diff --git a/2020/day21.go b/2020/day21.go
new file mode 100644
index 0000000..24a2922
--- /dev/null
+++ b/2020/day21.go
@@ -0,0 +1,134 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"regexp"
+	"sort"
+	"strings"
+
+	"git.annabunch.es/annabunches/adventofcode/2020/lib/util"
+)
+
+// could optimize this better with maps, but we'll just do this. Our data set should be small enough
+func intersection(a, b []string) (result []string) {
+	result = make([]string, 0)
+	for _, av := range a {
+		for _, bv := range b {
+			if av == bv {
+				result = append(result, av)
+				break
+			}
+		}
+	}
+	return
+}
+
+func includes(list []string, item string) bool {
+	for _, cmp := range list {
+		if cmp == item {
+			return true
+		}
+	}
+	return false
+}
+
+func remove(list []string, item string) []string {
+	for i, cmp := range list {
+		if cmp == item {
+			return append(list[:i], list[i+1:]...)
+		}
+	}
+	return list
+}
+
+func parseInput(input []string) (ingredients map[string]int, allergens map[string][]string) {
+	re := regexp.MustCompile("^(.*) \\(contains (.*)\\)")
+	allergens = make(map[string][]string)
+	ingredients = make(map[string]int)
+	for _, line := range input {
+		data := re.FindStringSubmatch(line)
+		is := strings.Split(data[1], " ")
+		as := strings.Split(data[2], ", ")
+
+		for _, ingredient := range is {
+			ingredients[ingredient]++
+		}
+
+		for _, allergen := range as {
+			if ingredients, ok := allergens[allergen]; ok {
+				allergens[allergen] = intersection(is, ingredients)
+			} else {
+				allergens[allergen] = is
+			}
+		}
+	}
+
+	return
+}
+
+func findUniqueSolution(aMap map[string][]string) {
+	done := false
+	for !done {
+		done = true
+		for a1, is1 := range aMap {
+			if len(is1) == 1 {
+				ingredient := is1[0]
+				for a2, is2 := range aMap {
+					if a1 == a2 {
+						continue
+					}
+					aMap[a2] = remove(is2, ingredient)
+				}
+			}
+		}
+
+		for _, is := range aMap {
+			if len(is) > 1 {
+				done = false
+				break
+			}
+		}
+	}
+}
+
+func main() {
+	step := os.Args[1]
+	values := util.InputParserStrings(os.Args[2])
+	ingredients, allergenMap := parseInput(values)
+
+	switch step {
+	case "1":
+		loose := make([]string, 0)
+		for ingredient, _ := range ingredients {
+			found := false
+			for _, is := range allergenMap {
+				if includes(is, ingredient) {
+					found = true
+					break
+				}
+			}
+			if !found {
+				loose = append(loose, ingredient)
+			}
+		}
+
+		total := 0
+		for _, i := range loose {
+			total += ingredients[i]
+		}
+		fmt.Println(total)
+	case "2":
+		findUniqueSolution(allergenMap)
+		keys := make([]string, 0)
+		for k, _ := range allergenMap {
+			keys = append(keys, k)
+		}
+		result := ""
+		sort.Strings(keys)
+		for _, k := range keys {
+			result = result + "," + allergenMap[k][0]
+		}
+		fmt.Println(strings.Trim(result, ","))
+	}
+}