import toDate from '../toDate/index.js'
import startOfWeek from '../startOfWeek/index.js'
import startOfWeekYear from '../startOfWeekYear/index.js'

var MILLISECONDS_IN_WEEK = 604800000

/**
 * @name getWeek
 * @category Week Helpers
 * @summary Get the local week index of the given date.
 *
 * @description
 * Get the local week index of the given date.
 * The exact calculation depends on the values of
 * `options.weekStartsOn` (which is the index of the first day of the week)
 * and `options.firstWeekContainsDate` (which is the day of January, which is always in
 * the first week of the week-numbering year)
 *
 * Week numbering: https://en.wikipedia.org/wiki/Week#Week_numbering
 *
 *
 * ### 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} date - the given 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}
 * @param {0|1|2|3|4|5|6} [options.weekStartsOn=0] - the index of the first day of the week (0 - Sunday)
 * @param {1|2|3|4|5|6|7} [options.firstWeekContainsDate=1] - the day of January, which is always in the first week of the year
 * @returns {Number} the week
 * @throws {TypeError} 1 argument required
 * @throws {RangeError} `options.additionalDigits` must be 0, 1 or 2
 * @throws {RangeError} `options.weekStartsOn` must be between 0 and 6
 * @throws {RangeError} `options.firstWeekContainsDate` must be between 1 and 7
 *
 * @example
 * // Which week of the local week numbering year is 2 January 2005 with default options?
 * var result = getISOWeek(new Date(2005, 0, 2))
 * //=> 2
 *
 * // Which week of the local week numbering year is 2 January 2005,
 * // if Monday is the first day of the week,
 * // and the first week of the year always contains 4 January?
 * var result = getISOWeek(new Date(2005, 0, 2), {
 *   weekStartsOn: 1,
 *   firstWeekContainsDate: 4
 * })
 * //=> 53
 */

export default function getWeek(dirtyDate, dirtyOptions) {
  if (arguments.length < 1) {
    throw new TypeError(
      '1 argument required, but only ' + arguments.length + ' present'
    )
  }

  var date = toDate(dirtyDate, dirtyOptions)
  var diff =
    startOfWeek(date, dirtyOptions).getTime() -
    startOfWeekYear(date, dirtyOptions).getTime()

  // 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(diff / MILLISECONDS_IN_WEEK) + 1
}
