5 ms·

It's probably the fact that using time() for seeding a PRNG is a really poor choice.

by detrino 12y ago

pdw 12y ago
Especially if your clock is synced with every other clock in the world.
aidenn0 12y ago
Here's what happens when you do that: http://alt.org/nethack/player-all.php?player=WowDeath http://alt.org/nethack/player-all.php?player=WowDeath Full article here: http://taeb-nethack.blogspot.com/2009/03/predicting-and-controlling-nethacks.html http://taeb-nethack.blogspot.com/2009/03/predicting-and-cont...
deleted 12y ago
[deleted]
kevin_thibedeau 12y ago
It isn't a problem if all you need is a deterministic sequence of numbers that are pseudo-random like what rand() provides. Not every use of a PRNG involves cryptographic security or potentially dangerous exposures of internal state.
- detrino 12y ago
  There are concerns with using time() as a seed other than security. Besides, consider what you are typically trying to express: initialize my PRNG with a random seed. Almost never are you trying to express: initialize my PRNG so that all runs in the same second will behave identically.
  - kevin_thibedeau 12y ago
    If you're generating random test case input on a single machine, time() is a perfectly reasonable way to seed a PRNG. A super-robust entropy source isn't a necessity when you just want something nominally random between intermittent runs.
    - deleted 12y ago
      [deleted]
    - nitrogen 12y ago
      ...nominally random between intermittent runs. That's a lot of caveats. Suppose you want to run multiple times in one second. Suppose you want statistically uniform random numbers to ensure even coverage of test cases.
      - xorcist 12y ago
        Also if your clock resets on boot for some reason, and boot time is predictable.
    - CHY872 12y ago
      For test case generation, I'd argue that you want the opposite of a time initialised random number generator. You'd want a deterministic seed for your random number generator, so you can repeat if required.
- panzi 12y ago
  Why don't you use this sequence in such a case?: 0, 1, 2, 3, 4, 5, 6, 7, 8, ...
craftkiller 12y ago
oh thats also a good point, but it would be solved with arc4random :-)
wglb 12y ago
In some cases, yes. Some simulations want to have the same sequence of numbers. Seems like a fundamental design misunderstanding in nethack, however.
- detrino 12y ago
  > Some simulations want to have the same sequence of numbers. srand(time(NULL)); does not provide that.
- jerf 12y ago
  The point is, if you want a repeatable sequence, ask for it. If you want a truly random sequence, ask for it. What you don't want is a repeatable random sequence that you, the programmer, incorrectly think is truly random. But rather a lot of invocations to srand turn out to be this case, rather than either of the legitimate two.

It's probably the fact that using time() for seeding a PRNG is a really poor choice.

by detrino 12y ago

It's probably the fact that using time() for seeding a PRNG is a really poor choice.

pdw 12y ago

Especially if your clock is synced with every other clock in the world.

aidenn0 12y ago

Here's what happens when you do that: http://alt.org/nethack/player-all.php?player=WowDeath http://alt.org/nethack/player-all.php?player=WowDeath Full article here: http://taeb-nethack.blogspot.com/2009/03/predicting-and-controlling-nethacks.html http://taeb-nethack.blogspot.com/2009/03/predicting-and-cont...

deleted 12y ago

[deleted]

kevin_thibedeau 12y ago

It isn't a problem if all you need is a deterministic sequence of numbers that are pseudo-random like what rand() provides. Not every use of a PRNG involves cryptographic security or potentially dangerous exposures of internal state.

detrino 12y ago
There are concerns with using time() as a seed other than security. Besides, consider what you are typically trying to express: initialize my PRNG with a random seed. Almost never are you trying to express: initialize my PRNG so that all runs in the same second will behave identically.
- kevin_thibedeau 12y ago
  If you're generating random test case input on a single machine, time() is a perfectly reasonable way to seed a PRNG. A super-robust entropy source isn't a necessity when you just want something nominally random between intermittent runs.
  - deleted 12y ago
    [deleted]
  - nitrogen 12y ago
    ...nominally random between intermittent runs. That's a lot of caveats. Suppose you want to run multiple times in one second. Suppose you want statistically uniform random numbers to ensure even coverage of test cases.
    - xorcist 12y ago
      Also if your clock resets on boot for some reason, and boot time is predictable.
  - CHY872 12y ago
    For test case generation, I'd argue that you want the opposite of a time initialised random number generator. You'd want a deterministic seed for your random number generator, so you can repeat if required.
panzi 12y ago
Why don't you use this sequence in such a case?: 0, 1, 2, 3, 4, 5, 6, 7, 8, ...

craftkiller 12y ago

oh thats also a good point, but it would be solved with arc4random :-)

wglb 12y ago

In some cases, yes. Some simulations want to have the same sequence of numbers. Seems like a fundamental design misunderstanding in nethack, however.

detrino 12y ago
> Some simulations want to have the same sequence of numbers. srand(time(NULL)); does not provide that.
jerf 12y ago
The point is, if you want a repeatable sequence, ask for it. If you want a truly random sequence, ask for it. What you don't want is a repeatable random sequence that you, the programmer, incorrectly think is truly random. But rather a lot of invocations to srand turn out to be this case, rather than either of the legitimate two.