var parse = require('../parse/index.js')
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var startOfISOWeek = require('../start_of_iso_week/index.js')
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var startOfISOYear = require('../start_of_iso_year/index.js')
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var MILLISECONDS_IN_WEEK = 604800000
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/**
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* @category ISO Week Helpers
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* @summary Get the ISO week of the given date.
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*
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* @description
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* Get the ISO week of the given date.
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*
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* ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
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*
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* @param {Date|String|Number} date - the given date
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* @returns {Number} the ISO week
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*
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* @example
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* // Which week of the ISO-week numbering year is 2 January 2005?
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* var result = getISOWeek(new Date(2005, 0, 2))
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* //=> 53
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*/
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function getISOWeek (dirtyDate) {
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var date = parse(dirtyDate)
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var diff = startOfISOWeek(date).getTime() - startOfISOYear(date).getTime()
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// Round the number of days to the nearest integer
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// because the number of milliseconds in a week is not constant
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// (e.g. it's different in the week of the daylight saving time clock shift)
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return Math.round(diff / MILLISECONDS_IN_WEEK) + 1
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}
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module.exports = getISOWeek
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