Source: https://doc.qt.io/qt-6.11/qtwebengine-3rdparty-general-purpose-fft-fast-fourier-cosine-sine-transform-package.html

General Purpose FFT (Fast Fourier/Cosine/Sine Transform) Package

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/*
 * http://www.kurims.kyoto-u.ac.jp/~ooura/fft.html
 * Copyright Takuya OOURA, 1996-2001
 *
 * You may use, copy, modify and distribute this code for any purpose (include
 * commercial use) and without fee. Please refer to this package when you modify
 * this code.
 * /