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@@ -831,10 +831,20 @@ static bool is_valid_time_string(StringView value)
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}
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// https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#week-number-of-the-last-day
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-static u32 week_number_of_the_last_day(u64)
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+static u32 week_number_of_the_last_day(u64 year)
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{
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- // FIXME: sometimes return 53 (!)
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// https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#weeks
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+ // NOTE: A year is considered to have 53 weeks if either of the following conditions are satisfied:
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+ // - January 1 of that year is a Thursday.
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+ // - January 1 of that year is a Wednesday and the year is divisible by 400, or divisible by 4, but not 100.
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+
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+ // Note: Gauss's algorithm for determining the day of the week with D = 1, and M = 0
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+ // https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week#Gauss's_algorithm
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+ u8 day_of_week = (1 + 5 * ((year - 1) % 4) + 4 * ((year - 1) % 100) + 6 * ((year - 1) % 400)) % 7;
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+
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+ if (day_of_week == 4 || (day_of_week == 3 && (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))))
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+ return 53;
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+
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return 52;
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}
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stelar7