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Simplify QDate::weekNumber() by looking at the right day of the week

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The Thursday in the same week has the same week-number and determines
the right year to use for the week. So calculate its week-number and
save a whole lot of complications.

Change-Id: I9c28267e6083afdd04a15245e1609c64beb82b37
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
This commit is contained in:
Edward Welbourne 2019-07-24 10:37:23 +02:00
parent 3183e428a9
commit eab533ae0d
1 changed files with 14 additions and 27 deletions
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41
src/corelib/time/qdatetime.cpp
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@ -570,16 +570,18 @@ int QDate::daysInYear() const
}
/*!
Returns the week number (1 to 53), and stores the year in
*\a{yearNumber} unless \a yearNumber is null (the default).
Returns the ISO 8601 week number (1 to 53).
Returns 0 if the date is invalid.
Returns 0 if the date is invalid. Otherwise, returns the week number for the
date. If \a yearNumber is not \nullptr (its default), stores the year as
*\a{yearNumber}.
In accordance with ISO 8601, weeks start on Monday and the first
Thursday of a year is always in week 1 of that year. Most years
have 52 weeks, but some have 53.
In accordance with ISO 8601, each week falls in the year to which most of
its days belong, in the Gregorian calendar. As ISO 8601's week starts on
Monday, this is the year in which the week's Thursday falls. Most years have
52 weeks, but some have 53.
*\a{yearNumber} is not always the same as year(). For example, 1
\note *\a{yearNumber} is not always the same as year(). For example, 1
January 2000 has week number 52 in the year 1999, and 31 December
2002 has week number 1 in the year 2003.
@ -591,26 +593,11 @@ int QDate::weekNumber(int *yearNumber) const
if (!isValid())
return 0;
int year = QDate::year();
int yday = dayOfYear();
int wday = dayOfWeek();
int week = (yday - wday + 10) / 7;
if (week == 0) {
// last week of previous year
--year;
week = (yday + 365 + (QDate::isLeapYear(year) ? 1 : 0) - wday + 10) / 7;
Q_ASSERT(week == 52 || week == 53);
} else if (week == 53) {
// maybe first week of next year
int w = (yday - 365 - (QDate::isLeapYear(year) ? 1 : 0) - wday + 10) / 7;
if (w > 0) {
++year;
week = w;
}
Q_ASSERT(week == 53 || week == 1);
}
// The Thursday of the same week determines our answer:
QDate thursday(addDays(4 - dayOfWeek()));
int year = thursday.year();
// Week n's Thurs's DOY has 1 <= DOY - 7*(n-1) < 8, so 0 <= DOY + 6 - 7*n < 7:
int week = (thursday.dayOfYear() + 6) / 7;
if (yearNumber)
*yearNumber = year;