pocketfft_hdronly.h 83.6 KB
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/*
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This file is part of pocketfft.

Copyright (C) 2010-2019 Max-Planck-Society
Author: Martin Reinecke

All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright notice, this
  list of conditions and the following disclaimer in the documentation and/or
  other materials provided with the distribution.
* Neither the name of the copyright holder nor the names of its contributors may
  be used to endorse or promote products derived from this software without
  specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
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#ifndef POCKETFFT_HDRONLY_H
#define POCKETFFT_HDRONLY_H

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#ifndef __cplusplus
#error This file is C++ and requires a C++ compiler
#endif

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#if !(__cplusplus >= 201103L || _MSVC_LANG+0L >= 201103L)
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#error This file requires at least C++11 support
#endif

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#include <cmath>
#include <cstring>
#include <cstdlib>
#include <stdexcept>
#include <memory>
#include <vector>
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#include <complex>
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#if defined(_WIN32)
#include <malloc.h>
#endif
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#ifdef POCKETFFT_OPENMP
#include <omp.h>
#endif

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#if defined(__GNUC__)
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#define NOINLINE __attribute__((noinline))
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#define RESTRICT __restrict__
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#elif defined(_MSC_VER)
#define NOINLINE __declspec(noinline)
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#define RESTRICT __restrict
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#else
#define NOINLINE
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#define RESTRICT
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#endif

namespace pocketfft {

namespace detail {

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using namespace std;

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using shape_t = vector<size_t>;
using stride_t = vector<ptrdiff_t>;

constexpr bool FORWARD  = true,
               BACKWARD = false;

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// only enable vector support for gcc>=5.0 and clang>=5.0
#ifndef POCKETFFT_NO_VECTORS
#define POCKETFFT_NO_VECTORS
#if defined(__INTEL_COMPILER)
// do nothing. This is necessary because this compiler also sets __GNUC__
#elif defined(__clang__)
#if __clang__>=5
#undef POCKETFFT_NO_VECTORS
#endif
#elif defined(__GNUC__)
#if __GNUC__>=5
#undef POCKETFFT_NO_VECTORS
#endif
#endif
#endif

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template<typename T> struct VLEN { static constexpr size_t val=1; };

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#ifndef POCKETFFT_NO_VECTORS
#if (defined(__AVX512F__))
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template<> struct VLEN<float> { static constexpr size_t val=16; };
template<> struct VLEN<double> { static constexpr size_t val=8; };
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#elif (defined(__AVX__))
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template<> struct VLEN<float> { static constexpr size_t val=8; };
template<> struct VLEN<double> { static constexpr size_t val=4; };
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#elif (defined(__SSE2__))
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template<> struct VLEN<float> { static constexpr size_t val=4; };
template<> struct VLEN<double> { static constexpr size_t val=2; };
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#elif (defined(__VSX__))
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template<> struct VLEN<float> { static constexpr size_t val=4; };
template<> struct VLEN<double> { static constexpr size_t val=2; };
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#else
#define POCKETFFT_NO_VECTORS
#endif
#endif

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template<typename T> class arr
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  {
  private:
    T *p;
    size_t sz;

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#if defined(POCKETFFT_NO_VECTORS)
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    static T *ralloc(size_t num)
      {
      if (num==0) return nullptr;
      void *res = malloc(num*sizeof(T));
      if (!res) throw bad_alloc();
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      return reinterpret_cast<T *>(res);
      }
    static void dealloc(T *ptr)
      { free(ptr); }
#elif __cplusplus >= 201703L
    static T *ralloc(size_t num)
      {
      if (num==0) return nullptr;
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      void *res = aligned_alloc(64,num*sizeof(T));
      if (!res) throw bad_alloc();
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      return reinterpret_cast<T *>(res);
      }
    static void dealloc(T *ptr)
      { free(ptr); }
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#elif defined(_WIN32)
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    static T *ralloc(size_t num)
      {
      if (num==0) return nullptr;
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      void *res = _aligned_malloc(num*sizeof(T), 64);
      if (!res) throw bad_alloc();
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      return reinterpret_cast<T *>(res);
      }
    static void dealloc(T *ptr)
      { _aligned_free(ptr); }
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#else
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    static T *ralloc(size_t num)
      {
      if (num==0) return nullptr;
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      void *res(nullptr);
      if (posix_memalign(&res, 64, num*sizeof(T))!=0)
        throw bad_alloc();
      return reinterpret_cast<T *>(res);
      }
    static void dealloc(T *ptr)
      { free(ptr); }
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#endif
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  public:
    arr() : p(0), sz(0) {}
    arr(size_t n) : p(ralloc(n)), sz(n) {}
    arr(arr &&other)
      : p(other.p), sz(other.sz)
      { other.p=nullptr; other.sz=0; }
    ~arr() { dealloc(p); }

    void resize(size_t n)
      {
      if (n==sz) return;
      dealloc(p);
      p = ralloc(n);
      sz = n;
      }

    T &operator[](size_t idx) { return p[idx]; }
    const T &operator[](size_t idx) const { return p[idx]; }

    T *data() { return p; }
    const T *data() const { return p; }

    size_t size() const { return sz; }
  };

template<typename T> struct cmplx {
  T r, i;
  cmplx() {}
  cmplx(T r_, T i_) : r(r_), i(i_) {}
  void Set(T r_, T i_) { r=r_; i=i_; }
  void Set(T r_) { r=r_; i=T(0); }
  cmplx &operator+= (const cmplx &other)
    { r+=other.r; i+=other.i; return *this; }
  template<typename T2>cmplx &operator*= (T2 other)
    { r*=other; i*=other; return *this; }
  cmplx operator+ (const cmplx &other) const
    { return cmplx(r+other.r, i+other.i); }
  cmplx operator- (const cmplx &other) const
    { return cmplx(r-other.r, i-other.i); }
  template<typename T2> auto operator* (const T2 &other) const
    -> cmplx<decltype(r*other)>
    { return {r*other, i*other}; }
  template<typename T2> auto operator* (const cmplx<T2> &other) const
    -> cmplx<decltype(r+other.r)>
    { return {r*other.r-i*other.i, r*other.i + i*other.r}; }
  template<bool bwd, typename T2> auto special_mul (const cmplx<T2> &other) const
    -> cmplx<decltype(r+other.r)>
    {
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    using Tres = cmplx<decltype(r+other.r)>;
    return bwd ? Tres(r*other.r-i*other.i, r*other.i + i*other.r)
               : Tres(r*other.r+i*other.i, i*other.r - r*other.i);
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    }
};
template<typename T> void PMC(cmplx<T> &a, cmplx<T> &b,
  const cmplx<T> &c, const cmplx<T> &d)
  { a = c+d; b = c-d; }
template<typename T> cmplx<T> conj(const cmplx<T> &a)
  { return {a.r, -a.i}; }

template<typename T> void ROT90(cmplx<T> &a)
  { auto tmp_=a.r; a.r=-a.i; a.i=tmp_; }
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template<bool bwd, typename T> void ROTX90(cmplx<T> &a)
  { auto tmp_= bwd ? a.r : -a.r; a.r = bwd ? -a.i : a.i; a.i=tmp_; }
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//
// twiddle factor section
//

template<typename T> class sincos_2pibyn
  {
  private:
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    template<typename Ta, typename Tb, bool bigger> struct TypeSelector
      {};
    template<typename Ta, typename Tb> struct TypeSelector<Ta, Tb, true>
      { using type = Ta; };
    template<typename Ta, typename Tb> struct TypeSelector<Ta, Tb, false>
      { using type = Tb; };

    using Thigh = typename TypeSelector<T, double, (sizeof(T)>sizeof(double))>::type;
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    arr<T> data;

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    void my_sincosm1pi (Thigh a_, Thigh *RESTRICT res)
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      {
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      if (sizeof(Thigh)>sizeof(double)) // don't have the code for long double
        {
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        constexpr Thigh pi = Thigh(3.141592653589793238462643383279502884197L);
        auto s = sin(pi*a_);
        res[1] = s;
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        res[0] = (s*s)/(-sqrt((1-s)*(1+s))-1);
        return;
        }
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      // adapted from https://stackoverflow.com/questions/42792939/
      // CAUTION: this function only works for arguments in the range
      //          [-0.25; 0.25]!
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      double a = double(a_);
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      double s = a * a;
      /* Approximate cos(pi*x)-1 for x in [-0.25,0.25] */
      double r =     -1.0369917389758117e-4;
      r = fma (r, s,  1.9294935641298806e-3);
      r = fma (r, s, -2.5806887942825395e-2);
      r = fma (r, s,  2.3533063028328211e-1);
      r = fma (r, s, -1.3352627688538006e+0);
      r = fma (r, s,  4.0587121264167623e+0);
      r = fma (r, s, -4.9348022005446790e+0);
      double c = r*s;
      /* Approximate sin(pi*x) for x in [-0.25,0.25] */
      r =             4.6151442520157035e-4;
      r = fma (r, s, -7.3700183130883555e-3);
      r = fma (r, s,  8.2145868949323936e-2);
      r = fma (r, s, -5.9926452893214921e-1);
      r = fma (r, s,  2.5501640398732688e+0);
      r = fma (r, s, -5.1677127800499516e+0);
      s = s * a;
      r = r * s;
      s = fma (a, 3.1415926535897931e+0, r);
      res[0] = c;
      res[1] = s;
      }

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    NOINLINE void calc_first_octant(size_t den, T * RESTRICT res)
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      {
      size_t n = (den+4)>>3;
      if (n==0) return;
      res[0]=1.; res[1]=0.;
      if (n==1) return;
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      size_t l1 = size_t(sqrt(n));
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      arr<Thigh> tmp(2*l1);
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      for (size_t i=1; i<l1; ++i)
        {
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        my_sincosm1pi(Thigh(2*i)/Thigh(den),&tmp[2*i]);
        res[2*i  ] = T(tmp[2*i]+1);
        res[2*i+1] = T(tmp[2*i+1]);
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        }
      size_t start=l1;
      while(start<n)
        {
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        Thigh cs[2];
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        my_sincosm1pi((Thigh(2*start))/Thigh(den),cs);
        res[2*start] = T(cs[0]+1);
        res[2*start+1] = T(cs[1]);
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        size_t end = l1;
        if (start+end>n) end = n-start;
        for (size_t i=1; i<end; ++i)
          {
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          Thigh csx[2]={tmp[2*i], tmp[2*i+1]};
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          res[2*(start+i)] = T(((cs[0]*csx[0] - cs[1]*csx[1] + cs[0]) + csx[0]) + 1);
          res[2*(start+i)+1] = T((cs[0]*csx[1] + cs[1]*csx[0]) + cs[1] + csx[1]);
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          }
        start += l1;
        }
      }

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    void calc_first_quadrant(size_t n, T * RESTRICT res)
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      {
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      T * RESTRICT p = res+n;
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      calc_first_octant(n<<1, p);
      size_t ndone=(n+2)>>2;
      size_t i=0, idx1=0, idx2=2*ndone-2;
      for (; i+1<ndone; i+=2, idx1+=2, idx2-=2)
        {
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        res[idx1] = p[2*i  ]; res[idx1+1] = p[2*i+1];
        res[idx2] = p[2*i+3]; res[idx2+1] = p[2*i+2];
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        }
      if (i!=ndone)
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        { res[idx1] = p[2*i]; res[idx1+1] = p[2*i+1]; }
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      }

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    void calc_first_half(size_t n, T * RESTRICT res)
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      {
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      int ndone=int(n+1)>>1;
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      T * p = res+n-1;
      calc_first_octant(n<<2, p);
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      int i4=0, in=int(n), i=0;
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      for (; i4<=in-i4; ++i, i4+=4) // octant 0
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        { res[2*i] = p[2*i4]; res[2*i+1] = p[2*i4+1]; }
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      for (; i4-in <= 0; ++i, i4+=4) // octant 1
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        { auto xm = in-i4; res[2*i] = p[2*xm+1]; res[2*i+1] = p[2*xm]; }
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      for (; i4<=3*in-i4; ++i, i4+=4) // octant 2
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        { auto xm = i4-in; res[2*i] = -p[2*xm+1]; res[2*i+1] = p[2*xm]; }
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      for (; i<ndone; ++i, i4+=4) // octant 3
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        { auto xm = 2*in-i4; res[2*i] = -p[2*xm]; res[2*i+1] = p[2*xm+1]; }
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      }

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    void fill_first_quadrant(size_t n, T * RESTRICT res)
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      {
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      constexpr T hsqt2 = T(0.707106781186547524400844362104849L);
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      size_t quart = n>>2;
      if ((n&7)==0)
        res[quart] = res[quart+1] = hsqt2;
      for (size_t i=2, j=2*quart-2; i<quart; i+=2, j-=2)
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        { res[j] = res[i+1]; res[j+1] = res[i]; }
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      }

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    NOINLINE void fill_first_half(size_t n, T * RESTRICT res)
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      {
      size_t half = n>>1;
      if ((n&3)==0)
        for (size_t i=0; i<half; i+=2)
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          { res[i+half] = -res[i+1]; res[i+half+1] = res[i]; }
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      else
        for (size_t i=2, j=2*half-2; i<half; i+=2, j-=2)
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          { res[j] = -res[i]; res[j+1] = res[i+1]; }
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      }

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    void fill_second_half(size_t n, T * RESTRICT res)
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      {
      if ((n&1)==0)
        for (size_t i=0; i<n; ++i)
          res[i+n] = -res[i];
      else
        for (size_t i=2, j=2*n-2; i<n; i+=2, j-=2)
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          { res[j] = res[i]; res[j+1] = -res[i+1]; }
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      }

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    NOINLINE void sincos_2pibyn_half(size_t n, T * RESTRICT res)
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      {
      if ((n&3)==0)
        {
        calc_first_octant(n, res);
        fill_first_quadrant(n, res);
        fill_first_half(n, res);
        }
      else if ((n&1)==0)
        {
        calc_first_quadrant(n, res);
        fill_first_half(n, res);
        }
      else
        calc_first_half(n, res);
      }

  public:
    NOINLINE sincos_2pibyn(size_t n, bool half)
      : data(2*n)
      {
      sincos_2pibyn_half(n, data.data());
      if (!half) fill_second_half(n, data.data());
      }

    T operator[](size_t idx) const { return data[idx]; }
    const T *rdata() const { return data; }
    const cmplx<T> *cdata() const
      { return reinterpret_cast<const cmplx<T> *>(data.data()); }
  };

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struct util // hack to avoid duplicate symbols
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  {
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  static NOINLINE size_t largest_prime_factor (size_t n)
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    {
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    size_t res=1;
    while ((n&1)==0)
      { res=2; n>>=1; }
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    size_t limit=size_t(sqrt(double(n)+0.01));
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    for (size_t x=3; x<=limit; x+=2)
    while ((n/x)*x==n)
      {
      res=x;
      n/=x;
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      limit=size_t(sqrt(double(n)+0.01));
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      }
    if (n>1) res=n;
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    return res;
    }

  static NOINLINE double cost_guess (size_t n)
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    {
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    constexpr double lfp=1.1; // penalty for non-hardcoded larger factors
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    size_t ni=n;
    double result=0.;
    while ((n&1)==0)
      { result+=2; n>>=1; }

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    size_t limit=size_t(sqrt(double(n)+0.01));
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    for (size_t x=3; x<=limit; x+=2)
    while ((n/x)*x==n)
      {
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      result+= (x<=5) ? double(x) : lfp*double(x); // penalize larger prime factors
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      n/=x;
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      limit=size_t(sqrt(double(n)+0.01));
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      }
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    if (n>1) result+=(n<=5) ? double(n) : lfp*double(n);
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    return result*double(ni);
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    }

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  /* returns the smallest composite of 2, 3, 5, 7 and 11 which is >= n */
  static NOINLINE size_t good_size(size_t n)
    {
    if (n<=12) return n;

    size_t bestfac=2*n;
    for (size_t f2=1; f2<bestfac; f2*=2)
      for (size_t f23=f2; f23<bestfac; f23*=3)
        for (size_t f235=f23; f235<bestfac; f235*=5)
          for (size_t f2357=f235; f2357<bestfac; f2357*=7)
            for (size_t f235711=f2357; f235711<bestfac; f235711*=11)
              if (f235711>=n) bestfac=f235711;
    return bestfac;
    }
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  static size_t prod(const shape_t &shape)
    {
    size_t res=1;
    for (auto sz: shape)
      res*=sz;
    return res;
    }
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  static NOINLINE void sanity_check(const shape_t &shape,
    const stride_t &stride_in, const stride_t &stride_out, bool inplace)
    {
    auto ndim = shape.size();
    if (ndim<1) throw runtime_error("ndim must be >= 1");
    if ((stride_in.size()!=ndim) || (stride_out.size()!=ndim))
      throw runtime_error("stride dimension mismatch");
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    if (inplace && (stride_in!=stride_out))
      throw runtime_error("stride mismatch");
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    }

  static NOINLINE void sanity_check(const shape_t &shape,
    const stride_t &stride_in, const stride_t &stride_out, bool inplace,
    const shape_t &axes)
    {
    sanity_check(shape, stride_in, stride_out, inplace);
    auto ndim = shape.size();
    shape_t tmp(ndim,0);
    for (auto ax : axes)
      {
      if (ax>=ndim) throw runtime_error("bad axis number");
      if (++tmp[ax]>1) throw runtime_error("axis specified repeatedly");
      }
    }

  static NOINLINE void sanity_check(const shape_t &shape,
    const stride_t &stride_in, const stride_t &stride_out, bool inplace,
    size_t axis)
    {
    sanity_check(shape, stride_in, stride_out, inplace);
    if (axis>=shape.size()) throw runtime_error("bad axis number");
    }
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#ifdef POCKETFFT_OPENMP
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    static size_t nthreads() { return size_t(omp_get_num_threads()); }
    static size_t thread_num() { return size_t(omp_get_thread_num()); }
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    static size_t thread_count (size_t nthreads, const shape_t &shape,
      size_t axis)
      {
      if (nthreads==1) return 1;
      if (prod(shape)/shape[axis] < 20) return 1;
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      return (nthreads==0) ? size_t(omp_get_max_threads()) : nthreads;
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      }
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#else
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    static size_t nthreads() { return 1; }
    static size_t thread_num() { return 0; }
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#endif
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  };
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#define CH(a,b,c) ch[(a)+ido*((b)+l1*(c))]
#define CC(a,b,c) cc[(a)+ido*((b)+cdim*(c))]
#define WA(x,i) wa[(i)-1+(x)*(ido-1)]

//
// complex FFTPACK transforms
//

template<typename T0> class cfftp
  {
  private:
    struct fctdata
      {
      size_t fct;
      cmplx<T0> *tw, *tws;
      };

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    size_t length;
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    arr<cmplx<T0>> mem;
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    vector<fctdata> fact;
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    void add_factor(size_t factor)
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      { fact.push_back({factor, nullptr, nullptr}); }
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template<bool bwd, typename T> void pass2 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
  constexpr size_t cdim=2;

  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      CH(0,k,0) = CC(0,0,k)+CC(0,1,k);
      CH(0,k,1) = CC(0,0,k)-CC(0,1,k);
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      CH(0,k,0) = CC(0,0,k)+CC(0,1,k);
      CH(0,k,1) = CC(0,0,k)-CC(0,1,k);
      for (size_t i=1; i<ido; ++i)
        {
        CH(i,k,0) = CC(i,0,k)+CC(i,1,k);
        CH(i,k,1) = (CC(i,0,k)-CC(i,1,k)).template special_mul<bwd>(WA(0,i));
        }
      }
  }

#define PREP3(idx) \
        T t0 = CC(idx,0,k), t1, t2; \
        PMC (t1,t2,CC(idx,1,k),CC(idx,2,k)); \
        CH(idx,k,0)=t0+t1;
#define PARTSTEP3a(u1,u2,twr,twi) \
        { \
        T ca,cb; \
        ca=t0+t1*twr; \
        cb=t2*twi; ROT90(cb); \
        PMC(CH(0,k,u1),CH(0,k,u2),ca,cb) ;\
        }
#define PARTSTEP3b(u1,u2,twr,twi) \
        { \
        T ca,cb,da,db; \
        ca=t0+t1*twr; \
        cb=t2*twi; ROT90(cb); \
        PMC(da,db,ca,cb); \
        CH(i,k,u1) = da.template special_mul<bwd>(WA(u1-1,i)); \
        CH(i,k,u2) = db.template special_mul<bwd>(WA(u2-1,i)); \
        }
template<bool bwd, typename T> void pass3 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
  constexpr size_t cdim=3;
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  constexpr T0 tw1r=-0.5,
               tw1i= (bwd ? 1: -1) * T0(0.8660254037844386467637231707529362L);
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  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      PREP3(0)
      PARTSTEP3a(1,2,tw1r,tw1i)
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      {
      PREP3(0)
      PARTSTEP3a(1,2,tw1r,tw1i)
      }
      for (size_t i=1; i<ido; ++i)
        {
        PREP3(i)
        PARTSTEP3b(1,2,tw1r,tw1i)
        }
      }
  }

#undef PARTSTEP3b
#undef PARTSTEP3a
#undef PREP3

template<bool bwd, typename T> void pass4 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
  constexpr size_t cdim=4;

  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      T t1, t2, t3, t4;
      PMC(t2,t1,CC(0,0,k),CC(0,2,k));
      PMC(t3,t4,CC(0,1,k),CC(0,3,k));
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      ROTX90<bwd>(t4);
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      PMC(CH(0,k,0),CH(0,k,2),t2,t3);
      PMC(CH(0,k,1),CH(0,k,3),t1,t4);
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      {
      T t1, t2, t3, t4;
      PMC(t2,t1,CC(0,0,k),CC(0,2,k));
      PMC(t3,t4,CC(0,1,k),CC(0,3,k));
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      ROTX90<bwd>(t4);
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      PMC(CH(0,k,0),CH(0,k,2),t2,t3);
      PMC(CH(0,k,1),CH(0,k,3),t1,t4);
      }
      for (size_t i=1; i<ido; ++i)
        {
        T c2, c3, c4, t1, t2, t3, t4;
        T cc0=CC(i,0,k), cc1=CC(i,1,k),cc2=CC(i,2,k),cc3=CC(i,3,k);
        PMC(t2,t1,cc0,cc2);
        PMC(t3,t4,cc1,cc3);
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        ROTX90<bwd>(t4);
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        PMC(CH(i,k,0),c3,t2,t3);
        PMC(c2,c4,t1,t4);
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        CH(i,k,1) = c2.template special_mul<bwd>(WA(0,i));
        CH(i,k,2) = c3.template special_mul<bwd>(WA(1,i));
        CH(i,k,3) = c4.template special_mul<bwd>(WA(2,i));
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        }
      }
  }

#define PREP5(idx) \
        T t0 = CC(idx,0,k), t1, t2, t3, t4; \
        PMC (t1,t4,CC(idx,1,k),CC(idx,4,k)); \
        PMC (t2,t3,CC(idx,2,k),CC(idx,3,k)); \
        CH(idx,k,0).r=t0.r+t1.r+t2.r; \
        CH(idx,k,0).i=t0.i+t1.i+t2.i;

#define PARTSTEP5a(u1,u2,twar,twbr,twai,twbi) \
        { \
        T ca,cb; \
        ca.r=t0.r+twar*t1.r+twbr*t2.r; \
        ca.i=t0.i+twar*t1.i+twbr*t2.i; \
        cb.i=twai*t4.r twbi*t3.r; \
        cb.r=-(twai*t4.i twbi*t3.i); \
        PMC(CH(0,k,u1),CH(0,k,u2),ca,cb); \
        }

#define PARTSTEP5b(u1,u2,twar,twbr,twai,twbi) \
        { \
        T ca,cb,da,db; \
        ca.r=t0.r+twar*t1.r+twbr*t2.r; \
        ca.i=t0.i+twar*t1.i+twbr*t2.i; \
        cb.i=twai*t4.r twbi*t3.r; \
        cb.r=-(twai*t4.i twbi*t3.i); \
        PMC(da,db,ca,cb); \
        CH(i,k,u1) = da.template special_mul<bwd>(WA(u1-1,i)); \
        CH(i,k,u2) = db.template special_mul<bwd>(WA(u2-1,i)); \
        }
template<bool bwd, typename T> void pass5 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
  constexpr size_t cdim=5;
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  constexpr T0 tw1r= T0(0.3090169943749474241022934171828191L),
               tw1i= (bwd ? 1: -1) * T0(0.9510565162951535721164393333793821L),
               tw2r= T0(-0.8090169943749474241022934171828191L),
               tw2i= (bwd ? 1: -1) * T0(0.5877852522924731291687059546390728L);
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  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      PREP5(0)
      PARTSTEP5a(1,4,tw1r,tw2r,+tw1i,+tw2i)
      PARTSTEP5a(2,3,tw2r,tw1r,+tw2i,-tw1i)
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      {
      PREP5(0)
      PARTSTEP5a(1,4,tw1r,tw2r,+tw1i,+tw2i)
      PARTSTEP5a(2,3,tw2r,tw1r,+tw2i,-tw1i)
      }
      for (size_t i=1; i<ido; ++i)
        {
        PREP5(i)
        PARTSTEP5b(1,4,tw1r,tw2r,+tw1i,+tw2i)
        PARTSTEP5b(2,3,tw2r,tw1r,+tw2i,-tw1i)
        }
      }
  }

#undef PARTSTEP5b
#undef PARTSTEP5a
#undef PREP5

#define PREP7(idx) \
        T t1 = CC(idx,0,k), t2, t3, t4, t5, t6, t7; \
        PMC (t2,t7,CC(idx,1,k),CC(idx,6,k)); \
        PMC (t3,t6,CC(idx,2,k),CC(idx,5,k)); \
        PMC (t4,t5,CC(idx,3,k),CC(idx,4,k)); \
        CH(idx,k,0).r=t1.r+t2.r+t3.r+t4.r; \
        CH(idx,k,0).i=t1.i+t2.i+t3.i+t4.i;

#define PARTSTEP7a0(u1,u2,x1,x2,x3,y1,y2,y3,out1,out2) \
        { \
        T ca,cb; \
        ca.r=t1.r+x1*t2.r+x2*t3.r+x3*t4.r; \
        ca.i=t1.i+x1*t2.i+x2*t3.i+x3*t4.i; \
        cb.i=y1*t7.r y2*t6.r y3*t5.r; \
        cb.r=-(y1*t7.i y2*t6.i y3*t5.i); \
        PMC(out1,out2,ca,cb); \
        }
#define PARTSTEP7a(u1,u2,x1,x2,x3,y1,y2,y3) \
        PARTSTEP7a0(u1,u2,x1,x2,x3,y1,y2,y3,CH(0,k,u1),CH(0,k,u2))
#define PARTSTEP7(u1,u2,x1,x2,x3,y1,y2,y3) \
        { \
        T da,db; \
        PARTSTEP7a0(u1,u2,x1,x2,x3,y1,y2,y3,da,db) \
        CH(i,k,u1) = da.template special_mul<bwd>(WA(u1-1,i)); \
        CH(i,k,u2) = db.template special_mul<bwd>(WA(u2-1,i)); \
        }

template<bool bwd, typename T> void pass7(size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
  constexpr size_t cdim=7;
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  constexpr T0 tw1r= T0(0.6234898018587335305250048840042398L),
               tw1i= (bwd ? 1 : -1) * T0(0.7818314824680298087084445266740578L),
               tw2r= T0(-0.2225209339563144042889025644967948L),
               tw2i= (bwd ? 1 : -1) * T0(0.9749279121818236070181316829939312L),
               tw3r= T0(-0.9009688679024191262361023195074451L),
               tw3i= (bwd ? 1 : -1) * T0(0.433883739117558120475768332848359L);
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  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      PREP7(0)
      PARTSTEP7a(1,6,tw1r,tw2r,tw3r,+tw1i,+tw2i,+tw3i)
      PARTSTEP7a(2,5,tw2r,tw3r,tw1r,+tw2i,-tw3i,-tw1i)
      PARTSTEP7a(3,4,tw3r,tw1r,tw2r,+tw3i,-tw1i,+tw2i)
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      {
      PREP7(0)
      PARTSTEP7a(1,6,tw1r,tw2r,tw3r,+tw1i,+tw2i,+tw3i)
      PARTSTEP7a(2,5,tw2r,tw3r,tw1r,+tw2i,-tw3i,-tw1i)
      PARTSTEP7a(3,4,tw3r,tw1r,tw2r,+tw3i,-tw1i,+tw2i)
      }
      for (size_t i=1; i<ido; ++i)
        {
        PREP7(i)
        PARTSTEP7(1,6,tw1r,tw2r,tw3r,+tw1i,+tw2i,+tw3i)
        PARTSTEP7(2,5,tw2r,tw3r,tw1r,+tw2i,-tw3i,-tw1i)
        PARTSTEP7(3,4,tw3r,tw1r,tw2r,+tw3i,-tw1i,+tw2i)
        }
      }
  }

#undef PARTSTEP7
#undef PARTSTEP7a0
#undef PARTSTEP7a
#undef PREP7

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template <bool bwd, typename T> void ROTX45(T &a)
  {
  constexpr T0 hsqt2=T0(0.707106781186547524400844362104849L);
  if (bwd)
    { auto tmp_=a.r; a.r=hsqt2*(a.r-a.i); a.i=hsqt2*(a.i+tmp_); }
  else
    { auto tmp_=a.r; a.r=hsqt2*(a.r+a.i); a.i=hsqt2*(a.i-tmp_); }
  }
template <bool bwd, typename T> void ROTX135(T &a)
  {
  constexpr T0 hsqt2=T0(0.707106781186547524400844362104849L);
  if (bwd)
    { auto tmp_=a.r; a.r=hsqt2*(-a.r-a.i); a.i=hsqt2*(tmp_-a.i); }
  else
    { auto tmp_=a.r; a.r=hsqt2*(a.i-a.r); a.i=hsqt2*(-tmp_-a.i); }
  }
template<typename T> inline void PMINPLACE(T &a, T &b)
  { T t = a; a.r+=b.r; a.i+=b.i; b.r=t.r-b.r; b.i=t.i-b.i; }

template<bool bwd, typename T> void pass8 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
  constexpr size_t cdim=8;

  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      T a0, a1, a2, a3, a4, a5, a6, a7;
      PMC(a0,a4,CC(0,0,k),CC(0,4,k));
      PMC(a1,a5,CC(0,1,k),CC(0,5,k));
      PMC(a2,a6,CC(0,2,k),CC(0,6,k));
      PMC(a3,a7,CC(0,3,k),CC(0,7,k));
      ROTX90<bwd>(a6);
      ROTX90<bwd>(a7);
      PMINPLACE(a0,a2);
      PMINPLACE(a1,a3);
      PMINPLACE(a4,a6);
      PMINPLACE(a5,a7);
      ROTX45<bwd>(a5);
      ROTX90<bwd>(a3);
      ROTX135<bwd>(a7);
      PMC(CH(0,k,0),CH(0,k,4),a0,a1);
      PMC(CH(0,k,1),CH(0,k,5),a4,a5);
      PMC(CH(0,k,2),CH(0,k,6),a2,a3);
      PMC(CH(0,k,3),CH(0,k,7),a6,a7);
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      T a0, a1, a2, a3, a4, a5, a6, a7;
      PMC(a0,a4,CC(0,0,k),CC(0,4,k));
      PMC(a1,a5,CC(0,1,k),CC(0,5,k));
      PMC(a2,a6,CC(0,2,k),CC(0,6,k));
      PMC(a3,a7,CC(0,3,k),CC(0,7,k));
      ROTX90<bwd>(a6);
      ROTX90<bwd>(a7);
      PMINPLACE(a0,a2);
      PMINPLACE(a1,a3);
      PMINPLACE(a4,a6);
      PMINPLACE(a5,a7);
      ROTX45<bwd>(a5);
      ROTX90<bwd>(a3);
      ROTX135<bwd>(a7);
      PMC(CH(0,k,0),CH(0,k,4),a0,a1);
      PMC(CH(0,k,1),CH(0,k,5),a4,a5);
      PMC(CH(0,k,2),CH(0,k,6),a2,a3);
      PMC(CH(0,k,3),CH(0,k,7),a6,a7);

      for (size_t i=1; i<ido; ++i)
        {
        T a0, a1, a2, a3, a4, a5, a6, a7;
        PMC(a0,a4,CC(i,0,k),CC(i,4,k));
        PMC(a1,a5,CC(i,1,k),CC(i,5,k));
        PMC(a2,a6,CC(i,2,k),CC(i,6,k));
        PMC(a3,a7,CC(i,3,k),CC(i,7,k));
        ROTX90<bwd>(a6);
        ROTX90<bwd>(a7);
        PMINPLACE(a0,a2);
        PMINPLACE(a1,a3);
        PMINPLACE(a4,a6);
        PMINPLACE(a5,a7);
        ROTX45<bwd>(a5);
        ROTX90<bwd>(a3);
        ROTX135<bwd>(a7);
        PMINPLACE(a0,a1);
        PMINPLACE(a2,a3);
        PMINPLACE(a4,a5);
        PMINPLACE(a6,a7);
        CH(i,k,0) = a0;
        CH(i,k,1) = a4.template special_mul<bwd>(WA(0,i));
        CH(i,k,2) = a2.template special_mul<bwd>(WA(1,i));
        CH(i,k,3) = a6.template special_mul<bwd>(WA(2,i));
        CH(i,k,4) = a1.template special_mul<bwd>(WA(3,i));
        CH(i,k,5) = a5.template special_mul<bwd>(WA(4,i));
        CH(i,k,6) = a3.template special_mul<bwd>(WA(5,i));
        CH(i,k,7) = a7.template special_mul<bwd>(WA(6,i));
        }
      }
   }


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#define PREP11(idx) \
        T t1 = CC(idx,0,k), t2, t3, t4, t5, t6, t7, t8, t9, t10, t11; \
        PMC (t2,t11,CC(idx,1,k),CC(idx,10,k)); \
        PMC (t3,t10,CC(idx,2,k),CC(idx, 9,k)); \
        PMC (t4,t9 ,CC(idx,3,k),CC(idx, 8,k)); \
        PMC (t5,t8 ,CC(idx,4,k),CC(idx, 7,k)); \
        PMC (t6,t7 ,CC(idx,5,k),CC(idx, 6,k)); \
        CH(idx,k,0).r=t1.r+t2.r+t3.r+t4.r+t5.r+t6.r; \
        CH(idx,k,0).i=t1.i+t2.i+t3.i+t4.i+t5.i+t6.i;

#define PARTSTEP11a0(u1,u2,x1,x2,x3,x4,x5,y1,y2,y3,y4,y5,out1,out2) \
        { \
        T ca = t1 + t2*x1 + t3*x2 + t4*x3 + t5*x4 +t6*x5, \
          cb; \
        cb.i=y1*t11.r y2*t10.r y3*t9.r y4*t8.r y5*t7.r; \
        cb.r=-(y1*t11.i y2*t10.i y3*t9.i y4*t8.i y5*t7.i ); \
        PMC(out1,out2,ca,cb); \
        }
#define PARTSTEP11a(u1,u2,x1,x2,x3,x4,x5,y1,y2,y3,y4,y5) \
        PARTSTEP11a0(u1,u2,x1,x2,x3,x4,x5,y1,y2,y3,y4,y5,CH(0,k,u1),CH(0,k,u2))
#define PARTSTEP11(u1,u2,x1,x2,x3,x4,x5,y1,y2,y3,y4,y5) \
        { \
        T da,db; \
        PARTSTEP11a0(u1,u2,x1,x2,x3,x4,x5,y1,y2,y3,y4,y5,da,db) \
        CH(i,k,u1) = da.template special_mul<bwd>(WA(u1-1,i)); \
        CH(i,k,u2) = db.template special_mul<bwd>(WA(u2-1,i)); \
        }

template<bool bwd, typename T> void pass11 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa)
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  {
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  constexpr size_t cdim=11;
  constexpr T0 tw1r= T0(0.8412535328311811688618116489193677L),
               tw1i= (bwd ? 1 : -1) * T0(0.5406408174555975821076359543186917L),
               tw2r= T0(0.4154150130018864255292741492296232L),
               tw2i= (bwd ? 1 : -1) * T0(0.9096319953545183714117153830790285L),
               tw3r= T0(-0.1423148382732851404437926686163697L),
               tw3i= (bwd ? 1 : -1) * T0(0.9898214418809327323760920377767188L),
               tw4r= T0(-0.6548607339452850640569250724662936L),
               tw4i= (bwd ? 1 : -1) * T0(0.7557495743542582837740358439723444L),
               tw5r= T0(-0.9594929736144973898903680570663277L),
               tw5i= (bwd ? 1 : -1) * T0(0.2817325568414296977114179153466169L);
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  if (ido==1)
    for (size_t k=0; k<l1; ++k)
      {
      PREP11(0)
      PARTSTEP11a(1,10,tw1r,tw2r,tw3r,tw4r,tw5r,+tw1i,+tw2i,+tw3i,+tw4i,+tw5i)
      PARTSTEP11a(2, 9,tw2r,tw4r,tw5r,tw3r,tw1r,+tw2i,+tw4i,-tw5i,-tw3i,-tw1i)
      PARTSTEP11a(3, 8,tw3r,tw5r,tw2r,tw1r,tw4r,+tw3i,-tw5i,-tw2i,+tw1i,+tw4i)
      PARTSTEP11a(4, 7,tw4r,tw3r,tw1r,tw5r,tw2r,+tw4i,-tw3i,+tw1i,+tw5i,-tw2i)
      PARTSTEP11a(5, 6,tw5r,tw1r,tw4r,tw2r,tw3r,+tw5i,-tw1i,+tw4i,-tw2i,+tw3i)
      }
  else
    for (size_t k=0; k<l1; ++k)
      {
      {
      PREP11(0)
      PARTSTEP11a(1,10,tw1r,tw2r,tw3r,tw4r,tw5r,+tw1i,+tw2i,+tw3i,+tw4i,+tw5i)
      PARTSTEP11a(2, 9,tw2r,tw4r,tw5r,tw3r,tw1r,+tw2i,+tw4i,-tw5i,-tw3i,-tw1i)
      PARTSTEP11a(3, 8,tw3r,tw5r,tw2r,tw1r,tw4r,+tw3i,-tw5i,-tw2i,+tw1i,+tw4i)
      PARTSTEP11a(4, 7,tw4r,tw3r,tw1r,tw5r,tw2r,+tw4i,-tw3i,+tw1i,+tw5i,-tw2i)
      PARTSTEP11a(5, 6,tw5r,tw1r,tw4r,tw2r,tw3r,+tw5i,-tw1i,+tw4i,-tw2i,+tw3i)
      }
      for (size_t i=1; i<ido; ++i)
        {
        PREP11(i)
        PARTSTEP11(1,10,tw1r,tw2r,tw3r,tw4r,tw5r,+tw1i,+tw2i,+tw3i,+tw4i,+tw5i)
        PARTSTEP11(2, 9,tw2r,tw4r,tw5r,tw3r,tw1r,+tw2i,+tw4i,-tw5i,-tw3i,-tw1i)
        PARTSTEP11(3, 8,tw3r,tw5r,tw2r,tw1r,tw4r,+tw3i,-tw5i,-tw2i,+tw1i,+tw4i)
        PARTSTEP11(4, 7,tw4r,tw3r,tw1r,tw5r,tw2r,+tw4i,-tw3i,+tw1i,+tw5i,-tw2i)
        PARTSTEP11(5, 6,tw5r,tw1r,tw4r,tw2r,tw3r,+tw5i,-tw1i,+tw4i,-tw2i,+tw3i)
        }
      }
  }

#undef PARTSTEP11
#undef PARTSTEP11a0
#undef PARTSTEP11a
#undef PREP11

#define CX(a,b,c) cc[(a)+ido*((b)+l1*(c))]
#define CX2(a,b) cc[(a)+idl1*(b)]
#define CH2(a,b) ch[(a)+idl1*(b)]

template<bool bwd, typename T> void passg (size_t ido, size_t ip,
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  size_t l1, T * RESTRICT cc, T * RESTRICT ch, const cmplx<T0> * RESTRICT wa,
  const cmplx<T0> * RESTRICT csarr)
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  {
  const size_t cdim=ip;
  size_t ipph = (ip+1)/2;
  size_t idl1 = ido*l1;

  arr<cmplx<T0>> wal(ip);
  wal[0] = cmplx<T0>(1., 0.);
  for (size_t i=1; i<ip; ++i)
    wal[i]=cmplx<T0>(csarr[i].r,bwd ? csarr[i].i : -csarr[i].i);

  for (size_t k=0; k<l1; ++k)
    for (size_t i=0; i<ido; ++i)
      CH(i,k,0) = CC(i,0,k);
  for (size_t j=1, jc=ip-1; j<ipph; ++j, --jc)
    for (size_t k=0; k<l1; ++k)
      for (size_t i=0; i<ido; ++i)
        PMC(CH(i,k,j),CH(i,k,jc),CC(i,j,k),CC(i,jc,k));
  for (size_t k=0; k<l1; ++k)
    for (size_t i=0; i<ido; ++i)
      {
      T tmp = CH(i,k,0);
      for (size_t j=1; j<ipph; ++j)
        tmp+=CH(i,k,j);
      CX(i,k,0) = tmp;
      }
  for (size_t l=1, lc=ip-1; l<ipph; ++l, --lc)
    {
    // j=0
    for (size_t ik=0; ik<idl1; ++ik)
      {
      CX2(ik,l).r = CH2(ik,0).r+wal[l].r*CH2(ik,1).r+wal[2*l].r*CH2(ik,2).r;
      CX2(ik,l).i = CH2(ik,0).i+wal[l].r*CH2(ik,1).i+wal[2*l].r*CH2(ik,2).i;
      CX2(ik,lc).r=-wal[l].i*CH2(ik,ip-1).i-wal[2*l].i*CH2(ik,ip-2).i;
      CX2(ik,lc).i=wal[l].i*CH2(ik,ip-1).r+wal[2*l].i*CH2(ik,ip-2).r;
      }

    size_t iwal=2*l;
    size_t j=3, jc=ip-3;
    for (; j<ipph-1; j+=2, jc-=2)
      {
      iwal+=l; if (iwal>ip) iwal-=ip;
      cmplx<T0> xwal=wal[iwal];
      iwal+=l; if (iwal>ip) iwal-=ip;
      cmplx<T0> xwal2=wal[iwal];
      for (size_t ik=0; ik<idl1; ++ik)
        {
        CX2(ik,l).r += CH2(ik,j).r*xwal.r+CH2(ik,j+1).r*xwal2.r;
        CX2(ik,l).i += CH2(ik,j).i*xwal.r+CH2(ik,j+1).i*xwal2.r;
        CX2(ik,lc).r -= CH2(ik,jc).i*xwal.i+CH2(ik,jc-1).i*xwal2.i;
        CX2(ik,lc).i += CH2(ik,jc).r*xwal.i+CH2(ik,jc-1).r*xwal2.i;
        }
      }
    for (; j<ipph; ++j, --jc)
      {
      iwal+=l; if (iwal>ip) iwal-=ip;
      cmplx<T0> xwal=wal[iwal];
      for (size_t ik=0; ik<idl1; ++ik)
        {
        CX2(ik,l).r += CH2(ik,j).r*xwal.r;
        CX2(ik,l).i += CH2(ik,j).i*xwal.r;
        CX2(ik,lc).r -= CH2(ik,jc).i*xwal.i;
        CX2(ik,lc).i += CH2(ik,jc).r*xwal.i;
        }
      }
    }

  // shuffling and twiddling
  if (ido==1)
    for (size_t j=1, jc=ip-1; j<ipph; ++j, --jc)
      for (size_t ik=0; ik<idl1; ++ik)
        {
        T t1=CX2(ik,j), t2=CX2(ik,jc);
        PMC(CX2(ik,j),CX2(ik,jc),t1,t2);
        }
  else
    {
    for (size_t j=1, jc=ip-1; j<ipph; ++j,--jc)
      for (size_t k=0; k<l1; ++k)
        {
        T t1=CX(0,k,j), t2=CX(0,k,jc);
        PMC(CX(0,k,j),CX(0,k,jc),t1,t2);
        for (size_t i=1; i<ido; ++i)
          {
          T x1, x2;
          PMC(x1,x2,CX(i,k,j),CX(i,k,jc));
          size_t idij=(j-1)*(ido-1)+i-1;
          CX(i,k,j) = x1.template special_mul<bwd>(wa[idij]);
          idij=(jc-1)*(ido-1)+i-1;
          CX(i,k,jc) = x2.template special_mul<bwd>(wa[idij]);
          }
        }
    }
  }

#undef CH2
#undef CX2
#undef CX

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template<bool bwd, typename T> void pass_all(T c[], T0 fct)
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  {
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  if (length==1) { c[0]*=fct; return; }
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  size_t l1=1;
  arr<T> ch(length);
  T *p1=c, *p2=ch.data();

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  for(size_t k1=0; k1<fact.size(); k1++)
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    {
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    size_t ip=fact[k1].fct;
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    size_t l2=ip*l1;
    size_t ido = length/l2;
    if     (ip==4)
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      pass4<bwd> (ido, l1, p1, p2, fact[k1].tw);
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    else if(ip==8)
      pass8<bwd>(ido, l1, p1, p2, fact[k1].tw);
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    else if(ip==2)
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      pass2<bwd>(ido, l1, p1, p2, fact[k1].tw);
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    else if(ip==3)
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      pass3<bwd> (ido, l1, p1, p2, fact[k1].tw);
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    else if(ip==5)
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      pass5<bwd> (ido, l1, p1, p2, fact[k1].tw);
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    else if(ip==7)
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      pass7<bwd> (ido, l1, p1, p2, fact[k1].tw);
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    else if(ip==11)
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      pass11<bwd> (ido, l1, p1, p2, fact[k1].tw);
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    else
      {
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      passg<bwd>(ido, ip, l1, p1, p2, fact[k1].tw, fact[k1].tws);
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      swap(p1,p2);
      }
    swap(p1,p2);
    l1=l2;
    }
  if (p1!=c)
    {
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    if (fct!=1.)
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      for (size_t i=0; i<length; ++i)
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        c[i] = ch[i]*fct;
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    else
      memcpy (c,p1,length*sizeof(T));
    }
  else
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    if (fct!=1.)
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      for (size_t i=0; i<length; ++i)
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        c[i] *= fct;
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  }

#undef WA
#undef CC
#undef CH

  public:
    template<typename T> void forward(T c[], T0 fct)
      { pass_all<false>(c, fct); }

    template<typename T> void backward(T c[], T0 fct)
      { pass_all<true>(c, fct); }

  private:
    NOINLINE void factorize()
      {
      size_t len=length;
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      while ((len&7)==0)
        { add_factor(8); len>>=3; }
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      while ((len&3)==0)
        { add_factor(4); len>>=2; }
      if ((len&1)==0)
        {
        len>>=1;
        // factor 2 should be at the front of the factor list
        add_factor(2);
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        swap(fact[0].fct, fact.back().fct);
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        }
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      size_t maxl = size_t(sqrt(double(len)))+1;
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      for (size_t divisor=3; (len>1)&&(divisor<maxl); divisor+=2)
        if ((len%divisor)==0)
          {
          while ((len%divisor)==0)
            {
            add_factor(divisor);
            len/=divisor;
            }
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          maxl=size_t(sqrt(double(len)))+1;
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          }
      if (len>1) add_factor(len);
      }

    size_t twsize() const
      {
      size_t twsize=0, l1=1;
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      for (size_t k=0; k<fact.size(); ++k)
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        {
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        size_t ip=fact[k].fct, ido= length/(l1*ip);
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        twsize+=(ip-1)*(ido-1);
        if (ip>11)
          twsize+=ip;
        l1*=ip;
        }
      return twsize;
      }

    void comp_twiddle()
      {
      sincos_2pibyn<T0> twid(length, false);
      auto twiddle = twid.cdata();
      size_t l1=1;
      size_t memofs=0;
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      for (size_t k=0; k<fact.size(); ++k)
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        {
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        size_t ip=fact[k].fct, ido=length/(l1*ip);
        fact[k].tw=mem.data()+memofs;
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        memofs+=(ip-1)*(ido-1);
        for (size_t j=1; j<ip; ++j)
          for (size_t i=1; i<ido; ++i)
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            fact[k].tw[(j-1)*(ido-1)+i-1] = twiddle[j*l1*i];
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        if (ip>11)
          {
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          fact[k].tws=mem.data()+memofs;
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          memofs+=ip;
          for (size_t j=0; j<ip; ++j)
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            fact[k].tws[j] = twiddle[j*l1*ido];
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          }
        l1*=ip;
        }
      }

  public:
    NOINLINE cfftp(size_t length_)
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      : length(length_)
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      {
      if (length==0) throw runtime_error("zero length FFT requested");
      if (length==1) return;
      factorize();
      mem.resize(twsize());
      comp_twiddle();
      }
  };

//
// real-valued FFTPACK transforms
//

template<typename T0> class rfftp
  {
  private:
    struct fctdata
      {
      size_t fct;
      T0 *tw, *tws;
      };

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    size_t length;
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    arr<T0> mem;
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    vector<fctdata> fact;
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    void add_factor(size_t factor)
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      { fact.push_back({factor, nullptr, nullptr}); }
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#define WA(x,i) wa[(i)+(x)*(ido-1)]
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template<typename T> inline void PM(T &a, T &b, T c, T d)
  { a=c+d; b=c-d; }

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/* (a+ib) = conj(c+id) * (e+if) */
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template<typename T1, typename T2, typename T3> inline void MULPM
  (T1 &a, T1 &b, T2 c, T2 d, T3 e, T3 f)
  {  a=c*e+d*f; b=c*f-d*e; }
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#define CC(a,b,c) cc[(a)+ido*((b)+l1*(c))]
#define CH(a,b,c) ch[(a)+ido*((b)+cdim*(c))]

template<typename T> void radf2 (size_t ido, size_t l1,
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  const T * RESTRICT cc, T * RESTRICT ch, const T0 * RESTRICT wa)
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  {
  constexpr size_t cdim=2;

  for (size_t k=0; k<l1; k++)
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    PM (CH(0,0,k),CH(ido-1,1,k),CC(0,k,0),CC(0,k,1));
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