From 08fa71a5917d1715ea40535b0b61d0c6208a3f34 Mon Sep 17 00:00:00 2001
From: Eric Mertens <emertens@galois.com>
Date: Thu, 17 Nov 2022 08:38:14 -0800
Subject: [PATCH] minor speedup through reused allocation

---
 2019/16.cpp | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/2019/16.cpp b/2019/16.cpp
index d515f90..ceaa856 100644
--- a/2019/16.cpp
+++ b/2019/16.cpp
@@ -11,17 +11,15 @@ using namespace aocpp;
 
 namespace {
 
-auto FFT(std::size_t const start, std::vector<std::int8_t> const& prev)
+auto FFT(std::size_t const start, std::vector<std::int32_t> & partial_sums, std::vector<std::int8_t> & signal)
 {
-  std::vector<std::int8_t> result;
-  std::vector<std::int32_t> partial_sums;
-  auto n = prev.size();
-  result.reserve(n);
+  auto n = signal.size();
+
+  partial_sums.clear();
   partial_sums.reserve(n+1);
   partial_sums.push_back(0);
-
   std::int32_t acc = 0;
-  for (auto const x : prev) {
+  for (auto const x : signal) {
     partial_sums.push_back(acc += x);
   }
 
@@ -29,12 +27,11 @@ auto FFT(std::size_t const start, std::vector<std::int8_t> const& prev)
     std::int32_t acc = 0;
     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)];
+      acc += partial_sums[std::min(n, offset + 1*step)] - partial_sums[std::min(n, offset + 0*step)]
+           + partial_sums[std::min(n, offset + 2*step)] - partial_sums[std::min(n, offset + 3*step)];
     }
-    result.push_back(std::abs(acc % 10));
+    signal[i] = std::abs(acc % 10);
   }
-  return result;
 }
 
 auto Parse(std::istream & in) -> std::vector<std::int8_t>
@@ -55,8 +52,9 @@ auto Compute(
   std::size_t offset = 0
 ) -> std::string
 {
+  std::vector<std::int32_t> partial_sums;
   for (std::size_t i = 0; i < iterations; i++) {
-    signal = FFT(offset, signal);
+    FFT(offset, partial_sums, signal);
   }
   signal.resize(8);
   std::string result;