@js-temporal/polyfill/lib/calendar.ts

Polyfill for Tc39 Stage 3 proposal Temporal (https://github.com/tc39/proposal-temporal)

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 } 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 ObjectEntries = Object.entries;
const ObjectKeys = Object.keys;

/**
 * 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;
  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: Temporal.Calendar
  ): Temporal.PlainDate;
  yearMonthFromFields(
    fields: Params['yearMonthFromFields'][0],
    options: NonNullable<Params['yearMonthFromFields'][1]>,
    calendar: Temporal.Calendar
  ): Temporal.PlainYearMonth;
  monthDayFromFields(
    fields: Params['monthDayFromFields'][0],
    options: NonNullable<Params['monthDayFromFields'][1]>,
    calendar: Temporal.Calendar
  ): Temporal.PlainMonthDay;
  dateAdd(
    date: Temporal.PlainDate,
    years: number,
    months: number,
    weeks: number,
    days: number,
    overflow: Overflow,
    calendar: Temporal.Calendar
  ): 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[];
  mergeFields(fields: Record<string, unknown>, additionalFields: Record<string, unknown>): Record<string, unknown>;
}

/**
 * Implementations for each calendar. Non-ISO calendars have an extra `helper`
 * property that provides additional per-calendar logic.
 */
const impl = {} as {
  iso8601: CalendarImpl;
} & {
  [id in Exclude<BuiltinCalendarId, 'iso8601'>]: NonIsoImpl;
};

/**
 * 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, 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 ES.ToString(this);
  }
  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);
    return impl[GetSlot(this, CALENDAR_ID)].dateFromFields(fields, options, this);
  }
  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);
    return impl[GetSlot(this, CALENDAR_ID)].yearMonthFromFields(fields, options, this);
  }
  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);
    return impl[GetSlot(this, CALENDAR_ID)].monthDayFromFields(fields, options, this);
  }
  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');
    return impl[GetSlot(this, CALENDAR_ID)].mergeFields(fields, additionalFields);
  }
  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'
    );
    return impl[GetSlot(this, CALENDAR_ID)].dateAdd(
      date,
      GetSlot(duration, YEARS),
      GetSlot(duration, MONTHS),
      GetSlot(duration, WEEKS),
      days,
      overflow,
      this
    );
  }
  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);
  }
  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 ES.ToString(this);
  }
  static from(item: Params['from'][0]): Return['from'] {
    return ES.ToTemporalCalendar(item);
  }
  [Symbol.toStringTag]!: 'Temporal.Calendar';
}

MakeIntrinsicClass(Calendar, 'Temporal.Calendar');
DefineIntrinsic('Temporal.Calendar.from', Calendar.from);

/**
 * 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, calendar) {
    const overflow = ES.ToTemporalOverflow(options);
    let fields = ES.PrepareTemporalFields(fieldsParam, ['day', 'month', 'monthCode', 'year'], ['year', 'day']);
    fields = resolveNonLunisolarMonth(fields);
    let { year, month, day } = fields;
    ({ year, month, day } = ES.RegulateISODate(year, month, day, overflow));
    return ES.CreateTemporalDate(year, month, day, calendar);
  },
  yearMonthFromFields(fieldsParam, options, calendar) {
    const overflow = ES.ToTemporalOverflow(options);
    let fields = ES.PrepareTemporalFields(fieldsParam, ['month', 'monthCode', 'year'], ['year']);
    fields = resolveNonLunisolarMonth(fields);
    let { year, month } = fields;
    ({ year, month } = ES.RegulateISOYearMonth(year, month, overflow));
    return ES.CreateTemporalYearMonth(year, month, calendar, /* referenceISODay = */ 1);
  },
  monthDayFromFields(fieldsParam, options, calendar) {
    const overflow = ES.ToTemporalOverflow(options);
    let fields = ES.PrepareTemporalFields(fieldsParam, ['day', 'month', 'monthCode', 'year'], ['day']);
    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, calendar, referenceISOYear);
  },
  fields(fields) {
    return fields;
  },
  mergeFields(fields, additionalFields) {
    const merged: typeof fields = {};
    for (const nextKey of ObjectKeys(fields)) {
      if (nextKey === 'month' || nextKey === 'monthCode') continue;
      merged[nextKey] = fields[nextKey];
    }
    const newKeys = ObjectKeys(additionalFields);
    for (const nextKey of newKeys) {
      merged[nextKey] = additionalFields[nextKey];
    }
    if (!ArrayIncludes.call(newKeys, 'month') && !ArrayIncludes.call(newKeys, 'monthCode')) {
      const { month, monthCode } = fields;
      if (month !== undefined) merged.month = month;
      if (monthCode !== undefined) merged.monthCode = monthCode;
    }
    return merged;
  },
  dateAdd(date, years, months, weeks, days, overflow, calendar) {
    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, calendar);
  },
  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));
  },
  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 powers both ISO and non-ISO calendars. That interface is extended
// (as `NonIsoImpl`) with a `helper` property that implements logic that varies
// between each non-ISO calendar.

/**
 * Interface for non-ISO calendar implementations. The `helper` is an abstract
 * base class that's extended for each non-ISO calendar, e.g. `HebrewHelper`.
 */
interface NonIsoImpl extends CalendarImpl {
  helper: HelperBase;
}

/**
 * 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: this.eraLength,
        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 as Partial<FullCalendarDate>;
    // 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}`);
      }
    }
  }
  /**
   * 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 the
      // only ICU calendars that currently skip days (japanese/roc/buddhist) is
      // a bug (https://bugs.chromium.org/p/chromium/issues/detail?id=1173158)
      // that's currently detected by `checkIcuBugs()` which will throw. 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;
    let maybeConstrained = false;
    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;
          // If the calendar day is larger than the minimal length for this
          // month, then it might be larger than the actual length of the month.
          // So we won't cache it as the correct calendar date for this ISO
          // date.
          maybeConstrained = date.day > this.minimumMonthLength(date);
        } 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);
              maybeConstrained = true;
              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');
    }
    if (!maybeConstrained) {
      // Also cache the reverse mapping
      const keyReverse = JSON.stringify({
        func: 'isoToCalendarDate',
        isoYear: isoEstimate.year,
        isoMonth: isoEstimate.month,
        isoDay: isoEstimate.day,
        id: this.id
      });
      cache.set(keyReverse, date);
    }
    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 diffYears = calendarTwo.year - calendarOne.year;
        const diffMonths = calendarTwo.month - calendarOne.month;
        const diffDays = calendarTwo.day - calendarOne.day;
        const sign = this.compareCalendarDates(calendarTwo, calendarOne);
        if (!sign) {
          return { years: 0, months: 0, weeks: 0, days: 0 };
        }
        if (largestUnit === 'year' && diffYears) {
          const isOneFurtherInYear = diffMonths * sign < 0 || (diffMonths === 0 && diffDays * 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 th