diff --git a/build.gradle.kts b/build.gradle.kts
index f62a589..b97fbab 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -5,6 +5,9 @@ plugins {
 repositories {
     mavenCentral()
 }
+dependencies {
+    implementation("org.junit.jupiter:junit-jupiter:5.7.0")
+}
 
 tasks {
     sourceSets {
diff --git a/src/year2021/day16/Day16.kt b/src/year2021/day16/Day16.kt
new file mode 100644
index 0000000..cf70042
--- /dev/null
+++ b/src/year2021/day16/Day16.kt
@@ -0,0 +1,150 @@
+package year2021.day16
+
+import readInput
+
+enum class PacketType(val id: Byte) {
+    Sum(0),
+    Product(1),
+    Minimum(2),
+    LiteralValue(4),
+    Maximum(3),
+    GreaterThan(5),
+    LessThan(6),
+    EqualTo(7),
+}
+
+enum class LengthType(val id: Byte) {
+    TotalLength(0),
+    Count(1)
+}
+
+private fun Byte.toPacketType(): PacketType = PacketType.values().first { it.id == this }
+private fun Byte.toLengthType(): LengthType = LengthType.values().first { it.id == this }
+
+data class Packet(
+    val version: Byte,
+    val type: PacketType,
+    val bytes: List<Byte> = emptyList(),
+    val packets: List<Packet> = emptyList()
+) {
+    fun versionSum(): Int = when (type) {
+        PacketType.LiteralValue -> version.toInt()
+        else -> version.toInt() + packets.sumOf(Packet::versionSum)
+    }
+
+    fun eval(): Long = when (type) {
+        PacketType.Sum -> packets.sumOf(Packet::eval)
+        PacketType.Product -> packets.fold(1) { result, packet -> result * packet.eval() }
+        PacketType.Minimum -> packets.minOf(Packet::eval)
+        PacketType.LiteralValue -> bytes.fold(0) { result, byte -> result.shl(4).or(byte.toLong()) }
+        PacketType.Maximum -> packets.maxOf(Packet::eval)
+        PacketType.GreaterThan -> if (packets[0].eval() > packets[1].eval()) 1 else 0
+        PacketType.LessThan -> if (packets[0].eval() < packets[1].eval()) 1 else 0
+        PacketType.EqualTo -> if (packets[0].eval() == packets[1].eval()) 1 else 0
+    }
+}
+
+fun List<Int>.chunk(from: Int, to: Int): UInt {
+    check(to - from <= 32)
+
+    var result = 0u
+    var usedBits = 0
+    var i = from.div(4)
+
+    // prefix
+    if (from.mod(4) != 0) {
+        val offset = 4 - from.mod(4)
+        val offsetMask = 1.shl(offset) - 1
+
+        result = this[i].and(offsetMask).toUInt()
+
+        usedBits += offset
+        i++
+    }
+
+    while (4 * i < to) {
+        val fromFirst = this[i].toUInt()
+        result = result.shl(4).or(fromFirst)
+
+        i++
+        usedBits += 4
+    }
+
+    return result.shr(usedBits - (to - from))
+}
+
+fun parseGroups(bytes: List<Int>, groups: MutableList<Byte>, from: Int): Int {
+    val mask = (1.shl(4) - 1).toUInt()
+
+    var i = 0
+    var lastByte = false
+    while (!lastByte) {
+        val byte = bytes.chunk(from + 5 * i, from + 5 * (i + 1))
+        groups.add(byte.and(mask).toByte())
+
+        i++
+        lastByte = byte.shr(4) == 0u
+    }
+
+    return from + 5 * i
+}
+
+fun parsePackets(bytes: List<Int>, packets: MutableList<Packet>, from: Int): Int {
+    var index = from + 1
+    when (bytes.chunk(from, from + 1).toByte().toLengthType()) {
+        LengthType.TotalLength -> {
+            val packetsSize = bytes.chunk(index, index + 15).toInt()
+            index += 15
+
+            var read = 0
+            while (read < packetsSize) {
+                val (packet, newIndex) = parsePacket(bytes, index)
+
+                packets.add(packet)
+                read += newIndex - index
+                index = newIndex
+            }
+        }
+        LengthType.Count -> {
+            val packetCount = bytes.chunk(index, index + 11).toInt()
+            index += 11
+
+            repeat(packetCount) {
+                val (packet, newIndex) = parsePacket(bytes, index)
+
+                packets.add(packet)
+                index = newIndex
+            }
+        }
+    }
+
+    return index
+}
+
+fun parsePacket(bytes: List<Int>, from: Int): Pair<Packet, Int> {
+    val version = bytes.chunk(from, from + 3).toByte()
+    val typeId = bytes.chunk(from + 3, from + 6).toByte().toPacketType()
+
+    val byteGroups = mutableListOf<Byte>()
+    val packets = mutableListOf<Packet>()
+
+    val offset = when (typeId) {
+        PacketType.LiteralValue -> parseGroups(bytes, byteGroups, from + 6)
+        else -> parsePackets(bytes, packets, from + 6)
+    }
+
+    return Pair(Packet(version, typeId, byteGroups, packets), offset)
+}
+
+fun String.toPacket(): Packet =
+    parsePacket(this.map { it.digitToInt(16) }, 0).first
+
+fun part1(input: String): Int = input.toPacket().versionSum()
+fun part2(input: String): Long = input.toPacket().eval()
+
+fun main() {
+    val input = readInput(16, "input").first()
+
+    println(part1(input))
+    println(part2(input))
+}
diff --git a/src/year2021/day16/Day16KtTest.kt b/src/year2021/day16/Day16KtTest.kt
new file mode 100644
index 0000000..ac6fc64
--- /dev/null
+++ b/src/year2021/day16/Day16KtTest.kt
@@ -0,0 +1,100 @@
+package year2021.day16
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+internal class Day16KtTest {
+
+    @Test
+    fun chunkWithOneElement() {
+        val list = listOf(15)
+        assertEquals(1u, list.chunk(0, 1))
+        assertEquals(3u, list.chunk(0, 2))
+        assertEquals(7u, list.chunk(0, 3))
+        assertEquals(15u, list.chunk(0, 4))
+    }
+
+    @Test
+    fun chunk() {
+        val list = listOf(15, 12, 10, 3)
+        assertEquals(15u.shl(4).or(12u).shl(4).or(10u).shl(4).or(3u), list.chunk(0, 16))
+    }
+
+    @Test
+    fun chunkWholeInt() {
+        val list = listOf(15, 12, 10, 3, 6)
+        assertEquals(15u.shl(4).or(12u).shl(4).or(10u).shl(4).or(3u).shl(4).or(6u), list.chunk(0, 20))
+    }
+
+    @Test
+    fun parsing() {
+        assertEquals(
+            Packet(
+                6.toByte(),
+                PacketType.LiteralValue,
+                bytes = listOf(7, 14, 5).map(Int::toByte),
+            ),
+            "D2FE28".toPacket()
+        )
+    }
+
+    @Test
+    fun part1a() {
+        assertEquals(16, part1("8A004A801A8002F478"))
+    }
+
+    @Test
+    fun part1b() {
+        assertEquals(12, part1("620080001611562C8802118E34"))
+    }
+
+    @Test
+    fun part1c() {
+        assertEquals(23, part1("C0015000016115A2E0802F182340"))
+    }
+
+    @Test
+    fun part1d() {
+        assertEquals(31, part1("A0016C880162017C3686B18A3D4780"))
+    }
+
+    @Test
+    fun part2Sum() {
+        assertEquals(3, part2("C200B40A82"))
+    }
+
+    @Test
+    fun part2Product() {
+        assertEquals(54, part2("04005AC33890"))
+    }
+
+    @Test
+    fun part2Minimum() {
+        assertEquals(7, part2("880086C3E88112"))
+    }
+
+    @Test
+    fun part2Maximum() {
+        assertEquals(9, part2("CE00C43D881120"))
+    }
+
+    @Test
+    fun part2LessThan() {
+        assertEquals(1, part2("D8005AC2A8F0"))
+    }
+
+    @Test
+    fun part2GreaterThan() {
+        assertEquals(0, part2("F600BC2D8F"))
+    }
+
+    @Test
+    fun part2EqualTo() {
+        assertEquals(0, part2("9C005AC2F8F0"))
+    }
+
+    @Test
+    fun part2Complex() {
+        assertEquals(1, part2("9C0141080250320F1802104A08"))
+    }
+}
\ No newline at end of file