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Partly simplify FFT bit-reversal

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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.
This commit is contained in:
Chris Robinson
2020-09-13 00:58:05 -07:00
parent 1d4b355622
commit 9a883f5046
1 changed files with 4 additions and 8 deletions
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common/alcomplex.cpp
+4 -8
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@@ -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++)
{