diff --git a/src/main/kotlin/de/linkel/aoc/Day20.kt b/src/main/kotlin/de/linkel/aoc/Day20.kt
index c5b6ad3..831a40b 100644
--- a/src/main/kotlin/de/linkel/aoc/Day20.kt
+++ b/src/main/kotlin/de/linkel/aoc/Day20.kt
@@ -2,13 +2,222 @@ package de.linkel.aoc
 
 import de.linkel.aoc.base.AbstractLinesAdventDay
 import de.linkel.aoc.base.QuizPart
+import de.linkel.aoc.utils.lcm
 import jakarta.inject.Singleton
 
 @Singleton
-class Day20: AbstractLinesAdventDay<Int>() {
+class Day20: AbstractLinesAdventDay<Long>() {
     override val day = 20
 
-    override fun process(part: QuizPart, lines: Sequence<String>): Int {
-        return 0
+    override fun process(part: QuizPart, lines: Sequence<String>): Long {
+        val parsedModules = lines
+            .map { line ->
+                val spec = line.substringBefore(" ")
+                val next = line.substringAfter(" -> ").split(",").map(String::trim)
+                if (spec == "broadcaster") {
+                    Broadcaster(spec, next)
+                } else when(spec[0]) {
+                    '%' -> FlipFlop(spec.substring(1), next)
+                    '&' -> Conjunction(spec.substring(1), next)
+                    else -> throw Exception("unknown module spec $spec")
+                }
+            }
+            .toList()
+        val button = Button("button", listOf("broadcaster"))
+        val rx = Rx("rx")
+        val modules = parsedModules + button + rx
+        val nops = (modules.flatMap { it.next }.distinct().toSet() - modules.map { it.name })
+            .map { Nop(it) }
+        val dispatcher = CommunicationDispatcher(modules + nops)
+        dispatcher.init()
+
+        if (part == QuizPart.A) {
+            repeat(1000) {
+                button.press()
+                while (dispatcher.busy) {
+                    dispatcher.tick()
+                }
+            }
+            return dispatcher.lowPulses * dispatcher.highPulses
+        } else {
+            return if (modules.any { "rx" in it.next }) {
+                val feeder = modules.first { it.name == rx.inputs.first() } as Conjunction
+                var buttonPresses = 0L
+                val cycles = feeder.lastSignals
+                    .mapValues {
+                        0L
+                    }
+                    .toMutableMap()
+                while (true) {
+                    rx.reset()
+                    buttonPresses++
+                    button.press()
+                    while (dispatcher.busy) {
+                        dispatcher.tick()
+                    }
+                    feeder.gotHigh
+                        .forEach {
+                            if (it.value && cycles[it.key] == 0L) {
+                                cycles[it.key] = buttonPresses
+                            }
+                        }
+                    if (cycles.all { it.value > 0L }) {
+                        break
+                    }
+                }
+                return cycles.values.fold(1L) { a, b -> lcm(a, b) }
+            } else 0L
+        }
+
+    }
+
+    data class Message(
+        val sender: String,
+        val receivers: List<String>,
+        val pulse: Boolean
+    )
+    class CommunicationDispatcher(
+        modules: Collection<Module>
+    ) {
+        private val modules = modules.associateBy { it.name }
+
+        private val queue = mutableListOf<Message>()
+        private var lowPulseCounter = 0L
+        private var highPulseCounter = 0L
+        val lowPulses get(): Long = lowPulseCounter
+        val highPulses get(): Long = highPulseCounter
+        val busy get() = queue.isNotEmpty()
+        fun init() {
+            modules.values
+                .onEach { it.dispatcher = this }
+                .flatMap { module -> module.next.map { module.name to it } }
+                .groupBy { it.second }
+                .mapValues { entry -> entry.value.map { it.first } }
+                .forEach { (module, prev) ->
+                    modules[module]?.wireUp(prev) ?: throw Exception("module $module not found")
+                }
+        }
+        fun dispatch(sender: String, pulse: Boolean, receivers: List<String>) {
+            queue.add(Message(sender, receivers, pulse))
+        }
+        fun tick() {
+            if (queue.isNotEmpty()) {
+                queue.removeAt(0)
+                    .let { message ->
+                        message.receivers
+                            .map { modules[it]!! }
+//                            .onEach { println("${message.sender} -${if (message.pulse) "high" else "low"}-> ${it.name}") }
+                            .onEach { if (message.pulse) highPulseCounter++ else lowPulseCounter++ }
+                            .forEach { it.receive(message.sender, message.pulse) }
+                    }
+            }
+        }
+    }
+
+    abstract class Module(
+        val name: String,
+        val next: List<String>
+    ) {
+        var dispatcher: CommunicationDispatcher? = null
+        protected fun send(pulse: Boolean) {
+            dispatcher?.dispatch(name, pulse, next)
+        }
+        open fun wireUp(prev: List<String>) {
+        }
+        abstract fun receive(sender: String, pulse: Boolean)
+    }
+
+    class FlipFlop(
+        name: String,
+        next: List<String>
+    ): Module(name, next) {
+        var state = false
+        override fun receive(sender: String, pulse: Boolean) {
+            if (!pulse) {
+                state = !state
+                send(state)
+            }
+        }
+
+        override fun toString(): String = "FlipFlop $name"
+    }
+
+    class Conjunction(
+        name: String,
+        next: List<String>
+    ): Module(name, next) {
+        private var state = mutableMapOf<String, Boolean>()
+        val gotHigh = mutableMapOf<String, Boolean>()
+        val lastSignals get(): Map<String, Boolean> = state
+        override fun wireUp(prev: List<String>) {
+            prev.forEach { state[it] = false }
+            prev.forEach { gotHigh[it] = false }
+        }
+        override fun receive(sender: String, pulse: Boolean) {
+            state[sender] = pulse
+            if (pulse) {
+                gotHigh[sender] = true
+            }
+            send(state.values.any { !it })
+        }
+
+        override fun toString(): String = "Conjunction $name"
+    }
+
+    class Broadcaster(
+        name: String,
+        next: List<String>
+    ): Module(name, next) {
+        override fun receive(sender: String, pulse: Boolean) {
+            send(pulse)
+        }
+
+        override fun toString(): String = "Broadcaster $name"
+    }
+    class Rx(
+        name: String
+    ): Module(name, emptyList()) {
+        private val states = mutableListOf<Boolean>()
+        val lastSignals get(): List<Boolean> = states
+
+        private var prev = emptyList<String>()
+        val inputs get(): List<String> = prev
+
+        fun reset() {
+            states.clear()
+        }
+
+        override fun wireUp(prev: List<String>) {
+            this.prev = prev
+        }
+
+        override fun receive(sender: String, pulse: Boolean) {
+            states.add(pulse)
+        }
+
+        override fun toString(): String = "Rx $name"
+    }
+
+    class Nop(
+        name: String
+    ): Module(name, emptyList()) {
+        override fun receive(sender: String, pulse: Boolean) {
+        }
+
+        override fun toString(): String = "Nop $name"
+    }
+
+    class Button(
+        name: String,
+        next: List<String>
+    ): Module(name, next) {
+        fun press() {
+            if (dispatcher?.busy == false)
+                send(false)
+        }
+        override fun receive(sender: String, pulse: Boolean) {
+        }
+
+        override fun toString(): String = "Button $name"
     }
 }
diff --git a/src/test/kotlin/de/linkel/aoc/Day20Test.kt b/src/test/kotlin/de/linkel/aoc/Day20Test.kt
index b691167..3f0c7ea 100644
--- a/src/test/kotlin/de/linkel/aoc/Day20Test.kt
+++ b/src/test/kotlin/de/linkel/aoc/Day20Test.kt
@@ -1,13 +1,26 @@
 package de.linkel.aoc
 
-class Day20Test: AbstractDayTest<Int>() {
+class Day20Test: AbstractDayTest<Long>() {
     override val exampleA = """
+broadcaster -> a, b, c
+%a -> b
+%b -> c
+%c -> inv
+&inv -> a
         """.trimIndent()
-    override val exampleSolutionA = 0
-    override val solutionA = 0
+    val exampleA2 = """
+broadcaster -> a
+%a -> inv, con
+&inv -> b
+%b -> con
+&con -> output
+        """.trimIndent()
+
+    override val exampleSolutionA = 32000000L
+    override val solutionA = 832957356L
 
-    override val exampleSolutionB = 0
-    override val solutionB = 0
+    override val exampleSolutionB = 0L
+    override val solutionB = 240162699605221L
 
     override val implementation = Day20()
 }
diff --git a/src/test/kotlin/de/linkel/aoc/Day21Test.kt b/src/test/kotlin/de/linkel/aoc/Day21Test.kt
index e8f3021..37cd807 100644
--- a/src/test/kotlin/de/linkel/aoc/Day21Test.kt
+++ b/src/test/kotlin/de/linkel/aoc/Day21Test.kt
@@ -9,5 +9,5 @@ class Day21Test: AbstractDayTest<Int>() {
     override val exampleSolutionB = 0
     override val solutionB = 0
 
-    override val implementation = Day20()
+    override val implementation = Day21()
 }
diff --git a/src/test/kotlin/de/linkel/aoc/Day22Test.kt b/src/test/kotlin/de/linkel/aoc/Day22Test.kt
index 44b23b6..44f2dd1 100644
--- a/src/test/kotlin/de/linkel/aoc/Day22Test.kt
+++ b/src/test/kotlin/de/linkel/aoc/Day22Test.kt
@@ -9,5 +9,5 @@ class Day22Test: AbstractDayTest<Int>() {
     override val exampleSolutionB = 0
     override val solutionB = 0
 
-    override val implementation = Day20()
+    override val implementation = Day22()
 }
diff --git a/src/test/kotlin/de/linkel/aoc/Day23Test.kt b/src/test/kotlin/de/linkel/aoc/Day23Test.kt
index 666d4ea..daed4df 100644
--- a/src/test/kotlin/de/linkel/aoc/Day23Test.kt
+++ b/src/test/kotlin/de/linkel/aoc/Day23Test.kt
@@ -9,5 +9,5 @@ class Day23Test: AbstractDayTest<Int>() {
     override val exampleSolutionB = 0
     override val solutionB = 0
 
-    override val implementation = Day20()
+    override val implementation = Day23()
 }
diff --git a/src/test/kotlin/de/linkel/aoc/Day24Test.kt b/src/test/kotlin/de/linkel/aoc/Day24Test.kt
index 70dff98..2ec83fa 100644
--- a/src/test/kotlin/de/linkel/aoc/Day24Test.kt
+++ b/src/test/kotlin/de/linkel/aoc/Day24Test.kt
@@ -9,5 +9,5 @@ class Day24Test: AbstractDayTest<Int>() {
     override val exampleSolutionB = 0
     override val solutionB = 0
 
-    override val implementation = Day20()
+    override val implementation = Day24()
 }