Function rfftp_factorize

numpy/fft/_pocketfft.c:1782–1812

  }

WARN_UNUSED_RESULT
static int rfftp_factorize (rfftp_plan plan)
  {
  size_t length=plan->length;
  size_t nfct=0;
  while ((length%4)==0)
    { if (nfct>=NFCT) return -1; plan->fct[nfct++].fct=4; length>>=2; }
  if ((length%2)==0)
    {
    length>>=1;
    // factor 2 should be at the front of the factor list
    if (nfct>=NFCT) return -1;
    plan->fct[nfct++].fct=2;
    SWAP(plan->fct[0].fct, plan->fct[nfct-1].fct,size_t);
    }
  size_t maxl=(size_t)(sqrt((double)length))+1;
  for (size_t divisor=3; (length>1)&&(divisor<maxl); divisor+=2)
    if ((length%divisor)==0)
      {
      while ((length%divisor)==0)
        {
        if (nfct>=NFCT) return -1;
        plan->fct[nfct++].fct=divisor;
        length/=divisor;
        }
      maxl=(size_t)(sqrt((double)length))+1;
      }
  if (length>1) plan->fct[nfct++].fct=length;
  plan->nfct=nfct;
  return 0;
  }

static size_t rfftp_twsize(rfftp_plan plan)
  {