random streams

[Brian Goetz]

Sharing random number generators is indeed a tricky thing.  But, this 
doesn't change the status quo, it just wraps it differently.  Doing:

   rng.ints().parallel().forEach(i -> { ... i ... })

is no different from submitting code like

     { ... i = rng.nextInt(); i ... })

to an FJP; you'll get exactly the same interleavings.

ThreadLocalRandom can override this to implement:

      public IntStream ints() {
          return PrimitiveStreams.repeatedly(
              () -> TLR.current().nextInt());
      }






On 12/31/2012 1:54 PM, Doug Lea wrote:
> On 12/31/12 13:41, Brian Goetz wrote:
>> On the list of requested stream sources is 'stream of random numbers'.
>>
>> Here's a one-line addition to Random:
>>
>>      public IntStream ints() {
>>          return PrimitiveStreams.repeatedly(this::nextInt);
>>      }
>>
>> Certainly the implementation is straightforward enough (modulo
>> renaming of
>> PrimitiveStreams and repeatedly, which are not yet nailed down.)
>
> This is not so straightforward under parallel operations.
> This is a surprisingly deep topic with a lot of technical papers etc.
> As a first pass, you'd just use ThreadLocalRandom() as sources
> (to avoid horrible update contention), and make no promises
> about the aggregate randomness across parallel operations.
> However, people will come to expect that if you start off
> computations with a common seed, then you get both replicability
> and independence, which is not easy to deliver.
> As a start, you'd need a better generator than the
> one in Random (which is and must be the same algorithm
> used in ThreadLocalRandom).
>
> -Doug
>
>