LibWeb/HTML: Implement <input> element valueAsNumber for 'week'

One point to note is that I am not entirely sure what the result
of the pre-existing valueAsNumber test should be for this strange
case which does not lie exactly on a week/day boundary. Chrome
gives a negative timestamp, which seems more wrong than the result
we give, and neither gecko or WebKit appear to support the 'week'
type. So I'm considering this result acceptable for now, and this
may be something that will need more WPT tests added in the future.

Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -2079,6 +2079,20 @@ static Optional<double> convert_month_string_to_number(StringView input)
     return number_of_months_since_unix_epoch(maybe_year_and_month.value());
 }
 
+// https://html.spec.whatwg.org/multipage/input.html#week-state-(type=week):concept-input-value-string-number
+static Optional<double> convert_week_string_to_number(StringView input)
+{
+    // The algorithm to convert a string to a number, given a string input, is as follows: If parsing a week
+    // string from input results in an error, then return an error; otherwise, return the number of
+    // milliseconds elapsed from midnight UTC on the morning of 1970-01-01 (the time represented by the value
+    // "1970-01-01T00:00:00.0Z") to midnight UTC on the morning of the Monday of the parsed week, ignoring
+    // leap seconds.
+    auto parsed_week = parse_a_week_string(input);
+    if (!parsed_week.has_value())
+        return {};
+    return UnixDateTime::from_iso8601_week(parsed_week->week_year, parsed_week->week).milliseconds_since_epoch();
+}
+
 // https://html.spec.whatwg.org/multipage/input.html#concept-input-value-string-number
 Optional<double> HTMLInputElement::convert_string_to_number(StringView input) const
 {
@@ -2093,6 +2107,9 @@ Optional<double> HTMLInputElement::convert_string_to_number(StringView input) co
     if (type_state() == TypeAttributeState::Month)
         return convert_month_string_to_number(input);
 
+    if (type_state() == TypeAttributeState::Week)
+        return convert_week_string_to_number(input);
+
     dbgln("HTMLInputElement::convert_string_to_number() not implemented for input type {}", type());
     return {};
 }
@@ -2108,6 +2125,36 @@ static String convert_number_to_month_string(double input)
     return MUST(String::formatted("{:04d}-{:02d}", static_cast<int>(year), static_cast<int>(months) + 1));
 }
 
+// https://html.spec.whatwg.org/multipage/input.html#week-state-(type=week):concept-input-value-string-number
+static String convert_number_to_week_string(double input)
+{
+    // The algorithm to convert a number to a string, given a number input, is as follows: Return a valid week string that
+    // that represents the week that, in UTC, is current input milliseconds after midnight UTC on the morning of 1970-01-01
+    // (the time represented by the value "1970-01-01T00:00:00.0Z").
+
+    int days_since_epoch = static_cast<int>(input / AK::ms_per_day);
+    int year = 1970;
+
+    while (true) {
+        auto days = days_in_year(year);
+        if (days_since_epoch < days)
+            break;
+        days_since_epoch -= days;
+        ++year;
+    }
+
+    auto january_1_weekday = day_of_week(year, 1, 1);
+    int offset_to_week_start = (january_1_weekday <= 3) ? january_1_weekday : january_1_weekday - 7;
+    int week = (days_since_epoch + offset_to_week_start) / 7 + 1;
+
+    if (week < 0) {
+        --year;
+        week = weeks_in_year(year) + week;
+    }
+
+    return MUST(String::formatted("{:04d}-W{:02d}", year, week));
+}
+
 // https://html.spec.whatwg.org/multipage/input.html#concept-input-value-string-number
 String HTMLInputElement::convert_number_to_string(double input) const
 {
@@ -2122,6 +2169,9 @@ String HTMLInputElement::convert_number_to_string(double input) const
     if (type_state() == TypeAttributeState::Month)
         return convert_number_to_month_string(input);
 
+    if (type_state() == TypeAttributeState::Week)
+        return convert_number_to_week_string(input);
+
     dbgln("HTMLInputElement::convert_number_to_string() not implemented for input type {}", type());
     return {};
 }