diff --git a/input/day16.txt b/input/day16.txt
new file mode 100644
index 0000000..3b5092c
--- /dev/null
+++ b/input/day16.txt
@@ -0,0 +1 @@
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
diff --git a/src/day16.rs b/src/day16.rs
new file mode 100644
index 0000000..567ca9a
--- /dev/null
+++ b/src/day16.rs
@@ -0,0 +1,240 @@
+use std::str::Chars;
+
+use itertools::Itertools;
+
+pub fn day16() {
+    assert_eq!(
+        parse_hex_str("D2FE28"),
+        Packet {
+            version: 6,
+            type_id: 4,
+            length: 21,
+            payload: Payload::Literal(2021),
+        },
+    );
+    assert_eq!(
+        parse_hex_str("38006F45291200"),
+        Packet {
+            version: 1,
+            type_id: 6,
+            length: 49,
+            payload: Payload::Operator(vec![
+                Packet {
+                    version: 6,
+                    type_id: 4,
+                    length: 11,
+                    payload: Payload::Literal(10),
+                },
+                Packet {
+                    version: 2,
+                    type_id: 4,
+                    length: 16,
+                    payload: Payload::Literal(20),
+                },
+            ]),
+        },
+    );
+    dbg!(parse_hex_str("EE00D40C823060"));
+
+    let sums = &[
+        "8A004A801A8002F478",
+        "620080001611562C8802118E34",
+        "C0015000016115A2E0802F182340",
+        "A0016C880162017C3686B18A3D4780",
+    ]
+    .map(|s| parse_hex_str(s).sum_versions());
+    assert_eq!(sums, &[16, 12, 23, 31]);
+
+    let input = include_str!("../input/day16.txt");
+    let packet = parse_hex_str(input);
+    println!("part 1: {}", packet.sum_versions());
+
+    let values = &[
+        "C200B40A82",
+        "04005AC33890",
+        "880086C3E88112",
+        "CE00C43D881120",
+        "D8005AC2A8F0",
+        "F600BC2D8F",
+        "9C005AC2F8F0",
+        "9C0141080250320F1802104A08",
+    ]
+    .map(|s| parse_hex_str(s).eval());
+    assert_eq!(values, &[3, 54, 7, 9, 1, 0, 0, 1]);
+
+    println!("part 2: {}", packet.eval());
+}
+
+fn parse_hex_str(hex_str: &str) -> Packet {
+    let mut bits = HexToBits::new(hex_str.chars());
+    parse_packet(&mut bits, 0)
+}
+
+fn parse_packet<'a>(bits: &mut HexToBits<'a>, depth: u32) -> Packet {
+    let mut packet_len = 6;
+    let version = bits.read(3);
+    let type_id = bits.read(3);
+    let payload: Payload;
+    // println!("type id {} at depth {}", type_id, depth);
+    if type_id == 4 {
+        let mut val = 0;
+        loop {
+            let is_last = !bits.next().unwrap();
+            val <<= 4;
+            val |= bits.read(4);
+            packet_len += 5;
+            if is_last {
+                break;
+            }
+        }
+        payload = Payload::Literal(val);
+    } else {
+        packet_len += 1;
+        let length_type = bits.next().unwrap();
+        let length = if length_type {
+            packet_len += 11;
+            bits.read(11)
+        } else {
+            packet_len += 15;
+            bits.read(15)
+        };
+        // println!("length type: {}, length: {}", length_type, length);
+        let mut sub_packets = vec![];
+        let mut sub_packets_len = 0;
+        loop {
+            if length_type && sub_packets.len() >= length {
+                break;
+            } else if !length_type && sub_packets_len >= length {
+                break;
+            }
+            let packet = parse_packet(bits, depth + 1);
+            sub_packets_len += packet.length;
+            // println!(
+            //     "read packet of length {} at depth {}",
+            //     packet.length,
+            //     depth + 1
+            // );
+            sub_packets.push(packet);
+        }
+        payload = Payload::Operator(sub_packets);
+        packet_len += sub_packets_len;
+    }
+    Packet {
+        version,
+        type_id,
+        length: packet_len,
+        payload,
+    }
+}
+
+struct HexToBits<'a> {
+    hex_chars: Chars<'a>,
+    current: u32,
+    current_bits_remaining: u32,
+    read_so_far: usize,
+}
+
+impl<'a> HexToBits<'a> {
+    fn new(chars: Chars<'a>) -> Self {
+        Self {
+            hex_chars: chars,
+            current: 0,
+            current_bits_remaining: 0,
+            read_so_far: 0,
+        }
+    }
+
+    fn read(&mut self, len: u32) -> usize {
+        let mut res = 0;
+        for _ in 0..len {
+            res <<= 1;
+            res |= if self.next().unwrap() { 1 } else { 0 };
+        }
+        res
+    }
+}
+
+impl<'a> Iterator for HexToBits<'a> {
+    type Item = bool;
+
+    fn next(&mut self) -> Option<bool> {
+        if self.current_bits_remaining > 0 {
+            let res = Some((self.current >> self.current_bits_remaining - 1) & 1 == 1);
+            self.current_bits_remaining -= 1;
+            self.read_so_far += 1;
+            res
+        } else if let Some(c) = self.hex_chars.next() {
+            self.current = c.to_digit(16).unwrap();
+            self.current_bits_remaining = 3;
+            self.read_so_far += 1;
+            Some((self.current >> 3) & 1 == 1)
+        } else {
+            None
+        }
+    }
+}
+
+#[derive(Debug, PartialEq)]
+struct Packet {
+    version: usize,
+    type_id: usize,
+    length: usize,
+    payload: Payload,
+}
+
+impl Packet {
+    fn sum_versions(&self) -> usize {
+        match self.payload {
+            Payload::Literal(_) => self.version,
+            Payload::Operator(ref sub) => {
+                self.version + sub.iter().map(|p| p.sum_versions()).sum::<usize>()
+            }
+        }
+    }
+
+    fn eval(&self) -> usize {
+        match self.payload {
+            Payload::Literal(n) => n,
+            Payload::Operator(ref sub) => {
+                let sub_vals = sub.iter().map(|p| p.eval());
+                match self.type_id {
+                    0 => sub_vals.sum(),
+                    1 => sub_vals.product(),
+                    2 => sub_vals.min().unwrap(),
+                    3 => sub_vals.max().unwrap(),
+                    5 => {
+                        let (a, b) = sub_vals.collect_tuple().unwrap();
+                        if a > b {
+                            1
+                        } else {
+                            0
+                        }
+                    }
+                    6 => {
+                        let (a, b) = sub_vals.collect_tuple().unwrap();
+                        if a < b {
+                            1
+                        } else {
+                            0
+                        }
+                    }
+                    7 => {
+                        let (a, b) = sub_vals.collect_tuple().unwrap();
+                        if a == b {
+                            1
+                        } else {
+                            0
+                        }
+                    }
+                    _ => panic!(),
+                }
+            }
+        }
+    }
+}
+
+#[derive(Debug, PartialEq)]
+enum Payload {
+    Literal(usize),
+    Operator(Vec<Packet>),
+}
diff --git a/src/main.rs b/src/main.rs
index 172002c..8a003d7 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -14,7 +14,8 @@ mod day12;
 mod day13;
 mod day14;
 mod day15;
+mod day16;
 
 fn main() {
-    day15::day15();
+    day16::day16();
 }