diff --git a/day16.py b/day16.py
new file mode 100644
index 0000000..adcd4c8
--- /dev/null
+++ b/day16.py
@@ -0,0 +1,43 @@
+#!/usr/bin/env python3
+
+from intcode import defaultlist
+from operator import mul
+
+
+def str_of_signal(signal):
+    return ''.join(map(str, signal))
+
+def apply_pattern(signal, offset=0):
+    sums = [None] * (len(signal)-1) + [signal[-1]]
+    for i in range(len(signal)-2, -1, -1):
+        sums[i] = sums[i+1] + signal[i]
+
+    sums = defaultlist(sums, lambda : 0)
+    new = []
+    for i in range(len(signal)):
+        step = i + offset + 1
+        n = 0
+        for block in range(i, len(signal), 4*step):
+            n += sums[block] - sums[block + step]
+            n -= sums[block + 2*step] - sums[block + 3*step]
+        new.append(abs(n)%10)
+    return new
+
+def phase(signal, n, offset=0):
+    signal = signal[offset:]
+    for i in range(n):
+        signal = apply_pattern(signal, offset)
+    return signal
+
+def preproc(puzzle_input):
+    return list(map(int, puzzle_input))
+
+def partI(signal):
+    return str_of_signal(phase(signal, 100)[:8])
+
+def partII(signal):
+    signal = signal * 10**4
+    offset = int(str_of_signal(signal[:7]))
+
+    return str_of_signal(phase(signal, 100, offset)[:8])
+