diff --git a/2021/src/day16.rs b/2021/src/day16.rs
new file mode 100644
index 0000000..4690b2b
--- /dev/null
+++ b/2021/src/day16.rs
@@ -0,0 +1,127 @@
+use color_eyre::eyre;
+
+use crate::lib::{BitVec, BitSlice};
+
+
+#[derive(Debug)]
+struct Packet {
+    version: u8,
+    type_id: u8,
+    content: PacketContent,
+}
+
+#[derive(Debug)]
+enum PacketContent {
+    Literal(u64),
+    Operator(Vec<Packet>),
+}
+
+struct Parser {
+    data: BitVec,
+    idx: usize,
+}
+
+impl Parser {
+    fn from_hex(hex: &str) -> eyre::Result<Self> {
+        let mut data = BitVec::with_capacity(hex.len() * 4);
+        for i in 0 .. (hex.len() / 2) { // we are assuming valid hex
+            let chunk = &hex[i * 2 .. (i + 1) * 2];
+            let byte = u8::from_str_radix(chunk, 16)?;
+            data.push_byte(byte);
+        }
+
+        Ok(Parser {data, idx: 0})
+    }
+
+    fn next(&mut self, n: usize) -> BitSlice {
+        let range = self.idx .. self.idx + n;
+        self.idx += n;
+        self.data.slice(range)
+    }
+
+    fn parse_value(&mut self) -> u64 {
+        let mut value = 0;
+        loop {
+            let group = self.next(5);
+            value = (value << 4) | group.slice(1..5).as_u64();
+            if !group[0] {break;}
+        }
+        value
+    }
+
+    fn parse_children(&mut self) -> Vec<Packet> {
+        let mut children = Vec::new();
+        if self.next(1).as_bool() {
+            // count refers to number of sub-packets
+            let count = self.next(11).as_u16();
+            children = Vec::with_capacity(count as usize);
+            for _i in 0..count {
+                children.push(self.parse());
+            }
+        }
+        else {
+            // len refers to content length in bits
+            let len = self.next(15).as_u16();
+            let end = self.idx + (len as usize);
+            while self.idx < end {
+                children.push(self.parse());
+            }
+        }
+        children
+    }
+
+    fn parse(&mut self) -> Packet {
+        let version = self.next(3).as_u8();
+        let type_id = self.next(3).as_u8();
+        let content = match type_id {
+            4 => PacketContent::Literal(self.parse_value()),
+            _ => PacketContent::Operator(self.parse_children()),
+        };
+
+        Packet {version, type_id, content}
+    }
+}
+
+
+fn sum_versions(root: &Packet) -> usize {
+    let mut total = root.version as usize;
+    if let PacketContent::Operator(children) = &root.content {
+        for child in children {
+            total += sum_versions(child);
+        }
+    }
+    total
+}
+
+
+fn evaluate(packet: &Packet) -> usize {
+    let children = match &packet.content {
+        PacketContent::Literal(value) => {return *value as usize;}, // early exit here
+        PacketContent::Operator(children) => children,
+    };
+
+    let mut values = children.iter().map(|p| evaluate(p));
+    match packet.type_id {
+        0 => values.sum(),
+        1 => values.product(),
+        2 => values.min().unwrap(),
+        3 => values.max().unwrap(),
+        // it's guaranteed that comparisons will have exactly two children
+        5 => if values.next().unwrap() > values.next().unwrap() {1} else {0},
+        6 => if values.next().unwrap() < values.next().unwrap() {1} else {0},
+        7 => if values.next().unwrap() == values.next().unwrap() {1} else {0},
+        // the version number was a 3-bit value, so:
+        _ => unreachable!(),
+    }
+}
+
+
+pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
+    let mut parser = Parser::from_hex(data)?;
+    let root = parser.parse();
+
+    let one = sum_versions(&root);
+    let two = evaluate(&root);
+
+    Ok((one, two))
+}
diff --git a/2021/src/lib.rs b/2021/src/lib.rs
index 77c970d..6940ade 100644
--- a/2021/src/lib.rs
+++ b/2021/src/lib.rs
@@ -4,7 +4,7 @@ use std::collections::HashMap;
 use std::fmt::{Display, Formatter};
 use std::fs::File;
 use std::hash::Hash;
-use std::ops::{Index, IndexMut};
+use std::ops::{Index, IndexMut, Range};
 use std::str::FromStr;
 
 use color_eyre::{eyre};
@@ -341,3 +341,128 @@ impl<K> Counter<K>
         self.data.iter().map(|(k, v)| (k, *v))
     }
 }
+
+
+pub struct BitVec {
+    data: Vec<u8>,
+    len: usize,
+}
+
+#[allow(dead_code)]
+impl BitVec {
+    fn new() -> Self {
+        BitVec {data: vec![], len: 0}
+    }
+
+    pub fn with_capacity(cap: usize) -> Self {
+        let true_cap = (cap + 8 - 1) / 8; // ceiling division
+        BitVec {
+            data: Vec::with_capacity(true_cap),
+            len: 0
+        }
+    }
+    
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    pub fn push(&mut self, b: bool) {
+        let offset = self.len % 8;
+        let v = (b as u8) << (7 - offset);
+        match offset {
+            0 => self.data.push(v),
+            _ => *self.data.last_mut().unwrap() |= v,
+        }
+        self.len += 1;
+    }
+
+    pub fn push_byte(&mut self, byte: u8) {
+        if self.len() % 8 != 0 {
+            panic!("Attempted to push full byte across byte boundary");
+        }
+        self.data.push(byte);
+        self.len += 8;
+    }
+
+    pub fn slice(&self, range: Range<usize>) -> BitSlice {
+        if range.end > self.len {
+            panic!("Slice out of bounds: {}..{}", range.start, range.end);
+        }
+        BitSlice {
+            src: self,
+            start: range.start,
+            end: range.end,
+        }
+    }
+}
+
+impl Index<usize> for BitVec {
+    type Output = bool;
+    fn index(&self, index: usize) -> &bool {
+        let (idx, offset) = (index / 8, index % 8);
+        let bit = self.data[idx] & (128 >> offset);
+        if bit == 0 {&false} else {&true}
+    }
+}
+
+
+#[derive(Copy, Clone)]
+pub struct BitSlice<'a> {
+    src: &'a BitVec,
+    start: usize,
+    end: usize,
+}
+
+#[allow(dead_code)]
+impl BitSlice<'_> {
+    pub fn len(&self) -> usize {
+        self.end - self.start
+    }
+
+    pub fn as_u64(&self) -> u64 {
+        if self.len() > 64 {panic!("Slice too large for u64");}
+
+        let mut value = 0;
+        for i in self.start..self.end {
+            let offset = self.end - 1 - i;
+            value |= (self.src[i] as u64) << offset;
+        }
+        value
+    }
+
+    pub fn as_u16(&self) -> u16 {
+        if self.len() > 16 {panic!("Slice too large for u16");}
+        self.as_u64() as u16
+    }
+
+    pub fn as_u8(&self) -> u8 {
+        if self.len() > 8 {panic!("Slice too large for u8");}
+        self.as_u64() as u8
+    }
+
+    pub fn as_bool(&self) -> bool {
+        if self.len() != 1 {
+            panic!("Only slices of length 1 can be converted to bool");
+        }
+        self.as_u64() == 1
+    }
+
+    pub fn slice(&self, range: Range<usize>) -> BitSlice {
+        if range.end > self.len() {
+            panic!("Slice out of bounds: {}..{}", range.start, range.end);
+        }
+
+        BitSlice {
+            src: &self.src,
+            start: self.start + range.start,
+            end: self.start + range.end,
+        }
+    }
+}
+
+impl Index<usize> for BitSlice<'_> {
+    type Output = bool;
+    fn index(&self, index: usize) -> &bool {
+        self.src.index(self.start + index)
+    }
+}
diff --git a/2021/src/main.rs b/2021/src/main.rs
index 1c18e6b..424d2c9 100644
--- a/2021/src/main.rs
+++ b/2021/src/main.rs
@@ -4,14 +4,14 @@ use color_eyre::eyre;
 
 mod lib;
 use lib::load;
-mod day15;
+mod day16;
 
 
 fn main() -> eyre::Result<()> {
-    let data = load("data/15.txt")?;
+    let data = load("data/16.txt")?;
 
     let start = Instant::now();
-    let (one, two) = day15::run(&data)?;
+    let (one, two) = day16::run(&data)?;
     let (dur, unit) = format_ns(start.elapsed().as_nanos());
 
     let precision = 2.0 - dur.log10().floor();