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42 lines
1.6 KiB
42 lines
1.6 KiB
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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