diff --git a/day16.py b/day16.py
index adcd4c8..b526e7d 100644
--- a/day16.py
+++ b/day16.py
@@ -1,8 +1,5 @@
 #!/usr/bin/env python3
 
-from intcode import defaultlist
-from operator import mul
-
 
 def str_of_signal(signal):
     return ''.join(map(str, signal))
@@ -12,32 +9,40 @@ def apply_pattern(signal, offset=0):
     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
+    i = 0 
+    step = offset + 1
+
+    while i + step < len(signal):
+        n = sum(sums[i::4*step])
+        n -= sum(sums[i+step::4*step])
+        n -= sum(sums[i+2*step::4*step])
+        n += sum(sums[i+3*step::4*step])
+
+        signal[i] = abs(n)%10
+
+        i += 1
+        step += 1
+
+    signal[i:] = [s % 10 for s in sums[i:]]
+
 
 def phase(signal, n, offset=0):
-    signal = signal[offset:]
     for i in range(n):
-        signal = apply_pattern(signal, offset)
-    return signal
+        apply_pattern(signal, offset)
 
 def preproc(puzzle_input):
     return list(map(int, puzzle_input))
 
 def partI(signal):
-    return str_of_signal(phase(signal, 100)[:8])
+    signal = signal.copy()
+    phase(signal, 100)
+    return str_of_signal(signal[:8])
 
 def partII(signal):
     signal = signal * 10**4
     offset = int(str_of_signal(signal[:7]))
 
-    return str_of_signal(phase(signal, 100, offset)[:8])
+    signal = signal[offset:]
+    phase(signal, 100, offset)
+    return str_of_signal(signal[:8])