Re: [boost] [chrono/date] conversion between concrete dates

Subject: Re: [boost] [chrono/date] conversion between concrete dates
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2013-05-09 06:16:43

Le 08/05/13 19:29, Howard Hinnant a écrit :
> On May 8, 2013, at 12:08 PM, Vicente J. Botet Escriba <vicente.botet_at_[hidden]> wrote:
>
>> Hi,
>>
>> as the conversion between concrete dates could be expensive I guess that these conversion must be explicit.
>>
>> But this has some consequences when used the implicit conversion was hidden a not efficient implementation, e.g.
>>
>>   date ISO_week_start = mon <= jan/day(4)/y;
>>
>> jan/day(4)/y should be ymd_date as is the efficient representation.
>>
>> The date generator was declared as
>>
>>    date operator<=(weekday wd, date x);
>>
>> but it works efficiently only for days_date. If we provide only the functions that are efficient we should declare it as
>>
>>   days_date operator<=(weekday wd, days_date x);
>>
>> So the preceding expression would need an explicit conversion
>>
>>   days_date ISO_week_start = mon <= days_date(jan/day(4)/y);
>>
>> Do we want to go on this direction?
>>
>> A radical alternative to the explicit construction, if we want to make evident that the conversion operation could be expensive, is to use a compute_ factory
>>
>>   days_date ISO_week_start = mon <= compute_days_date(jan/day(4)/y);
>>
>> Best,
>> Vicente
>>
>> P.S. the generator function is just an example needing explicit conversion.
>
> I think we should consider the route of an explicit conversion between the serial date and the ymd date, and see where field experience takes us.
I agree. The implicit conversion from ymd dates is needed to maintain 
the code readable.
> In some timings I did last weekend I was getting about 1.2ns for a field->serial conversion averaged over 200 years, and 18ns for a serial->field conversion averaged over the same range.  I was just timing the conversion function, and not any validation checks.
>
> If experience holds that serial->field is 15X the cost of field->serial, a possibility is for a hybrid approach: implicit in one direction and explicit in the other.  However I wouldn't take these numbers as fact.  This is just one report from one machine and one implementation.
>
>
Could you share your test program so that I can run it with my 
implementation and on other platforms?

Best,
Vicente