diff --git a/challenge/day16/a.go b/challenge/day16/a.go
new file mode 100644
index 0000000..9d7b593
--- /dev/null
+++ b/challenge/day16/a.go
@@ -0,0 +1,64 @@
+package day16
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+
+	"github.com/nlowe/aoc2019/util"
+
+	"github.com/nlowe/aoc2019/challenge"
+	"github.com/spf13/cobra"
+)
+
+var A = &cobra.Command{
+	Use:   "16a",
+	Short: "Day 16, Problem A",
+	Run: func(_ *cobra.Command, _ []string) {
+		fmt.Printf("Answer: %s\n", a(challenge.FromFile()))
+	},
+}
+
+const iterations = 100
+
+var pattern = [4]int{0, 1, 0, -1}
+
+func a(challenge *challenge.Input) string {
+	in := <-challenge.Lines()
+
+	data := make([]int, len(in))
+	for i, r := range in {
+		data[i] = util.MustAtoI(string(r))
+	}
+
+	for i := 0; i < iterations; i++ {
+		data = fft(data)
+	}
+
+	result := strings.Builder{}
+	for i := 0; i < 8; i++ {
+		result.WriteString(strconv.Itoa(data[i]))
+	}
+
+	return result.String()
+}
+
+func fft(in []int) []int {
+	result := make([]int, len(in))
+
+	for i := 0; i < len(in); i++ {
+		sum := 0
+		for j := i; j < len(in); j++ {
+			sum += in[j] * mixin(j, i)
+		}
+
+		result[i] = int(math.Abs(float64(sum))) % 10
+	}
+
+	return result
+}
+
+func mixin(offset, iteration int) int {
+	return pattern[((1+offset)/(1+iteration))%len(pattern)]
+}
diff --git a/challenge/day16/a_test.go b/challenge/day16/a_test.go
new file mode 100644
index 0000000..085d3c0
--- /dev/null
+++ b/challenge/day16/a_test.go
@@ -0,0 +1,25 @@
+package day16
+
+import (
+	"testing"
+
+	"github.com/nlowe/aoc2019/challenge"
+	"github.com/stretchr/testify/require"
+)
+
+func TestA(t *testing.T) {
+	tests := []struct {
+		in  string
+		out string
+	}{
+		{in: "80871224585914546619083218645595", out: "24176176"},
+		{in: "19617804207202209144916044189917", out: "73745418"},
+		{in: "69317163492948606335995924319873", out: "52432133"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			require.Equal(t, tt.out, a(challenge.FromLiteral(tt.in)))
+		})
+	}
+}
diff --git a/challenge/day16/b.go b/challenge/day16/b.go
new file mode 100644
index 0000000..22c1d03
--- /dev/null
+++ b/challenge/day16/b.go
@@ -0,0 +1,59 @@
+package day16
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/nlowe/aoc2019/challenge"
+	"github.com/nlowe/aoc2019/util"
+	"github.com/spf13/cobra"
+)
+
+var B = &cobra.Command{
+	Use:   "16b",
+	Short: "Day 16, Problem B",
+	Run: func(_ *cobra.Command, _ []string) {
+		fmt.Printf("Answer: %s\n", b(challenge.FromFile()))
+	},
+}
+
+func b(challenge *challenge.Input) string {
+	in := strings.Repeat(<-challenge.Lines(), 10000)
+
+	data := make([]int, len(in))
+	for i, r := range in {
+		data[i] = util.MustAtoI(string(r))
+	}
+
+	offset := util.MustAtoI(in[:7])
+	data = data[offset:]
+	for i := 0; i < iterations; i++ {
+		data = shortcut(data)
+	}
+
+	result := strings.Builder{}
+	for i := 0; i < 8; i++ {
+		result.WriteString(strconv.Itoa(data[i]))
+	}
+
+	return result.String()
+}
+
+// TODO: Here be vodo. I need to read up on why this works.
+//       this was mostly pieced together from posts on the
+//       subreddit.
+func shortcut(in []int) []int {
+	cumulativeSum := 0
+	for _, v := range in {
+		cumulativeSum += v
+	}
+
+	var result []int
+	for i := range in {
+		result = append(result, ((cumulativeSum%10)+10)%10)
+		cumulativeSum -= in[i]
+	}
+
+	return result
+}
diff --git a/challenge/day16/b_test.go b/challenge/day16/b_test.go
new file mode 100644
index 0000000..ba51fe6
--- /dev/null
+++ b/challenge/day16/b_test.go
@@ -0,0 +1,25 @@
+package day16
+
+import (
+	"testing"
+
+	"github.com/nlowe/aoc2019/challenge"
+	"github.com/stretchr/testify/require"
+)
+
+func TestB(t *testing.T) {
+	tests := []struct {
+		in  string
+		out string
+	}{
+		{in: "03036732577212944063491565474664", out: "84462026"},
+		{in: "02935109699940807407585447034323", out: "78725270"},
+		{in: "03081770884921959731165446850517", out: "53553731"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			require.Equal(t, tt.out, b(challenge.FromLiteral(tt.in)))
+		})
+	}
+}
diff --git a/challenge/day16/input.txt b/challenge/day16/input.txt
new file mode 100644
index 0000000..d723c14
--- /dev/null
+++ b/challenge/day16/input.txt
@@ -0,0 +1 @@
+59776034095811644545367793179989602140948714406234694972894485066523525742503986771912019032922788494900655855458086979764617375580802558963587025784918882219610831940992399201782385674223284411499237619800193879768668210162176394607502218602633153772062973149533650562554942574593878073238232563649673858167635378695190356159796342204759393156294658366279922734213385144895116649768185966866202413314939692174223210484933678866478944104978890019728562001417746656699281992028356004888860103805472866615243544781377748654471750560830099048747570925902575765054898899512303917159138097375338444610809891667094051108359134017128028174230720398965960712
diff --git a/main.go b/main.go
index 05a6f6f..ccbf17c 100644
--- a/main.go
+++ b/main.go
@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"time"
 
+	"github.com/nlowe/aoc2019/challenge/day16"
+
 	"github.com/nlowe/aoc2019/challenge/day15"
 
 	"github.com/nlowe/aoc2019/challenge/day14"
@@ -74,6 +76,7 @@ func init() {
 		day13.A, day13.B,
 		day14.A, day14.B,
 		day15.A, day15.B,
+		day16.A, day16.B,
 	)
 
 	flags := rootCmd.PersistentFlags()