diff --git a/2019/16.cpp b/2019/16.cpp
index 30634a5..598fd0b 100644
--- a/2019/16.cpp
+++ b/2019/16.cpp
@@ -8,12 +8,12 @@ using namespace aocpp;
 
 namespace {
 
-auto FFT(std::size_t start, std::vector<std::int8_t> const& prev)
+auto FFT(std::size_t const start, std::vector<std::int8_t> const& prev)
 {
   std::vector<std::int8_t> result;
   std::vector<std::int32_t> partial_sums;
   auto n = prev.size();
-  result.resize(n);
+  result.reserve(n);
   partial_sums.reserve(n+1);
   partial_sums.push_back(0);
 
@@ -22,14 +22,14 @@ auto FFT(std::size_t start, std::vector<std::int8_t> const& prev)
     partial_sums.push_back(acc += x);
   }
 
-  for (std::size_t i = start; i < n+start; i++) {
+  for (std::size_t i = 0; i < n; i++) {
     std::int32_t acc = 0;
-    for (std::size_t offset = i; offset < n+start; offset += 4*(i+1)) {
-      acc += partial_sums[std::min(n,offset-start + i + 1)] - partial_sums[std::min(n, offset-start)];
-      acc -= partial_sums[std::min(n, offset-start + 3*(i + 1))] - partial_sums[std::min(n, offset-start + 2*(i + 1))];
+    auto step = i+start+1;
+    for (std::size_t offset = i; offset < n; offset += 4*step) {
+      acc += partial_sums[std::min(n, offset + 1*step)] - partial_sums[std::min(n, offset         )];
+      acc -= partial_sums[std::min(n, offset + 3*step)] - partial_sums[std::min(n, offset + 2*step)];
     }
-
-    result[i-start] = std::abs(acc % 10);
+    result.push_back(std::abs(acc % 10));
   }
   return result;
 }