JulianDay.js

/*
 * JulianDay.js - A Julian Day object.
 *
 * Copyright © 2012-2015, JEDLSoft
 *
 * 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.
 */

/**
 * @class
 * A Julian Day class. A Julian Day is a date based on the Julian Day count
 * of time invented by Joseph Scaliger in 1583 for use with astronomical calculations.
 * Do not confuse it with a date in the Julian calendar, which it has very
 * little in common with. The naming is unfortunately close, and comes from history.<p>
 *
 *
 * @constructor
 * @param {number} num the Julian Day expressed as a floating point number
 */
var JulianDay = function(num) {
    this.jd = num;
    this.days = Math.floor(this.jd);
    this.frac = num - this.days;
};

JulianDay.prototype = {
    /**
     * Return the integral portion of this Julian Day instance. This corresponds to
     * the number of days since the beginning of the epoch.
     *
     * @return {number} the integral portion of this Julian Day
     */
    getDays: function() {
        return this.days;
    },

    /**
     * Set the date of this Julian Day instance.
     *
     * @param {number} days the julian date expressed as a floating point number
     */
    setDays: function(days) {
        this.days = Math.floor(days);
        this.jd = this.days + this.frac;
    },

    /**
     * Return the fractional portion of this Julian Day instance. This portion
     * corresponds to the time of day for the instance.
     */
    getDayFraction: function() {
        return this.frac;
    },

    /**
     * Set the fractional part of the Julian Day. The fractional part represents
     * the portion of a fully day. Julian dates start at noon, and proceed until
     * noon of the next day. That would mean midnight is represented as a fractional
     * part of 0.5.
     *
     * @param {number} fraction The fractional part of the Julian date
     */
    setDayFraction: function(fraction) {
        var t = Math.floor(fraction);
        this.frac = fraction - t;
        this.jd = this.days + this.frac;
    },

    /**
     * Return the Julian Day expressed as a floating point number.
     * @return {number} the Julian Day as a number
     */
    getDate: function () {
        return this.jd;
    },

    /**
     * Set the date of this Julian Day instance.
     *
     * @param {number} num the numeric Julian Day to set into this instance
     */
    setDate: function (num) {
        this.jd = num;
    },

    /**
     * Add an offset to the current date instance. The offset should be expressed in
     * terms of Julian days. That is, each integral unit represents one day of time, and
     * fractional part represents a fraction of a regular 24-hour day.
     *
     * @param {number} offset an amount to add (or subtract) to the current result instance.
     */
    addDate: function(offset) {
        if (typeof(offset) === 'number') {
            this.jd += offset;
            this.days = Math.floor(this.jd);
            this.frac = this.jd - this.days;
        }
    }
};

module.exports = JulianDay;