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43 lines
1.6 KiB
43 lines
1.6 KiB
4 years ago
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var parse = require('../parse/index.js')
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var differenceInCalendarISOYears = require('../difference_in_calendar_iso_years/index.js')
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var compareAsc = require('../compare_asc/index.js')
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var subISOYears = require('../sub_iso_years/index.js')
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/**
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* @category ISO Week-Numbering Year Helpers
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* @summary Get the number of full ISO week-numbering years between the given dates.
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*
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* @description
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* Get the number of full ISO week-numbering years between the given dates.
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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} dateLeft - the later date
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* @param {Date|String|Number} dateRight - the earlier date
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* @returns {Number} the number of full ISO week-numbering years
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*
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* @example
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* // How many full ISO week-numbering years are between 1 January 2010 and 1 January 2012?
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* var result = differenceInISOYears(
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* new Date(2012, 0, 1),
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* new Date(2010, 0, 1)
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* )
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* //=> 1
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*/
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function differenceInISOYears (dirtyDateLeft, dirtyDateRight) {
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var dateLeft = parse(dirtyDateLeft)
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var dateRight = parse(dirtyDateRight)
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var sign = compareAsc(dateLeft, dateRight)
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var difference = Math.abs(differenceInCalendarISOYears(dateLeft, dateRight))
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dateLeft = subISOYears(dateLeft, sign * difference)
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// Math.abs(diff in full ISO years - diff in calendar ISO years) === 1
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// if last calendar ISO year is not full
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// If so, result must be decreased by 1 in absolute value
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var isLastISOYearNotFull = compareAsc(dateLeft, dateRight) === -sign
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return sign * (difference - isLastISOYearNotFull)
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}
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module.exports = differenceInISOYears
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