diff --git a/21/21.go b/21/21.go
new file mode 100644
index 0000000..8edcfcd
--- /dev/null
+++ b/21/21.go
@@ -0,0 +1,196 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+type rule struct {
+	input  []string
+	output []string
+}
+
+// cat input | go run 21.go
+
+func main() {
+
+	iterations := 18
+	var rules []rule
+	grid := []string{".#.", "..#", "###"}
+
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		t := strings.Split(scanner.Text(), " => ")
+		i := strings.Split(t[0], "/")
+		o := strings.Split(t[1], "/")
+
+		r := rule{}
+		r.output = o
+		r.input = i
+		rules = append(rules, r)
+		r.input = FlipH(i)
+		rules = append(rules, r)
+		r.input = FlipV(i)
+		rules = append(rules, r)
+
+		for j := 1; j <= 3; j++ {
+			r := rule{}
+			i = Rotate90(i)
+			r.output = o
+			r.input = i
+			rules = append(rules, r)
+			r.input = FlipH(i)
+			rules = append(rules, r)
+			r.input = FlipV(i)
+			rules = append(rules, r)
+		}
+
+	}
+
+	size := 0
+	for i := 0; i < iterations; i++ {
+		fmt.Println("============ iter:", i)
+		fmt.Println("grid:", grid)
+		grid, size = Embiggen(grid)
+		fmt.Println("embiggened:", grid, size)
+		grid = Cromulate(grid, size, rules)
+		fmt.Println("cromulated:", grid, size)
+	}
+
+	countstr := strings.Join(grid, "")
+	count := 0
+	for i := range countstr {
+		if countstr[i] == '#' {
+			count++
+		}
+	}
+	fmt.Println(count)
+
+}
+
+func Cromulate(grid []string, osize int, rules []rule) []string {
+	size := osize + 1
+	//steps := (len(grid) / size) - 1
+
+	//fmt.Println("cromulating:", osize, len(grid), steps)
+
+	for yo := 0; yo < len(grid); yo += size {
+		for xo := 0; xo < len(grid); xo += size {
+			//fmt.Println("crom:", xo, yo)
+
+			var match []string
+			for i := 0; i < osize; i++ {
+				m := grid[yo+i][xo : xo+osize]
+				match = append(match, m)
+			}
+			result := FindMatch(match, rules)
+			//fmt.Println("match:", match, result)
+
+			for y := 0; y < len(result); y++ {
+				yarr := []byte(grid[yo+y])
+
+				for x := 0; x < len(result); x++ {
+					yarr[xo+x] = byte(result[y][x])
+				}
+				//fmt.Println("swap:", grid[yo+y], string(yarr))
+				grid[yo+y] = string(yarr)
+			}
+
+		}
+	}
+
+	return grid
+}
+
+func FindMatch(match []string, rules []rule) []string {
+	m := strings.Join(match, "")
+	for r := range rules {
+		c := strings.Join(rules[r].input, "")
+		if m == c {
+			return rules[r].output
+		}
+	}
+	return match
+}
+
+func Embiggen(grid []string) ([]string, int) {
+	var out []string
+
+	size := 3
+	if len(grid)%2 == 0 {
+		size = 2
+	}
+
+	for y := 0; y < len(grid); y++ {
+		s := ""
+		for x := 0; x < len(grid[y]); x++ {
+			b := grid[y][x]
+			s += string(b)
+			if x%size == size-1 {
+				s += "X"
+			}
+		}
+		out = append(out, s)
+
+		if y%size == size-1 {
+			tmpstr := "X"
+			for ts := 0; ts < len(out[0])-1; ts++ {
+				tmpstr += "X"
+			}
+			out = append(out, tmpstr)
+		}
+	}
+
+	return out, size
+}
+
+func Rotate90(in []string) []string {
+	var out []string
+	for y := 0; y < len(in); y++ {
+		s := ""
+
+		for x := len(in[y]) - 1; x >= 0; x-- {
+			b := in[x][y]
+			s += string(b)
+
+		}
+
+		out = append(out, s)
+	}
+	return out
+}
+
+func FlipH(in []string) []string {
+	var out []string
+	for y := 0; y < len(in); y++ {
+		s := ""
+
+		for x := len(in[y]) - 1; x >= 0; x-- {
+			b := in[y][x]
+			s += string(b)
+
+		}
+
+		out = append(out, s)
+	}
+	return out
+}
+
+func FlipV(in []string) []string {
+	var out []string
+
+	for y := len(in) - 1; y >= 0; y-- {
+		s := ""
+
+		for x := 0; x < len(in); x++ {
+			b := in[y][x]
+			s += string(b)
+
+		}
+
+		out = append(out, s)
+	}
+	return out
+}