diff --git a/2021/day20/main.go b/2021/day20/main.go
new file mode 100644
index 0000000..d76f6b8
--- /dev/null
+++ b/2021/day20/main.go
@@ -0,0 +1,154 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/fatih/color"
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	text := aoc.Input().ReadFile("day20.txt")
+	alg, img := parseInput(text)
+	fmt.Println(run(alg, img, 2))
+	fmt.Println(run(alg, img, 50))
+}
+
+func parseInput(text string) (Alg, Image) {
+	alg, img := aoc.Cut(text, "\n\n")
+	lines := strings.Split(img, "\n")
+	return NewAlg(alg), NewImage(lines)
+}
+
+func run(alg Alg, img Image, n int) int {
+	for i := 0; i < n; i++ {
+		img = tick(alg, img)
+	}
+	return len(img.light)
+}
+
+func tick(alg Alg, img Image) Image {
+	light := make(map[RC]struct{})
+	for r := img.rmin - 1; r <= img.rmax+1; r++ {
+		for c := img.cmin - 1; c <= img.cmax+1; c++ {
+			rc := RC{r, c}
+			x := img.Next(rc)
+			if alg.Apply(x) {
+				light[rc] = struct{}{}
+			}
+		}
+	}
+
+	return Image{
+		light,
+		!img.border,
+		img.rmin - 1, img.rmax + 1,
+		img.cmin - 1, img.cmax + 1,
+	}
+}
+
+type RC struct{ r, c int }
+
+type Image struct {
+	light      map[RC]struct{}
+	border     bool
+	rmin, rmax int
+	cmin, cmax int
+}
+
+func NewImage(lines []string) (img Image) {
+	img.rmin, img.rmax = 0, len(lines)
+	img.cmin, img.cmax = 0, len(lines[0])
+	img.light = make(map[RC]struct{})
+	for r, line := range lines {
+		for c, char := range line {
+			if lightChar(char) {
+				img.light[RC{r, c}] = struct{}{}
+			}
+		}
+	}
+	return
+}
+
+func (i Image) String() string {
+	var sb strings.Builder
+	pad := 4
+	dim, bright := color.New(color.FgWhite), color.New(color.FgHiWhite)
+	for r := i.rmin - pad; r <= i.rmax+pad; r++ {
+		for c := i.cmin - pad; c <= i.cmax+pad; c++ {
+			rc := RC{r, c}
+			f := dim
+			if i.Inbounds(rc) {
+				f = bright
+			}
+			if i.IsLight(rc) {
+				f.Fprint(&sb, "#")
+			} else {
+				f.Fprint(&sb, ".")
+			}
+		}
+		sb.WriteString("\n")
+	}
+	return sb.String()
+}
+
+func (i Image) IsLight(rc RC) bool {
+	if i.Inbounds(rc) {
+		_, ok := i.light[rc]
+		return ok
+	}
+	return i.border
+}
+
+func (i Image) Inbounds(rc RC) bool {
+	r, c := rc.r, rc.c
+	return i.rmin <= r && r <= i.rmax && i.cmin <= c && c <= i.cmax
+}
+
+func (i Image) Next(rc RC) uint {
+	var x uint
+	for j, n := range rc.Neighbors() {
+		if i.IsLight(n) {
+			x |= 1 << (8 - j)
+		}
+	}
+	return x
+}
+
+func (rc RC) Neighbors() []RC {
+	r, c := rc.r, rc.c
+	return []RC{
+		{r - 1, c - 1}, {r - 1, c}, {r - 1, c + 1},
+		{r, c - 1}, {r, c}, {r, c + 1},
+		{r + 1, c - 1}, {r + 1, c}, {r + 1, c + 1},
+	}
+}
+
+type Alg map[uint]struct{}
+
+func NewAlg(line string) Alg {
+	alg := make(Alg)
+	for i := uint(0); i < uint(len(line)); i++ {
+		if lightChar(rune(line[i])) {
+			alg[i] = struct{}{}
+		}
+	}
+	return alg
+}
+
+func (a Alg) Apply(n uint) (light bool) {
+	_, ok := a[n]
+	return ok
+}
+
+func lightChar(r rune) bool {
+	switch r {
+	case '#':
+		return true
+	case '.':
+		return false
+	default:
+		panic(r)
+	}
+}