zffti1.f

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      subroutine zffti1 (n,wa,ifac)
      implicit double precision (a-h,o-z)
      dimension       wa(*)      ,ifac(*)    ,ntryh(4)
      data ntryh(1),ntryh(2),ntryh(3),ntryh(4)/3,4,2,5/
      nl = n
      nf = 0
      j = 0
  101 j = j+1
      if (j-4) 102,102,103
  102 ntry = ntryh(j)
      go to 104
  103 ntry = ntry+2
  104 nq = nl/ntry
      nr = nl-ntry*nq
      if (nr) 101,105,101
  105 nf = nf+1
      ifac(nf+2) = ntry
      nl = nq
      if (ntry .ne. 2) go to 107
      if (nf .eq. 1) go to 107
      do 106 i=2,nf
         ib = nf-i+2
         ifac(ib+2) = ifac(ib+1)
  106 continue
      ifac(3) = 2
  107 if (nl .ne. 1) go to 104
      ifac(1) = n
      ifac(2) = nf
      tpi = 6.28318530717959d0
      argh = tpi/dble(n)
      i = 2
      l1 = 1
      do 110 k1=1,nf
         ip = ifac(k1+2)
         ld = 0
         l2 = l1*ip
         ido = n/l2
         idot = ido+ido+2
         ipm = ip-1
         do 109 j=1,ipm
            i1 = i
            wa(i-1) = 1.
            wa(i) = 0.
            ld = ld+l1
            fi = 0.
            argld = dble(ld)*argh
            do 108 ii=4,idot,2
               i = i+2
               fi = fi+1.
               arg = fi*argld
               wa(i-1) = cos(arg)
               wa(i) = sin(arg)
  108       continue
            if (ip .le. 5) go to 109
            wa(i1-1) = wa(i-1)
            wa(i1) = wa(i)
  109    continue
         l1 = l2
  110 continue
      return
      end