subroutine indexx(arr,n,indx)

  parameter (M=7,NSTACK=50)
  integer n,indx(n)
  integer arr(n)
  integer i,indxt,ir,itemp,j,jstack,k,l,istack(NSTACK)
  real a

  do j=1,n
     indx(j)=j
  enddo

  jstack=0
  l=1
  ir=n
1 if(ir-l.lt.M) then
     do j=l+1,ir
        indxt=indx(j)
        a=arr(indxt)
        do i=j-1,1,-1
           if(arr(indx(i)).le.a) goto 2
           indx(i+1)=indx(i)
        enddo
        i=0
2       indx(i+1)=indxt
     enddo
     if(jstack.eq.0) return

     ir=istack(jstack)
     l=istack(jstack-1)
     jstack=jstack-2

  else
     k=(l+ir)/2
     itemp=indx(k)
     indx(k)=indx(l+1)
     indx(l+1)=itemp

     if(arr(indx(l+1)).gt.arr(indx(ir))) then
        itemp=indx(l+1)
        indx(l+1)=indx(ir)
        indx(ir)=itemp
     endif

     if(arr(indx(l)).gt.arr(indx(ir))) then
        itemp=indx(l)
        indx(l)=indx(ir)
        indx(ir)=itemp
     endif

     if(arr(indx(l+1)).gt.arr(indx(l))) then
        itemp=indx(l+1)
        indx(l+1)=indx(l)
        indx(l)=itemp
     endif

     i=l+1
     j=ir
     indxt=indx(l)
     a=arr(indxt)
3    continue
     i=i+1
     if(arr(indx(i)).lt.a) goto 3

4    continue
     j=j-1
     if(arr(indx(j)).gt.a) goto 4
     if(j.lt.i) goto 5
     itemp=indx(i)
     indx(i)=indx(j)
     indx(j)=itemp
     goto 3

5    indx(l)=indx(j)
     indx(j)=indxt
     jstack=jstack+2
     if(jstack.gt.NSTACK) stop 'NSTACK too small in indexx'
     if(ir-i+1.ge.j-l)then
        istack(jstack)=ir
        istack(jstack-1)=i
        ir=j-1
     else
        istack(jstack)=j-1
        istack(jstack-1)=l
        l=i
     endif
  endif
  goto 1

end subroutine indexx