diff --git a/day20.py b/day20.py
new file mode 100644
index 0000000..ff5c09c
--- /dev/null
+++ b/day20.py
@@ -0,0 +1,105 @@
+from collections import defaultdict, deque
+import math
+from pprint import pprint
+import sys
+import re
+
+modules = {}
+conjunctions = {}
+flipflops = {}
+
+firstit = {}
+cycles = {}
+
+def simulate(it):
+    queue = deque([(None, "button", 0)])
+    pulses = [0, 0]
+
+    while queue:
+        (source, target, level) = queue.popleft()
+        if target not in modules:
+            continue
+
+        (type, destinations) = modules[target]
+
+        if type == "":
+            pass
+        elif type == "%":
+            if level == 1:
+                continue
+
+            flipflops[target] = not flipflops[target]
+            level = int(flipflops[target])
+        elif type == "&":
+            conjunction = conjunctions[target]
+            conjunction[source] = level
+            level = 0 if all(conjunction.values()) else 1
+        else:
+            continue
+
+        for destination in destinations:
+            queue.append((target, destination, level))
+            pulses[level] += 1
+
+            if destination == "dt" and level and target not in cycles:
+                if target not in firstit:
+                    firstit[target] = it
+                    continue
+
+                cycle = it - firstit[target]
+                cycles[target] = (firstit[target], cycle)
+
+                if len(cycles) == len(conjunctions["dt"]):
+                    lcm = math.lcm(*(v[1] for v in cycles.values()))
+                    return (pulses, lcm)
+
+    return (pulses, None)
+
+def main():
+    has_rx = False
+    with open(sys.argv[1]) as f:
+        for line in f:
+            (type, name, destinations) = re.match(r"([%\&]?)(.+) -> (.+)", line).groups()
+            destinations = destinations.split(", ")
+            modules[name] = (type, destinations)
+
+            if "rx" in destinations:
+                has_rx = True
+
+            if type == "%":
+                flipflops[name] = False
+            elif type == "&":
+                conjunctions[name] = {}
+
+    modules["button"] = ("", ["broadcaster"])
+
+    for (name, (type, dests)) in modules.items():
+        for dest in dests:
+            if dest not in modules or modules[dest][0] != "&":
+                continue
+            conjunctions[dest][name] = 0
+
+    (lows, highs) = (0, 0)
+    (part1, part2) = (None, None)
+    it = 0
+
+    while part1 is None or part2 is None:
+        ([low, high], cycle) = simulate(it)
+
+        if it < 1000:
+            lows += low
+            highs += high
+        elif it == 1000:
+            part1 = lows * highs
+            if not has_rx:
+                break
+
+        if cycle:
+            part2 = cycle
+
+        it += 1
+
+    print(part1, part2)
+
+if __name__ == "__main__":
+    main()
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