fft.hpp

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/// @file spectral/fft.hpp
/// @brief FFT interface with backend dispatch.
///
/// Forward transform convention:
/// \f[
///   X_k=\sum_{j=0}^{n-1} x_j e^{-2\pi i jk/n}.
/// \f]
/// The inverse transform is unnormalized.
#pragma once
 
#include "core/types.hpp"
#include "core/vector.hpp"
 
namespace num {
namespace spectral {
 
enum class FFTBackend {
    seq,
    simd,
    stdsimd,
    fftw,
};
 
inline constexpr FFTBackend seq = FFTBackend::seq;
inline constexpr FFTBackend fftw = FFTBackend::fftw;
inline constexpr FFTBackend fft_simd = FFTBackend::simd;
inline constexpr FFTBackend fft_stdsimd = FFTBackend::stdsimd;
 
inline constexpr bool has_fftw =
#ifdef NUMERICS_HAS_FFTW
    true;
#else
    false;
#endif
 
inline constexpr bool has_fft_simd =
#if defined(NUMERICS_HAS_AVX2) || defined(NUMERICS_HAS_NEON)
    true;
#else
    false;
#endif
 
inline constexpr bool has_fft_stdsimd =
#ifdef NUMERICS_HAS_STD_SIMD
    true;
#else
    false;
#endif
 
inline constexpr FFTBackend default_fft_backend =
#ifdef NUMERICS_HAS_FFTW
    FFTBackend::fftw;
#elif defined(NUMERICS_HAS_AVX2) || defined(NUMERICS_HAS_NEON)
    FFTBackend::simd;
#else
                FFTBackend::seq;
#endif
 
void fft(const CVector& in, CVector& out, FFTBackend b = default_fft_backend);
 
void ifft(const CVector& in, CVector& out, FFTBackend b = default_fft_backend);
 
void rfft(const Vector& in, CVector& out, FFTBackend b = default_fft_backend);
 
void irfft(const CVector& in, int n, Vector& out, FFTBackend b = default_fft_backend);
 
/// @brief Precomputed complex transform plan.
class FFTPlan {
  public:
    explicit FFTPlan(int n, bool forward = true, FFTBackend b = default_fft_backend);
    ~FFTPlan();
 
    FFTPlan(const FFTPlan&) = delete;
    FFTPlan& operator=(const FFTPlan&) = delete;
 
    void execute(const CVector& in, CVector& out) const;
 
    int size() const { return n_; }
    FFTBackend backend() const { return backend_; }
 
  private:
    int n_;
    FFTBackend backend_;
    void* impl_;
};
 
} // namespace spectral
} // namespace num