn from_input<'a>(input: impl Iterator<Item = &'a [u8]>) -> Self {
+        let mut input = input.peekable();
+
+        let width = input.peek().unwrap().len();
+
+        let mut finite = BitSet::new();
+
+        let len = input
+            .flatten()
+            .enumerate()
+            .map(|(index, &c)| {
+                if c == b'#' {
+                    finite.insert(index);
+                }
+            })
+            .count();
+
+        debug_assert_eq!(len % width, 0);
+        let height = len / width;
+
+        Self {
+            width,
+            height,
+            finite,
+            infinity: false,
+        }
+    }
+
+    pub fn advance(&mut self, translation: &[bool; 512]) {
+        const INDEX_MASK: usize = (1 << 9) - 1;
+
+        let new_width = self.width + 2;
+        let new_height = self.height + 2;
+
+        let mut new_finite = BitSet::with_capacity(new_width * new_height);
+
+        // Now we can just do a normal loop
+        for y in 0..new_height {
+            for x in 0..new_width {
+                let mut mask = if self.infinity { INDEX_MASK } else { 0 };
+
+                for y in y.saturating_sub(2)..=y {
+                    if x < 2 {
+                        for _ in 0..(2 - x) {
+                            mask = self.infinity as usize | (mask << 1);
+                        }
+                    }
+
+                    for x in x.saturating_sub(2)..=x {
+                        mask = (mask << 1) | (self[(x, y)] as usize);
+                    }
+                }
+
+                if translation[mask & INDEX_MASK] {
+                    let index = x + y * new_width;
+                    new_finite.insert(index);
+                }
+            }
+        }
+
+        self.width += 2;
+        self.height += 2;
+        self.finite = new_finite;
+        self.infinity = translation[if self.infinity { INDEX_MASK } else { 0 }];
+    }
+
+    pub fn len(&self) -> usize {
+        assert!(!self.infinity);
+        self.finite.len()
+    }
+}
+
+impl Index<(usize, usize)> for Field {
+    type Output = bool;
+
+    #[inline]
+    fn index(&self, (x, y): (usize, usize)) -> &Self::Output {
+        if x >= self.width || y >= self.height {
+            return &self.infinity;
+        }
+
+        let index = x + y * self.width;
+
+        if self.finite.contains(index) {
+            &true
+        } else {
+            &false
+        }
+    }
+}
+
+impl Display for Field {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        for y in 0..self.height {
+            for x in 0..self.width {
+                if self[(x, y)] {
+                    write!(f, "#")?
+                } else {
+                    write!(f, ".")?
+                }
+            }
+            writeln!(f)?;
+        }
+        Ok(())
+    }
+}
 
 fn read_input(input: &mut dyn Read) -> (Translation, Field) {
     let mut buffer = Vec::new();
@@ -19,67 +134,16 @@ fn read_input(input: &mut dyn Read) -> (Translation, Field) {
         .zip(it.next().unwrap())
         .for_each(|(t, &c)| *t = c == b'#');
 
-    let mut field = Field::default();
-
-    for (y, line) in it.skip(1).enumerate() {
-        for (x, _) in line.iter().enumerate().filter(|(_, &c)| c == b'#') {
-            field.insert((x as i32, y as i32));
-        }
-    }
+    let field = Field::from_input(it.skip(1));
 
     (translation, field)
 }
 
-fn find_dimensions(field: &Field) -> ((i32, i32), (i32, i32)) {
-    field
-        .iter()
-        .fold(((0, 0), (0, 0)), |((xmin, xmax), (ymin, ymax)), &(x, y)| {
-            ((xmin.min(x), xmax.max(x)), (ymin.min(y), ymax.max(y)))
-        })
-}
-
-fn advance(translation: &Translation, field: &Field, new_field: &mut Field, infinity: &mut bool) {
-    const INDEX_MASK: usize = (1 << 9) - 1;
-    new_field.clear();
-
-    let ((xmin, xmax), (ymin, ymax)) = find_dimensions(field);
-
-    for x in (xmin - 1)..=(xmax + 1) {
-        let mut index = if *infinity { INDEX_MASK } else { 0 };
-
-        for y in (ymin - 1)..=(ymax + 1) {
-            for dx in -1..=1 {
-                index <<= 1;
-
-                let nx = x + dx;
-                let ny = y + 1;
-
-                if nx < xmin || nx > xmax || ny < ymin || ny > ymax {
-                    index |= *infinity as usize;
-                } else if field.contains(&(nx, ny)) {
-                    index |= 1;
-                }
-            }
-
-            index &= INDEX_MASK;
-
-            if translation[index] {
-                new_field.insert((x, y));
-            }
-        }
-    }
-
-    *infinity = translation[if *infinity { 511 } else { 0 }]
-}
-
 fn parts_common(input: &mut dyn Read, count: usize) -> String {
     let (translation, mut field) = read_input(input);
-    let mut new_field = Field::new();
-    let mut infinity = false;
 
     for _ in 0..count {
-        advance(&translation, &field, &mut new_field, &mut infinity);
-        swap(&mut field, &mut new_field);
+        field.advance(&translation);
     }
 
     field.len().to_string()