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      subroutine sswapf (n,sx,incx,sy,incy)
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c
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c     interchanges two vectors.
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c     uses unrolled loops for increments equal to 1.
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c     jack dongarra, linpack, 3/11/78.
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c     modified 12/3/93, array(1) declarations changed to array(*)
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c
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      real sx(*),sy(*),stemp
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      integer i,incx,incy,ix,iy,m,mp1,n
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c
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      if(n.le.0)return
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      if(incx.eq.1.and.incy.eq.1)go to 20
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c
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c       code for unequal increments or equal increments not equal
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c         to 1
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c
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      ix = 1
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      iy = 1
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      if(incx.lt.0)ix = (-n+1)*incx + 1
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      if(incy.lt.0)iy = (-n+1)*incy + 1
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      do 10 i = 1,n
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        stemp = sx(ix)
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        sx(ix) = sy(iy)
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        sy(iy) = stemp
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        ix = ix + incx
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        iy = iy + incy
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   10 continue
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      return
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c
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c       code for both increments equal to 1
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c
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c
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c       clean-up loop
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c
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   20 m = mod(n,3)
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      if( m .eq. 0 ) go to 40
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      do 30 i = 1,m
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        stemp = sx(i)
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        sx(i) = sy(i)
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        sy(i) = stemp
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   30 continue
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      if( n .lt. 3 ) return
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   40 mp1 = m + 1
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      do 50 i = mp1,n,3
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        stemp = sx(i)
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        sx(i) = sy(i)
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        sy(i) = stemp
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        stemp = sx(i + 1)
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        sx(i + 1) = sy(i + 1)
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        sy(i + 1) = stemp
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        stemp = sx(i + 2)
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        sx(i + 2) = sy(i + 2)
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        sy(i + 2) = stemp
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   50 continue
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      return
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      end