1/*
2 * Copyright (C) 2010 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.android.server;
18
19import android.text.format.DateUtils;
20
21/** @hide */
22public class TwilightCalculator {
23
24    /** Value of {@link #mState} if it is currently day */
25    public static final int DAY = 0;
26
27    /** Value of {@link #mState} if it is currently night */
28    public static final int NIGHT = 1;
29
30    private static final float DEGREES_TO_RADIANS = (float) (Math.PI / 180.0f);
31
32    // element for calculating solar transit.
33    private static final float J0 = 0.0009f;
34
35    // correction for civil twilight
36    private static final float ALTIDUTE_CORRECTION_CIVIL_TWILIGHT = -0.104719755f;
37
38    // coefficients for calculating Equation of Center.
39    private static final float C1 = 0.0334196f;
40    private static final float C2 = 0.000349066f;
41    private static final float C3 = 0.000005236f;
42
43    private static final float OBLIQUITY = 0.40927971f;
44
45    // Java time on Jan 1, 2000 12:00 UTC.
46    private static final long UTC_2000 = 946728000000L;
47
48    /**
49     * Time of sunset (civil twilight) in milliseconds or -1 in the case the day
50     * or night never ends.
51     */
52    public long mSunset;
53
54    /**
55     * Time of sunrise (civil twilight) in milliseconds or -1 in the case the
56     * day or night never ends.
57     */
58    public long mSunrise;
59
60    /** Current state */
61    public int mState;
62
63    /**
64     * calculates the civil twilight bases on time and geo-coordinates.
65     *
66     * @param time time in milliseconds.
67     * @param latiude latitude in degrees.
68     * @param longitude latitude in degrees.
69     */
70    public void calculateTwilight(long time, double latiude, double longitude) {
71        final float daysSince2000 = (float) (time - UTC_2000) / DateUtils.DAY_IN_MILLIS;
72
73        // mean anomaly
74        final float meanAnomaly = 6.240059968f + daysSince2000 * 0.01720197f;
75
76        // true anomaly
77        final double trueAnomaly = meanAnomaly + C1 * Math.sin(meanAnomaly) + C2
78                * Math.sin(2 * meanAnomaly) + C3 * Math.sin(3 * meanAnomaly);
79
80        // ecliptic longitude
81        final double solarLng = trueAnomaly + 1.796593063d + Math.PI;
82
83        // solar transit in days since 2000
84        final double arcLongitude = -longitude / 360;
85        float n = Math.round(daysSince2000 - J0 - arcLongitude);
86        double solarTransitJ2000 = n + J0 + arcLongitude + 0.0053d * Math.sin(meanAnomaly)
87                + -0.0069d * Math.sin(2 * solarLng);
88
89        // declination of sun
90        double solarDec = Math.asin(Math.sin(solarLng) * Math.sin(OBLIQUITY));
91
92        final double latRad = latiude * DEGREES_TO_RADIANS;
93
94        double cosHourAngle = (Math.sin(ALTIDUTE_CORRECTION_CIVIL_TWILIGHT) - Math.sin(latRad)
95                * Math.sin(solarDec)) / (Math.cos(latRad) * Math.cos(solarDec));
96        // The day or night never ends for the given date and location, if this value is out of
97        // range.
98        if (cosHourAngle >= 1) {
99            mState = NIGHT;
100            mSunset = -1;
101            mSunrise = -1;
102            return;
103        } else if (cosHourAngle <= -1) {
104            mState = DAY;
105            mSunset = -1;
106            mSunrise = -1;
107            return;
108        }
109
110        float hourAngle = (float) (Math.acos(cosHourAngle) / (2 * Math.PI));
111
112        mSunset = Math.round((solarTransitJ2000 + hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000;
113        mSunrise = Math.round((solarTransitJ2000 - hourAngle) * DateUtils.DAY_IN_MILLIS) + UTC_2000;
114
115        if (mSunrise < time && mSunset > time) {
116            mState = DAY;
117        } else {
118            mState = NIGHT;
119        }
120    }
121
122}
123