import getTimezoneOffsetInMilliseconds from '../_lib/getTimezoneOffsetInMilliseconds/index.js'
import startOfISOWeek from '../startOfISOWeek/index.js'

var MILLISECONDS_IN_WEEK = 604800000

/**
 * @name differenceInCalendarISOWeeks
 * @category ISO Week Helpers
 * @summary Get the number of calendar ISO weeks between the given dates.
 *
 * @description
 * Get the number of calendar ISO weeks between the given dates.
 *
 * ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
 *
 *
 * ### v2.0.0 breaking changes:
 *
 * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
 *
 * @param {Date|String|Number} dateLeft - the later date
 * @param {Date|String|Number} dateRight - the earlier date
 * @param {Options} [options] - the object with options. See [Options]{@link https://date-fns.org/docs/Options}
 * @param {0|1|2} [options.additionalDigits=2] - passed to `toDate`. See [toDate]{@link https://date-fns.org/docs/toDate}
 * @returns {Number} the number of calendar ISO weeks
 * @throws {TypeError} 2 arguments required
 * @throws {RangeError} `options.additionalDigits` must be 0, 1 or 2
 *
 * @example
 * // How many calendar ISO weeks are between 6 July 2014 and 21 July 2014?
 * var result = differenceInCalendarISOWeeks(
 *   new Date(2014, 6, 21),
 *   new Date(2014, 6, 6)
 * )
 * //=> 3
 */
export default function differenceInCalendarISOWeeks (dirtyDateLeft, dirtyDateRight, dirtyOptions) {
  if (arguments.length < 2) {
    throw new TypeError('2 arguments required, but only ' + arguments.length + ' present')
  }

  var startOfISOWeekLeft = startOfISOWeek(dirtyDateLeft, dirtyOptions)
  var startOfISOWeekRight = startOfISOWeek(dirtyDateRight, dirtyOptions)

  var timestampLeft = startOfISOWeekLeft.getTime() -
    getTimezoneOffsetInMilliseconds(startOfISOWeekLeft)
  var timestampRight = startOfISOWeekRight.getTime() -
    getTimezoneOffsetInMilliseconds(startOfISOWeekRight)

  // Round the number of days to the nearest integer
  // because the number of milliseconds in a week is not constant
  // (e.g. it's different in the week of the daylight saving time clock shift)
  return Math.round((timestampLeft - timestampRight) / MILLISECONDS_IN_WEEK)
}
