diff --git a/input/day20.txt b/input/day20.txt
new file mode 100644
index 0000000..551873f
--- /dev/null
+++ b/input/day20.txt
@@ -0,0 +1,58 @@
+%bc -> ph
+%hr -> zq
+%sn -> sj
+%df -> dn, hf
+%lp -> dm
+%lf -> sn, vk
+&fv -> sq
+%gd -> vm, gr
+%jt -> vm, lz
+%xf -> vk
+%nf -> lp, kb
+%dl -> nn
+&sq -> rx
+%vb -> vm, lg
+%zq -> kb, fl
+%fk -> fz
+%gj -> pq
+%jx -> jh
+%pq -> kb, nf
+&dn -> kg, vt, tg, fs, pn, jx
+%ln -> dn, fs
+%fz -> vm, jt
+%fs -> df
+%dm -> kb, hr
+%ds -> kb
+&kk -> sq
+%tg -> tv
+&vt -> sq
+%fl -> zl, kb
+&vk -> bc, sj, jd, lf, xr, sn
+%jd -> pp
+%tv -> dn, pn
+%sb -> gf, vk
+&kb -> kk, gj, gt, hr, lp
+%pp -> vk, bc
+%pn -> pf
+%zc -> vm
+&vm -> dl, fk, nn, fv, gd, lg
+%rn -> dn
+%gr -> vb, vm
+%sj -> kq
+%zl -> kb, ds
+%lz -> vm, zc
+%jh -> dn, ln
+%pf -> dn, jx
+%kq -> sb, vk
+%ph -> md, vk
+%nc -> gj, kb
+%kg -> tg, dn
+%hf -> dn, rn
+%nn -> fk
+%gf -> jd, vk
+%lg -> pv
+broadcaster -> gd, kg, gt, lf
+%gt -> nc, kb
+%pv -> dl, vm
+&xr -> sq
+%md -> vk, xf
diff --git a/src/day08.rs b/src/day08.rs
index bdc2592..a7e6788 100644
--- a/src/day08.rs
+++ b/src/day08.rs
@@ -128,7 +128,7 @@ fn count_steps_until_end(
     steps
 }
 
-fn lcm(xs: impl Iterator<Item = usize>) -> usize {
+pub fn lcm(xs: impl Iterator<Item = usize>) -> usize {
     let mut a = 1;
     for x in xs {
         a = a * x / gcd(a, x);
diff --git a/src/day20.rs b/src/day20.rs
new file mode 100644
index 0000000..6eb2ec5
--- /dev/null
+++ b/src/day20.rs
@@ -0,0 +1,324 @@
+use std::{
+    collections::{HashMap, HashSet, VecDeque},
+    fmt::Display,
+};
+
+use itertools::Itertools;
+
+pub fn run() {
+    let input = include_str!("../input/day20.txt");
+    // let input = r#"
+    // broadcaster -> a, b, c
+    // %a -> b
+    // %b -> c
+    // %c -> inv
+    // &inv -> a
+    //     "#;
+
+    // let input = r#"
+    // broadcaster -> a
+    // %a -> inv, con
+    // &inv -> b
+    // %b -> con
+    // &con -> output
+    //     "#;
+
+    let modules = parse_modules(input);
+    dbg!(simulate_one_thousand(modules.clone()));
+
+    let rx_input = find_inputs(&modules, "rx");
+    assert_eq!(rx_input.len(), 1);
+    let rx_input = rx_input[0];
+    assert!(modules[rx_input].as_any().is::<Conjunction>());
+
+    let rx_input_inputs = find_inputs(&modules, rx_input);
+    let cycles = rx_input_inputs
+        .into_iter()
+        .map(|s| find_output_cycle(modules.clone(), s))
+        .collect::<Vec<_>>();
+
+    dbg!(crate::day08::lcm(cycles.into_iter()));
+}
+
+fn parse_modules(s: &'static str) -> HashMap<&'static str, Box<dyn Module>> {
+    let mut m = HashMap::<&'static str, Box<dyn Module>>::new();
+    m.insert("output", Box::new(Output));
+    let mut conjunctions = vec![];
+    for l in s.trim().lines() {
+        let l = l.trim();
+        let (name, receivers) = l.split_once(" -> ").unwrap();
+        let receivers = receivers.split(", ").collect::<Vec<_>>();
+        let trimmed_name: &'static str;
+        let module: Box<dyn Module>;
+        if name == "broadcaster" {
+            trimmed_name = "broadcaster";
+            module = Box::new(Broadcaster { receivers });
+        } else if name.starts_with("%") {
+            trimmed_name = &name[1..];
+            module = Box::new(FlipFlop {
+                receivers,
+                state: false,
+            });
+        } else if name.starts_with("&") {
+            trimmed_name = &name[1..];
+            conjunctions.push(trimmed_name);
+            module = Box::new(Conjunction {
+                receivers,
+                inputs: HashMap::new(),
+            });
+        } else {
+            panic!("invalid name {}", name);
+        }
+        m.insert(trimmed_name, module);
+    }
+    for name in conjunctions {
+        let inputs = m
+            .iter()
+            .filter(|(_, v)| v.get_destinations().contains(&name))
+            .map(|(k, _)| (*k, false))
+            .collect::<HashMap<&'static str, bool>>();
+        let any = m.get_mut(name).unwrap().as_any_mut();
+        let conjunction = any.downcast_mut::<Conjunction>().unwrap();
+        conjunction.inputs = inputs;
+    }
+    m
+}
+
+#[derive(Debug, Clone, Copy)]
+struct Pulse {
+    destination: &'static str,
+    value: bool,
+}
+
+trait Module: std::fmt::Debug {
+    fn get_destinations(&self) -> &[&'static str];
+    fn handle_pulse(&mut self, source: &'static str, value: bool) -> Vec<Pulse>;
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any;
+    fn as_any(&self) -> &dyn std::any::Any;
+    fn clone_module(&self) -> Box<dyn Module>;
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct Broadcaster {
+    receivers: Vec<&'static str>,
+}
+
+impl Module for Broadcaster {
+    fn handle_pulse(&mut self, _source: &'static str, value: bool) -> Vec<Pulse> {
+        self.receivers
+            .iter()
+            .map(|r| Pulse {
+                destination: r,
+                value,
+            })
+            .collect()
+    }
+
+    fn get_destinations(&self) -> &[&'static str] {
+        &self.receivers
+    }
+
+    fn clone_module(&self) -> Box<dyn Module> {
+        Box::new(self.clone())
+    }
+
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct FlipFlop {
+    receivers: Vec<&'static str>,
+    state: bool,
+}
+
+impl Module for FlipFlop {
+    fn handle_pulse(&mut self, _source: &'static str, value: bool) -> Vec<Pulse> {
+        if value {
+            return vec![];
+        }
+        self.state = !self.state;
+        self.receivers
+            .iter()
+            .map(|r| Pulse {
+                destination: r,
+                value: self.state,
+            })
+            .collect()
+    }
+
+    fn get_destinations(&self) -> &[&'static str] {
+        &self.receivers
+    }
+
+    fn clone_module(&self) -> Box<dyn Module> {
+        Box::new(self.clone())
+    }
+
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+struct Conjunction {
+    receivers: Vec<&'static str>,
+    inputs: HashMap<&'static str, bool>,
+}
+
+impl Module for Conjunction {
+    fn handle_pulse(&mut self, source: &'static str, value: bool) -> Vec<Pulse> {
+        self.inputs.insert(source, value);
+        let output = !self.inputs.values().all(|b| *b);
+        self.receivers
+            .iter()
+            .map(|r| Pulse {
+                destination: r,
+                value: output,
+            })
+            .collect()
+    }
+
+    fn get_destinations(&self) -> &[&'static str] {
+        &self.receivers
+    }
+
+    fn clone_module(&self) -> Box<dyn Module> {
+        Box::new(self.clone())
+    }
+
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+struct Output;
+
+impl Module for Output {
+    fn get_destinations(&self) -> &[&'static str] {
+        &[]
+    }
+
+    fn handle_pulse(&mut self, _source: &'static str, _value: bool) -> Vec<Pulse> {
+        vec![]
+    }
+
+    fn clone_module(&self) -> Box<dyn Module> {
+        Box::new(Output)
+    }
+
+    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
+        self
+    }
+
+    fn as_any(&self) -> &dyn std::any::Any {
+        self
+    }
+}
+
+fn simulate(modules: &mut HashMap<&'static str, Box<dyn Module>>) -> (usize, usize) {
+    let mut pulses_to_send = VecDeque::from([(
+        "button",
+        Pulse {
+            destination: "broadcaster",
+            value: false,
+        },
+    )]);
+    let mut high = 0;
+    let mut low = 0;
+    // println!("button -low-> broadcaster");
+    while let Some((source, p)) = pulses_to_send.pop_front() {
+        if p.value {
+            high += 1;
+        } else {
+            low += 1;
+        }
+        let handler = p.destination;
+        let handling_module = modules.get_mut(handler);
+        if handling_module.is_none() {
+            // println!("no module {}", handler);
+            continue;
+        }
+        let new = handling_module.unwrap().handle_pulse(source, p.value);
+        for p in new {
+            // let value = if p.value { "high" } else { "low" };
+            // println!("{} -{}-> {}", handler, value, p.destination);
+            pulses_to_send.push_back((handler, p));
+        }
+    }
+    (high, low)
+}
+
+impl Clone for Box<dyn Module> {
+    fn clone(&self) -> Self {
+        self.clone_module()
+    }
+}
+
+fn simulate_one_thousand(mut modules: HashMap<&'static str, Box<dyn Module>>) -> usize {
+    let mut high = 0;
+    let mut low = 0;
+    for _ in 0..1000 {
+        let r = simulate(&mut modules);
+        high += r.0;
+        low += r.1;
+    }
+    high * low
+}
+
+fn find_inputs(
+    modules: &HashMap<&'static str, Box<dyn Module>>,
+    dest: &'static str,
+) -> Vec<&'static str> {
+    let mut inputs = vec![];
+    for (name, module) in modules.iter() {
+        if module.get_destinations().contains(&dest) {
+            inputs.push(*name);
+        }
+    }
+    inputs
+}
+
+fn find_output_cycle(
+    mut modules: HashMap<&'static str, Box<dyn Module>>,
+    name: &'static str,
+) -> usize {
+    let mut iterations = 0;
+    loop {
+        iterations += 1;
+
+        let mut pulses_to_send = VecDeque::from([(
+            "button",
+            Pulse {
+                destination: "broadcaster",
+                value: false,
+            },
+        )]);
+        while let Some((source, p)) = pulses_to_send.pop_front() {
+            if !p.value && p.destination == name {
+                return iterations;
+            }
+            let handler = p.destination;
+            if let Some(handling_module) = modules.get_mut(handler) {
+                let new = handling_module.handle_pulse(source, p.value);
+                for p in new {
+                    pulses_to_send.push_back((handler, p));
+                }
+            }
+        }
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 7d7a681..8601e3f 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -20,7 +20,8 @@ mod day16;
 mod day17;
 mod day18;
 mod day19;
+mod day20;
 
 fn main() {
-    day19::run();
+    day20::run();
 }