LibJS+LibUnicode: Implement the Intl.DateTimeFormat constructor

Userland/Libraries/LibJS/Runtime/Intl/DateTimeFormat.cpp

@@ -4,7 +4,11 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/NumericLimits.h>
+#include <LibJS/Runtime/Intl/AbstractOperations.h>
 #include <LibJS/Runtime/Intl/DateTimeFormat.h>
+#include <LibJS/Runtime/Temporal/TimeZone.h>
+#include <LibUnicode/Locale.h>
 
 namespace JS::Intl {
 
@@ -51,4 +55,612 @@ StringView DateTimeFormat::style_to_string(Style style)
     }
 }
 
+// 11.1.1 InitializeDateTimeFormat ( dateTimeFormat, locales, options ), https://tc39.es/ecma402/#sec-initializedatetimeformat
+ThrowCompletionOr<DateTimeFormat*> initialize_date_time_format(GlobalObject& global_object, DateTimeFormat& date_time_format, Value locales_value, Value options_value)
+{
+    auto& vm = global_object.vm();
+
+    // 1. Let requestedLocales be ? CanonicalizeLocaleList(locales).
+    auto requested_locales = TRY(canonicalize_locale_list(global_object, locales_value));
+
+    // 2. Let options be ? ToDateTimeOptions(options, "any", "date").
+    auto* options = TRY(to_date_time_options(global_object, options_value, OptionRequired::Any, OptionDefaults::Date));
+
+    // 3. Let opt be a new Record.
+    LocaleOptions opt {};
+
+    // 4. Let matcher be ? GetOption(options, "localeMatcher", "string", « "lookup", "best fit" », "best fit").
+    auto matcher = TRY(get_option(global_object, *options, vm.names.localeMatcher, Value::Type::String, AK::Array { "lookup"sv, "best fit"sv }, "best fit"sv));
+
+    // 5. Set opt.[[localeMatcher]] to matcher.
+    opt.locale_matcher = matcher;
+
+    // 6. Let calendar be ? GetOption(options, "calendar", "string", undefined, undefined).
+    auto calendar = TRY(get_option(global_object, *options, vm.names.calendar, Value::Type::String, {}, Empty {}));
+
+    // 7. If calendar is not undefined, then
+    if (!calendar.is_undefined()) {
+        // a. If calendar does not match the Unicode Locale Identifier type nonterminal, throw a RangeError exception.
+        if (!Unicode::is_type_identifier(calendar.as_string().string()))
+            return vm.throw_completion<RangeError>(global_object, ErrorType::OptionIsNotValidValue, calendar, "calendar"sv);
+
+        // 8. Set opt.[[ca]] to calendar.
+        opt.ca = calendar.as_string().string();
+    }
+
+    // 9. Let numberingSystem be ? GetOption(options, "numberingSystem", "string", undefined, undefined).
+    auto numbering_system = TRY(get_option(global_object, *options, vm.names.numberingSystem, Value::Type::String, {}, Empty {}));
+
+    // 10. If numberingSystem is not undefined, then
+    if (!numbering_system.is_undefined()) {
+        // a. If numberingSystem does not match the Unicode Locale Identifier type nonterminal, throw a RangeError exception.
+        if (!Unicode::is_type_identifier(numbering_system.as_string().string()))
+            return vm.throw_completion<RangeError>(global_object, ErrorType::OptionIsNotValidValue, numbering_system, "numberingSystem"sv);
+
+        // 11. Set opt.[[nu]] to numberingSystem.
+        opt.nu = numbering_system.as_string().string();
+    }
+
+    // 12. Let hour12 be ? GetOption(options, "hour12", "boolean", undefined, undefined).
+    auto hour12 = TRY(get_option(global_object, *options, vm.names.hour12, Value::Type::Boolean, {}, Empty {}));
+
+    // 13. Let hourCycle be ? GetOption(options, "hourCycle", "string", « "h11", "h12", "h23", "h24" », undefined).
+    auto hour_cycle = TRY(get_option(global_object, *options, vm.names.hourCycle, Value::Type::String, AK::Array { "h11"sv, "h12"sv, "h23"sv, "h24"sv }, Empty {}));
+
+    // 14. If hour12 is not undefined, then
+    if (!hour12.is_undefined()) {
+        // a. Let hourCycle be null.
+        hour_cycle = js_null();
+    }
+
+    // 15. Set opt.[[hc]] to hourCycle.
+    if (!hour_cycle.is_nullish())
+        opt.hc = hour_cycle.as_string().string();
+
+    // 16. Let localeData be %DateTimeFormat%.[[LocaleData]].
+    // 17. Let r be ResolveLocale(%DateTimeFormat%.[[AvailableLocales]], requestedLocales, opt, %DateTimeFormat%.[[RelevantExtensionKeys]], localeData).
+    auto result = resolve_locale(requested_locales, opt, DateTimeFormat::relevant_extension_keys());
+
+    // 18. Set dateTimeFormat.[[Locale]] to r.[[locale]].
+    date_time_format.set_locale(move(result.locale));
+
+    // 19. Let calendar be r.[[ca]].
+    // 20. Set dateTimeFormat.[[Calendar]] to calendar.
+    date_time_format.set_calendar(result.ca.release_value());
+
+    // 21. Set dateTimeFormat.[[HourCycle]] to r.[[hc]].
+    date_time_format.set_hour_cycle(result.hc.release_value());
+
+    // 22. Set dateTimeFormat.[[NumberingSystem]] to r.[[nu]].
+    date_time_format.set_numbering_system(result.nu.release_value());
+
+    // 23. Let dataLocale be r.[[dataLocale]].
+    auto data_locale = move(result.data_locale);
+
+    // 24. Let timeZone be ? Get(options, "timeZone").
+    auto time_zone_value = TRY(options->get(vm.names.timeZone));
+    String time_zone;
+
+    // 25. If timeZone is undefined, then
+    if (time_zone_value.is_undefined()) {
+        // a. Let timeZone be DefaultTimeZone().
+        time_zone = Temporal::default_time_zone();
+    }
+    // 26. Else,
+    else {
+        // a. Let timeZone be ? ToString(timeZone).
+        time_zone = TRY(time_zone_value.to_string(global_object));
+
+        // b. If the result of IsValidTimeZoneName(timeZone) is false, then
+        if (!Temporal::is_valid_time_zone_name(time_zone)) {
+            // i. Throw a RangeError exception.
+            return vm.throw_completion<RangeError>(global_object, ErrorType::OptionIsNotValidValue, time_zone, vm.names.timeZone);
+        }
+
+        // c. Let timeZone be CanonicalizeTimeZoneName(timeZone).
+        time_zone = Temporal::canonicalize_time_zone_name(time_zone);
+    }
+
+    // 27. Set dateTimeFormat.[[TimeZone]] to timeZone.
+    date_time_format.set_time_zone(move(time_zone));
+
+    // 28. Let opt be a new Record.
+    Unicode::CalendarPattern format_options {};
+
+    // 29. For each row of Table 4, except the header row, in table order, do
+    TRY(for_each_calendar_field(global_object, format_options, [&](auto& option, auto const& property, auto const& defaults) -> ThrowCompletionOr<void> {
+        using ValueType = typename RemoveReference<decltype(option)>::ValueType;
+
+        // a. Let prop be the name given in the Property column of the row.
+
+        // b. If prop is "fractionalSecondDigits", then
+        if constexpr (IsIntegral<ValueType>) {
+            // i. Let value be ? GetNumberOption(options, "fractionalSecondDigits", 1, 3, undefined).
+            auto value = TRY(get_number_option(global_object, *options, property, 1, 3, {}));
+
+            // d. Set opt.[[<prop>]] to value.
+            if (value.has_value())
+                option = static_cast<ValueType>(value.value());
+        }
+        // c. Else,
+        else {
+            // i. Let value be ? GetOption(options, prop, "string", « the strings given in the Values column of the row », undefined).
+            auto value = TRY(get_option(global_object, *options, property, Value::Type::String, defaults, Empty {}));
+
+            // d. Set opt.[[<prop>]] to value.
+            if (!value.is_undefined())
+                option = Unicode::calendar_pattern_style_from_string(value.as_string().string());
+        }
+
+        return {};
+    }));
+
+    // 30. Let dataLocaleData be localeData.[[<dataLocale>]].
+
+    // 31. Let matcher be ? GetOption(options, "formatMatcher", "string", « "basic", "best fit" », "best fit").
+    matcher = TRY(get_option(global_object, *options, vm.names.formatMatcher, Value::Type::String, AK::Array { "basic"sv, "best fit"sv }, "best fit"sv));
+
+    // 32. Let dateStyle be ? GetOption(options, "dateStyle", "string", « "full", "long", "medium", "short" », undefined).
+    auto date_style = TRY(get_option(global_object, *options, vm.names.dateStyle, Value::Type::String, AK::Array { "full"sv, "long"sv, "medium"sv, "short"sv }, Empty {}));
+
+    // 33. Set dateTimeFormat.[[DateStyle]] to dateStyle.
+    if (!date_style.is_undefined())
+        date_time_format.set_date_style(date_style.as_string().string());
+
+    // 34. Let timeStyle be ? GetOption(options, "timeStyle", "string", « "full", "long", "medium", "short" », undefined).
+    auto time_style = TRY(get_option(global_object, *options, vm.names.timeStyle, Value::Type::String, AK::Array { "full"sv, "long"sv, "medium"sv, "short"sv }, Empty {}));
+
+    // 35. Set dateTimeFormat.[[TimeStyle]] to timeStyle.
+    if (!time_style.is_undefined())
+        date_time_format.set_time_style(time_style.as_string().string());
+
+    Optional<Unicode::CalendarPattern> best_format {};
+
+    // 36. If dateStyle is not undefined or timeStyle is not undefined, then
+    if (date_time_format.has_date_style() || date_time_format.has_time_style()) {
+        // a. For each row in Table 4, except the header row, do
+        TRY(for_each_calendar_field(global_object, format_options, [&](auto const& option, auto const& property, auto const&) -> ThrowCompletionOr<void> {
+            // i. Let prop be the name given in the Property column of the row.
+            // ii. Let p be opt.[[<prop>]].
+            // iii. If p is not undefined, then
+            if (option.has_value()) {
+                // 1. Throw a TypeError exception.
+                return vm.throw_completion<TypeError>(global_object, ErrorType::IntlInvalidDateTimeFormatOption, property, "dateStyle or timeStyle"sv);
+            }
+
+            return {};
+        }));
+
+        // b. Let styles be dataLocaleData.[[styles]].[[<calendar>]].
+        // c. Let bestFormat be DateTimeStyleFormat(dateStyle, timeStyle, styles).
+        best_format = date_time_style_format(data_locale, date_time_format);
+    }
+    // 37. Else,
+    else {
+        // a. Let formats be dataLocaleData.[[formats]].[[<calendar>]].
+        auto formats = Unicode::get_calendar_available_formats(data_locale, date_time_format.calendar());
+
+        // b. If matcher is "basic", then
+        if (matcher.as_string().string() == "basic"sv) {
+            // i. Let bestFormat be BasicFormatMatcher(opt, formats).
+            best_format = basic_format_matcher(format_options, move(formats));
+        }
+        // c. Else,
+        else {
+            // i. Let bestFormat be BestFitFormatMatcher(opt, formats).
+            best_format = best_fit_format_matcher(format_options, move(formats));
+        }
+    }
+
+    // Non-standard, best_format will be empty if Unicode data generation is disabled.
+    if (!best_format.has_value())
+        return &date_time_format;
+
+    // 38. For each row in Table 4, except the header row, in table order, do
+    date_time_format.for_each_calendar_field_zipped_with(*best_format, [&](auto& date_time_format_field, auto const& best_format_field) {
+        // a. Let prop be the name given in the Property column of the row.
+        // b. If bestFormat has a field [[<prop>]], then
+        if (best_format_field.has_value()) {
+            // i. Let p be bestFormat.[[<prop>]].
+            // ii. Set dateTimeFormat's internal slot whose name is the Internal Slot column of the row to p.
+            date_time_format_field = best_format_field;
+        }
+    });
+
+    String pattern;
+
+    // 39. If dateTimeFormat.[[Hour]] is undefined, then
+    if (!date_time_format.has_hour()) {
+        // a. Set dateTimeFormat.[[HourCycle]] to undefined.
+        date_time_format.clear_hour_cycle();
+
+        // b. Let pattern be bestFormat.[[pattern]].
+        pattern = move(best_format->pattern);
+
+        // FIXME: Implement step c when range formats are parsed by LibUnicode.
+        // c. Let rangePatterns be bestFormat.[[rangePatterns]].
+    }
+    // 40. Else,
+    else {
+        // a. Let hcDefault be dataLocaleData.[[hourCycle]].
+        auto default_hour_cycle = Unicode::get_default_regional_hour_cycle(data_locale);
+        VERIFY(default_hour_cycle.has_value());
+
+        // b. Let hc be dateTimeFormat.[[HourCycle]].
+        // c. If hc is null, then
+        //     i. Set hc to hcDefault.
+        auto hour_cycle = date_time_format.has_hour_cycle() ? date_time_format.hour_cycle() : *default_hour_cycle;
+
+        // d. If hour12 is not undefined, then
+        if (!hour12.is_undefined()) {
+            // i. If hour12 is true, then
+            if (hour12.as_bool()) {
+                // 1. If hcDefault is "h11" or "h23", then
+                if ((default_hour_cycle == Unicode::HourCycle::H11) || (default_hour_cycle == Unicode::HourCycle::H23)) {
+                    // a. Set hc to "h11".
+                    hour_cycle = Unicode::HourCycle::H11;
+                }
+                // 2. Else,
+                else {
+                    // a. Set hc to "h12".
+                    hour_cycle = Unicode::HourCycle::H12;
+                }
+            }
+            // ii. Else,
+            else {
+                // 1. Assert: hour12 is false.
+                // 2. If hcDefault is "h11" or "h23", then
+                if ((default_hour_cycle == Unicode::HourCycle::H11) || (default_hour_cycle == Unicode::HourCycle::H23)) {
+                    // a. Set hc to "h23".
+                    hour_cycle = Unicode::HourCycle::H23;
+                }
+                // 3. Else,
+                else {
+                    // a. Set hc to "h24".
+                    hour_cycle = Unicode::HourCycle::H24;
+                }
+            }
+        }
+
+        // e. Set dateTimeFormat.[[HourCycle]] to hc.
+        date_time_format.set_hour_cycle(hour_cycle);
+
+        // FIXME: Implement steps f.ii and g.ii when range formats are parsed by LibUnicode.
+
+        // f. If dateTimeformat.[[HourCycle]] is "h11" or "h12", then
+        if ((hour_cycle == Unicode::HourCycle::H11) || (hour_cycle == Unicode::HourCycle::H12)) {
+            // i. Let pattern be bestFormat.[[pattern12]].
+            if (best_format->pattern12.has_value())
+                pattern = best_format->pattern12.release_value();
+
+            // ii. Let rangePatterns be bestFormat.[[rangePatterns12]].
+        }
+        // g. Else,
+        else {
+            // i. Let pattern be bestFormat.[[pattern]].
+            pattern = move(best_format->pattern);
+
+            // ii. Let rangePatterns be bestFormat.[[rangePatterns]].
+        }
+    }
+
+    // 41. Set dateTimeFormat.[[Pattern]] to pattern.
+    date_time_format.set_pattern(move(pattern));
+
+    // FIXME: Implement step 42 when range formats are parsed by LibUnicode.
+    // 42. Set dateTimeFormat.[[RangePatterns]] to rangePatterns.
+
+    // 43. Return dateTimeFormat.
+    return &date_time_format;
+}
+
+// 11.1.2 ToDateTimeOptions ( options, required, defaults ), https://tc39.es/ecma402/#sec-todatetimeoptions
+ThrowCompletionOr<Object*> to_date_time_options(GlobalObject& global_object, Value options_value, OptionRequired required, OptionDefaults defaults)
+{
+    auto& vm = global_object.vm();
+
+    // 1. If options is undefined, let options be null; otherwise let options be ? ToObject(options).
+    Object* options = nullptr;
+    if (!options_value.is_undefined())
+        options = TRY(options_value.to_object(global_object));
+
+    // 2. Let options be OrdinaryObjectCreate(options).
+    options = Object::create(global_object, options);
+
+    // 3. Let needDefaults be true.
+    bool needs_defaults = true;
+
+    // 4. If required is "date" or "any", then
+    if ((required == OptionRequired::Date) || (required == OptionRequired::Any)) {
+        // a. For each property name prop of « "weekday", "year", "month", "day" », do
+        for (auto const& property : AK::Array { vm.names.weekday, vm.names.year, vm.names.month, vm.names.day }) {
+            // i. Let value be ? Get(options, prop).
+            auto value = TRY(options->get(property));
+
+            // ii. If value is not undefined, let needDefaults be false.
+            if (!value.is_undefined())
+                needs_defaults = false;
+        }
+    }
+
+    // 5. If required is "time" or "any", then
+    if ((required == OptionRequired::Time) || (required == OptionRequired::Any)) {
+        // a. For each property name prop of « "dayPeriod", "hour", "minute", "second", "fractionalSecondDigits" », do
+        for (auto const& property : AK::Array { vm.names.dayPeriod, vm.names.hour, vm.names.minute, vm.names.second, vm.names.fractionalSecondDigits }) {
+            // i. Let value be ? Get(options, prop).
+            auto value = TRY(options->get(property));
+
+            // ii. If value is not undefined, let needDefaults be false.
+            if (!value.is_undefined())
+                needs_defaults = false;
+        }
+    }
+
+    // 6. Let dateStyle be ? Get(options, "dateStyle").
+    auto date_style = TRY(options->get(vm.names.dateStyle));
+
+    // 7. Let timeStyle be ? Get(options, "timeStyle").
+    auto time_style = TRY(options->get(vm.names.timeStyle));
+
+    // 8. If dateStyle is not undefined or timeStyle is not undefined, let needDefaults be false.
+    if (!date_style.is_undefined() || !time_style.is_undefined())
+        needs_defaults = false;
+
+    // 9. If required is "date" and timeStyle is not undefined, then
+    if ((required == OptionRequired::Date) && !time_style.is_undefined()) {
+        // a. Throw a TypeError exception.
+        return vm.throw_completion<TypeError>(global_object, ErrorType::IntlInvalidDateTimeFormatOption, "timeStyle"sv, "date"sv);
+    }
+
+    // 10. If required is "time" and dateStyle is not undefined, then
+    if ((required == OptionRequired::Time) && !date_style.is_undefined()) {
+        // a. Throw a TypeError exception.
+        return vm.throw_completion<TypeError>(global_object, ErrorType::IntlInvalidDateTimeFormatOption, "dateStyle"sv, "time"sv);
+    }
+
+    // 11. If needDefaults is true and defaults is either "date" or "all", then
+    if (needs_defaults && ((defaults == OptionDefaults::Date) || (defaults == OptionDefaults::All))) {
+        // a. For each property name prop of « "year", "month", "day" », do
+        for (auto const& property : AK::Array { vm.names.year, vm.names.month, vm.names.day }) {
+            // i. Perform ? CreateDataPropertyOrThrow(options, prop, "numeric").
+            TRY(options->create_data_property_or_throw(property, js_string(vm, "numeric"sv)));
+        }
+    }
+
+    // 12. If needDefaults is true and defaults is either "time" or "all", then
+    if (needs_defaults && ((defaults == OptionDefaults::Time) || (defaults == OptionDefaults::All))) {
+        // a. For each property name prop of « "hour", "minute", "second" », do
+        for (auto const& property : AK::Array { vm.names.hour, vm.names.minute, vm.names.second }) {
+            // i. Perform ? CreateDataPropertyOrThrow(options, prop, "numeric").
+            TRY(options->create_data_property_or_throw(property, js_string(vm, "numeric"sv)));
+        }
+    }
+
+    // 13. Return options.
+    return options;
+}
+
+// 11.1.3 DateTimeStyleFormat ( dateStyle, timeStyle, styles ), https://tc39.es/ecma402/#sec-date-time-style-format
+Optional<Unicode::CalendarPattern> date_time_style_format(StringView data_locale, DateTimeFormat& date_time_format)
+{
+    Unicode::CalendarPattern time_format {};
+    Unicode::CalendarPattern date_format {};
+
+    auto get_pattern = [&](auto type, auto style) -> Optional<Unicode::CalendarPattern> {
+        auto formats = Unicode::get_calendar_format(data_locale, date_time_format.calendar(), type);
+
+        if (formats.has_value()) {
+            switch (style) {
+            case DateTimeFormat::Style::Full:
+                return formats->full_format;
+            case DateTimeFormat::Style::Long:
+                return formats->long_format;
+            case DateTimeFormat::Style::Medium:
+                return formats->medium_format;
+            case DateTimeFormat::Style::Short:
+                return formats->short_format;
+            }
+        }
+
+        return {};
+    };
+
+    // 1. If timeStyle is not undefined, then
+    if (date_time_format.has_time_style()) {
+        // a. Assert: timeStyle is one of "full", "long", "medium", or "short".
+        // b. Let timeFormat be styles.[[TimeFormat]].[[<timeStyle>]].
+        auto pattern = get_pattern(Unicode::CalendarFormatType::Time, date_time_format.time_style());
+        if (!pattern.has_value())
+            return {};
+
+        time_format = pattern.release_value();
+    }
+
+    // 2. If dateStyle is not undefined, then
+    if (date_time_format.has_date_style()) {
+        // a. Assert: dateStyle is one of "full", "long", "medium", or "short".
+        // b. Let dateFormat be styles.[[DateFormat]].[[<dateStyle>]].
+        auto pattern = get_pattern(Unicode::CalendarFormatType::Date, date_time_format.date_style());
+        if (!pattern.has_value())
+            return {};
+
+        date_format = pattern.release_value();
+    }
+
+    // 3. If dateStyle is not undefined and timeStyle is not undefined, then
+    if (date_time_format.has_date_style() && date_time_format.has_time_style()) {
+        // a. Let format be a new Record.
+        Unicode::CalendarPattern format {};
+
+        // b. Add to format all fields from dateFormat except [[pattern]] and [[rangePatterns]].
+        format.for_each_calendar_field_zipped_with(date_format, [](auto& format_field, auto const& date_format_field) {
+            format_field = date_format_field;
+        });
+
+        // c. Add to format all fields from timeFormat except [[pattern]], [[rangePatterns]], [[pattern12]], and [[rangePatterns12]], if present.
+        format.for_each_calendar_field_zipped_with(time_format, [](auto& format_field, auto const& time_format_field) {
+            if (time_format_field.has_value())
+                format_field = time_format_field;
+        });
+
+        // d. Let connector be styles.[[DateTimeFormat]].[[<dateStyle>]].
+        auto connector = get_pattern(Unicode::CalendarFormatType::DateTime, date_time_format.date_style());
+        if (!connector.has_value())
+            return {};
+
+        // e. Let pattern be the string connector with the substring "{0}" replaced with timeFormat.[[pattern]] and the substring "{1}" replaced with dateFormat.[[pattern]].
+        auto pattern = connector->pattern.replace("{0}"sv, time_format.pattern).replace("{1}"sv, date_format.pattern);
+
+        // f. Set format.[[pattern]] to pattern.
+        format.pattern = move(pattern);
+
+        // g. If timeFormat has a [[pattern12]] field, then
+        if (time_format.pattern12.has_value()) {
+            // i. Let pattern12 be the string connector with the substring "{0}" replaced with timeFormat.[[pattern12]] and the substring "{1}" replaced with dateFormat.[[pattern]].
+            auto pattern12 = connector->pattern.replace("{0}"sv, *time_format.pattern12).replace("{1}"sv, date_format.pattern);
+
+            // ii. Set format.[[pattern12]] to pattern12.
+            format.pattern12 = move(pattern12);
+        }
+
+        // FIXME: Implement steps h-j when range formats are parsed by LibUnicode.
+        // h. Let dateTimeRangeFormat be styles.[[DateTimeRangeFormat]].[[<dateStyle>]].[[<timeStyle>]].
+        // i. Set format.[[rangePatterns]] to dateTimeRangeFormat.[[rangePatterns]].
+        // j. If dateTimeRangeFormat has a [[rangePatterns12]] field, then
+        //     i. Set format.[[rangePatterns12]] to dateTimeRangeFormat.[[rangePatterns12]].
+
+        // k. Return format.
+        return format;
+    }
+
+    // 4. If timeStyle is not undefined, then
+    if (date_time_format.has_time_style()) {
+        // a. Return timeFormat.
+        return time_format;
+    }
+
+    // 5. Assert: dateStyle is not undefined.
+    VERIFY(date_time_format.has_date_style());
+
+    // 6. Return dateFormat.
+    return date_format;
+}
+
+// 11.1.4 BasicFormatMatcher ( options, formats ), https://tc39.es/ecma402/#sec-basicformatmatcher
+Optional<Unicode::CalendarPattern> basic_format_matcher(Unicode::CalendarPattern const& options, Vector<Unicode::CalendarPattern> formats)
+{
+    // 1. Let removalPenalty be 120.
+    constexpr int removal_penalty = 120;
+
+    // 2. Let additionPenalty be 20.
+    constexpr int addition_penalty = 20;
+
+    // 3. Let longLessPenalty be 8.
+    constexpr int long_less_penalty = 8;
+
+    // 4. Let longMorePenalty be 6.
+    constexpr int long_more_penalty = 6;
+
+    // 5. Let shortLessPenalty be 6.
+    constexpr int short_less_penalty = 6;
+
+    // 6. Let shortMorePenalty be 3.
+    constexpr int short_more_penalty = 3;
+
+    // 7. Let bestScore be -Infinity.
+    int best_score = NumericLimits<int>::min();
+
+    // 8. Let bestFormat be undefined.
+    Optional<Unicode::CalendarPattern> best_format;
+
+    // 9. Assert: Type(formats) is List.
+    // 10. For each element format of formats, do
+    for (auto& format : formats) {
+        // a. Let score be 0.
+        int score = 0;
+
+        // b. For each property name property shown in Table 4, do
+        format.for_each_calendar_field_zipped_with(options, [&](auto const& format_prop, auto const& options_prop) {
+            using ValueType = typename RemoveReference<decltype(options_prop)>::ValueType;
+
+            // i. If options has a field [[<property>]], let optionsProp be options.[[<property>]]; else let optionsProp be undefined.
+            // ii. If format has a field [[<property>]], let formatProp be format.[[<property>]]; else let formatProp be undefined.
+
+            // iii. If optionsProp is undefined and formatProp is not undefined, decrease score by additionPenalty.
+            if (!options_prop.has_value() && format_prop.has_value()) {
+                score -= addition_penalty;
+            }
+            // iv. Else if optionsProp is not undefined and formatProp is undefined, decrease score by removalPenalty.
+            else if (options_prop.has_value() && !format_prop.has_value()) {
+                score -= removal_penalty;
+            }
+            // v. Else if optionsProp ≠ formatProp, then
+            else if (options_prop != format_prop) {
+                using ValuesType = Conditional<IsIntegral<ValueType>, AK::Array<u8, 3>, AK::Array<Unicode::CalendarPatternStyle, 5>>;
+                ValuesType values {};
+
+                // 1. If property is "fractionalSecondDigits", then
+                if constexpr (IsIntegral<ValueType>) {
+                    // a. Let values be « 1𝔽, 2𝔽, 3𝔽 ».
+                    values = { 1, 2, 3 };
+                }
+                // 2. Else,
+                else {
+                    // a. Let values be « "2-digit", "numeric", "narrow", "short", "long" ».
+                    values = {
+                        Unicode::CalendarPatternStyle::TwoDigit,
+                        Unicode::CalendarPatternStyle::Numeric,
+                        Unicode::CalendarPatternStyle::Narrow,
+                        Unicode::CalendarPatternStyle::Short,
+                        Unicode::CalendarPatternStyle::Long,
+                    };
+                }
+
+                // 3. Let optionsPropIndex be the index of optionsProp within values.
+                auto options_prop_index = static_cast<int>(find_index(values.begin(), values.end(), *options_prop));
+
+                // 4. Let formatPropIndex be the index of formatProp within values.
+                auto format_prop_index = static_cast<int>(find_index(values.begin(), values.end(), *format_prop));
+
+                // 5. Let delta be max(min(formatPropIndex - optionsPropIndex, 2), -2).
+                int delta = max(min(format_prop_index - options_prop_index, 2), -2);
+
+                // 6. If delta = 2, decrease score by longMorePenalty.
+                if (delta == 2)
+                    score -= long_more_penalty;
+                // 7. Else if delta = 1, decrease score by shortMorePenalty.
+                else if (delta == 1)
+                    score -= short_more_penalty;
+                // 8. Else if delta = -1, decrease score by shortLessPenalty.
+                else if (delta == -1)
+                    score -= short_less_penalty;
+                // 9. Else if delta = -2, decrease score by longLessPenalty.
+                else if (delta == -2)
+                    score -= long_less_penalty;
+            }
+        });
+
+        // c. If score > bestScore, then
+        if (score > best_score) {
+            // i. Let bestScore be score.
+            best_score = score;
+
+            // ii. Let bestFormat be format.
+            best_format = format;
+        }
+    }
+
+    // 11. Return bestFormat.
+    return best_format;
+}
+
+// 11.1.5 BestFitFormatMatcher ( options, formats ), https://tc39.es/ecma402/#sec-bestfitformatmatcher
+Optional<Unicode::CalendarPattern> best_fit_format_matcher(Unicode::CalendarPattern const& options, Vector<Unicode::CalendarPattern> formats)
+{
+    // When the BestFitFormatMatcher abstract operation is called with two arguments options and formats, it performs
+    // implementation dependent steps, which should return a set of component representations that a typical user of
+    // the selected locale would perceive as at least as good as the one returned by BasicFormatMatcher.
+    return basic_format_matcher(options, move(formats));
+}
+
 }