diff --git a/README.md b/README.md
index ac0ec64..71cd7b8 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,8 @@
   - [solution](src/day18.rs)
 - [Day 19: Aplenty](https://adventofcode.com/2023/day/19)
   - [solution](src/day19.rs)
+- [Day 20: Pulse Propagation](https://adventofcode.com/2023/day/20)
+  - [solution](src/day20.rs)
 <!-- Insert before -->
 
 See:
diff --git a/input b/input
index 5cecd8b..39fbf42 160000
--- a/input
+++ b/input
@@ -1 +1 @@
-Subproject commit 5cecd8b34b697dc79dd368fad4a6f27472ce3dc7
+Subproject commit 39fbf42f1250e70f5bf69b6f40cafae94696deec
diff --git a/src/day20.rs b/src/day20.rs
new file mode 100644
index 0000000..156b037
--- /dev/null
+++ b/src/day20.rs
@@ -0,0 +1,296 @@
+use std::collections::VecDeque;
+
+use ahash::{HashMap, HashMapExt};
+use aoc_runner_derive::{aoc, aoc_generator};
+use arrayvec::ArrayVec;
+use nom::{
+    bytes::complete::{tag, take_while1},
+    character::complete::{newline, one_of},
+    multi::separated_list0,
+    IResult,
+};
+use num::Integer;
+type String = smallstr::SmallString<[u8; 2]>;
+
+use crate::common::nom::{fold_separated_list0, process_input};
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum Signal {
+    Low,
+    High,
+}
+
+impl std::ops::Not for Signal {
+    type Output = Self;
+
+    fn not(self) -> Self::Output {
+        match self {
+            Signal::Low => Signal::High,
+            Signal::High => Signal::Low,
+        }
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub enum Module {
+    Broadcaster,
+    FlipFlop(bool),
+    Conjunction(HashMap<String, Signal>),
+}
+
+#[derive(Debug, PartialEq, Eq, Clone)]
+pub struct Machine {
+    m_type: Module,
+    id: String,
+    output: ArrayVec<String, 8>,
+}
+
+impl Machine {
+    fn pulse(
+        &mut self,
+        sender: &String,
+        signal: Signal,
+        queue: &mut impl Extend<(String, String, Signal)>,
+    ) {
+        match &mut self.m_type {
+            Module::Broadcaster => self.generate_output(signal, queue),
+            Module::FlipFlop(status) => {
+                if signal == Signal::Low {
+                    let new_status = !*status;
+                    *status = new_status;
+
+                    self.generate_output(
+                        if new_status {
+                            Signal::High
+                        } else {
+                            Signal::Low
+                        },
+                        queue,
+                    )
+                }
+            }
+            Module::Conjunction(memory) => {
+                *memory.get_mut(sender).unwrap() = signal;
+                let new_signal = if memory.values().all(|s| s == &Signal::High) {
+                    Signal::Low
+                } else {
+                    Signal::High
+                };
+
+                self.generate_output(new_signal, queue)
+            }
+        }
+    }
+
+    fn generate_output(&self, signal: Signal, queue: &mut impl Extend<(String, String, Signal)>) {
+        queue.extend(
+            self.output
+                .iter()
+                .map(|id| (self.id.clone(), id.clone(), signal)),
+        )
+    }
+}
+
+fn parse_connection(s: &str) -> IResult<&str, (String, ArrayVec<String, 8>)> {
+    let (s, name) = take_while1(|c: char| c.is_alphabetic())(s)?;
+    let (s, _) = tag(" -> ")(s)?;
+    let (s, res) = fold_separated_list0(
+        tag(", "),
+        take_while1(|c: char| c.is_alphabetic()),
+        ArrayVec::new,
+        |mut v, x: &str| {
+            v.push(x.into());
+            v
+        },
+    )(s)?;
+
+    Ok((s, (name.into(), res)))
+}
+
+fn parse_module(s: &str) -> IResult<&str, Machine> {
+    let (s, typ) = one_of("%&b")(s)?;
+    let (s, (key, output)) = parse_connection(s)?;
+    let res = match typ {
+        'b' => Machine {
+            m_type: Module::Broadcaster,
+            id: "br".into(),
+            output,
+        },
+        '%' => Machine {
+            m_type: Module::FlipFlop(false),
+            id: key.to_owned(),
+            output,
+        },
+        '&' => Machine {
+            m_type: Module::Conjunction(HashMap::new()),
+            id: key.to_owned(),
+            output,
+        },
+        _ => panic!("foo"),
+    };
+
+    Ok((s, res))
+}
+
+#[aoc_generator(day20)]
+pub fn generator(input: &str) -> Vec<Machine> {
+    let mut machines = process_input(separated_list0(newline, parse_module))(input);
+
+    // Find all conjuctions, and the HashMap
+    let mut conjunctions: HashMap<String, HashMap<String, Signal>> = machines
+        .iter()
+        .filter_map(|m| {
+            if let Module::Conjunction(_) = m.m_type {
+                Some((m.id.clone(), HashMap::new()))
+            } else {
+                None
+            }
+        })
+        .collect();
+
+    // Run though each machine, if the outputs are contained in the conjunctions hashmap, then add the id
+    for m in machines.iter() {
+        for out in m.output.iter() {
+            if let Some(hm) = conjunctions.get_mut(out) {
+                hm.insert(m.id.clone(), Signal::Low);
+            }
+        }
+    }
+
+    // Run through the machines again, and assign the HashMap in conjunctions to the conjunction memory
+    for m in machines.iter_mut() {
+        if let Module::Conjunction(memory) = &mut m.m_type {
+            let id = m.id.clone();
+            if let Some(hm) = conjunctions.remove(&id) {
+                *memory = hm;
+            }
+        }
+    }
+
+    machines
+}
+
+#[aoc(day20, part1)]
+pub fn part1(inputs: &[Machine]) -> u64 {
+    let mut machines = inputs.iter().fold(HashMap::new(), |mut hm, m| {
+        hm.insert(m.id.clone(), m.clone());
+        hm
+    });
+    let mut queue = VecDeque::with_capacity(64);
+    let mut high = 0;
+    let mut low = 0;
+
+    for _ in 0..1000 {
+        assert!(queue.is_empty());
+        queue.push_back((String::from("bu"), String::from("br"), Signal::Low));
+
+        while let Some((fr, to, signal)) = queue.pop_front() {
+            match signal {
+                Signal::High => {
+                    high += 1;
+                }
+                Signal::Low => {
+                    low += 1;
+                }
+            }
+
+            if let Some(m) = machines.get_mut(&to) {
+                m.pulse(&fr, signal, &mut queue);
+            }
+        }
+    }
+
+    high * low
+}
+
+#[aoc(day20, part2)]
+pub fn part2(inputs: &[Machine]) -> u64 {
+    let mut machines = inputs.iter().fold(HashMap::new(), |mut hm, m| {
+        hm.insert(m.id.clone(), m.clone());
+        hm
+    });
+
+    let key_conjunction = inputs
+        .iter()
+        .find(|m| m.output.contains(&String::from("rx")))
+        .unwrap();
+
+    let mut penultimate: Vec<String> = inputs
+        .iter()
+        .filter(|m| m.output.contains(&key_conjunction.id))
+        .map(|m| m.id.clone())
+        .collect();
+
+    let mut frequency = Vec::with_capacity(penultimate.len());
+
+    let mut queue = VecDeque::with_capacity(64);
+    for i in 1.. {
+        assert!(queue.is_empty());
+        queue.push_back((String::from("bu"), String::from("br"), Signal::Low));
+
+        while let Some((fr, to, signal)) = queue.pop_front() {
+            if let Some(pos) = penultimate.iter().position(|k| k == &to) {
+                if signal == Signal::Low {
+                    frequency.push(i);
+
+                    penultimate.remove(pos);
+                    if penultimate.is_empty() {
+                        // println!("{frequency:?}");
+                        return frequency.iter().fold(1, |lcm, v| lcm.lcm(v));
+                    }
+                }
+            }
+
+            if let Some(m) = machines.get_mut(&to) {
+                m.pulse(&fr, signal, &mut queue);
+            }
+        }
+    }
+
+    panic!("shouldn't reach here")
+    // println!("{:?}", frequency);
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE: &str = r"broadcaster -> a, b, c
+%a -> b
+%b -> c
+%c -> inv
+&inv -> a";
+
+    #[test]
+    pub fn input_test() {
+        println!("{:?}", generator(SAMPLE));
+
+        // assert_eq!(generator(SAMPLE), Object());
+    }
+
+    #[test]
+    pub fn part1_test() {
+        assert_eq!(part1(&generator(SAMPLE)), 32000000);
+    }
+
+    #[test]
+    pub fn part2_test() {
+        // assert_eq!(part2(&generator(SAMPLE)), 336);
+    }
+
+    mod regression {
+        use super::*;
+
+        const INPUT: &str = include_str!("../input/2023/day20.txt");
+        const ANSWERS: (u64, u64) = (788848550, 228300182686739);
+
+        #[test]
+        pub fn test() {
+            let input = INPUT.trim_end_matches('\n');
+            let output = generator(input);
+
+            assert_eq!(part1(&output), ANSWERS.0);
+            assert_eq!(part2(&output), ANSWERS.1);
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index b717e47..90d03cb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -20,5 +20,6 @@ pub mod day16;
 pub mod day17;
 pub mod day18;
 pub mod day19;
+pub mod day20;
 // Insert before
 aoc_lib! { year = 2023 }