This corrects for some drift in our shadowed copy of libshared's
Datetime::validate (in our own DatetimeParser class with function of
same name), from libshared commit 7dffb13c

Signed-off-by: Scott Mcdermott <[email protected]>

…== 1

This is a duplicate of the changes from commit with same subject line in
GothenburgBitFactory/libshared#81 and also duplicates the commit
message below.  Timewarrior has copies of many functions from
libshared's Datetime class in its own DatetimeParser class, and until
such time as these classes are integrated, we have to maintain copies of
these functions here, and the changes need to be duplicated.  See
discussion in aforementioned PR.

The one difference with the patch over there is, this one is using the
public Datetime::dayOfWeek() and Datetime::daysInYear() methods from
libshared, rather than its own implementation.  The copy in Timewarrior
already was doing this before, but it's worth noting it's the only
difference with the corresponding patch in libshared PR 81, and only
amounts to a change in the namespace qualifier.

Copied commit message from libshared follows.

                           *     *    *

This patch makes the parsing of week numbers in dates ISO-8601 compliant
in the case that Datetime::weekstart == 1, while the existing behavior
remains available if Datetime::weekstart == 0.

The previous code parsed week numbers (given as "yyyy-Www") into dates
starting on Sunday.  Although the code had a "Datetime::weekstart"
variable, and this value was initialized to 1 (which is Monday) in
libshared, nonetheless week specifications would be parsed into calendar
days starting on Sunday.

Furthermore, week days in such given weeks ('d' in "yyyy-Www-d") used 0-6
for Sunday-Monday, while ISO8601 specifies 1-7 Monday-Sunday.
Therefore, yyyy-Www-0 was treated as valid (and should not be), while
yyyy-Www-7 was invalid (and should be valid).

Note that neither Taskwarrior nor Timewarrior ever set the value of
Datetime::weekstart.  Taskwarrior has an rc.weekstart, but this is only
used for "task calendar" output, not for parsing dates.

The patch does the following:

- Initialize "Datetime" instances with a weekday value from
  Datetime::weekstart.  This helps the case when weekday is not
  supplied, it won't default to zero and fail validation (when
  weekstart is '1').  Note that mktime() is usually used in the code
  to convert populated "struct tm" broken-down times into epoch-times,
  and this operation does not use tm.tm_wday for input, only as an
  output field, recomputed as a normalized value, so it appears to be
  safe to initialize it with a 1 (which we might wonder about since
  .tm_wday is supposed to be 0-6 Sunday based).

- Use the already-existing Datetime::parse_weekday() to parse the
  'ww' in "yyyy-Www" input dates (the function was not used by any
  caller previously; guessing it may have been intended for eventual
  use in order to respect weekstart(?))

- Teach parse_weekday() about weekstart.  Previously this assumed
  1-7, which is the reason I'm guessing this was intended to be used
  for ISO weeks eventually.  Now it can also use 0-6 if weekstart 0.

- Teach Datetime::validate to respect Datetime::weekstart also.
  Previously only 0-6 Sunday-Monday was considered valid.

- Default the weekday to Datetime::weekstart if not supplied, ie for
  "yyyy-Www-X" if the "-X" is not supplied, as recognized when
  Datetime::standaloneDateEnabled = true, which is the case for (1)
  tests, (2) timewarrior, but NOT for taskwarrior at this time
  (both the standalone 'calc' and 'task calc' (ie Context.cpp) set
  this to false).

- Implement the complete ISO week calculation algorithm in
  Datetime::resolve() if weekstart is '1', and keeps the existing
  algorithm if weekstart is '0'.  This will allow Taskwarrior and
  Timewarrior to offer the option of the old behavior for those
  who want to use Sunday-based weeks and ignore ISO week calculations
  (such as 2023-01-01 being in ISO week 2022-W52).

Signed-off-by: Scott Mcdermott <[email protected]>

Brings in new dayOfWeek() code from GothenburgBitFactory/libshared#81

Signed-off-by: Scott Mcdermott <[email protected]>