calendar.ts

import { DEBUG } from './debug';
import * as ES from './ecmascript';
import { GetIntrinsic, MakeIntrinsicClass, DefineIntrinsic } from './intrinsicclass';
import {
  CALENDAR_ID,
  ISO_YEAR,
  ISO_MONTH,
  ISO_DAY,
  YEARS,
  MONTHS,
  WEEKS,
  DAYS,
  HOURS,
  MINUTES,
  SECONDS,
  MILLISECONDS,
  MICROSECONDS,
  NANOSECONDS,
  CreateSlots,
  GetSlot,
  HasSlot,
  SetSlot
} from './slots';
import type { Temporal } from '..';
import type {
  BuiltinCalendarId,
  CalendarParams as Params,
  CalendarReturn as Return,
  AnyTemporalKey,
  CalendarSlot
} from './internaltypes';

const ArrayIncludes = Array.prototype.includes;
const ArrayPrototypePush = Array.prototype.push;
const IntlDateTimeFormat = globalThis.Intl.DateTimeFormat;
const ArraySort = Array.prototype.sort;
const MathAbs = Math.abs;
const MathFloor = Math.floor;
const ObjectCreate = Object.create;
const ObjectEntries = Object.entries;
const OriginalSet = Set;
const ReflectOwnKeys = Reflect.ownKeys;
const SetPrototypeAdd = Set.prototype.add;
const SetPrototypeValues = Set.prototype.values;

/**
 * Shape of internal implementation of each built-in calendar. Note that
 * parameter types are simpler than CalendarProtocol because the `Calendar`
 * class performs validation and parameter normalization before handing control
 * over to CalendarImpl.
 *
 * There are two instances of this interface: one for the ISO calendar and
 * another that handles logic that's the same across all non-ISO calendars. The
 * latter is cloned for each non-ISO calendar at the end of this file.
 */
interface CalendarImpl {
  year(date: Temporal.PlainDate | Temporal.PlainYearMonth): number;
  month(date: Temporal.PlainDate | Temporal.PlainYearMonth | Temporal.PlainMonthDay): number;
  monthCode(date: Temporal.PlainDate | Temporal.PlainYearMonth | Temporal.PlainMonthDay): string;
  day(date: Temporal.PlainDate | Temporal.PlainMonthDay): number;
  era(date: Temporal.PlainDate | Temporal.PlainYearMonth): string | undefined;
  eraYear(date: Temporal.PlainDate | Temporal.PlainYearMonth): number | undefined;
  dayOfWeek(date: Temporal.PlainDate): number;
  dayOfYear(date: Temporal.PlainDate): number;
  weekOfYear(date: Temporal.PlainDate): number;
  yearOfWeek(date: Temporal.PlainDate): number;
  daysInWeek(date: Temporal.PlainDate): number;
  daysInMonth(date: Temporal.PlainDate | Temporal.PlainYearMonth): number;
  daysInYear(date: Temporal.PlainDate | Temporal.PlainYearMonth): number;
  monthsInYear(date: Temporal.PlainDate | Temporal.PlainYearMonth): number;
  inLeapYear(date: Temporal.PlainDate | Temporal.PlainYearMonth): boolean;
  dateFromFields(
    fields: Params['dateFromFields'][0],
    options: NonNullable<Params['dateFromFields'][1]>,
    calendar: string
  ): Temporal.PlainDate;
  yearMonthFromFields(
    fields: Params['yearMonthFromFields'][0],
    options: NonNullable<Params['yearMonthFromFields'][1]>,
    calendar: string
  ): Temporal.PlainYearMonth;
  monthDayFromFields(
    fields: Params['monthDayFromFields'][0],
    options: NonNullable<Params['monthDayFromFields'][1]>,
    calendar: string
  ): Temporal.PlainMonthDay;
  dateAdd(
    date: Temporal.PlainDate,
    years: number,
    months: number,
    weeks: number,
    days: number,
    overflow: Overflow,
    calendar: string
  ): Temporal.PlainDate;
  dateUntil(
    one: Temporal.PlainDate,
    two: Temporal.PlainDate,
    largestUnit: 'year' | 'month' | 'week' | 'day'
  ): { years: number; months: number; weeks: number; days: number };
  fields(fields: string[]): string[];
  fieldKeysToIgnore(keys: string[]): string[];
}

type CalendarImplementations = {
  [k in BuiltinCalendarId]: CalendarImpl;
};

/**
 * Implementations for each calendar.
 * Registration for each of these calendars happens throughout this file. The ISO and non-ISO calendars are registered
 * separately - look for 'iso8601' for the ISO calendar registration, and all non-ISO calendar registrations happens
 * at the bottom of the file.
 */
const impl: CalendarImplementations = {} as unknown as CalendarImplementations;

/**
 * Thin wrapper around the implementation of each built-in calendar. This
 * class's methods follow a similar pattern:
 * 1. Validate parameters
 * 2. Fill in default options (for methods where options are present)
 * 3. Simplify and/or normalize parameters. For example, some methods accept
 *    PlainDate, PlainDateTime, ZonedDateTime, etc. and these are normalized to
 *    PlainDate.
 * 4. Look up the ID of the built-in calendar
 * 5. Fetch the implementation object for that ID.
 * 6. Call the corresponding method in the implementation object.
 */
export class Calendar implements Temporal.Calendar {
  constructor(idParam: Params['constructor'][0]) {
    // Note: if the argument is not passed, IsBuiltinCalendar("undefined") will fail. This check
    //       exists only to improve the error message.
    if (arguments.length < 1) {
      throw new RangeError('missing argument: id is required');
    }

    const id = ES.ToString(idParam);
    if (!ES.IsBuiltinCalendar(id)) throw new RangeError(`invalid calendar identifier ${id}`);
    CreateSlots(this);
    SetSlot(this, CALENDAR_ID, ES.ASCIILowercase(id));

    if (DEBUG) {
      Object.defineProperty(this, '_repr_', {
        value: `${this[Symbol.toStringTag]} <${id}>`,
        writable: false,
        enumerable: false,
        configurable: false
      });
    }
  }
  get id(): Return['id'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    return GetSlot(this, CALENDAR_ID);
  }
  dateFromFields(
    fields: Params['dateFromFields'][0],
    optionsParam: Params['dateFromFields'][1] = undefined
  ): Return['dateFromFields'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsObject(fields)) throw new TypeError('invalid fields');
    const options = ES.GetOptionsObject(optionsParam);
    const id = GetSlot(this, CALENDAR_ID);
    return impl[id].dateFromFields(fields, options, id);
  }
  yearMonthFromFields(
    fields: Params['yearMonthFromFields'][0],
    optionsParam: Params['yearMonthFromFields'][1] = undefined
  ): Return['yearMonthFromFields'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsObject(fields)) throw new TypeError('invalid fields');
    const options = ES.GetOptionsObject(optionsParam);
    const id = GetSlot(this, CALENDAR_ID);
    return impl[id].yearMonthFromFields(fields, options, id);
  }
  monthDayFromFields(
    fields: Params['monthDayFromFields'][0],
    optionsParam: Params['monthDayFromFields'][1] = undefined
  ): Return['monthDayFromFields'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsObject(fields)) throw new TypeError('invalid fields');
    const options = ES.GetOptionsObject(optionsParam);
    const id = GetSlot(this, CALENDAR_ID);
    return impl[id].monthDayFromFields(fields, options, id);
  }
  fields(fields: Params['fields'][0]): Return['fields'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const fieldsArray = [] as string[];
    const allowed = new Set([
      'year',
      'month',
      'monthCode',
      'day',
      'hour',
      'minute',
      'second',
      'millisecond',
      'microsecond',
      'nanosecond'
    ]);
    for (const name of fields) {
      if (typeof name !== 'string') throw new TypeError('invalid fields');
      if (!allowed.has(name)) throw new RangeError(`invalid field name ${name}`);
      allowed.delete(name);
      ArrayPrototypePush.call(fieldsArray, name);
    }
    return impl[GetSlot(this, CALENDAR_ID)].fields(fieldsArray);
  }
  mergeFields(fields: Params['mergeFields'][0], additionalFields: Params['mergeFields'][1]): Return['mergeFields'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const fieldsCopy = ES.SnapshotOwnProperties(fields, null, [], [undefined]);
    const additionalFieldsCopy = ES.SnapshotOwnProperties(additionalFields, null, [], [undefined]);
    const additionalKeys = ReflectOwnKeys(additionalFieldsCopy) as (keyof typeof additionalFields)[];
    const overriddenKeys = impl[GetSlot(this, CALENDAR_ID)].fieldKeysToIgnore(additionalKeys);
    const merged = ObjectCreate(null);
    const fieldsKeys = ReflectOwnKeys(fieldsCopy);
    ES.uncheckedAssertNarrowedType<string[]>(fieldsKeys, 'Reflect.ownKeys does not respect the type of its input');
    for (const key of fieldsKeys) {
      let propValue = undefined;
      if (ES.Call(ArrayIncludes, overriddenKeys, [key])) propValue = additionalFieldsCopy[key];
      else propValue = fieldsCopy[key];
      if (propValue !== undefined) merged[key] = propValue;
    }
    ES.CopyDataProperties(merged, additionalFieldsCopy, []);
    return merged;
  }
  dateAdd(
    dateParam: Params['dateAdd'][0],
    durationParam: Params['dateAdd'][1],
    optionsParam: Params['dateAdd'][2] = undefined
  ): Return['dateAdd'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const date = ES.ToTemporalDate(dateParam);
    const duration = ES.ToTemporalDuration(durationParam);
    const options = ES.GetOptionsObject(optionsParam);
    const overflow = ES.ToTemporalOverflow(options);
    const { days } = ES.BalanceDuration(
      GetSlot(duration, DAYS),
      GetSlot(duration, HOURS),
      GetSlot(duration, MINUTES),
      GetSlot(duration, SECONDS),
      GetSlot(duration, MILLISECONDS),
      GetSlot(duration, MICROSECONDS),
      GetSlot(duration, NANOSECONDS),
      'day'
    );
    const id = GetSlot(this, CALENDAR_ID);
    return impl[id].dateAdd(
      date,
      GetSlot(duration, YEARS),
      GetSlot(duration, MONTHS),
      GetSlot(duration, WEEKS),
      days,
      overflow,
      id
    );
  }
  dateUntil(
    oneParam: Params['dateUntil'][0],
    twoParam: Params['dateUntil'][1],
    optionsParam: Params['dateUntil'][2] = undefined
  ): Return['dateUntil'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const one = ES.ToTemporalDate(oneParam);
    const two = ES.ToTemporalDate(twoParam);
    const options = ES.GetOptionsObject(optionsParam);
    let largestUnit = ES.GetTemporalUnit(options, 'largestUnit', 'date', 'auto');
    if (largestUnit === 'auto') largestUnit = 'day';
    const { years, months, weeks, days } = impl[GetSlot(this, CALENDAR_ID)].dateUntil(one, two, largestUnit);
    const Duration = GetIntrinsic('%Temporal.Duration%');
    return new Duration(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
  }
  year(dateParam: Params['year'][0]): Return['year'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].year(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  month(dateParam: Params['month'][0]): Return['month'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (ES.IsTemporalMonthDay(date)) throw new TypeError('use monthCode on PlainMonthDay instead');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].month(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  monthCode(dateParam: Params['monthCode'][0]): Return['monthCode'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date) && !ES.IsTemporalMonthDay(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].monthCode(
      date as Temporal.PlainDate | Temporal.PlainMonthDay | Temporal.PlainYearMonth
    );
  }
  day(dateParam: Params['day'][0]): Return['day'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalMonthDay(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].day(date as Temporal.PlainDate | Temporal.PlainMonthDay);
  }
  era(dateParam: Params['era'][0]): Return['era'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].era(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  eraYear(dateParam: Params['eraYear'][0]): Return['eraYear'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].eraYear(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  dayOfWeek(dateParam: Params['dayOfWeek'][0]): Return['dayOfWeek'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const date = ES.ToTemporalDate(dateParam);
    return impl[GetSlot(this, CALENDAR_ID)].dayOfWeek(date);
  }
  dayOfYear(dateParam: Params['dayOfYear'][0]): Return['dayOfYear'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const date = ES.ToTemporalDate(dateParam);
    return impl[GetSlot(this, CALENDAR_ID)].dayOfYear(date);
  }
  weekOfYear(dateParam: Params['weekOfYear'][0]): Return['weekOfYear'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const date = ES.ToTemporalDate(dateParam);
    return impl[GetSlot(this, CALENDAR_ID)].weekOfYear(date);
  }
  yearOfWeek(dateParam: Params['yearOfWeek'][0]): Return['yearOfWeek'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const date = ES.ToTemporalDate(dateParam);
    return impl[GetSlot(this, CALENDAR_ID)].yearOfWeek(date);
  }
  daysInWeek(dateParam: Params['daysInWeek'][0]): Return['daysInWeek'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    const date = ES.ToTemporalDate(dateParam);
    return impl[GetSlot(this, CALENDAR_ID)].daysInWeek(date);
  }
  daysInMonth(dateParam: Params['daysInMonth'][0]): Return['daysInMonth'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].daysInMonth(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  daysInYear(dateParam: Params['daysInYear'][0]): Return['daysInYear'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].daysInYear(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  monthsInYear(dateParam: Params['monthsInYear'][0]): Return['monthsInYear'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].monthsInYear(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  inLeapYear(dateParam: Params['inLeapYear'][0]): Return['inLeapYear'] {
    let date = dateParam;
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    if (!ES.IsTemporalYearMonth(date)) date = ES.ToTemporalDate(date);
    return impl[GetSlot(this, CALENDAR_ID)].inLeapYear(date as Temporal.PlainDate | Temporal.PlainYearMonth);
  }
  toString(): string {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    return GetSlot(this, CALENDAR_ID);
  }
  toJSON(): Return['toJSON'] {
    if (!ES.IsTemporalCalendar(this)) throw new TypeError('invalid receiver');
    return GetSlot(this, CALENDAR_ID);
  }
  static from(item: Params['from'][0]): Return['from'] {
    const calendarSlotValue = ES.ToTemporalCalendarSlotValue(item);
    return ES.ToTemporalCalendarObject(calendarSlotValue);
  }
  [Symbol.toStringTag]!: 'Temporal.Calendar';
}

MakeIntrinsicClass(Calendar, 'Temporal.Calendar');
DefineIntrinsic('Temporal.Calendar.from', Calendar.from);
DefineIntrinsic('Temporal.Calendar.prototype.dateAdd', Calendar.prototype.dateAdd);
DefineIntrinsic('Temporal.Calendar.prototype.dateFromFields', Calendar.prototype.dateFromFields);
DefineIntrinsic('Temporal.Calendar.prototype.dateUntil', Calendar.prototype.dateUntil);
DefineIntrinsic('Temporal.Calendar.prototype.day', Calendar.prototype.day);
DefineIntrinsic('Temporal.Calendar.prototype.dayOfWeek', Calendar.prototype.dayOfWeek);
DefineIntrinsic('Temporal.Calendar.prototype.dayOfYear', Calendar.prototype.dayOfYear);
DefineIntrinsic('Temporal.Calendar.prototype.daysInMonth', Calendar.prototype.daysInMonth);
DefineIntrinsic('Temporal.Calendar.prototype.daysInWeek', Calendar.prototype.daysInWeek);
DefineIntrinsic('Temporal.Calendar.prototype.daysInYear', Calendar.prototype.daysInYear);
DefineIntrinsic('Temporal.Calendar.prototype.era', Calendar.prototype.era);
DefineIntrinsic('Temporal.Calendar.prototype.eraYear', Calendar.prototype.eraYear);
DefineIntrinsic('Temporal.Calendar.prototype.fields', Calendar.prototype.fields);
DefineIntrinsic('Temporal.Calendar.prototype.inLeapYear', Calendar.prototype.inLeapYear);
DefineIntrinsic('Temporal.Calendar.prototype.mergeFields', Calendar.prototype.mergeFields);
DefineIntrinsic('Temporal.Calendar.prototype.month', Calendar.prototype.month);
DefineIntrinsic('Temporal.Calendar.prototype.monthCode', Calendar.prototype.monthCode);
DefineIntrinsic('Temporal.Calendar.prototype.monthDayFromFields', Calendar.prototype.monthDayFromFields);
DefineIntrinsic('Temporal.Calendar.prototype.monthsInYear', Calendar.prototype.monthsInYear);
DefineIntrinsic('Temporal.Calendar.prototype.weekOfYear', Calendar.prototype.weekOfYear);
DefineIntrinsic('Temporal.Calendar.prototype.year', Calendar.prototype.year);
DefineIntrinsic('Temporal.Calendar.prototype.yearMonthFromFields', Calendar.prototype.yearMonthFromFields);
DefineIntrinsic('Temporal.Calendar.prototype.yearOfWeek', Calendar.prototype.yearOfWeek);

/**
 * Implementation for the ISO 8601 calendar. This is the only calendar that's
 * guaranteed to be supported by all ECMAScript implementations, including those
 * without Intl (ECMA-402) support.
 */
impl['iso8601'] = {
  dateFromFields(fieldsParam, options, calendarSlotValue) {
    let fields = ES.PrepareTemporalFields(fieldsParam, ['day', 'month', 'monthCode', 'year'], ['year', 'day']);
    const overflow = ES.ToTemporalOverflow(options);
    fields = resolveNonLunisolarMonth(fields);
    let { year, month, day } = fields;
    ({ year, month, day } = ES.RegulateISODate(year, month, day, overflow));
    return ES.CreateTemporalDate(year, month, day, calendarSlotValue);
  },
  yearMonthFromFields(fieldsParam, options, calendarSlotValue) {
    let fields = ES.PrepareTemporalFields(fieldsParam, ['month', 'monthCode', 'year'], ['year']);
    const overflow = ES.ToTemporalOverflow(options);
    fields = resolveNonLunisolarMonth(fields);
    let { year, month } = fields;
    ({ year, month } = ES.RegulateISOYearMonth(year, month, overflow));
    return ES.CreateTemporalYearMonth(year, month, calendarSlotValue, /* referenceISODay = */ 1);
  },
  monthDayFromFields(fieldsParam, options, calendarSlotValue) {
    let fields = ES.PrepareTemporalFields(fieldsParam, ['day', 'month', 'monthCode', 'year'], ['day']);
    const overflow = ES.ToTemporalOverflow(options);
    if (fields.month !== undefined && fields.year === undefined && fields.monthCode === undefined) {
      throw new TypeError('either year or monthCode required with month');
    }
    const useYear = fields.monthCode === undefined;
    const referenceISOYear = 1972;
    fields = resolveNonLunisolarMonth(fields);
    let { month, day, year } = fields;
    ({ month, day } = ES.RegulateISODate(useYear ? year : referenceISOYear, month, day, overflow));
    return ES.CreateTemporalMonthDay(month, day, calendarSlotValue, referenceISOYear);
  },
  fields(fields) {
    return fields;
  },
  fieldKeysToIgnore(keys) {
    const result = new OriginalSet();
    for (let ix = 0; ix < keys.length; ix++) {
      const key = keys[ix];
      ES.Call(SetPrototypeAdd, result, [key]);
      if (key === 'month') {
        ES.Call(SetPrototypeAdd, result, ['monthCode']);
      } else if (key === 'monthCode') {
        ES.Call(SetPrototypeAdd, result, ['month']);
      }
    }
    return [...ES.Call(SetPrototypeValues, result, [])];
  },
  dateAdd(date, years, months, weeks, days, overflow, calendarSlotValue) {
    let year = GetSlot(date, ISO_YEAR);
    let month = GetSlot(date, ISO_MONTH);
    let day = GetSlot(date, ISO_DAY);
    ({ year, month, day } = ES.AddISODate(year, month, day, years, months, weeks, days, overflow));
    return ES.CreateTemporalDate(year, month, day, calendarSlotValue);
  },
  dateUntil(one, two, largestUnit) {
    return ES.DifferenceISODate(
      GetSlot(one, ISO_YEAR),
      GetSlot(one, ISO_MONTH),
      GetSlot(one, ISO_DAY),
      GetSlot(two, ISO_YEAR),
      GetSlot(two, ISO_MONTH),
      GetSlot(two, ISO_DAY),
      largestUnit
    );
  },
  year(date) {
    return GetSlot(date, ISO_YEAR);
  },
  era() {
    return undefined;
  },
  eraYear() {
    return undefined;
  },
  month(date) {
    return GetSlot(date, ISO_MONTH);
  },
  monthCode(date) {
    return buildMonthCode(GetSlot(date, ISO_MONTH));
  },
  day(date) {
    return GetSlot(date, ISO_DAY);
  },
  dayOfWeek(date) {
    return ES.DayOfWeek(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY));
  },
  dayOfYear(date) {
    return ES.DayOfYear(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY));
  },
  weekOfYear(date) {
    return ES.WeekOfYear(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY)).week;
  },
  yearOfWeek(date) {
    return ES.WeekOfYear(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH), GetSlot(date, ISO_DAY)).year;
  },
  daysInWeek() {
    return 7;
  },
  daysInMonth(date) {
    return ES.ISODaysInMonth(GetSlot(date, ISO_YEAR), GetSlot(date, ISO_MONTH));
  },
  daysInYear(dateParam) {
    let date = dateParam;
    if (!HasSlot(date, ISO_YEAR)) date = ES.ToTemporalDate(date);
    return ES.LeapYear(GetSlot(date, ISO_YEAR)) ? 366 : 365;
  },
  monthsInYear() {
    return 12;
  },
  inLeapYear(dateParam) {
    let date = dateParam;
    if (!HasSlot(date, ISO_YEAR)) date = ES.ToTemporalDate(date);
    return ES.LeapYear(GetSlot(date, ISO_YEAR));
  }
};

// Note: Built-in calendars other than iso8601 are not part of the Temporal
// proposal for ECMA-262. These calendars will be standardized as part of
// ECMA-402. Code below here includes an implementation of these calendars to
// validate the Temporal API and to get feedback. However, native non-ISO
// calendar behavior is at least somewhat implementation-defined, so may not
// match this polyfill's output exactly.
//
// Some ES implementations don't include ECMA-402. For this reason, it's helpful
// to ensure a clean separation between the ISO calendar implementation which is
// a part of ECMA-262 and the non-ISO calendar implementation which requires
// ECMA-402.
//
// To ensure this separation, the implementation is split. A `CalendarImpl`
// interface defines the common operations between both ISO and non-ISO
// calendars.

/**
 * This type is passed through from Calendar#dateFromFields().
 * `monthExtra` is additional information used internally to identify lunisolar leap months.
 */
type CalendarDateFields = Params['dateFromFields'][0] & { monthExtra?: string };

/**
 * This is a "fully populated" calendar date record. It's only lacking
 * `era`/`eraYear` (which may not be present in all calendars) and `monthExtra`
 * which is only used in some cases.
 */
type FullCalendarDate = {
  era?: string;
  eraYear?: number;
  year: number;
  month: number;
  monthCode: string;
  day: number;
  monthExtra?: string;
};

// The types below are various subsets of calendar dates
type CalendarYMD = { year: number; month: number; day: number };
type CalendarYM = { year: number; month: number };
type CalendarYearOnly = { year: number };
type EraAndEraYear = { era: string; eraYear: number };

/** Record representing YMD of an ISO calendar date */
type IsoYMD = { year: number; month: number; day: number };

type Overflow = NonNullable<Temporal.AssignmentOptions['overflow']>;

function monthCodeNumberPart(monthCode: string) {
  if (!monthCode.startsWith('M')) {
    throw new RangeError(`Invalid month code: ${monthCode}.  Month codes must start with M.`);
  }
  const month = +monthCode.slice(1);
  if (isNaN(month)) throw new RangeError(`Invalid month code: ${monthCode}`);
  return month;
}

function buildMonthCode(month: number | string, leap = false) {
  return `M${month.toString().padStart(2, '0')}${leap ? 'L' : ''}`;
}

/**
 * Safely merge a month, monthCode pair into an integer month.
 * If both are present, make sure they match.
 * This logic doesn't work for lunisolar calendars!
 * */
function resolveNonLunisolarMonth<T extends { monthCode?: string; month?: number }>(
  calendarDate: T,
  overflow: Overflow | undefined = undefined,
  monthsPerYear = 12
) {
  let { month, monthCode } = calendarDate;
  if (monthCode === undefined) {
    if (month === undefined) throw new TypeError('Either month or monthCode are required');
    // The ISO calendar uses the default (undefined) value because it does
    // constrain/reject after this method returns. Non-ISO calendars, however,
    // rely on this function to constrain/reject out-of-range `month` values.
    if (overflow === 'reject') ES.RejectToRange(month, 1, monthsPerYear);
    if (overflow === 'constrain') month = ES.ConstrainToRange(month, 1, monthsPerYear);
    monthCode = buildMonthCode(month);
  } else {
    const numberPart = monthCodeNumberPart(monthCode);
    if (month !== undefined && month !== numberPart) {
      throw new RangeError(`monthCode ${monthCode} and month ${month} must match if both are present`);
    }
    if (monthCode !== buildMonthCode(numberPart)) {
      throw new RangeError(`Invalid month code: ${monthCode}`);
    }
    month = numberPart;
    if (month < 1 || month > monthsPerYear) throw new RangeError(`Invalid monthCode: ${monthCode}`);
  }
  return { ...calendarDate, month, monthCode };
}

type CachedTypes = Temporal.PlainYearMonth | Temporal.PlainDate | Temporal.PlainMonthDay;

/**
 * This prototype implementation of non-ISO calendars makes many repeated calls
 * to Intl APIs which may be slow (e.g. >0.2ms). This trivial cache will speed
 * up these repeat accesses. Each cache instance is associated (via a WeakMap)
 * to a specific Temporal object, which speeds up multiple calendar calls on the
 * same Temporal object instance.  No invalidation or pruning is necessary
 * because each object's cache is thrown away when the object is GC-ed.
 */
class OneObjectCache {
  map = new Map();
  calls = 0;
  now: number;
  hits = 0;
  misses = 0;
  constructor(cacheToClone?: OneObjectCache) {
    this.now = globalThis.performance ? globalThis.performance.now() : Date.now();
    if (cacheToClone !== undefined) {
      let i = 0;
      for (const entry of cacheToClone.map.entries()) {
        if (++i > OneObjectCache.MAX_CACHE_ENTRIES) break;
        this.map.set(...entry);
      }
    }
  }
  get(key: string) {
    const result = this.map.get(key);
    if (result) {
      this.hits++;
      this.report();
    }
    this.calls++;
    return result;
  }
  set(key: string, value: unknown) {
    this.map.set(key, value);
    this.misses++;
    this.report();
  }
  report() {
    /*
    if (this.calls === 0) return;
    const ms = (globalThis.performance ? globalThis.performance.now() : Date.now()) - this.now;
    const hitRate = ((100 * this.hits) / this.calls).toFixed(0);
    console.log(`${this.calls} calls in ${ms.toFixed(2)}ms. Hits: ${this.hits} (${hitRate}%). Misses: ${this.misses}.`);
    */
  }
  setObject(obj: CachedTypes) {
    if (OneObjectCache.objectMap.get(obj)) throw new RangeError('object already cached');
    OneObjectCache.objectMap.set(obj, this);
    this.report();
  }

  static objectMap = new WeakMap();
  static MAX_CACHE_ENTRIES = 1000;

  /**
   * Returns a WeakMap-backed cache that's used to store expensive results
   * that are associated with a particular Temporal object instance.
   *
   * @param obj - object to associate with the cache
   */
  static getCacheForObject(obj: CachedTypes) {
    let cache = OneObjectCache.objectMap.get(obj);
    if (!cache) {
      cache = new OneObjectCache();
      OneObjectCache.objectMap.set(obj, cache);
    }
    return cache;
  }
}

function toUtcIsoDateString({ isoYear, isoMonth, isoDay }: { isoYear: number; isoMonth: number; isoDay: number }) {
  const yearString = ES.ISOYearString(isoYear);
  const monthString = ES.ISODateTimePartString(isoMonth);
  const dayString = ES.ISODateTimePartString(isoDay);
  return `${yearString}-${monthString}-${dayString}T00:00Z`;
}

function simpleDateDiff(one: CalendarYMD, two: CalendarYMD) {
  return {
    years: one.year - two.year,
    months: one.month - two.month,
    days: one.day - two.day
  };
}

/**
 * Implementation helper that's common to all non-ISO calendars
 */
abstract class HelperBase {
  abstract id: BuiltinCalendarId;
  abstract monthsInYear(calendarDate: CalendarYearOnly, cache?: OneObjectCache): number;
  abstract maximumMonthLength(calendarDate?: CalendarYM): number;
  abstract minimumMonthLength(calendarDate?: CalendarYM): number;
  abstract estimateIsoDate(calendarDate: CalendarYMD): IsoYMD;
  abstract inLeapYear(calendarDate: CalendarYearOnly, cache?: OneObjectCache): boolean;
  abstract calendarType: 'solar' | 'lunar' | 'lunisolar';
  reviseIntlEra?<T extends Partial<EraAndEraYear>>(calendarDate: T, isoDate: IsoYMD): T;
  constantEra?: string;
  checkIcuBugs?(isoDate: IsoYMD): void;
  private formatter?: globalThis.Intl.DateTimeFormat;
  getFormatter() {
    // `new Intl.DateTimeFormat()` is amazingly slow and chews up RAM. Per
    // https://bugs.chromium.org/p/v8/issues/detail?id=6528#c4, we cache one
    // DateTimeFormat instance per calendar. Caching is lazy so we only pay for
    // calendars that are used. Note that the nonIsoHelperBase object is spread
    // into each each calendar's implementation before any cache is created, so
    // each calendar gets its own separate cached formatter.
    if (typeof this.formatter === 'undefined') {
      this.formatter = new IntlDateTimeFormat(`en-US-u-ca-${this.id}`, {
        day: 'numeric',
        month: 'numeric',
        year: 'numeric',
        era: 'short',
        timeZone: 'UTC'
      });
    }
    return this.formatter;
  }
  isoToCalendarDate(isoDate: IsoYMD, cache: OneObjectCache): FullCalendarDate {
    const { year: isoYear, month: isoMonth, day: isoDay } = isoDate;
    const key = JSON.stringify({ func: 'isoToCalendarDate', isoYear, isoMonth, isoDay, id: this.id });
    const cached = cache.get(key);
    if (cached) return cached;

    const dateTimeFormat = this.getFormatter();
    let parts, isoString;
    try {
      isoString = toUtcIsoDateString({ isoYear, isoMonth, isoDay });
      parts = dateTimeFormat.formatToParts(new Date(isoString));
    } catch (e: unknown) {
      throw new RangeError(`Invalid ISO date: ${JSON.stringify({ isoYear, isoMonth, isoDay })}`);
    }
    const result: Partial<FullCalendarDate> = {};
    for (let { type, value } of parts) {
      if (type === 'year') result.eraYear = +value;
      // TODO: remove this type annotation when `relatedYear` gets into TS lib types
      if (type === ('relatedYear' as Intl.DateTimeFormatPartTypes)) result.eraYear = +value;
      if (type === 'month') {
        const matches = /^([0-9]*)(.*?)$/.exec(value);
        if (!matches || matches.length != 3 || (!matches[1] && !matches[2])) {
          throw new RangeError(`Unexpected month: ${value}`);
        }
        // If the month has no numeric part (should only see this for the Hebrew
        // calendar with newer FF / Chromium versions; see
        // https://bugzilla.mozilla.org/show_bug.cgi?id=1751833) then set a
        // placeholder month index of `1` and rely on the derived class to
        // calculate the correct month index from the month name stored in
        // `monthExtra`.
        result.month = matches[1] ? +matches[1] : 1;
        if (result.month < 1) {
          throw new RangeError(
            `Invalid month ${value} from ${isoString}[u-ca-${this.id}]` +
              ' (probably due to https://bugs.chromium.org/p/v8/issues/detail?id=10527)'
          );
        }
        if (result.month > 13) {
          throw new RangeError(
            `Invalid month ${value} from ${isoString}[u-ca-${this.id}]` +
              ' (probably due to https://bugs.chromium.org/p/v8/issues/detail?id=10529)'
          );
        }

        // The ICU formats for the Hebrew calendar no longer support a numeric
        // month format. So we'll rely on the derived class to interpret it.
        // `monthExtra` is also used on the Chinese calendar to handle a suffix
        // "bis" indicating a leap month.
        if (matches[2]) result.monthExtra = matches[2];
      }
      if (type === 'day') result.day = +value;
      if (this.hasEra && type === 'era' && value != null && value !== '') {
        // The convention for Temporal era values is lowercase, so following
        // that convention in this prototype. Punctuation is removed, accented
        // letters are normalized, and spaces are replaced with dashes.
        // E.g.: "ERA0" => "era0", "Before R.O.C." => "before-roc", "En’ō" => "eno"
        // The call to normalize() and the replacement regex deals with era
        // names that contain non-ASCII characters like Japanese eras. Also
        // ignore extra content in parentheses like JPN era date ranges.
        value = value.split(' (')[0];
        result.era = value
          .normalize('NFD')
          .replace(/[^-0-9 \p{L}]/gu, '')
          .replace(' ', '-')
          .toLowerCase();
      }
    }
    if (result.eraYear === undefined) {
      // Node 12 has outdated ICU data that lacks the `relatedYear` field in the
      // output of Intl.DateTimeFormat.formatToParts.
      throw new RangeError(
        `Intl.DateTimeFormat.formatToParts lacks relatedYear in ${this.id} calendar. Try Node 14+ or modern browsers.`
      );
    }
    // Translate eras that may be handled differently by Temporal vs. by Intl
    // (e.g. Japanese pre-Meiji eras). See https://github.com/tc39/proposal-temporal/issues/526.
    if (this.reviseIntlEra) {
      const { era, eraYear } = this.reviseIntlEra(result, isoDate);
      result.era = era;
      result.eraYear = eraYear;
    }
    if (this.checkIcuBugs) this.checkIcuBugs(isoDate);

    const calendarDate = this.adjustCalendarDate(result, cache, 'constrain', true);
    if (calendarDate.year === undefined) throw new RangeError(`Missing year converting ${JSON.stringify(isoDate)}`);
    if (calendarDate.month === undefined) throw new RangeError(`Missing month converting ${JSON.stringify(isoDate)}`);
    if (calendarDate.day === undefined) throw new RangeError(`Missing day converting ${JSON.stringify(isoDate)}`);
    cache.set(key, calendarDate);
    // Also cache the reverse mapping
    ['constrain', 'reject'].forEach((overflow) => {
      const keyReverse = JSON.stringify({
        func: 'calendarToIsoDate',
        year: calendarDate.year,
        month: calendarDate.month,
        day: calendarDate.day,
        overflow,
        id: this.id
      });
      cache.set(keyReverse, isoDate);
    });
    return calendarDate;
  }
  validateCalendarDate(calendarDate: Partial<FullCalendarDate>): asserts calendarDate is FullCalendarDate {
    const { era, month, year, day, eraYear, monthCode, monthExtra } = calendarDate;
    // When there's a suffix (e.g. "5bis" for a leap month in Chinese calendar)
    // the derived class must deal with it.
    if (monthExtra !== undefined) throw new RangeError('Unexpected `monthExtra` value');
    if (year === undefined && eraYear === undefined) throw new TypeError('year or eraYear is required');
    if (month === undefined && monthCode === undefined) throw new TypeError('month or monthCode is required');
    if (day === undefined) throw new RangeError('Missing day');
    if (monthCode !== undefined) {
      if (typeof monthCode !== 'string') {
        throw new RangeError(`monthCode must be a string, not ${typeof monthCode}`);
      }
      if (!/^M([01]?\d)(L?)$/.test(monthCode)) throw new RangeError(`Invalid monthCode: ${monthCode}`);
    }
    if (this.constantEra) {
      if (era !== undefined && era !== this.constantEra) {
        throw new RangeError(`era must be ${this.constantEra}, not ${era}`);
      }
      if (eraYear !== undefined && year !== undefined && eraYear !== year) {
        throw new RangeError(`eraYear ${eraYear} does not match year ${year}`);
      }
    }
    if (this.hasEra) {
      if ((calendarDate['era'] === undefined) !== (calendarDate['eraYear'] === undefined)) {
        throw new RangeError("properties 'era' and 'eraYear' must be provided together");
      }
    }
  }
  /**
   * Allows derived calendars to add additional fields and/or to make
   * adjustments e.g. to set the era based on the date or to revise the month
   * number in lunisolar calendars per
   * https://github.com/tc39/proposal-temporal/issues/1203.
   *
   * The base implementation fills in missing values by assuming the simplest
   * possible calendar:
   * - no eras or a constant era defined in `.constantEra`
   * - non-lunisolar calendar (no leap months)
   * */
  adjustCalendarDate(
    calendarDateParam: Partial<FullCalendarDate>,
    cache: OneObjectCache | undefined = undefined,
    overflow: Overflow = 'constrain',
    // This param is only used by derived classes
    // eslint-disable-next-line @typescript-eslint/no-unused-vars
    fromLegacyDate = false
  ): FullCalendarDate {
    if (this.calendarType === 'lunisolar') throw new RangeError('Override required for lunisolar calendars');
    let calendarDate = calendarDateParam;
    this.validateCalendarDate(calendarDate);
    // For calendars that always use the same era, set it here so that derived
    // calendars won't need to implement this method simply to set the era.
    if (this.constantEra) {
      // year and eraYear always match when there's only one possible era
      const { year, eraYear } = calendarDate;
      calendarDate = {
        ...calendarDate,
        era: this.constantEra,
        year: year !== undefined ? year : eraYear,
        eraYear: eraYear !== undefined ? eraYear : year
      };
    }

    const largestMonth = this.monthsInYear(calendarDate as CalendarYearOnly, cache);
    let { month, monthCode } = calendarDate;

    ({ month, monthCode } = resolveNonLunisolarMonth(calendarDate, overflow, largestMonth));
    return { ...(calendarDate as typeof calendarDate & CalendarYMD), month, monthCode };
  }
  regulateMonthDayNaive(calendarDate: FullCalendarDate, overflow: Overflow, cache: OneObjectCache): FullCalendarDate {
    const largestMonth = this.monthsInYear(calendarDate, cache);
    let { month, day } = calendarDate;
    if (overflow === 'reject') {
      ES.RejectToRange(month, 1, largestMonth);
      ES.RejectToRange(day, 1, this.maximumMonthLength(calendarDate));
    } else {
      month = ES.ConstrainToRange(month, 1, largestMonth);
      day = ES.ConstrainToRange(day, 1, this.maximumMonthLength({ ...calendarDate, month }));
    }
    return { ...calendarDate, month, day };
  }
  calendarToIsoDate(dateParam: CalendarDateFields, overflow: Overflow = 'constrain', cache: OneObjectCache): IsoYMD {
    const originalDate = dateParam as Partial<FullCalendarDate>;
    // First, normalize the calendar date to ensure that (year, month, day)
    // are all present, converting monthCode and eraYear if needed.
    let date = this.adjustCalendarDate(dateParam, cache, overflow, false);

    // Fix obviously out-of-bounds values. Values that are valid generally, but
    // not in this particular year, may not be caught here for some calendars.
    // If so, these will be handled lower below.
    date = this.regulateMonthDayNaive(date, overflow, cache);

    const { year, month, day } = date;
    const key = JSON.stringify({ func: 'calendarToIsoDate', year, month, day, overflow, id: this.id });
    let cached = cache.get(key);
    if (cached) return cached;
    // If YMD are present in the input but the input has been constrained
    // already, then cache both the original value and the constrained value.
    let keyOriginal;
    if (
      originalDate.year !== undefined &&
      originalDate.month !== undefined &&
      originalDate.day !== undefined &&
      (originalDate.year !== date.year || originalDate.month !== date.month || originalDate.day !== date.day)
    ) {
      keyOriginal = JSON.stringify({
        func: 'calendarToIsoDate',
        year: originalDate.year,
        month: originalDate.month,
        day: originalDate.day,
        overflow,
        id: this.id
      });
      cached = cache.get(keyOriginal);
      if (cached) return cached;
    }

    // First, try to roughly guess the result
    let isoEstimate = this.estimateIsoDate({ year, month, day });
    const calculateSameMonthResult = (diffDays: number) => {
      // If the estimate is in the same year & month as the target, then we can
      // calculate the result exactly and short-circuit any additional logic.
      // This optimization assumes that months are continuous. It would break if
      // a calendar skipped days, like the Julian->Gregorian switchover. But
      // current ICU calendars only skip days (japanese/roc/buddhist) because of
      // a bug (https://bugs.chromium.org/p/chromium/issues/detail?id=1173158)
      // that's currently worked around by a custom calendarToIsoDate
      // implementation in those calendars. So this optimization should be safe
      // for all ICU calendars.
      let testIsoEstimate = this.addDaysIso(isoEstimate, diffDays);
      if (date.day > this.minimumMonthLength(date)) {
        // There's a chance that the calendar date is out of range. Throw or
        // constrain if so.
        let testCalendarDate = this.isoToCalendarDate(testIsoEstimate, cache);
        while (testCalendarDate.month !== month || testCalendarDate.year !== year) {
          if (overflow === 'reject') {
            throw new RangeError(`day ${day} does not exist in month ${month} of year ${year}`);
          }
          // Back up a day at a time until we're not hanging over the month end
          testIsoEstimate = this.addDaysIso(testIsoEstimate, -1);
          testCalendarDate = this.isoToCalendarDate(testIsoEstimate, cache);
        }
      }
      return testIsoEstimate;
    };
    let sign = 0;
    let roundtripEstimate = this.isoToCalendarDate(isoEstimate, cache);
    let diff = simpleDateDiff(date, roundtripEstimate);
    if (diff.years !== 0 || diff.months !== 0 || diff.days !== 0) {
      const diffTotalDaysEstimate = diff.years * 365 + diff.months * 30 + diff.days;
      isoEstimate = this.addDaysIso(isoEstimate, diffTotalDaysEstimate);
      roundtripEstimate = this.isoToCalendarDate(isoEstimate, cache);
      diff = simpleDateDiff(date, roundtripEstimate);
      if (diff.years === 0 && diff.months === 0) {
        isoEstimate = calculateSameMonthResult(diff.days);
      } else {
        sign = this.compareCalendarDates(date, roundtripEstimate);
      }
    }
    // If the initial guess is not in the same month, then then bisect the
    // distance to the target, starting with 8 days per step.
    let increment = 8;
    while (sign) {
      isoEstimate = this.addDaysIso(isoEstimate, sign * increment);
      const oldRoundtripEstimate = roundtripEstimate;
      roundtripEstimate = this.isoToCalendarDate(isoEstimate, cache);
      const oldSign = sign;
      sign = this.compareCalendarDates(date, roundtripEstimate);
      if (sign) {
        diff = simpleDateDiff(date, roundtripEstimate);
        if (diff.years === 0 && diff.months === 0) {
          isoEstimate = calculateSameMonthResult(diff.days);
          // Signal the loop condition that there's a match.
          sign = 0;
        } else if (oldSign && sign !== oldSign) {
          if (increment > 1) {
            // If the estimate overshot the target, try again with a smaller increment
            // in the reverse direction.
            increment /= 2;
          } else {
            // Increment is 1, and neither the previous estimate nor the new
            // estimate is correct. The only way that can happen is if the
            // original date was an invalid value that will be constrained or
            // rejected here.
            if (overflow === 'reject') {
              throw new RangeError(`Can't find ISO date from calendar date: ${JSON.stringify({ ...originalDate })}`);
            } else {
              // To constrain, pick the earliest value
              const order = this.compareCalendarDates(roundtripEstimate, oldRoundtripEstimate);
              // If current value is larger, then back up to the previous value.
              if (order > 0) isoEstimate = this.addDaysIso(isoEstimate, -1);
              sign = 0;
            }
          }
        }
      }
    }
    cache.set(key, isoEstimate);
    if (keyOriginal) cache.set(keyOriginal, isoEstimate);
    if (
      date.year === undefined ||
      date.month === undefined ||
      date.day === undefined ||
      date.monthCode === undefined ||
      (this.hasEra && (date.era === undefined || date.eraYear === undefined))
    ) {
      throw new RangeError('Unexpected missing property');
    }
    return isoEstimate;
  }
  temporalToCalendarDate(
    date: Temporal.PlainDate | Temporal.PlainMonthDay | Temporal.PlainYearMonth,
    cache: OneObjectCache
  ): FullCalendarDate {
    const isoDate = { year: GetSlot(date, ISO_YEAR), month: GetSlot(date, ISO_MONTH), day: GetSlot(date, ISO_DAY) };
    const result = this.isoToCalendarDate(isoDate, cache);
    return result;
  }
  compareCalendarDates(date1Param: Partial<CalendarYMD>, date2Param: Partial<CalendarYMD>): 0 | 1 | -1 {
    // `date1` and `date2` are already records. The calls below simply validate
    // that all three required fields are present.
    const date1 = ES.PrepareTemporalFields(date1Param, ['day', 'month', 'year'], ['day', 'month', 'year']);
    const date2 = ES.PrepareTemporalFields(date2Param, ['day', 'month', 'year'], ['day', 'month', 'year']);
    if (date1.year !== date2.year) return ES.ComparisonResult(date1.year - date2.year);
    if (date1.month !== date2.month) return ES.ComparisonResult(date1.month - date2.month);
    if (date1.day !== date2.day) return ES.ComparisonResult(date1.day - date2.day);
    return 0;
  }
  /** Ensure that a calendar date actually exists. If not, return the closest earlier date. */
  regulateDate(calendarDate: CalendarYMD, overflow: Overflow = 'constrain', cache: OneObjectCache): FullCalendarDate {
    const isoDate = this.calendarToIsoDate(calendarDate, overflow, cache);
    return this.isoToCalendarDate(isoDate, cache);
  }
  addDaysIso(isoDate: IsoYMD, days: number): IsoYMD {
    const added = ES.AddISODate(isoDate.year, isoDate.month, isoDate.day, 0, 0, 0, days, 'constrain');
    return added;
  }
  addDaysCalendar(calendarDate: CalendarYMD, days: number, cache: OneObjectCache): FullCalendarDate {
    const isoDate = this.calendarToIsoDate(calendarDate, 'constrain', cache);
    const addedIso = this.addDaysIso(isoDate, days);
    const addedCalendar = this.isoToCalendarDate(addedIso, cache);
    return addedCalendar;
  }
  addMonthsCalendar(
    calendarDateParam: CalendarYMD,
    months: number,
    overflow: Overflow,
    cache: OneObjectCache
  ): CalendarYMD {
    let calendarDate = calendarDateParam;
    const { day } = calendarDate;
    for (let i = 0, absMonths = MathAbs(months); i < absMonths; i++) {
      const { month } = calendarDate;
      const oldCalendarDate = calendarDate;
      const days =
        months < 0
          ? -Math.max(day, this.daysInPreviousMonth(calendarDate, cache))
          : this.daysInMonth(calendarDate, cache);
      const isoDate = this.calendarToIsoDate(calendarDate, 'constrain', cache);
      let addedIso = this.addDaysIso(isoDate, days);
      calendarDate = this.isoToCalendarDate(addedIso, cache);

      // Normally, we can advance one month by adding the number of days in the
      // current month. However, if we're at the end of the current month and
      // the next month has fewer days, then we rolled over to the after-next
      // month. Below we detect this condition and back up until we're back in
      // the desired month.
      if (months > 0) {
        const monthsInOldYear = this.monthsInYear(oldCalendarDate, cache);
        while (calendarDate.month - 1 !== month % monthsInOldYear) {
          addedIso = this.addDaysIso(addedIso, -1);
          calendarDate = this.isoToCalendarDate(addedIso, cache);
        }
      }

      if (calendarDate.day !== day) {
        // try to retain the original day-of-month, if possible
        calendarDate = this.regulateDate({ ...calendarDate, day }, 'constrain', cache);
      }
    }
    if (overflow === 'reject' && calendarDate.day !== day) {
      throw new RangeError(`Day ${day} does not exist in resulting calendar month`);
    }
    return calendarDate;
  }
  addCalendar(
    calendarDate: CalendarYMD & { monthCode: string },
    { years = 0, months = 0, weeks = 0, days = 0 },
    overflow: Overflow,
    cache: OneObjectCache
  ): FullCalendarDate {
    const { year, day, monthCode } = calendarDate;
    const addedYears = this.adjustCalendarDate({ year: year + years, monthCode, day }, cache);
    const addedMonths = this.addMonthsCalendar(addedYears, months, overflow, cache);
    const initialDays = days + weeks * 7;
    const addedDays = this.addDaysCalendar(addedMonths, initialDays, cache);
    return addedDays;
  }
  untilCalendar(
    calendarOne: FullCalendarDate,
    calendarTwo: FullCalendarDate,
    largestUnit: Temporal.DateUnit,
    cache: OneObjectCache
  ): { years: number; months: number; weeks: number; days: number } {
    let days = 0;
    let weeks = 0;
    let months = 0;
    let years = 0;
    switch (largestUnit) {
      case 'day':
        days = this.calendarDaysUntil(calendarOne, calendarTwo, cache);
        break;
      case 'week': {
        const totalDays = this.calendarDaysUntil(calendarOne, calendarTwo, cache);
        days = totalDays % 7;
        weeks = (totalDays - days) / 7;
        break;
      }
      case 'month':
      case 'year': {
        const sign = this.compareCalendarDates(calendarTwo, calendarOne);
        if (!sign) {
          return { years: 0, months: 0, weeks: 0, days: 0 };
        }
        const diffYears = calendarTwo.year - calendarOne.year;
        const diffDays = calendarTwo.day - calendarOne.day;
        if (largestUnit === 'year' && diffYears) {
          let diffInYearSign = 0;
          if (calendarTwo.monthCode > calendarOne.monthCode) diffInYearSign = 1;
          if (calendarTwo.monthCode < calendarOne.monthCode) diffInYearSign = -1;
          if (!diffInYearSign) diffInYearSign = Math.sign(diffDays);
          const isOneFurtherInYear = diffInYearSign * sign < 0;
          years = isOneFurtherInYear ? diffYears - sign : diffYears;
        }
        const yearsAdded = years ? this.addCalendar(calendarOne, { years }, 'constrain', cache) : calendarOne;
        // Now we have less than one year remaining. Add one month at a time
        // until we go over the target, then back up one month and calculate
        // remaining days and weeks.
        let current;
        let next: CalendarYMD = yearsAdded;
        do {
          months += sign;
          current = next;
          next = this.addMonthsCalendar(current, sign, 'constrain', cache);
          if (next.day !== calendarOne.day) {
            // In case the day was constrained down, try to un-constrain it
            next = this.regulateDate({ ...next, day: calendarOne.day }, 'constrain', cache);
          }
        } while (this.compareCalendarDates(calendarTwo, next) * sign >= 0);
        months -= sign; // correct for loop above which overshoots by 1
        const remainingDays = this.calendarDaysUntil(current, calendarTwo, cache);
        days = remainingDays;
        break;
      }
    }
    return { years, months, weeks, days };
  }
  daysInMonth(calendarDate: CalendarYMD, cache: OneObjectCache): number {
    // Add enough days to roll over to the next month. One we're in the next
    // month, we can calculate the length of the current month. NOTE: This
    // algorithm assumes that months are continuous. It would break if a
    // calendar skipped days, like the Julian->Gregorian switchover. But current
    // ICU calendars only skip days (japanese/roc/buddhist) because of a bug
    // (https://bugs.chromium.org/p/chromium/issues/detail?id=1173158) that's
    // currently worked around by a custom calendarToIsoDate implementation in
    // those calendars. So this code should be safe for all ICU calendars.
    const { day } = calendarDate;
    const max = this.maximumMonthLength(calendarDate);
    const min = this.minimumMonthLength(calendarDate);
    // easiest case: we already know the month length if min and max are the same.
    if (min === max) return min;

    // Add enough days to get into the next month, without skipping it
    const increment = day <= max - min ? max : min;
    const isoDate = this.calendarToIsoDate(calendarDate, 'constrain', cache);
    const addedIsoDate = this.addDaysIso(isoDate, increment);
    const addedCalendarDate = this.isoToCalendarDate(addedIsoDate, cache);

    // Now back up to the last day of the original month
    const endOfMonthIso = this.addDaysIso(addedIsoDate, -addedCalendarDate.day);
    const endOfMonthCalendar = this.isoToCalendarDate(endOfMonthIso, cache);
    return endOfMonthCalendar.day;
  }
  daysInPreviousMonth(calendarDate: CalendarYMD, cache: OneObjectCache): number {
    const { day, month, year } = calendarDate;

    // Check to see if we already know the month length, and return it if so
    const previousMonthYear = month > 1 ? year : year - 1;
    let previousMonthDate = { year: previousMonthYear, month, day: 1 };
    const previousMonth = month > 1 ? month - 1 : this.monthsInYear(previousMonthDate, cache);
    previousMonthDate = { ...previousMonthDate, month: previousMonth };
    const min = this.minimumMonthLength(previousMonthDate);
    const max = this.maximumMonthLength(previousMonthDate);
    if (min === max) return max;

    const isoDate = this.calendarToIsoDate(calendarDate, 'constrain', cache);
    const lastDayOfPreviousMonthIso = this.addDaysIso(isoDate, -day);
    const lastDayOfPreviousMonthCalendar = this.isoToCalendarDate(lastDayOfPreviousMonthIso, cache);
    return lastDayOfPreviousMonthCalendar.day;
  }
  startOfCalendarYear(calendarDate: CalendarYearOnly): CalendarYMD & { monthCode: string } {
    return { year: calendarDate.year, month: 1, monthCode: 'M01', day: 1 };
  }
  startOfCalendarMonth(calendarDate: CalendarYM): CalendarYMD {
    return { year: calendarDate.year, month: calendarDate.month, day: 1 };
  }
  calendarDaysUntil(calendarOne: CalendarYMD, calendarTwo: CalendarYMD, cache: OneObjectCache): number {
    const oneIso = this.calendarToIsoDate(calendarOne, 'constrain', cache);
    const twoIso = this.calendarToIsoDate(calendarTwo, 'constrain', cache);
    return this.isoDaysUntil(oneIso, twoIso);
  }
  isoDaysUntil(oneIso: IsoYMD, twoIso: IsoYMD): number {
    const duration = ES.DifferenceISODate(
      oneIso.year,
      oneIso.month,
      oneIso.day,
      twoIso.year,
      twoIso.month,
      twoIso.day,
      'day'
    );
    return duration.days;
  }
  // All built-in calendars except Chinese/Dangi and Hebrew use an era
  hasEra = true;
  // See https://github.com/tc39/proposal-temporal/issues/1784
  erasBeginMidYear = false;
  monthDayFromFields(fields: FullCalendarDate, overflow: Overflow, cache: OneObjectCache): IsoYMD {
    let { monthCode, day } = fields;
    if (monthCode === undefined) {
      let { year, era, eraYear } = fields;
      if (year === undefined && (era === undefined || eraYear === undefined)) {
        throw new TypeError('when `monthCode` is omitted, `year` (or `era` and `eraYear`) and `month` are required');
      }
      // Apply overflow behaviour to year/month/day, to get correct monthCode/day
      ({ monthCode, day } = this.isoToCalendarDate(this.calendarToIsoDate(fields, overflow, cache), cache));
    }

    let isoYear, isoMonth, isoDay;
    let closestCalendar, closestIso;
    // Look backwards starting from one of the calendar years spanning ISO year
    // 1972, up to 100 calendar years prior, to find a year that has this month
    // and day. Normal months and days will match immediately, but for leap days
    // and leap months we may have to look for a while.
    const startDateIso = { year: 1972, month: 12, day: 31 };
    const calendarOfStartDateIso = this.isoToCalendarDate(startDateIso, cache);
    // Note: relies on lexicographical ordering of monthCodes
    const calendarYear =
      calendarOfStartDateIso.monthCode > monthCode ||
      (calendarOfStartDateIso.monthCode === monthCode && calendarOfStartDateIso.day >= day)
        ? calendarOfStartDateIso.year
        : calendarOfStartDateIso.year - 1;
    for (let i = 0; i < 100; i++) {
      const testCalendarDate: FullCalendarDate = this.adjustCalendarDate(
        { day, monthCode, year: calendarYear - i },
        cache
      );
      const isoDate = this.calendarToIsoDate(testCalendarDate, 'constrain', cache);
      const roundTripCalendarDate = this.isoToCalendarDate(isoDate, cache);
      ({ year: isoYear, month: isoMonth, day: isoDay } = isoDate);
      if (roundTripCalendarDate.monthCode === monthCode && roundTripCalendarDate.day === day) {
        return { month: isoMonth, day: isoDay, year: isoYear };
      } else if (overflow === 'constrain') {
        // non-ISO constrain algorithm tries to find the closest date in a matching month
        if (
          closestCalendar === undefined ||
          (roundTripCalendarDate.monthCode === closestCalendar.monthCode &&
            roundTripCalendarDate.day > closestCalendar.day)
        ) {
          closestCalendar = roundTripCalendarDate;
          closestIso = isoDate;
        }
      }
    }
    if (overflow === 'constrain' && closestIso !== undefined) return closestIso;
    throw new RangeError(`No recent ${this.id} year with monthCode ${monthCode} and day ${day}`);
  }
}

interface HebrewMonthInfo {
  [m: string]: (
    | {
        leap: undefined;
        regular: number;
      }
    | {
        leap: number;
        regular: undefined;
      }
    | {
        leap: number;
        regular: number;
      }
  ) & {
    monthCode: string;
    days:
      | number
      | {
          min: number;
          max: number;
        };
  };
}

class HebrewHelper extends HelperBase {
  id = 'hebrew' as const;
  calendarType = 'lunisolar' as const;
  inLeapYear(calendarDate: CalendarYearOnly) {
    const { year } = calendarDate;
    // FYI: In addition to adding a month in leap years, the Hebrew calendar
    // also has per-year changes to the number of days of Heshvan and Kislev.
    // Given that these can be calculated by counting the number of days in
    // those months, I assume that these DO NOT need to be exposed as
    // Hebrew-only prototype fields or methods.
    return (7 * year + 1) % 19 < 7;
  }
  monthsInYear(calendarDate: CalendarYearOnly) {
    return this.inLeapYear(calendarDate) ? 13 : 12;
  }
  minimumMonthLength(calendarDate: CalendarYM) {
    return this.minMaxMonthLength(calendarDate, 'min');
  }
  maximumMonthLength(calendarDate: CalendarYM) {
    return this.minMaxMonthLength(calendarDate, 'max');
  }
  minMaxMonthLength(calendarDate: CalendarYM, minOrMax: 'min' | 'max') {
    const { month, year } = calendarDate;
    const monthCode = this.getMonthCode(year, month);
    const monthInfo = ObjectEntries(this.months).find((m) => m[1].monthCode === monthCode);
    if (monthInfo === undefined) throw new RangeError(`unmatched Hebrew month: ${month}`);
    const daysInMonth = monthInfo[1].days;
    return typeof daysInMonth === 'number' ? daysInMonth : daysInMonth[minOrMax];
  }
  /** Take a guess at what ISO date a particular calendar date corresponds to */
  estimateIsoDate(calendarDate: CalendarYMD) {
    const { year } = calendarDate;
    return { year: year - 3760, month: 1, day: 1 };
  }
  months: HebrewMonthInfo = {
    Tishri: { leap: 1, regular: 1, monthCode: 'M01', days: 30 },
    Heshvan: { leap: 2, regular: 2, monthCode: 'M02', days: { min: 29, max: 30 } },
    Kislev: { leap: 3, regular: 3, monthCode: 'M03', days: { min: 29, max: 30 } },
    Tevet: { leap: 4, regular: 4, monthCode: 'M04', days: 29 },
    Shevat: { leap: 5, regular: 5, monthCode: 'M05', days: 30 },
    Adar: { leap: undefined, regular: 6, monthCode: 'M06', days: 29 },
    'Adar I': { leap: 6, regular: undefined, monthCode: 'M05L', days: 30 },
    'Adar II': { leap: 7, regular: undefined, monthCode: 'M06', days: 29 },
    Nisan: { leap: 8, regular: 7, monthCode: 'M07', days: 30 },
    Iyar: { leap: 9, regular: 8, monthCode: 'M08', days: 29 },
    Sivan: { leap: 10, regular: 9, monthCode: 'M09', days: 30 },
    Tamuz: { leap: 11, regular: 10, monthCode: 'M10', days: 29 },
    Av: { leap: 12, regular: 11, monthCode: 'M11', days: 30 },
    Elul: { leap: 13, regular: 12, monthCode: 'M12', days: 29 }
  };
  getMonthCode(year: number, month: number) {
    if (this.inLeapYear({ year })) {
      return month === 6 ? buildMonthCode(5, true) : buildMonthCode(month < 6 ? month : month - 1);
    } else {
      return buildMonthCode(month);
    }
  }
  override adjustCalendarDate(
    calendarDate: Partial<FullCalendarDate>,
    cache?: OneObjectCache,
    overflow: Overflow = 'constrain',
    fromLegacyDate = false
  ): FullCalendarDate {
    // The incoming type is actually CalendarDate (same as args to
    // Calendar.dateFromParams) but TS isn't smart enough to follow all the
    // reassignments below, so as an alternative to 10+ type casts, we'll lie
    // here and claim that the type has `day` and `year` filled in already.
    let { year, eraYear, month, monthCode, day, monthExtra } = calendarDate as Omit<
      typeof calendarDate,
      'year' | 'day'
    > & { year: number; day: number };
    if (year === undefined && eraYear !== undefined) year = eraYear;
    if (eraYear === undefined && year !== undefined) eraYear = year;
    if (fromLegacyDate) {
      // In Pre Node-14 V8, DateTimeFormat.formatToParts `month: 'numeric'`
      // output returns the numeric equivalent of `month` as a string, meaning
      // that `'6'` in a leap year is Adar I, while `'6'` in a non-leap year
      // means Adar. In this case, `month` will already be correct and no action
      // is needed. However, in Node 14 and later formatToParts returns the name
      // of the Hebrew month (e.g. "Tevet"), so we'll need to look up the
      // correct `month` using the string name as a key.
      if (monthExtra) {
        const monthInfo = this.months[monthExtra];
        if (!monthInfo) throw new RangeError(`Unrecognized month from formatToParts: ${monthExtra}`);
        month = this.inLeapYear({ year }) ? monthInfo.leap : monthInfo.regular;
      }
      // Because we're getting data from legacy Date, then `month` will always be present
      monthCode = this.getMonthCode(year, month as number);
      const result = { year, month: month as number, day, era: undefined as string | undefined, eraYear, monthCode };
      return result;
    } else {
      // When called without input coming from legacy Date output, simply ensure
      // that all fields are present.
      this.validateCalendarDate(calendarDate);
      if (month === undefined) {
        if ((monthCode as string).endsWith('L')) {
          if (monthCode !== 'M05L') {
            throw new RangeError(`Hebrew leap month must have monthCode M05L, not ${monthCode}`);
          }
          month = 6;
          if (!this.inLeapYear({ year })) {
            if (overflow === 'reject') {
              throw new RangeError(`Hebrew monthCode M05L is invalid in year ${year} which is not a leap year`);
            } else {
              // constrain to same day of next month (Adar)
              month = 6;
              monthCode = 'M06';
            }
          }
        } else {
          month = monthCodeNumberPart(monthCode as string);
          // if leap month is before this one, the month index is one more than the month code
          if (this.inLeapYear({ year }) && month >= 6) month++;
          const largestMonth = this.monthsInYear({ year });
          if (month < 1 || month > largestMonth) throw new RangeError(`Invalid monthCode: ${monthCode}`);
        }
      } else {
        if (overflow === 'reject') {
          ES.RejectToRange(month, 1, this.monthsInYear({ year }));
          ES.RejectToRange(day, 1, this.maximumMonthLength({ year, month }));
        } else {
          month = ES.ConstrainToRange(month, 1, this.monthsInYear({ year }));
          day = ES.ConstrainToRange(day, 1, this.maximumMonthLength({ year, month }));
        }
        if (monthCode === undefined) {
          monthCode = this.getMonthCode(year, month);
        } else {
          const calculatedMonthCode = this.getMonthCode(year, month);
          if (calculatedMonthCode !== monthCode) {
            throw new RangeError(`monthCode ${monthCode} doesn't correspond to month ${month} in Hebrew year ${year}`);
          }
        }
      }
      return { ...calendarDate, day, month, monthCode: monthCode as string, year, eraYear };
    }
  }
  // All built-in calendars except Chinese/Dangi and Hebrew use an era
  override hasEra = false;
}

/**
 * For Temporal purposes, the Islamic calendar is simple because it's always the
 * same 12 months in the same order.
 */
abstract class IslamicBaseHelper extends HelperBase {
  abstract override id: BuiltinCalendarId;
  calendarType = 'lunar' as const;
  inLeapYear(calendarDate: CalendarYearOnly, cache: OneObjectCache) {
    // In leap years, the 12th month has 30 days. In non-leap years: 29.
    const days = this.daysInMonth({ year: calendarDate.year, month: 12, day: 1 }, cache);
    return days === 30;
  }
  monthsInYear(/* calendarYear, cache */) {
    return 12;
  }
  minimumMonthLength(/* calendarDate */) {
    return 29;
  }
  maximumMonthLength(/* calendarDate */) {
    return 30;
  }
  DAYS_PER_ISLAMIC_YEAR = 354 + 11 / 30;
  DAYS_PER_ISO_YEAR = 365.2425;
  override constantEra = 'ah';
  estimateIsoDate(calendarDate: CalendarYMD) {
    const { year } = this.adjustCalendarDate(calendarDate);
    return { year: MathFloor((year * this.DAYS_PER_ISLAMIC_YEAR) / this.DAYS_PER_ISO_YEAR) + 622, month: 1, day: 1 };
  }
}

// There are 6 Islamic calendars with the same implementation in this polyfill.
// They vary only in their ID. They do emit different output from the underlying
// Intl implementation, but our code for each of them is identical.
class IslamicHelper extends IslamicBaseHelper {
  id = 'islamic' as const;
}
class IslamicUmalquraHelper extends IslamicBaseHelper {
  id = 'islamic-umalqura' as const;
}
class IslamicTblaHelper extends IslamicBaseHelper {
  id = 'islamic-tbla' as const;
}
class IslamicCivilHelper extends IslamicBaseHelper {
  id = 'islamic-civil' as const;
}
class IslamicRgsaHelper extends IslamicBaseHelper {
  id = 'islamic-rgsa' as const;
}
class IslamicCcHelper extends IslamicBaseHelper {
  id = 'islamicc' as const;
}

class PersianHelper extends HelperBase {
  id = 'persian' as const;
  calendarType = 'solar' as const;
  inLeapYear(calendarDate: CalendarYearOnly, cache: OneObjectCache) {
    // Same logic (count days in the last month) for Persian as for Islamic,
    // even though Persian is solar and Islamic is lunar.
    return IslamicHelper.prototype.inLeapYear.call(this, calendarDate, cache);
  }
  monthsInYear(/* calendarYear, cache */) {
    return 12;
  }
  minimumMonthLength(calendarDate: CalendarYM) {
    const { month } = calendarDate;
    if (month === 12) return 29;
    return month <= 6 ? 31 : 30;
  }
  maximumMonthLength(calendarDate: CalendarYM) {
    const { month } = calendarDate;
    if (month === 12) return 30;
    return month <= 6 ? 31 : 30;
  }
  override constantEra = 'ap';
  estimateIsoDate(calendarDate: CalendarYMD) {
    const { year } = this.adjustCalendarDate(calendarDate);
    return { year: year + 621, month: 1, day: 1 };
  }
}

interface IndianMonthInfo {
  [month: number]: {
    length: number;
    month: number;
    day: number;
    leap?: {
      length: number;
      month: number;
      day: number;
    };
    nextYear?: true | undefined;
  };
}

class IndianHelper extends HelperBase {
  id = 'indian' as const;
  calendarType = 'solar' as const;
  inLeapYear(calendarDate: CalendarYearOnly) {
    // From https://en.wikipedia.org/wiki/Indian_national_calendar:
    // Years are counted in the Saka era, which starts its year 0 in the year 78
    // of the Common Era. To determine leap years, add 78 to the Saka year – if
    // the result is a leap year in the Gregorian calendar, then the Saka year
    // is a leap year as well.
    return isGregorianLeapYear(calendarDate.year + 78);
  }
  monthsInYear(/* calendarYear, cache */) {
    return 12;
  }
  minimumMonthLength(calendarDate: CalendarYM) {
    return this.getMonthInfo(calendarDate).length;
  }
  maximumMonthLength(calendarDate: CalendarYM) {
    return this.getMonthInfo(calendarDate).length;
  }
  override constantEra = 'saka';
  // Indian months always start at the same well-known Gregorian month and
  // day. So this conversion is easy and fast. See
  // https://en.wikipedia.org/wiki/Indian_national_calendar
  months: IndianMonthInfo = {
    1: { length: 30, month: 3, day: 22, leap: { length: 31, month: 3, day: 21 } },
    2: { length: 31, month: 4, day: 21 },
    3: { length: 31, month: 5, day: 22 },
    4: { length: 31, month: 6, day: 22 },
    5: { length: 31, month: 7, day: 23 },
    6: { length: 31, month: 8, day: 23 },
    7: { length: 30, month: 9, day: 23 },
    8: { length: 30, month: 10, day: 23 },
    9: { length: 30, month: 11, day: 22 },
    10: { length: 30, month: 12, day: 22 },
    11: { length: 30, month: 1, nextYear: true, day: 21 },
    12: { length: 30, month: 2, nextYear: true, day: 20 }
  };
  getMonthInfo(calendarDate: CalendarYM) {
    const { month } = calendarDate;
    let monthInfo = this.months[month];
    if (monthInfo === undefined) throw new RangeError(`Invalid month: ${month}`);
    if (this.inLeapYear(calendarDate) && monthInfo.leap) monthInfo = monthInfo.leap;
    return monthInfo;
  }
  estimateIsoDate(calendarDateParam: CalendarYMD) {
    // FYI, this "estimate" is always the exact ISO date, which makes the Indian
    // calendar fast!
    const calendarDate = this.adjustCalendarDate(calendarDateParam);
    const monthInfo = this.getMonthInfo(calendarDate);
    const isoYear = calendarDate.year + 78 + (monthInfo.nextYear ? 1 : 0);
    const isoMonth = monthInfo.month;
    const isoDay = monthInfo.day;
    const isoDate = ES.AddISODate(isoYear, isoMonth, isoDay, 0, 0, 0, calendarDate.day - 1, 'constrain');
    return isoDate;
  }
  // https://bugs.chromium.org/p/v8/issues/detail?id=10529 causes Intl's Indian
  // calendar output to fail for all dates before 0001-01-01 ISO.  For example,
  // in Node 12 0000-01-01 is calculated as 6146/12/-583 instead of 10/11/-79 as
  // expected.
  vulnerableToBceBug =
    new Date('0000-01-01T00:00Z').toLocaleDateString('en-US-u-ca-indian', { timeZone: 'UTC' }) !== '10/11/-79 Saka';
  override checkIcuBugs(isoDate: IsoYMD) {
    if (this.vulnerableToBceBug && isoDate.year < 1) {
      throw new RangeError(
        `calendar '${this.id}' is broken for ISO dates before 0001-01-01` +
          ' (see https://bugs.chromium.org/p/v8/issues/detail?id=10529)'
      );
    }
  }
}

/**
 * Era metadata defined for each calendar.
 * TODO: instead of optional properties, this should really have rules
 * encoded in the type, e.g. isoEpoch is required unless reverseOf is present.
 *  */
interface InputEra {
  /** name of the era */
  name: string;

  /**
   * Signed calendar year where this era begins.Will be
   * 1 (or 0 for zero-based eras) for the anchor era assuming that `year`
   * numbering starts at the beginning of the anchor era, which is true
   * for all ICU calendars except Japanese. If an era starts mid-year
   * then a calendar month and day are included. Otherwise
   * `{ month: 1, day: 1 }` is assumed.
   */
  anchorEpoch?: CalendarYearOnly | CalendarYMD;

  /** ISO date of the first day of this era */
  isoEpoch?: { year: number; month: number; day: number };

  /**
   * If present, then this era counts years backwards like BC
   * and this property points to the forward era. This must be
   * the last (oldest) era in the array.
   * */
  reverseOf?: string;

  /**
   * If true, the era's years are 0-based. If omitted or false,
   * then the era's years are 1-based.
   * */
  hasYearZero?: boolean;

  /**
   * Override if this era is the anchor. Not normally used because
   * anchor eras are inferred.
   * */
  isAnchor?: boolean;
}
/**
 * Transformation of the `InputEra` type with all fields filled in by
 * `adjustEras()`
 * */
interface Era {
  /** name of the era */
  name: string;

  /**
   * alternate name of the era used in old versions of ICU data
   * format is `era{n}` where n is the zero-based index of the era
   * with the oldest era being 0.
   * */
  genericName: string;

  /**
   * Signed calendar year where this era begins. Will be 1 (or 0 for zero-based
   * eras) for the anchor era assuming that `year` numbering starts at the
   * beginning of the anchor era, which is true for all ICU calendars except
   * Japanese. For input, the month and day are optional. If an era starts
   * mid-year then a calendar month and day are included.
   * Otherwise `{ month: 1, day: 1 }` is assumed.
   */
  anchorEpoch: CalendarYMD;

  /** ISO date of the first day of this era */
  isoEpoch: IsoYMD;

  /**
   * If present, then this era counts years backwards like BC
   * and this property points to the forward era. This must be
   * the last (oldest) era in the array.
   * */
  reverseOf?: Era;

  /**
   * If true, the era's years are 0-based. If omitted or false,
   * then the era's years are 1-based.
   * */
  hasYearZero?: boolean;

  /**
   * Override if this era is the anchor. Not normally used because
   * anchor eras are inferred.
   * */
  isAnchor?: boolean;
}

/**
 * This function adds additional metadata that makes it easier to work with
 * eras. Note that it mutates and normalizes the original era objects, which is
 * OK because this is non-observable, internal-only metadata.
 *
 * The result is an array of eras with the shape defined above.
 * */
function adjustEras(erasParam: InputEra[]): { eras: Era[]; anchorEra: Era } {
  let eras: (InputEra | Era)[] = erasParam;
  if (eras.length === 0) {
    throw new RangeError('Invalid era data: eras are required');
  }
  if (eras.length === 1 && eras[0].reverseOf) {
    throw new RangeError('Invalid era data: anchor era cannot count years backwards');
  }
  if (eras.length === 1 && !eras[0].name) {
    throw new RangeError('Invalid era data: at least one named era is required');
  }
  if (eras.filter((e) => e.reverseOf != null).length > 1) {
    throw new RangeError('Invalid era data: only one era can count years backwards');
  }

  // Find the "anchor era" which is the era used for (era-less) `year`. Reversed
  // eras can never be anchors. The era without an `anchorEpoch` property is the
  // anchor.
  let anchorEra: Era | InputEra | undefined;
  eras.forEach((e) => {
    if (e.isAnchor || (!e.anchorEpoch && !e.reverseOf)) {
      if (anchorEra) throw new RangeError('Invalid era data: cannot have multiple anchor eras');
      anchorEra = e;
      e.anchorEpoch = { year: e.hasYearZero ? 0 : 1 };
    } else if (!e.name) {
      throw new RangeError('If era name is blank, it must be the anchor era');
    }
  });

  // If the era name is undefined, then it's an anchor that doesn't interact
  // with eras at all. For example, Japanese `year` is always the same as ISO
  // `year`.  So this "era" is the anchor era but isn't used for era matching.
  // Strip it from the list that's returned.
  eras = eras.filter((e) => e.name);

  eras.forEach((e) => {
    // Some eras are mirror images of another era e.g. B.C. is the reverse of A.D.
    // Replace the string-valued "reverseOf" property with the actual era object
    // that's reversed.
    const { reverseOf } = e;
    if (reverseOf) {
      const reversedEra = eras.find((era) => era.name === reverseOf);
      if (reversedEra === undefined) throw new RangeError(`Invalid era data: unmatched reverseOf era: ${reverseOf}`);
      e.reverseOf = reversedEra as Era;
      e.anchorEpoch = reversedEra.anchorEpoch;
      e.isoEpoch = reversedEra.isoEpoch;
    }
    type YMD = {
      year: number;
      month: number;
      day: number;
    };
    if ((e.anchorEpoch as YMD).month === undefined) (e.anchorEpoch as YMD).month = 1;
    if ((e.anchorEpoch as YMD).day === undefined) (e.anchorEpoch as YMD).day = 1;
  });

  // Ensure that the latest epoch is first in the array. This lets us try to
  // match eras in index order, with the last era getting the remaining older
  // years. Any reverse-signed era must be at the end.
  ArraySort.call(eras, (e1, e2) => {
    if (e1.reverseOf) return 1;
    if (e2.reverseOf) return -1;
    if (!e1.isoEpoch || !e2.isoEpoch) throw new RangeError('Invalid era data: missing ISO epoch');
    return e2.isoEpoch.year - e1.isoEpoch.year;
  });

  // If there's a reversed era, then the one before it must be the era that's
  // being reversed.
  const lastEraReversed = eras[eras.length - 1].reverseOf;
  if (lastEraReversed) {
    if (lastEraReversed !== eras[eras.length - 2]) throw new RangeError('Invalid era data: invalid reverse-sign era');
  }

  // Finally, add a "genericName" property in the format "era{n} where `n` is
  // zero-based index, with the oldest era being zero. This format is used by
  // older versions of ICU data.
  eras.forEach((e, i) => {
    (e as Era).genericName = `era${eras.length - 1 - i}`;
  });

  return { eras: eras as Era[], anchorEra: (anchorEra || eras[0]) as Era };
}

function isGregorianLeapYear(year: number) {
  return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
}

/** Base for all Gregorian-like calendars. */
abstract class GregorianBaseHelper extends HelperBase {
  id: BuiltinCalendarId;
  eras: Era[];
  anchorEra: Era;

  constructor(id: BuiltinCalendarId, originalEras: InputEra[]) {
    super();
    this.id = id;
    const { eras, anchorEra } = adjustEras(originalEras);
    this.anchorEra = anchorEra;
    this.eras = eras;
  }
  calendarType = 'solar' as const;
  inLeapYear(calendarDate: CalendarYearOnly) {
    // Calendars that don't override this method use the same months and leap
    // years as Gregorian. Once we know the ISO year corresponding to the
    // calendar year, we'll know if it's a leap year or not.
    const { year } = this.estimateIsoDate({ month: 1, day: 1, year: calendarDate.year });
    return isGregorianLeapYear(year);
  }
  monthsInYear(/* calendarDate */) {
    return 12;
  }
  minimumMonthLength(calendarDate: CalendarYM): number {
    const { month } = calendarDate;
    if (month === 2) return this.inLeapYear(calendarDate) ? 29 : 28;
    return [4, 6, 9, 11].indexOf(month) >= 0 ? 30 : 31;
  }
  maximumMonthLength(calendarDate: CalendarYM): number {
    return this.minimumMonthLength(calendarDate);
  }
  /** Fill in missing parts of the (year, era, eraYear) tuple */
  completeEraYear<T extends Partial<FullCalendarDate>>(
    calendarDate: T
  ): T & { year: number; eraYear: number; era: string } {
    const checkField = (name: keyof FullCalendarDate, value: string | number | undefined) => {
      const currentValue = calendarDate[name];
      if (currentValue != null && currentValue != value) {
        throw new RangeError(`Input ${name} ${currentValue} doesn't match calculated value ${value}`);
      }
    };
    const eraFromYear = (year: number) => {
      let eraYear;
      const adjustedCalendarDate = { ...calendarDate, year };
      const matchingEra = this.eras.find((e, i) => {
        if (i === this.eras.length - 1) {
          if (e.reverseOf) {
            // This is a reverse-sign era (like BCE) which must be the oldest
            // era. Count years backwards.
            if (year > 0) throw new RangeError(`Signed year ${year} is invalid for era ${e.name}`);
            eraYear = e.anchorEpoch.year - year;
            return true;
          }
          // last era always gets all "leftover" (older than epoch) years,
          // so no need for a comparison like below.
          eraYear = year - e.anchorEpoch.year + (e.hasYearZero ? 0 : 1);
          return true;
        }
        const comparison = this.compareCalendarDates(adjustedCalendarDate, e.anchorEpoch);
        if (comparison >= 0) {
          eraYear = year - e.anchorEpoch.year + (e.hasYearZero ? 0 : 1);
          return true;
        }
        return false;
      });
      if (!matchingEra) throw new RangeError(`Year ${year} was not matched by any era`);
      return { eraYear: eraYear as unknown as number, era: matchingEra.name };
    };

    let { year, eraYear, era } = calendarDate;
    if (year != null) {
      ({ eraYear, era } = eraFromYear(year));
      checkField('era', era);
      checkField('eraYear', eraYear);
    } else if (eraYear != null) {
      const matchingEra =
        era === undefined ? undefined : this.eras.find((e) => e.name === era || e.genericName === era);
      if (!matchingEra) throw new RangeError(`Era ${era} (ISO year ${eraYear}) was not matched by any era`);
      if (eraYear < 1 && matchingEra.reverseOf) {
        throw new RangeError(`Years in ${era} era must be positive, not ${year}`);
      }
      if (matchingEra.reverseOf) {
        year = matchingEra.anchorEpoch.year - eraYear;
      } else {
        year = eraYear + matchingEra.anchorEpoch.year - (matchingEra.hasYearZero ? 0 : 1);
      }
      checkField('year', year);
      // We'll accept dates where the month/day is earlier than the start of
      // the era or after its end as long as it's in the same year. If that
      // happens, we'll adjust the era/eraYear pair to be the correct era for
      // the `year`.
      ({ eraYear, era } = eraFromYear(year));
    } else {
      throw new RangeError('Either `year` or `eraYear` and `era` are required');
    }
    return { ...calendarDate, year, eraYear, era };
  }
  override adjustCalendarDate(
    calendarDateParam: Partial<FullCalendarDate>,
    cache?: OneObjectCache,
    overflow: Overflow = 'constrain'
  ): FullCalendarDate {
    let calendarDate = calendarDateParam;
    // Because this is not a lunisolar calendar, it's safe to convert monthCode to a number
    const { month, monthCode } = calendarDate;
    if (month === undefined) calendarDate = { ...calendarDate, month: monthCodeNumberPart(monthCode as string) };
    this.validateCalendarDate(calendarDate);
    calendarDate = this.completeEraYear(calendarDate);
    return super.adjustCalendarDate(calendarDate, cache, overflow);
  }
  estimateIsoDate(calendarDateParam: CalendarYMD) {
    const calendarDate = this.adjustCalendarDate(calendarDateParam);
    const { year, month, day } = calendarDate;
    const { anchorEra } = this;
    const isoYearEstimate = year + anchorEra.isoEpoch.year - (anchorEra.hasYearZero ? 0 : 1);
    return ES.RegulateISODate(isoYearEstimate, month, day, 'constrain');
  }
}

/**
 * Some calendars are identical to Gregorian except era and year. For these
 * calendars, we can avoid using Intl.DateTimeFormat and just calculate the
 * year, era, and eraYear. This is faster (because Intl.DateTimeFormat is slow
 * and uses a huge amount of RAM), and it avoids ICU bugs like
 * https://bugs.chromium.org/p/chromium/issues/detail?id=1173158.
 */
abstract class SameMonthDayAsGregorianBaseHelper extends GregorianBaseHelper {
  constructor(id: BuiltinCalendarId, originalEras: InputEra[]) {
    super(id, originalEras);
  }
  override isoToCalendarDate(isoDate: IsoYMD): FullCalendarDate {
    // Month and day are same as ISO, so bypass Intl.DateTimeFormat and
    // calculate the year, era, and eraYear here.
    const { year: isoYear, month, day } = isoDate;
    const monthCode = buildMonthCode(month);
    const year = isoYear - this.anchorEra.isoEpoch.year + 1;
    return this.completeEraYear({ year, month, monthCode, day });
  }
}
abstract class OrthodoxBaseHelper extends GregorianBaseHelper {
  constructor(id: BuiltinCalendarId, originalEras: InputEra[]) {
    super(id, originalEras);
  }
  override inLeapYear(calendarDate: CalendarYearOnly) {
    // Leap years happen one year before the Julian leap year. Note that this
    // calendar is based on the Julian calendar which has a leap year every 4
    // years, unlike the Gregorian calendar which doesn't have leap years on
    // years divisible by 100 except years divisible by 400.
    //
    // Note that we're assuming that leap years in before-epoch times match
    // how leap years are defined now. This is probably not accurate but I'm
    // not sure how better to do it.
    const { year } = calendarDate;
    return (year + 1) % 4 === 0;
  }
  override monthsInYear(/* calendarDate */) {
    return 13;
  }
  override minimumMonthLength(calendarDate: CalendarYM) {
    const { month } = calendarDate;
    // Ethiopian/Coptic calendars have 12 30-day months and an extra 5-6 day 13th month.
    if (month === 13) return this.inLeapYear(calendarDate) ? 6 : 5;
    return 30;
  }
  override maximumMonthLength(calendarDate: CalendarYM) {
    return this.minimumMonthLength(calendarDate);
  }
}

// `coptic` and `ethiopic` calendars are very similar to `ethioaa` calendar,
// with the following differences:
// - Coptic uses BCE-like positive numbers for years before its epoch (the other
//   two use negative year numbers before epoch)
// - Coptic has a different epoch date
// - Ethiopic has an additional second era that starts at the same date as the
//   zero era of ethioaa.
class EthioaaHelper extends OrthodoxBaseHelper {
  constructor() {
    super('ethioaa', [{ name: 'era0', isoEpoch: { year: -5492, month: 7, day: 17 } }]);
  }
}
class CopticHelper extends OrthodoxBaseHelper {
  constructor() {
    super('coptic', [
      { name: 'era1', isoEpoch: { year: 284, month: 8, day: 29 } },
      { name: 'era0', reverseOf: 'era1' }
    ]);
  }
}

// Anchor is currently the older era to match ethioaa, but should it be the newer era?
// See https://github.com/tc39/ecma402/issues/534 for discussion.
class EthiopicHelper extends OrthodoxBaseHelper {
  constructor() {
    super('ethiopic', [
      { name: 'era0', isoEpoch: { year: -5492, month: 7, day: 17 } },
      { name: 'era1', isoEpoch: { year: 8, month: 8, day: 27 }, anchorEpoch: { year: 5501 } }
    ]);
  }
}

class RocHelper extends SameMonthDayAsGregorianBaseHelper {
  constructor() {
    super('roc', [
      { name: 'minguo', isoEpoch: { year: 1912, month: 1, day: 1 } },
      { name: 'before-roc', reverseOf: 'minguo' }
    ]);
  }
}

class BuddhistHelper extends SameMonthDayAsGregorianBaseHelper {
  constructor() {
    super('buddhist', [{ name: 'be', hasYearZero: true, isoEpoch: { year: -543, month: 1, day: 1 } }]);
  }
}

class GregoryHelper extends SameMonthDayAsGregorianBaseHelper {
  constructor() {
    super('gregory', [
      { name: 'ce', isoEpoch: { year: 1, month: 1, day: 1 } },
      { name: 'bce', reverseOf: 'ce' }
    ]);
  }
  override reviseIntlEra<T extends Partial<EraAndEraYear>>(calendarDate: T /*, isoDate: IsoDate*/): T {
    let { era, eraYear } = calendarDate;
    // Firefox 96 introduced a bug where the `'short'` format of the era
    // option mistakenly returns the one-letter (narrow) format instead. The
    // code below handles either the correct or Firefox-buggy format. See
    // https://bugzilla.mozilla.org/show_bug.cgi?id=1752253
    if (era === 'bc' || era === 'b') era = 'bce';
    if (era === 'ad' || era === 'a') era = 'ce';
    return { era, eraYear } as T;
  }
}

// NOTE: Only the 5 modern eras (Meiji and later) are included. For dates
// before Meiji 1, the `ce` and `bce` eras are used. Challenges with pre-Meiji
// eras include:
// - Start/end dates of older eras are not precisely defined, which is
//   challenging given Temporal's need for precision
// - Some era dates and/or names are disputed by historians
// - As historical research proceeds, new eras are discovered and existing era
//   dates are modified, leading to considerable churn which is not good for
//   Temporal use.
//  - The earliest era (in 645 CE) may not end up being the earliest depending
//    on future historical scholarship
//  - Before Meiji, Japan used a lunar (or lunisolar?) calendar but AFAIK
//    that's not reflected in the ICU implementation.
//
// For more discussion: https://github.com/tc39/proposal-temporal/issues/526.
//
// Here's a full list of CLDR/ICU eras:
// https://github.com/unicode-org/icu/blob/master/icu4c/source/data/locales/root.txt#L1582-L1818
// https://github.com/unicode-org/cldr/blob/master/common/supplemental/supplementalData.xml#L4310-L4546
//
// NOTE: Japan started using the Gregorian calendar in 6 Meiji, replacing a
// lunisolar calendar. So the day before January 1 of 6 Meiji (1873) was not
// December 31, but December 2, of 5 Meiji (1872). The existing Ecma-402
// Japanese calendar doesn't seem to take this into account, so neither do we:
// > args = ['en-ca-u-ca-japanese', { era: 'short' }]
// > new Date('1873-01-01T12:00').toLocaleString(...args)
// '1 1, 6 Meiji, 12:00:00 PM'
// > new Date('1872-12-31T12:00').toLocaleString(...args)
// '12 31, 5 Meiji, 12:00:00 PM'
class JapaneseHelper extends SameMonthDayAsGregorianBaseHelper {
  constructor() {
    super('japanese', [
      // The Japanese calendar `year` is just the ISO year, because (unlike other
      // ICU calendars) there's no obvious "default era", we use the ISO year.
      { name: 'reiwa', isoEpoch: { year: 2019, month: 5, day: 1 }, anchorEpoch: { year: 2019, month: 5, day: 1 } },
      { name: 'heisei', isoEpoch: { year: 1989, month: 1, day: 8 }, anchorEpoch: { year: 1989, month: 1, day: 8 } },
      { name: 'showa', isoEpoch: { year: 1926, month: 12, day: 25 }, anchorEpoch: { year: 1926, month: 12, day: 25 } },
      { name: 'taisho', isoEpoch: { year: 1912, month: 7, day: 30 }, anchorEpoch: { year: 1912, month: 7, day: 30 } },
      { name: 'meiji', isoEpoch: { year: 1868, month: 9, day: 8 }, anchorEpoch: { year: 1868, month: 9, day: 8 } },
      { name: 'ce', isoEpoch: { year: 1, month: 1, day: 1 } },
      { name: 'bce', reverseOf: 'ce' }
    ]);
  }

  override erasBeginMidYear = true;

  override reviseIntlEra<T extends Partial<EraAndEraYear>>(calendarDate: T, isoDate: IsoYMD): T {
    const { era, eraYear } = calendarDate;
    const { year: isoYear } = isoDate;
    if (this.eras.find((e) => e.name === era)) return { era, eraYear } as T;
    return (isoYear < 1 ? { era: 'bce', eraYear: 1 - isoYear } : { era: 'ce', eraYear: isoYear }) as T;
  }
}

interface ChineseMonthInfo {
  [key: string]: { monthIndex: number; daysInMonth: number };
}
interface ChineseDraftMonthInfo {
  [key: string]: { monthIndex: number; daysInMonth?: number };
}

abstract class ChineseBaseHelper extends HelperBase {
  abstract override id: BuiltinCalendarId;
  calendarType = 'lunisolar' as const;
  inLeapYear(calendarDate: CalendarYearOnly, cache: OneObjectCache) {
    const months = this.getMonthList(calendarDate.year, cache);
    return ObjectEntries(months).length === 13;
  }
  monthsInYear(calendarDate: CalendarYearOnly, cache: OneObjectCache) {
    return this.inLeapYear(calendarDate, cache) ? 13 : 12;
  }
  minimumMonthLength(/* calendarDate */) {
    return 29;
  }
  maximumMonthLength(/* calendarDate */) {
    return 30;
  }
  getMonthList(calendarYear: number, cache: OneObjectCache): ChineseMonthInfo {
    if (calendarYear === undefined) {
      throw new TypeError('Missing year');
    }
    const key = JSON.stringify({ func: 'getMonthList', calendarYear, id: this.id });
    const cached = cache.get(key);
    if (cached) return cached;
    const dateTimeFormat = this.getFormatter();
    const getCalendarDate = (isoYear: number, daysPastFeb1: number) => {
      const isoStringFeb1 = toUtcIsoDateString({ isoYear, isoMonth: 2, isoDay: 1 });
      const legacyDate = new Date(isoStringFeb1);
      // Now add the requested number of days, which may wrap to the next month.
      legacyDate.setUTCDate(daysPastFeb1 + 1);
      const newYearGuess = dateTimeFormat.formatToParts(legacyDate);
      const calendarMonthString = (newYearGuess.find((tv) => tv.type === 'month') as Intl.DateTimeFormatPart).value;
      const calendarDay = +(newYearGuess.find((tv) => tv.type === 'day') as Intl.DateTimeFormatPart).value;
      let calendarYearToVerify: globalThis.Intl.DateTimeFormatPart | number | undefined = newYearGuess.find(
        (tv) => (tv.type as string) === 'relatedYear'
      );
      if (calendarYearToVerify !== undefined) {
        calendarYearToVerify = +calendarYearToVerify.value;
      } else {
        // Node 12 has outdated ICU data that lacks the `relatedYear` field in the
        // output of Intl.DateTimeFormat.formatToParts.
        throw new RangeError(
          `Intl.DateTimeFormat.formatToParts lacks relatedYear in ${this.id} calendar. Try Node 14+ or modern browsers.`
        );
      }
      return { calendarMonthString, calendarDay, calendarYearToVerify };
    };

    // First, find a date close to Chinese New Year. Feb 17 will either be in
    // the first month or near the end of the last month of the previous year.
    let isoDaysDelta = 17;
    let { calendarMonthString, calendarDay, calendarYearToVerify } = getCalendarDate(calendarYear, isoDaysDelta);

    // If we didn't guess the first month correctly, add (almost in some months)
    // a lunar month
    if (calendarMonthString !== '1') {
      isoDaysDelta += 29;
      ({ calendarMonthString, calendarDay } = getCalendarDate(calendarYear, isoDaysDelta));
    }

    // Now back up to near the start of the first month, but not too near that
    // off-by-one issues matter.
    isoDaysDelta -= calendarDay - 5;
    const result = {} as ChineseDraftMonthInfo;
    let monthIndex = 1;
    let oldCalendarDay: number | undefined;
    let oldMonthString: string | undefined;
    let done = false;
    do {
      ({ calendarMonthString, calendarDay, calendarYearToVerify } = getCalendarDate(calendarYear, isoDaysDelta));
      if (oldCalendarDay) {
        result[oldMonthString as string].daysInMonth = oldCalendarDay + 30 - calendarDay;
      }
      if (calendarYearToVerify !== calendarYear) {
        done = true;
      } else {
        result[calendarMonthString] = { monthIndex: monthIndex++ };
        // Move to the next month. Because months are sometimes 29 days, the day of the
        // calendar month will move forward slowly but not enough to flip over to a new
        // month before the loop ends at 12-13 months.
        isoDaysDelta += 30;
      }
      oldCalendarDay = calendarDay;
      oldMonthString = calendarMonthString;
    } while (!done);
    result[oldMonthString].daysInMonth = oldCalendarDay + 30 - calendarDay;

    cache.set(key, result);
    return result as ChineseMonthInfo;
  }
  estimateIsoDate(calendarDate: CalendarYMD) {
    const { year, month } = calendarDate;
    return { year, month: month >= 12 ? 12 : month + 1, day: 1 };
  }
  override adjustCalendarDate(
    calendarDate: Partial<FullCalendarDate>,
    cache: OneObjectCache,
    overflow: Overflow = 'constrain',
    fromLegacyDate = false
  ): FullCalendarDate {
    let { year, month, monthExtra, day, monthCode, eraYear } = calendarDate;
    if (fromLegacyDate) {
      // Legacy Date output returns a string that's an integer with an optional
      // "bis" suffix used only by the Chinese/Dangi calendar to indicate a leap
      // month. Below we'll normalize the output.
      year = eraYear;
      if (monthExtra && monthExtra !== 'bis') throw new RangeError(`Unexpected leap month suffix: ${monthExtra}`);
      const monthCode = buildMonthCode(month as number, monthExtra !== undefined);
      const monthString = `${month}${monthExtra || ''}`;
      const months = this.getMonthList(year as number, cache);
      const monthInfo = months[monthString];
      if (monthInfo === undefined) throw new RangeError(`Unmatched month ${monthString} in Chinese year ${year}`);
      month = monthInfo.monthIndex;
      return { year: year as number, month, day: day as number, era: undefined, eraYear, monthCode };
    } else {
      // When called without input coming from legacy Date output,
      // simply ensure that all fields are present.
      this.validateCalendarDate(calendarDate);
      if (year === undefined) year = eraYear;
      if (eraYear === undefined) eraYear = year;
      if (month === undefined) {
        ES.assertExists(monthCode);
        const months = this.getMonthList(year as number, cache);
        let numberPart = monthCode.replace('L', 'bis').slice(1);
        if (numberPart[0] === '0') numberPart = numberPart.slice(1);
        let monthInfo = months[numberPart];
        month = monthInfo && monthInfo.monthIndex;

        // If this leap month isn't present in this year, constrain to the same
        // day of the previous month.
        if (month === undefined && monthCode.endsWith('L') && monthCode != 'M13L' && overflow === 'constrain') {
          let withoutML = monthCode.slice(1, -1);
          if (withoutML[0] === '0') withoutML = withoutML.slice(1);
          monthInfo = months[withoutML];
          if (monthInfo) {
            month = monthInfo.monthIndex;
            monthCode = buildMonthCode(withoutML);
          }
        }
        if (month === undefined) {
          throw new RangeError(`Unmatched month ${monthCode} in Chinese year ${year}`);
        }
      } else if (monthCode === undefined) {
        const months = this.getMonthList(year as number, cache);
        const monthEntries = ObjectEntries(months);
        const largestMonth = monthEntries.length;
        if (overflow === 'reject') {
          ES.RejectToRange(month, 1, largestMonth);
          ES.RejectToRange(day as number, 1, this.maximumMonthLength());
        } else {
          month = ES.ConstrainToRange(month, 1, largestMonth);
          day = ES.ConstrainToRange(day, 1, this.maximumMonthLength());
        }
        const matchingMonthEntry = monthEntries.find(([, v]) => v.monthIndex === month);
        if (matchingMonthEntry === undefined) {
          throw new RangeError(`Invalid month ${month} in Chinese year ${year}`);
        }
        monthCode = buildMonthCode(
          matchingMonthEntry[0].replace('bis', ''),
          matchingMonthEntry[0].indexOf('bis') !== -1
        );
      } else {
        // Both month and monthCode are present. Make sure they don't conflict.
        const months = this.getMonthList(year as number, cache);
        let numberPart = monthCode.replace('L', 'bis').slice(1);
        if (numberPart[0] === '0') numberPart = numberPart.slice(1);
        const monthInfo = months[numberPart];
        if (!monthInfo) throw new RangeError(`Unmatched monthCode ${monthCode} in Chinese year ${year}`);
        if (month !== monthInfo.monthIndex) {
          throw new RangeError(`monthCode ${monthCode} doesn't correspond to month ${month} in Chinese year ${year}`);
        }
      }
      return {
        ...calendarDate,
        year: year as number,
        eraYear,
        month,
        monthCode: monthCode,
        day: day as number
      };
    }
  }
  // All built-in calendars except Chinese/Dangi and Hebrew use an era
  override hasEra = false;
}

class ChineseHelper extends ChineseBaseHelper {
  id = 'chinese' as const;
}

// Dangi (Korean) calendar has same implementation as Chinese
class DangiHelper extends ChineseBaseHelper {
  id = 'dangi' as const;
}

/**
 * Common implementation of all non-ISO calendars.
 * Per-calendar id and logic live in `id` and `helper` properties attached later.
 * This split allowed an easy separation between code that was similar between
 * ISO and non-ISO implementations vs. code that was very different.
 */
class NonIsoCalendar implements CalendarImpl {
  constructor(private readonly helper: HelperBase) {}
  dateFromFields(
    fieldsParam: Params['dateFromFields'][0],
    options: NonNullable<Params['dateFromFields'][1]>,
    calendarSlotValue: string
  ): Temporal.PlainDate {
    const cache = new OneObjectCache();
    const fieldNames = this.fields(['day', 'month', 'monthCode', 'year']) as AnyTemporalKey[];
    const fields = ES.PrepareTemporalFields(fieldsParam, fieldNames, []);
    const overflow = ES.ToTemporalOverflow(options);
    const { year, month, day } = this.helper.calendarToIsoDate(fields, overflow, cache);
    const result = ES.CreateTemporalDate(year, month, day, calendarSlotValue);
    cache.setObject(result);
    return result;
  }
  yearMonthFromFields(
    fieldsParam: Params['yearMonthFromFields'][0],
    options: NonNullable<Params['yearMonthFromFields'][1]>,
    calendarSlotValue: CalendarSlot
  ): Temporal.PlainYearMonth {
    const cache = new OneObjectCache();
    const fieldNames = this.fields(['month', 'monthCode', 'year']) as AnyTemporalKey[];
    const fields = ES.PrepareTemporalFields(fieldsParam, fieldNames, []);
    const overflow = ES.ToTemporalOverflow(options);
    const { year, month, day } = this.helper.calendarToIsoDate({ ...fields, day: 1 }, overflow, cache);
    const result = ES.CreateTemporalYearMonth(year, month, calendarSlotValue, /* referenceISODay = */ day);
    cache.setObject(result);
    return result;
  }
  monthDayFromFields(
    fieldsParam: Params['monthDayFromFields'][0],
    options: NonNullable<Params['monthDayFromFields'][1]>,
    calendarSlotValue: CalendarSlot
  ): Temporal.PlainMonthDay {
    const cache = new OneObjectCache();
    // For lunisolar calendars, either `monthCode` or `year` must be provided
    // because `month` is ambiguous without a year or a code.
    const fieldNames = this.fields(['day', 'month', 'monthCode', 'year']) as AnyTemporalKey[];
    const fields = ES.PrepareTemporalFields(fieldsParam, fieldNames, []);
    const overflow = ES.ToTemporalOverflow(options);
    const { year, month, day } = this.helper.monthDayFromFields(fields, overflow, cache);
    // `year` is a reference year where this month/day exists in this calendar
    const result = ES.CreateTemporalMonthDay(month, day, calendarSlotValue, /* referenceISOYear = */ year);
    cache.setObject(result);
    return result;
  }
  fields(fieldsParam: string[]): string[] {
    let fields = fieldsParam;
    if (ArrayIncludes.call(fields, 'year')) fields = [...fields, 'era', 'eraYear'];
    return fields;
  }
  fieldKeysToIgnore(
    keys: Exclude<keyof Temporal.PlainDateLike, 'calendar'>[]
  ): Exclude<keyof Temporal.PlainDateLike, 'calendar'>[] {
    const result = new OriginalSet();
    for (let ix = 0; ix < keys.length; ix++) {
      const key = keys[ix];
      ES.Call(SetPrototypeAdd, result, [key]);
      switch (key) {
        case 'era':
          ES.Call(SetPrototypeAdd, result, ['eraYear']);
          ES.Call(SetPrototypeAdd, result, ['year']);
          break;
        case 'eraYear':
          ES.Call(SetPrototypeAdd, result, ['era']);
          ES.Call(SetPrototypeAdd, result, ['year']);
          break;
        case 'year':
          ES.Call(SetPrototypeAdd, result, ['era']);
          ES.Call(SetPrototypeAdd, result, ['eraYear']);
          break;
        case 'month':
          ES.Call(SetPrototypeAdd, result, ['monthCode']);
          // See https://github.com/tc39/proposal-temporal/issues/1784
          if (this.helper.erasBeginMidYear) {
            ES.Call(SetPrototypeAdd, result, ['era']);
            ES.Call(SetPrototypeAdd, result, ['eraYear']);
          }
          break;
        case 'monthCode':
          ES.Call(SetPrototypeAdd, result, ['month']);
          if (this.helper.erasBeginMidYear) {
            ES.Call(SetPrototypeAdd, result, ['era']);
            ES.Call(SetPrototypeAdd, result, ['eraYear']);
          }
          break;
        case 'day':
          if (this.helper.erasBeginMidYear) {
            ES.Call(SetPrototypeAdd, result, ['era']);
            ES.Call(SetPrototypeAdd, result, ['eraYear']);
          }
          break;
      }
    }
    return [...ES.Call(SetPrototypeValues, result, [])];
  }
  dateAdd(
    date: Temporal.PlainDate,
    years: number,
    months: number,
    weeks: number,
    days: number,
    overflow: Overflow,
    calendarSlotValue: CalendarSlot
  ): Temporal.PlainDate {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    const added = this.helper.addCalendar(calendarDate, { years, months, weeks, days }, overflow, cache);
    const isoAdded = this.helper.calendarToIsoDate(added, 'constrain', cache);
    const { year, month, day } = isoAdded;
    const newTemporalObject = ES.CreateTemporalDate(year, month, day, calendarSlotValue);
    // The new object's cache starts with the cache of the old object
    const newCache = new OneObjectCache(cache);
    newCache.setObject(newTemporalObject);
    return newTemporalObject;
  }
  dateUntil(one: Temporal.PlainDate, two: Temporal.PlainDate, largestUnit: Temporal.DateUnit) {
    const cacheOne = OneObjectCache.getCacheForObject(one);
    const cacheTwo = OneObjectCache.getCacheForObject(two);
    const calendarOne = this.helper.temporalToCalendarDate(one, cacheOne);
    const calendarTwo = this.helper.temporalToCalendarDate(two, cacheTwo);
    const result = this.helper.untilCalendar(calendarOne, calendarTwo, largestUnit, cacheOne);
    return result;
  }
  year(date: Temporal.PlainDate | Temporal.PlainYearMonth): number {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    return calendarDate.year;
  }
  month(date: Temporal.PlainDate | Temporal.PlainYearMonth | Temporal.PlainMonthDay): number {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    return calendarDate.month;
  }
  day(date: Temporal.PlainDate | Temporal.PlainMonthDay): number {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    return calendarDate.day;
  }
  era(date: Temporal.PlainDate | Temporal.PlainYearMonth): string | undefined {
    if (!this.helper.hasEra) return undefined;
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    return calendarDate.era;
  }
  eraYear(date: Temporal.PlainDate | Temporal.PlainYearMonth): number | undefined {
    if (!this.helper.hasEra) return undefined;
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    return calendarDate.eraYear;
  }
  monthCode(date: Temporal.PlainDate | Temporal.PlainYearMonth | Temporal.PlainMonthDay): string {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    return calendarDate.monthCode;
  }
  dayOfWeek(date: Temporal.PlainDate): number {
    return impl['iso8601'].dayOfWeek(date);
  }
  dayOfYear(date: Temporal.PlainDate): number {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.isoToCalendarDate(date, cache);
    const startOfYear = this.helper.startOfCalendarYear(calendarDate);
    const diffDays = this.helper.calendarDaysUntil(startOfYear, calendarDate, cache);
    return diffDays + 1;
  }
  weekOfYear(date: Temporal.PlainDate): number {
    return impl['iso8601'].weekOfYear(date);
  }
  yearOfWeek(date: Temporal.PlainDate): number {
    return impl['iso8601'].yearOfWeek(date);
  }
  daysInWeek(date: Temporal.PlainDate): number {
    return impl['iso8601'].daysInWeek(date);
  }
  daysInMonth(date: Temporal.PlainDate | Temporal.PlainYearMonth): number {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);

    // Easy case: if the helper knows the length without any heavy calculation.
    const max = this.helper.maximumMonthLength(calendarDate);
    const min = this.helper.minimumMonthLength(calendarDate);
    if (max === min) return max;

    // The harder case is where months vary every year, e.g. islamic calendars.
    // Find the answer by calculating the difference in days between the first
    // day of the current month and the first day of the next month.
    const startOfMonthCalendar = this.helper.startOfCalendarMonth(calendarDate);
    const startOfNextMonthCalendar = this.helper.addMonthsCalendar(startOfMonthCalendar, 1, 'constrain', cache);
    const result = this.helper.calendarDaysUntil(startOfMonthCalendar, startOfNextMonthCalendar, cache);
    return result;
  }
  daysInYear(dateParam: Temporal.PlainDate | Temporal.PlainYearMonth): number {
    let date = dateParam;
    if (!HasSlot(date, ISO_YEAR)) date = ES.ToTemporalDate(date);
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    const startOfYearCalendar = this.helper.startOfCalendarYear(calendarDate);
    const startOfNextYearCalendar = this.helper.addCalendar(startOfYearCalendar, { years: 1 }, 'constrain', cache);
    const result = this.helper.calendarDaysUntil(startOfYearCalendar, startOfNextYearCalendar, cache);
    return result;
  }
  monthsInYear(date: Temporal.PlainDate | Temporal.PlainYearMonth): number {
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    const result = this.helper.monthsInYear(calendarDate, cache);
    return result;
  }
  inLeapYear(dateParam: Temporal.PlainDate | Temporal.PlainYearMonth): boolean {
    let date = dateParam;
    if (!HasSlot(date, ISO_YEAR)) date = ES.ToTemporalDate(date);
    const cache = OneObjectCache.getCacheForObject(date);
    const calendarDate = this.helper.temporalToCalendarDate(date, cache);
    const result = this.helper.inLeapYear(calendarDate, cache);
    return result;
  }
}

for (const Helper of [
  HebrewHelper,
  PersianHelper,
  EthiopicHelper,
  EthioaaHelper,
  CopticHelper,
  ChineseHelper,
  DangiHelper,
  RocHelper,
  IndianHelper,
  BuddhistHelper,
  GregoryHelper,
  JapaneseHelper,
  IslamicHelper,
  IslamicUmalquraHelper,
  IslamicTblaHelper,
  IslamicCivilHelper,
  IslamicRgsaHelper,
  IslamicCcHelper
]) {
  const helper = new Helper();
  // Construct a new NonIsoCalendar instance with the given Helper implementation that contains
  // per-calendar logic.
  impl[helper.id] = new NonIsoCalendar(helper);
}