diff --git a/src/year2021/day20/Day20.kt b/src/year2021/day20/Day20.kt
new file mode 100644
index 0000000..0b1d706
--- /dev/null
+++ b/src/year2021/day20/Day20.kt
@@ -0,0 +1,77 @@
+package year2021.day20
+
+import readInput
+
+data class Point(val x: Int, val y: Int)
+
+fun getSquare(pixel: Point): Iterable<Point> =
+    (pixel.y - 1..pixel.y + 1).flatMap { y -> (pixel.x - 1..pixel.x + 1).map { x -> Point(x, y) } }
+
+data class TrenchMap(
+    val algorithm: List<Char>,
+    val map: Map<Point, Char>,
+    val default: Char = '.'
+) {
+    fun get(key: Point): Char = map.getOrDefault(key, default)
+    fun get(x: Int, y: Int): Char = get(Point(x, y))
+
+    private val xs: IntRange
+        get() = (map.keys.minOf { it.x } - 2..map.keys.maxOf { it.x } + 2)
+
+    private val ys: IntRange
+        get() = (map.keys.minOf { it.y } - 2..map.keys.maxOf { it.y } + 2)
+
+    private val indices: Iterable<Point>
+        get() = ys.flatMap { y -> xs.map { x -> Point(x, y) } }
+
+    fun print() {
+        for (y in ys) {
+            for (x in xs) {
+                print(get(x, y))
+            }
+            println()
+        }
+    }
+
+    private fun toIndex(square: Iterable<Point>): Int =
+        square.map { if (get(it) == '#') 1 else 0 }.fold(0) { acc, bit -> acc * 2 + bit }
+
+    private fun enhancePixel(pixel: Point): Char = algorithm[toIndex(getSquare(pixel))]
+
+    fun enhance(): TrenchMap =
+        TrenchMap(
+            algorithm,
+            indices.associateWith { enhancePixel(it) },
+            when (default) {
+                '.' -> algorithm.first()
+                '#' -> algorithm.last()
+                else -> error("Invalid char found")
+            }
+        )
+}
+
+fun List<String>.toTrenchMap(): TrenchMap =
+    TrenchMap(
+        this.first().toList(),
+        this.drop(2)
+            .flatMapIndexed { y, row ->
+                row.mapIndexed { x, cell -> Point(x, y) to cell }
+            }.toMap()
+    )
+
+fun part1(input: TrenchMap): Int =
+    (1..2).fold(input) { acc, i -> acc.enhance() }.map.values.count { it == '#' }
+
+fun part2(input: TrenchMap): Int =
+    (1..50).fold(input) { acc, i -> acc.enhance() }.map.values.count { it == '#' }
+
+fun main() {
+    val sample = readInput(20, "sample").toTrenchMap()
+    val input = readInput(20, "input").toTrenchMap()
+
+    check(part1(sample) == 35)
+    println(part1(input))
+
+    check(part2(sample) == 3351)
+    println(part2(input))
+}
diff --git a/src/year2021/day20/sample.txt b/src/year2021/day20/sample.txt
new file mode 100644
index 0000000..000a554
--- /dev/null
+++ b/src/year2021/day20/sample.txt
@@ -0,0 +1,7 @@
+..#.#..#####.#.#.#.###.##.....###.##.#..###.####..#####..#....#..#..##..###..######.###...####..#..#####..##..#.#####...##.#.#..#.##..#.#......#.###.######.###.####...#.##.##..#..#..#####.....#.#....###..#.##......#.....#..#..#..##..#...##.######.####.####.#.#...#.......#..#.#.#...####.##.#......#..#...##.#.##..#...##.#.##..###.#......#.#.......#.#.#.####.###.##...#.....####.#..#..#.##.#....##..#.####....##...##..#...#......#.#.......#.......##..####..#...#.#.#...##..#.#..###..#####........#..####......#..#
+
+#..#.
+#....
+##..#
+..#..
+..###
\ No newline at end of file