export const toRadians = (deg) => deg * Math.PI / 180;
export const toDegrees = (rad) => rad * 180 / Math.PI;

export const azimuth = (longitude1, latitude1, longitude2, latitude2) =>{
    const [lon1, lat1, lon2, lat2] = [longitude1, latitude1, longitude2, latitude2].map(toRadians);
    const _deltaLon = Math.abs(lon2 - lon1);
    const PI = Math.PI;
    const deltaLon = (_deltaLon) <= PI ? _deltaLon : 2*PI - _deltaLon;
    const q = (Math.tan(PI/4 + lat2/2))/(Math.tan(PI/4 + lat1/2));
    const ln = 1/Math.log(q);
    const alpha = Math.atan( deltaLon*ln ) * 180/PI; // degree!
    const eastCourse = _deltaLon <= PI;
    const direction = ( lon2>=lon1 &&  eastCourse) || (lon2<lon1 && !eastCourse);
    if(lat2 > lat1 && direction) {
        return alpha;
    } else if(lat2 < lat1 && direction) {
        return 180 - alpha;
    } else if(lat2 < lat1 && !direction) {
        return 180 + alpha;
    } else if(lat2 > lat1 && !direction) {
        return 360 - alpha;
    }
    if(lat2 === lat1) {
        return direction ? 90 : 270;
    }
}