Function calc_first_half

numpy/fft/_pocketfft.c:124–149

  }

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

NOINLINE static void fill_first_quadrant(size_t n, double * restrict res)
  {