/* Terminator.js -- Overlay day/night region on a Leaflet map */

Date.prototype.getJulian = function() {
    /* Calculate the present UTC Julian Date. Function is valid after
     * the beginning of the UNIX epoch 1970-01-01 and ignores leap
     * seconds. */
    return (this / 86400000) + 2440587.5;
}

Date.prototype.getGMST = function() {
    /* Calculate Greenwich Mean Sidereal Time according to
       http://aa.usno.navy.mil/faq/docs/GAST.php */
    var julianDay = this.getJulian();
    var d = julianDay - 2451545.0;
    // Low precision equation is good enough for our purposes.
    return (18.697374558 + 24.06570982441908 * d) % 24;
}

L.Terminator = L.Polygon.extend({
    options: {
    color: '#00',
    opacity: 0.1,
    fillColor: '#00',
    fillOpacity: 0.1,
    resolution: 2
    },

    initialize: function(options) {
    this.version = '0.1.0';
    this._R2D = 180 / Math.PI;
    this._D2R = Math.PI / 180;
    L.Util.setOptions(this, options);
    var latLng = this._compute(this.options.time || null)
    this.setLatLngs(latLng);
    },

    _sunEclipticPosition: function(julianDay) {
    /* Compute the position of the Sun in ecliptic coordinates at
       julianDay.  Following
       http://en.wikipedia.org/wiki/Position_of_the_Sun */
    // Days since start of J2000.0
    var n = julianDay - 2451545.0;
    // mean longitude of the Sun
    var L = 280.460 + 0.9856474 * n;
    L %= 360;
    // mean anomaly of the Sun
    var g = 357.528 + 0.9856003 * n;
    g %= 360;
    // ecliptic longitude of Sun
    var lambda = L + 1.915 * Math.sin(g * this._D2R) +
        0.02 * Math.sin(2 * g * this._D2R);
    // distance from Sun in AU
    var R = 1.00014 - 0.01671 * Math.cos(g * this._D2R) -
        0.0014 * Math.cos(2 * g * this._D2R);
    return {"lambda": lambda, "R": R};
    },

    _eclipticObliquity: function(julianDay) {
    // Following the short term expression in
    // http://en.wikipedia.org/wiki/Axial_tilt#Obliquity_of_the_ecliptic_.28Earth.27s_axial_tilt.29
    var n = julianDay - 2451545.0;
    // Julian centuries since J2000.0
    var T = n / 36525;
    var epsilon = 23.43929111 -
        T * (46.836769 / 3600
         - T * (0.0001831 / 3600
            + T * (0.00200340 / 3600
                   - T * (0.576e-6 / 3600
                      - T * 4.34e-8 / 3600))));
    return epsilon;
    },

    _sunEquatorialPosition: function(sunEclLng, eclObliq) {
    /* Compute the Sun's equatorial position from its ecliptic
     * position. Inputs are expected in degrees. Outputs are in
     * degrees as well. */
    var alpha = Math.atan(Math.cos(eclObliq * this._D2R)
                  * Math.tan(sunEclLng * this._D2R)) * this._R2D;
    var delta = Math.asin(Math.sin(eclObliq * this._D2R)
                  * Math.sin(sunEclLng * this._D2R)) * this._R2D;

    var lQuadrant  = Math.floor(sunEclLng / 90) * 90;
    var raQuadrant = Math.floor(alpha / 90) * 90;
    alpha = alpha + (lQuadrant - raQuadrant);

    return {"alpha": alpha, "delta": delta};
    },

    _hourAngle: function(lng, sunPos, gst) {
    /* Compute the hour angle of the sun for a longitude on
     * Earth. Return the hour angle in degrees. */
    var lst = gst + lng / 15;
    return lst * 15 - sunPos.alpha;
    },

    _latitude: function(ha, sunPos) {
    /* For a given hour angle and sun position, compute the
     * latitude of the terminator in degrees. */
    var lat = Math.atan(-Math.cos(ha * this._D2R) /
                Math.tan(sunPos.delta * this._D2R)) * this._R2D;
    return lat;
    },

    _compute: function(time) {
    if (time == null)
        var today = new Date();
    else
        var today = new Date(time);
    var julianDay = today.getJulian();
    var gst = today.getGMST();
    var latLng = [];
    var ha, lat;

    var sunEclPos = this._sunEclipticPosition(julianDay);
    var eclObliq = this._eclipticObliquity(julianDay);
    var sunEqPos = this._sunEquatorialPosition(sunEclPos.lambda, eclObliq);
    for (var i = 0; i <= 720 * this.options.resolution; i++) {
        lng = -360 + i / this.options.resolution;
        ha = this._hourAngle(lng, sunEqPos, gst);
        lat = this._latitude(ha, sunEqPos);
        latLng[i+1] = [lat, lng];
    }
    if (sunEqPos.delta < 0) {
        latLng[0] = [90, -360];
        latLng[latLng.length] = [90, 360];
    } else {
        latLng[0] = [-90, -360];
        latLng[latLng.length] = [-90, 360];
    }
    return latLng;
    }
});

L.terminator = function(options) {
    return new L.Terminator(options);
};