var startOfISOYear = require('../start_of_iso_year/index.js')
var addWeeks = require('../add_weeks/index.js')

var MILLISECONDS_IN_WEEK = 604800000

/**
 * @category ISO Week-Numbering Year Helpers
 * @summary Get the number of weeks in an ISO week-numbering year of the given date.
 *
 * @description
 * Get the number of weeks in an ISO week-numbering year of the given date.
 *
 * ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
 *
 * @param {Date|String|Number} date - the given date
 * @returns {Number} the number of ISO weeks in a year
 *
 * @example
 * // How many weeks are in ISO week-numbering year 2015?
 * var result = getISOWeeksInYear(new Date(2015, 1, 11))
 * //=> 53
 */
function getISOWeeksInYear (dirtyDate) {
  var thisYear = startOfISOYear(dirtyDate)
  var nextYear = startOfISOYear(addWeeks(thisYear, 60))
  var diff = nextYear.valueOf() - thisYear.valueOf()
  // Round the number of weeks 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)
}

module.exports = getISOWeeksInYear