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