diff --git a/21/main.go b/21/main.go
new file mode 100644
index 0000000..9913102
--- /dev/null
+++ b/21/main.go
@@ -0,0 +1,138 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	partOne()
+	duration := time.Since(start)
+	partTwo()
+	duration2 := time.Since(start)
+	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
+}
+
+type food struct {
+	allergens   []string
+	ingredients []string
+}
+
+func intersection(a, b []string) []string {
+	// special-case starting out
+	if len(a) == 0 {
+		return b
+	} else if len(b) == 0 {
+		return a
+	}
+	result := []string{}
+	for _, s := range b {
+		for _, t := range a {
+			if s == t {
+				result = append(result, s)
+			}
+		}
+	}
+	return result
+}
+
+func countSafeIngredients(f food, allergens map[string][]string) int {
+	for c, i := range f.ingredients {
+		for _, l := range allergens {
+			for _, a := range l {
+				if i == a {
+					f.ingredients[c] = ""
+				}
+			}
+		}
+	}
+	result := 0
+	for _, i := range f.ingredients {
+		if i != "" {
+			result = result + 1
+		}
+	}
+	return result
+}
+
+func remove(ingredients []string, ingredient string) []string {
+	for i, j := range ingredients {
+		if j == ingredient {
+			ingredients[i] = ingredients[len(ingredients)-1]
+			ingredients[len(ingredients)-1] = ""
+			return ingredients[:len(ingredients)-1]
+		}
+	}
+	return ingredients
+}
+
+func partOne() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	foods := []food{}
+	allergens := map[string][]string{}
+
+	for scanner.Scan() {
+		line := strings.Split(scanner.Text(), " (contains ")
+		f := food{
+			ingredients: strings.Split(line[0], " "),
+			allergens:   strings.Split(strings.TrimSuffix(line[1], ")"), ", "),
+		}
+		foods = append(foods, f)
+		for _, a := range f.allergens {
+			allergens[a] = intersection(f.ingredients, allergens[a])
+		}
+	}
+
+	sum := 0
+	for _, f := range foods {
+		sum = sum + countSafeIngredients(f, allergens)
+	}
+	fmt.Println(sum)
+}
+
+func partTwo() {
+	f, _ := os.Open("input")
+	reader := bufio.NewReader(f)
+	scanner := bufio.NewScanner(reader)
+
+	foods := []food{}
+	allergens := map[string][]string{}
+
+	for scanner.Scan() {
+		line := strings.Split(scanner.Text(), " (contains ")
+		f := food{
+			ingredients: strings.Split(line[0], " "),
+			allergens:   strings.Split(strings.TrimSuffix(line[1], ")"), ", "),
+		}
+		foods = append(foods, f)
+		for _, a := range f.allergens {
+			allergens[a] = intersection(f.ingredients, allergens[a])
+		}
+	}
+
+	// hey, this is the same reduction logic as day 16!
+	trueAllergens := map[string]string{}
+	var lastAllergenSolved string
+	for i := 0; i < len(allergens); i = i + 1 {
+		for i, a := range allergens {
+			if len(a) == 1 {
+				trueAllergens[i] = a[0]
+				lastAllergenSolved = a[0]
+			}
+		}
+		for i := range allergens {
+			allergens[i] = remove(allergens[i], lastAllergenSolved)
+		}
+	}
+
+	// formatting the result correctly is left as an exercise for the reader
+	// because I don't want to deal with go's map order randomization
+	fmt.Println(trueAllergens)
+}