import toDate from '../toDate/index'
import startOfISOWeek from '../startOfISOWeek/index'
import startOfISOWeekYear from '../startOfISOWeekYear/index'
import requiredArgs from '../_lib/requiredArgs/index'

const MILLISECONDS_IN_WEEK = 604800000

/**
 * @name getISOWeek
 * @category ISO Week Helpers
 * @summary Get the ISO week of the given date.
 *
 * @description
 * Get the ISO week of the given date.
 *
 * ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
 *
 * @param {Date|Number} date - the given date
 * @returns {Number} the ISO week
 * @throws {TypeError} 1 argument required
 *
 * @example
 * // Which week of the ISO-week numbering year is 2 January 2005?
 * const result = getISOWeek(new Date(2005, 0, 2))
 * //=> 53
 */
export default function getISOWeek(dirtyDate: Date | number): number {
  requiredArgs(1, arguments)

  const date = toDate(dirtyDate)
  const diff =
    startOfISOWeek(date).getTime() - startOfISOWeekYear(date).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
}