Add DateIterator and make consistent date type APIs

[jmh530]

Will use with between function to add support for period function (to return slice of Dates).

Had to make some adjustments to make each of the date types have a consistent API. I changed the addXXX to an incrementCopy, but really the important thing is the increment part of it. I left date._addDays since that was the only remaining addXXX function in `date and didn't want to get too ahead of myself.

[9il]

why not use iota?

date + 100.iota!uint;

[jmh530]

date + 100.iota!uint;

That only works for days. Even if you did something a stride, then it is constant by days. So for instance, skipping every 30 days is not the same as skipping every month.

I want to add a function, call it period or sequenceDate or iotaDate or something else, that takes a Date/YearMonth/YearQuarter and a length N and returns a 1-dimensional slice of length N that returns that starting date and the subsequent values incorporating units. So for instance, period!"months"(Date(2020, 1, 1), 4) would return [Date(2020, 1, 1), Date(2020, 2, 1), Date(2020, 3, 1), Date(2020, 4, 1)]. You can't easily replicate that behavior with your solution because the length of months changes and you have leap years.

[jmh530]

I'm also open to changing the name to something else, like `DateLikeIterator`

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[jmh530]

One thing that might make this somewhat simpler for YearMonth and YearQuarter is to only store _months and _quarters similar to _julianDays in Date. This would enable you to take an iota-like approach for these. There would still be a problem for Date that I mentioned above.

[9il]

The following solution requires much less changes and additional.

map!Date(YearMonth(2020, 1) + 4.iota!uint)

One thing that might make this somewhat simpler for YearMonth and YearQuarter is to only store _months and _quarters similar to _julianDays in Date. This would enable you to take an iota-like approach for these. There would still be a problem for Date that I mentioned above.

Not really. += -= + - operation can be defined for YearMonth and YearQuarter without changing their representation. Just convert the pair to int adjust, convert back to a pair.

[9il]

Yes, there are some special cases like leap year and different amount of days in a month. However, this day convention are much more complex and requires. There are multiple conventions of adding a month to a date that will result different adjusted dates.

[jmh530]

map!Date(YearMonth(2020, 1) + 4.iota!uint)

I'm open to suggestions, but this doesn't work currently because opBinary for YearMonth is const. If you remove const from it, then the result is not correct. It goes January, February, April, July. It also says you can make add const or inout, but that has its own issues.

[9il]

I would suggest making an MR from scratch and adding only opBinary and opOpAssign to YearMonth and YearQuarter.

[jmh530]

@9il You added opBinary for YearMonth as part of PR #431. The problem is that it uses the add function, which returns a reference to this. I assumed that you did it this way for a reason, for instance to be compatible with the code base you primarily work on, so I didn't want to modify it. This is why I added the increment and incrementCopy functions, because adding with a reference doesn't work when iterating. It also wouldn't break anything that you do that depends on this behavior.

version(mir_test_date)
unittest
{
    auto x = YearMonth(2020, 1);
    auto x1 = x + 1;
    auto x2 = x + 2;
    auto x3 = x + 3;
    import std.stdio: writeln;
    writeln(x3); // prints YearMonth(2020, 7)
}

[9il]

That is because opBinary bug. It should use a temporal variable instead. BTW, we have two implementation addMonth and add!months. For Date it is needed for the historical reasons. But for other types we can use only one of them. I would prefer to remove add! for other types because it can cause bugs.

[jmh530]

Closed in favor of PR #438