[m-rev.] time additions (was Re: for review: Add random.init/3)

[Peter Wang]

On Thu, 26 May 2016 12:15:52 +1000 (AEST), Julien Fischer <jfischer at opturion.com> wrote:
> 
> On Thu, 26 May 2016, Paul Bone wrote:
> 
> > On Thu, May 26, 2016 at 11:25:16AM +1000, Peter Wang wrote:
> >> On Wed, 25 May 2016 16:28:32 +1000, Peter Wang <novalazy at gmail.com> wrote:
> >> 
> >> A common way to seed the PRNG in C is
> >>
> >>     srand(time(0));
> >> 
> >> The time in seconds-since-the-Unix-epoch is useful for many other
> >> purposes, in addition to seeding any PRNG implementation.
> >> 
> >> I would like to see an easy way to get an integer (simple)
> >> representation of Unix time, as well as a way to convert between an
> >> integer representation and the time_t values.
> >> 
> >> The main problem is probably that a signed 32-bit int cannot represent
> >> Unix times past some time in 2038.  Apart from 32-bit machines, this
> >> problem also affects C# and Java backends.
> >> 
> >> difftime/2 returns a float.  You can get the current unix time as a
> >> float with
> >>
> >>     time(Now, !IO),
> >>     Secs = difftime(Now, unix_epoch)
> >> 
> >> where `unix_epoch' returns a time_t.
> >> 
> >> Using floats is not very satisfying but in most cases they will be
> >> double precision, and capable of representing any reasonable time
> >> value precisely.
> 
> I think we should define Mercury's float type to always be a double
> precision type and just do away with the spf grade component entirely.
> That would deal with the "most cases" issue above.

Fine by me.

> > Eww,
> >
> > My reaction to the use of floats for this kind of thing is rater visceral.

I understand, but doubles can represent values to 2^53 precisely.

    % date -d @$(( 2**53 ))
    Mon Nov 12 18:36:32 AEDT 285428751

That ought to be enough for anybody...

> > I agree that it may be useful to expose time in seconds since the epoch for
> > other reasons.  And as an aside, gettimeofday() could be useful if portable
> > too.

Yes.

> 4. Seed the PRNG using something other than time, for example:
> 
>     :- pred generate_random_int(int::out, io::di, io::uo) is det.
> 
> and generate the int using /dev/random (on Unix) or CryptGenRandom (on
> Windows), or whatever the appropriate OS / platform specific source of
> randomness is.  (Most OSs -- and certainly the ones that Mercury
> actually runs on -- will provide something along those lines.)

That would be useful, too.

Peter