From 9a883f50462f22474601204b0052ca9ef4fce676 Mon Sep 17 00:00:00 2001
From: Chris Robinson <chris.kcat@gmail.com>
Date: Sun, 13 Sep 2020 00:58:05 -0700
Subject: [PATCH] Partly simplify FFT bit-reversal

This can almost certainly be improved further, as less than half of the indices
really need their reversed bit-pattern calculated and elements swapped (any
symetrical bit pattern would just swap with itself, and indices whose reversed
bit-pattern has already been traversed is already swapped).

It may also prove beneficial to provide the base-2 log of the fft buffer size
(number of bits to represent the indices), as that could help make the reversal
more efficient with a known bit/loop count.
---
 common/alcomplex.cpp | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/common/alcomplex.cpp b/common/alcomplex.cpp
index e5cbe8a0..0af9fc98 100644
--- a/common/alcomplex.cpp
+++ b/common/alcomplex.cpp
@@ -18,12 +18,8 @@ void complex_fft(const al::span<std::complex<double>> buffer, const double sign)
     for(size_t i{1u};i < fftsize-1;i++)
     {
         size_t j{0u};
-        for(size_t mask{1u};mask < fftsize;mask <<= 1)
-        {
-            if((i&mask) != 0)
-                j++;
-            j <<= 1;
-        }
+        for(size_t imask{i + fftsize};imask;imask >>= 1)
+            j = (j<<1) + (imask&1);
         j >>= 1;
 
         if(i < j)
@@ -35,9 +31,9 @@ void complex_fft(const al::span<std::complex<double>> buffer, const double sign)
     for(size_t i{1u};i < fftsize;i<<=1, step<<=1)
     {
         const size_t step2{step >> 1};
-        double arg{al::MathDefs<double>::Pi() / static_cast<double>(step2)};
+        const double arg{al::MathDefs<double>::Pi() / static_cast<double>(step2)};
 
-        std::complex<double> w{std::cos(arg), std::sin(arg)*sign};
+        const std::complex<double> w{std::cos(arg), std::sin(arg)*sign};
         std::complex<double> u{1.0, 0.0};
         for(size_t j{0};j < step2;j++)
         {