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fix: correct %V ISO week at leap year boundaries #133

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fix: correct %V ISO week at leap year boundaries #133

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11 changes: 8 additions & 3 deletions lib/Date/Format/Generic.pm
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Original file line number Diff line number Diff line change
Expand Up @@ -252,12 +252,17 @@ sub format_V {
# Day-of-year of the Thursday in this ISO week
my $thu = $yday - $mwday + 3;
if ($thu < 0) {
# Thursday falls in the previous year
# Thursday falls in the previous year — this date belongs to
# the last ISO week of the previous year, skip year-end check
my $py = $year - 1;
$thu += (($py % 4 == 0 && $py % 100 != 0) || $py % 400 == 0) ? 366 : 365;
}
my $ylen = (($year % 4 == 0 && $year % 100 != 0) || $year % 400 == 0) ? 366 : 365;
return sprintf("%02d", 1) if $thu >= $ylen;
else {
# Thursday falls in the next year — this date belongs to
# ISO week 1 of the following year
my $ylen = (($year % 4 == 0 && $year % 100 != 0) || $year % 400 == 0) ? 366 : 365;
return sprintf("%02d", 1) if $thu >= $ylen;
}
sprintf("%02d", int($thu / 7) + 1);
}

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12 changes: 12 additions & 0 deletions t/iso-week-number.t
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Expand Up @@ -43,6 +43,18 @@ my @cases = (
[timegm(0,0,0,27,11,104), "53", "2004-12-27 (Mon) is week 53"],
[timegm(0,0,0, 2,0,105), "53", "2005-01-02 (Sun) is week 53 of 2004"],
[timegm(0,0,0, 3,0,105), "01", "2005-01-03 (Mon) is week 01 of 2005"],

# 2021: Jan 1 is Friday, previous year (2020) is leap year
# This is a regression test: the leap-year adjustment pushed $thu to
# exactly equal the current year's day count, triggering the year-end
# check incorrectly and returning week 01 instead of 53.
[timegm(0,0,0, 1,0,121), "53", "2021-01-01 (Fri) is week 53 of 2020 (leap year boundary)"],
[timegm(0,0,0, 2,0,121), "53", "2021-01-02 (Sat) is week 53 of 2020"],
[timegm(0,0,0, 3,0,121), "53", "2021-01-03 (Sun) is week 53 of 2020"],
[timegm(0,0,0, 4,0,121), "01", "2021-01-04 (Mon) is week 01 of 2021"],

# 2017: Jan 1 is Sunday → week 52 of 2016 (2016 is leap year)
[timegm(0,0,0, 1,0,117), "52", "2017-01-01 (Sun) is week 52 of 2016 (leap year)"],
);

for my $case (@cases) {
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