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!=======================================================================
! This is part of the 2DECOMP&FFT library
!
! 2DECOMP&FFT is a software framework for general-purpose 2D (pencil)
! decomposition. It also implements a highly scalable distributed
! three-dimensional Fast Fourier Transform (FFT).
!
! Copyright (C) 2009-2012 Ning Li, the Numerical Algorithms Group (NAG)
!
!=======================================================================
! A few utility routines to find factors of integer numbers
subroutine findfactor(num, factors, nfact)
implicit none
integer, intent(IN) :: num
integer, intent(OUT), dimension(*) :: factors
integer, intent(OUT) :: nfact
integer :: i, m
! find the factors <= sqrt(num)
m = int(sqrt(real(num)))
nfact = 1
do i=1,m
if (num/i*i == num) then
factors(nfact) = i
nfact = nfact + 1
end if
end do
nfact = nfact - 1
! derive those > sqrt(num)
if (factors(nfact)**2/=num) then
do i=nfact+1, 2*nfact
factors(i) = num / factors(2*nfact-i+1)
end do
nfact = nfact * 2
else
do i=nfact+1, 2*nfact-1
factors(i) = num / factors(2*nfact-i)
end do
nfact = nfact * 2 - 1
endif
return
end subroutine findfactor
subroutine primefactors(num, factors, nfact)
implicit none
integer, intent(IN) :: num
integer, intent(OUT), dimension(*) :: factors
integer, intent(INOUT) :: nfact
integer :: i, n
i = 2
nfact = 1
n = num
do
if (mod(n,i) == 0) then
factors(nfact) = i
nfact = nfact + 1
n = n / i
else
i = i + 1
end if
if (n == 1) then
nfact = nfact - 1
exit
end if
end do
return
end subroutine primefactors