[phobos] std.datetime and tick types

[Jonathan M Davis]

On Tuesday, September 28, 2010 07:40:50 Sean Kelly wrote:
> On Sep 27, 2010, at 10:08 PM, Robert Jacques wrote:
> > Well, if you make up a unit, then you can guarantee no one will have
> > heard of it before. And really, learning the hecto SI prefix means
> > someone will always understand what hectonanosecond means, as opposed to
> > StdTick or Duration, etc, which would have a tendency to be quickly
> > forgotten or confused with another library type from another language.
> > As for typing, I would expect the 100ns unit to be mainly an internal
> > thing. Most of the time I deal in microseconds/1000.0 (i.e. floating
> > point milliseconds), when dealing with std.perf today.
> 
> The Boost TimeDuration struct has a ticks() method as well as a few methods
> describing the tick resolution.  So someone wanting to possibly avoid a
> conversion could do:
> 
> if (7 == x.num_fractional_digits())
>     auto y = x.ticks();
> else
>     auto z = x.total_nanoseconds() / 100;
> 
> This is what I'm currently working with for durations in druntime (not yet
> committed).

I do believe, however, that Boost's definition of ticks varies with the 
resolution of the system clock - so it's really more along the lines of ticks 
per second like SHOO's Ticks struct uses than a fixed unit of time. That can be 
useful for some stuff, but it's not the sort of thing that I'd expect many 
programs to want to use directly. The datetime code needs to know how many ticks 
per seconds it's dealing with so that it can correctly convert high-precision 
time to known units - such as microseconds, nanoseconds, etc. - but doing stuff 
like creating a duration of a specific number of ticks doesn't strike me as being 
particularly useful - much as Boost does supply the means to do it. I certainly 
wouldn't want to have to worry about having a bunch of ifs or static ifs in my 
normal code to deal with varying tick resolution. Ideally, the date/time library 
code would abstract that out so that I don't have to worry about it. None of my 
datetime code uses variable unit sizes. The closest it does is to translate 
SHOO's Ticks struct to hecto-nanseconds when it gets the current time. After 
that, all of the units are of known and fixed size.

- Jonathan M Davis