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1845 :: How do I get the system time or cpu counter in order to seed RNG?

• enjoysmath (orginal) [2015-12-06T07:18:32+01:00] view original

  Questions's in the title. time.cpuTime() wouldn't be helpful for seeding the RNG.

  So what should be used?

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• Arrrrrrrrr (orginal) [2015-12-06T09:51:49+01:00] view original

  This is what randomize() does:

  randomize(cast[int](epochTime()))
  Run

  Do you have any problem with using epochTime ?

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• enjoysmath (orginal) [2015-12-06T22:44:20+01:00] view original

  Thanks, Arrrrrrr. That should work for me

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• Jehan (orginal) [2015-12-07T08:12:46+01:00] view original

  Please do NOT use cast to convert between integer and floating point types. It doesn't do what you think it does (it reinterprets the underlying bitpattern). Use int(f) instead (use math.mod on the floating point value first if overflow is a possibility).

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• Arrrrrrrrr (orginal) [2015-12-07T11:31:49+01:00] view original

  But i think that, in this case, it is not a problem because it is used for seeding random https://github.com/nim-lang/Nim/blob/master/lib/pure/math.nim#L275 I suppose is alright for this usecase.

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• Jehan (orginal) [2015-12-07T13:36:44+01:00] view original

  It depends. On a 32-bit machine, this will typically extract the first four bytes of the floating point number, which contain the sign bit, the exponent, and the most significant 20 bits of the mantissa. This loses information and may or may not be suitably random. Here's a function that tries to avoid this problem:

  import times, math
  
  proc getTimeSeed(): int =
    let t = epochTime()
    let th = int(t mod 1e9)
    let tl = int((t mod 1) * 1e9)
    when sizeof(int) == 4:
      tl xor th
    else:
      th * 1_000_000_000 + tl
  
  echo getTimeSeed()
  echo getTimeSeed()
  Run

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