Re: [boost] [date_time] day_of_week function

From: Jeff Garland (jeff_at_[hidden])
Date: 2005-07-17 10:13:45

On Sun, 17 Jul 2005 15:57:21 +0200, Friedrich Wilckens wrote
> date_time::date::day_of_week() calls two functions from the calendar
> template parameter: the first to convert the internal integer
> representation into a ymd_type, and the second to calculate the
> day_of_week for this ymd_type. Both do some non-trivial arithmetic.

True.
 
> I wonder why the following simpler and more efficient approach is not
> used.
> 
> In date_time::date:
> 
>    day_of_week_type day_of_week() const
>    {
>       return calendar::day_of_week(days_);
>    }
> 
> In date_time::gregorian_calendar_base add a method:
> 
>    static unsigned short day_of_week(const date_int_type& d)
>    {
>       return (d + 1) % 7;
>    }
> 
> As far as I know, the sequence of weeks in the gregorian calendar is
> completely regular and this simple calculation is correct.

Yes that's true -- of course the above would have to be some adjustment for
the possibility that day 0 != Sunday...

> This approach would also not limit flexibility of the date template. 
> If for another calendar there is some irregularity, we could for 

And there is if you wanted to implement an 'accurate' gregorian calendar
instead of a propletic one.  You'd have to adjust for all sorts of interesting
adjustments in the 1580's.

> this calendar implement
> 
>    static unsigned short day_of_week(const date_int_type& d)
>    {
>       return day_of_week(from_day_number(d));
>    }
> 
> Is there any problem in this reasoning?

Probably not.  I'll put it on the todo list for 1.34.

Thx,

Jeff