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!
! Generate a seed for the RANDOM_NUMBER PRNG that is guaranteed to be
! unique even if many processes are started simultaneously
!
subroutine init_random_seed()
use iso_fortran_env, only: int64
implicit none
integer, allocatable :: seed(:)
integer :: i, n, un, istat, dt(8), pid
integer(int64) :: t
call random_seed(size = n)
allocate(seed(n))
! First try if the OS provides a random number generator
open(newunit=un, file="/dev/urandom", access="stream", &
form="unformatted", action="read", status="old", iostat=istat)
if (istat == 0) then
read(un) seed
close(un)
else
! Fallback to XOR:ing the current time and pid. The PID is
! useful in case one launches multiple instances of the same
! program in parallel.
call system_clock(t)
if (t == 0) then
call date_and_time(values=dt)
t = (dt(1) - 1970) * 365_int64 * 24 * 60 * 60 * 1000 &
+ dt(2) * 31_int64 * 24 * 60 * 60 * 1000 &
+ dt(3) * 24_int64 * 60 * 60 * 1000 &
+ dt(5) * 60 * 60 * 1000 &
+ dt(6) * 60 * 1000 + dt(7) * 1000 &
+ dt(8)
end if
pid = getpid()
t = ieor(t, int(pid, kind(t)))
do i = 1, n
seed(i) = lcg(t)
end do
end if
call random_seed(put=seed)
contains
! This simple PRNG might not be good enough for real work, but is
! sufficient for seeding a better PRNG.
function lcg(s)
integer :: lcg
integer(int64) :: s
if (s == 0) then
s = 104729
else
s = mod(s, 4294967296_int64)
end if
s = mod(s * 279470273_int64, 4294967291_int64)
lcg = int(mod(s, int(huge(0), int64)), kind(0))
end function lcg
end subroutine init_random_seed
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