diff --git a/2021/input/2021/day16.txt b/2021/input/2021/day16.txt
new file mode 100644
index 0000000..2e4a973
--- /dev/null
+++ b/2021/input/2021/day16.txt
@@ -0,0 +1 @@
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
diff --git a/2021/src/day16.rs b/2021/src/day16.rs
new file mode 100644
index 0000000..07ea179
--- /dev/null
+++ b/2021/src/day16.rs
@@ -0,0 +1,174 @@
+use std::{
+    fmt::{Debug, Error, Formatter},
+    io::Read,
+    num::ParseIntError,
+    ops::{Index, IndexMut},
+    str::FromStr,
+};
+
+use anyhow::Result;
+use aoc_runner_derive::{aoc, aoc_generator};
+use thiserror::Error;
+
+fn hex(b: &u8) -> u8 {
+    if *b >= b'A' {
+        10 + b - b'A'
+    } else {
+        b - b'0'
+    }
+}
+
+struct Parser<'a> {
+    bytes: &'a [u8],
+    tmp: u64,
+    tmp_len: usize,
+}
+
+impl<'a> Parser<'a> {
+    fn new(input: &str) -> Parser {
+        Parser {
+            bytes: input.as_bytes(),
+            tmp: 0,
+            tmp_len: 0,
+        }
+    }
+    fn read(&mut self, n: usize) -> u64 {
+        assert!(n < 32, "can't read more than 32 bits at time");
+        print!(
+            "   BEGIN n {0} tmp 0b{1:b} len {2}    - ",
+            n, self.tmp, self.tmp_len
+        );
+        while self.tmp_len < n {
+            let mut buf = [0; 1];
+            self.bytes.read_exact(&mut buf).expect("EOF");
+            // Convert the byte from hexdecimal to binary and merge with any leftover bits.
+            self.tmp = (self.tmp << 4) | hex(&buf[0]) as u64;
+            self.tmp_len += 4;
+        }
+
+        let mask = (1 << n) - 1;
+        self.tmp_len -= n;
+        let v = (self.tmp >> self.tmp_len) & mask;
+
+        let mask = (1 << self.tmp_len) - 1;
+        self.tmp = self.tmp & mask;
+
+        println!(
+            "   END n {0} tmp 0b{2:b} len {3} v 0b{1:00$b} ",
+            n, v, self.tmp, self.tmp_len
+        );
+        v as u64
+    }
+}
+fn read_packet(p: &mut Parser) -> (Vec<u64>, u64) {
+    let mut versions = Vec::new();
+    let mut bits_processed: u64 = 0;
+    let version = p.read(3);
+    versions.push(version);
+    bits_processed += 3;
+    let typ = p.read(3);
+    bits_processed += 3;
+    println!("version {} type {}", version, typ);
+    if typ == 4 {
+        // Literal, read 5 bits at a time until MSB is 0
+        println!("type 4 literal");
+        loop {
+            let l = p.read(5);
+            bits_processed += 5;
+            println!("literal 0b{:05b}", l);
+            if 0b10000 & l == 0 {
+                /*
+                // Read trailing 0s
+                let n = 4 - ((bits_processed) % 4) as usize;
+                println!(
+                    "bits processed {}, draining {} trailing 0s",
+                    bits_processed, n
+                );
+                let _ = p.read(n);
+                bits_processed += n as u64;
+                */
+
+                break;
+            }
+        }
+    } else {
+        // length type ID
+        let ltid = p.read(1);
+        bits_processed += 1;
+        if ltid == 0 {
+            // If the length type ID is 0, then the next 15 bits are a number that represents the total length in bits of the sub-packets contained by this packet.
+            let len = p.read(15);
+            bits_processed += 15;
+            println!("{} bits in subpacket", len);
+            let mut sub_bits = 0;
+            while sub_bits < len {
+                let (vs, bp) = read_packet(p);
+                versions.extend(vs.iter());
+                bits_processed += bp;
+                sub_bits += bp;
+            }
+        } else {
+            // If the length type ID is 1, then the next 11 bits are a number that represents the number of sub-packets immediately contained by this packet.
+            let num = p.read(11);
+            bits_processed += 11;
+            println!("{} subpackets", num);
+            for _ in 0..num {
+                let (vs, bp) = read_packet(p);
+                versions.extend(vs.iter());
+                bits_processed += bp;
+            }
+        }
+    }
+    return (versions, bits_processed);
+}
+
+#[aoc(day16, part1)]
+fn part1(input: &str) -> Result<u64> {
+    let mut p = Parser::new(input);
+    let (versions, _) = read_packet(&mut p);
+    Ok(versions.iter().sum())
+}
+
+/*
+#[aoc(day16, part2)]
+fn part2(input: &str) -> Result<usize> {
+todo!("part2");
+Ok(0)
+}
+*/
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part1() -> Result<()> {
+        let input = vec![
+            ("D2FE28", 6),
+            //("38006F45291200", 1 + 0 + 0),
+            ("8A004A801A8002F478", 16),
+            ("620080001611562C8802118E34", 12),
+            ("C0015000016115A2E0802F182340", 23),
+            ("A0016C880162017C3686B18A3D4780", 31),
+        ];
+        for (inp, want) in input {
+            print!("\nTesting '{}'\n - ", inp);
+            inp.as_bytes().iter().for_each(|c| print!("{:04b}", hex(c)));
+            println!();
+            assert_eq!(part1(inp)?, want);
+            println!("Passed '{}'", inp);
+        }
+        Ok(())
+    }
+
+    /*
+    #[test]
+    fn test_part2()->Result<()> {
+    let input = r#"
+    "#
+    .trim();
+    assert_eq!(part2(input)?, usize::MAX);
+    Ok(())
+    }
+    */
+}
diff --git a/2021/src/lib.rs b/2021/src/lib.rs
index b823e09..c959bfc 100644
--- a/2021/src/lib.rs
+++ b/2021/src/lib.rs
@@ -5,6 +5,7 @@ pub mod day12;
 pub mod day13;
 pub mod day14;
 pub mod day15;
+pub mod day16;
 pub mod day2;
 pub mod day3;
 pub mod day4;