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* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
'use strict';
// MODULES //
var isDateObject = require( '@stdlib/assert/is-date-object' );
var isInteger = require( '@stdlib/assert/is-integer' ).isPrimitive;
var currentYear = require( '@stdlib/time/current-year' );
// MAIN //
/**
* Tests whether a value corresponds to a leap year in the Gregorian calendar.
*
* ## Notes
*
* - According to the Gregorian calendar, every year that is exactly divisible by `4` is a leap year, except those years which are also divisible by `100` and not by `400` (e.g., `1900`).
*
* @param {*} [value] - input value
* @returns {boolean} boolean whether a value corresponds to a leap year
*
* @example
* var bool = isLeapYear();
* // returns <boolean>
*
* @example
* var bool = isLeapYear( new Date() );
* // returns <boolean>
*
* @example
* var bool = isLeapYear( 1996 );
* // returns true
*
* @example
* var bool = isLeapYear( 2001 );
* // returns false
*/
function isLeapYear( value ) {
var yr;
if ( arguments.length ) {
if ( isDateObject( value ) ) {
yr = value.getFullYear();
} else if ( isInteger( value ) ) {
yr = value;
} else {
return false;
}
} else {
// Note: cannot cache, as possible for application to cross into a new year:
yr = currentYear();
}
// Special case if year is a new century...
if ( (yr % 100) === 0 ) {
// Centuries are only leap years at the end of "leap cycles" which happen every `400` years:
return ( (yr % 400) === 0 );
}
// All other years which are exactly divisible by `4` are leap years:
return ( (yr % 4) === 0 );
}
// EXPORTS //
module.exports = isLeapYear;
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