diff --git a/2021/aoc/aoc.go b/2021/aoc/aoc.go
index 9ecfc27..69fa93a 100644
--- a/2021/aoc/aoc.go
+++ b/2021/aoc/aoc.go
@@ -39,6 +39,14 @@ func MustInt(s string) int {
 	return i
 }
 
+func MustHex(s string) int {
+	i, err := strconv.ParseInt(s, 16, 0)
+	if err != nil {
+		panic(err)
+	}
+	return int(i)
+}
+
 type Inputs struct {
 	fs fs.FS
 }
diff --git a/2021/day16/main.go b/2021/day16/main.go
new file mode 100644
index 0000000..687d515
--- /dev/null
+++ b/2021/day16/main.go
@@ -0,0 +1,154 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	text := aoc.Input().ReadFile("day16.txt")
+	fmt.Println(part1(parse(text)))
+}
+
+type bits []int
+
+func (b bits) String() string {
+	var sb strings.Builder
+	for _, x := range b {
+		fmt.Fprintf(&sb, "%b", x)
+	}
+	return sb.String()
+}
+
+func (b bits) Uint() uint {
+	var u uint
+	for _, i := range b {
+		u <<= 1
+		u += uint(i)
+	}
+	return u
+}
+
+func (b bits) Consume(n uint) (prefix uint, extra bits) {
+	return b[:n].Uint(), b[n:]
+}
+
+func parse(text string) (b bits) {
+	for _, char := range strings.Split(text, "") {
+		x := aoc.MustHex(char)
+		for i := 3; i >= 0; i-- {
+			b = append(b, (x>>i)&1)
+		}
+	}
+	return
+}
+
+func part1(b bits) uint {
+	p, _ := packet(b)
+	return sumVersions(p)
+}
+
+func sumVersions(p Packet) uint {
+	switch p := p.(type) {
+	case Literal: // no-op
+		return p.Version()
+	case Operator:
+		sum := p.Version()
+		for _, inner := range p.Packets {
+			sum += sumVersions(inner)
+		}
+		return sum
+	default:
+		panic(fmt.Sprintf("Unknown data type: %T", p))
+	}
+}
+
+type Packet interface {
+	Version() uint
+	Type() uint
+}
+
+type Literal struct {
+	V, T  uint
+	Value uint
+}
+
+func (l Literal) Version() uint { return l.V }
+func (l Literal) Type() uint    { return l.T }
+
+type Operator struct {
+	V, T    uint
+	Packets []Packet
+}
+
+func (o Operator) Version() uint { return o.V }
+func (o Operator) Type() uint    { return o.T }
+
+func packet(b bits) (Packet, bits) {
+	v, b := b.Consume(3)
+	t, b := b.Consume(3)
+	switch t {
+	case 4:
+		var val uint
+		val, b = literal(b)
+		return Literal{v, t, val}, b
+	default:
+		var ps []Packet
+		ps, b = operator(b)
+		return Operator{v, t, ps}, b
+	}
+}
+
+func literal(b bits) (uint, bits) {
+	var val uint
+	for {
+		var n uint
+		n, b = b.Consume(5)
+
+		val <<= 4
+		val += n & 0b01111
+
+		if n&0b10000 == 0 {
+			break
+		}
+	}
+	return val, b
+}
+
+func operator(b bits) ([]Packet, bits) {
+	lt, b := b.Consume(1)
+	var ps []Packet
+	if lt == 0 {
+		var l uint
+		l, b = b.Consume(15)
+		taken := uint(0)
+		for l-taken > 6 { // min length for a packet
+			var p Packet
+			oldlen := len(b)
+			p, b = packet(b)
+			ps = append(ps, p)
+			taken += uint(oldlen - len(b))
+		}
+		b = padding(b, l-taken)
+		return ps, b
+	} else {
+		var n uint
+		n, b = b.Consume(11)
+		for i := uint(0); i < n; i++ {
+			var p Packet
+			p, b = packet(b)
+			ps = append(ps, p)
+		}
+		return ps, b
+	}
+}
+
+func padding(b bits, n uint) bits {
+	u, b := b.Consume(n)
+	if u != 0 {
+		panic(fmt.Sprintf("Expected padding, got nonzero bits: %b", u))
+	}
+	return b
+}