/**
 * 度换成弧度
 * @param  d  度
 * @return   弧度
 */
function rad(d) {
  return d * Math.PI / 180.0;
}
 
/**
 * 弧度换成度
 * @param  x 弧度
 * @return   度
 */
function deg(x) {
  return x * 180 / Math.PI;
}

/**
 * 计算另一点经纬度
 * @param lnglat
 *            已知点经纬度
 * @param brng
 *            方位角 ---- 正北方：000°或360°  正东方：090° 正南方：180°  正西方：270°
 * @param dist
 *            距离（米）
 */
 ADestinceAngle_B = function (lnglat, brng, dist) {
    
    /*
    * 大地坐标系资料WGS-84 长半径a=6378137 短半径b=6356752.3142 扁率f=1/298.2572236
    */
    var ct = {
      a: 6378137,
      b: 6356752.3142,
      f: 1 / 298.257223563
    };
    var a = ct.a, b = ct.b, f = ct.f;
 
    var lng1 = lnglat.lng * 1;  //乘一（*1）是为了确保经纬度的数据类型为number
    var lat1 = lnglat.lat * 1;
 
    var s = dist;
    var alpha1 = rad(brng);
    var sinAlpha1 = Math.sin(alpha1);
    var cosAlpha1 = Math.cos(alpha1);
 
    var tanU1 = (1 - f) * Math.tan(rad(lat1));
    var cosU1 = 1 / Math.sqrt((1 + tanU1 * tanU1)), sinU1 = tanU1 * cosU1;
    var sigma1 = Math.atan2(tanU1, cosAlpha1);
    var sinAlpha = cosU1 * sinAlpha1;
    var cosSqAlpha = 1 - sinAlpha * sinAlpha;
    var uSq = cosSqAlpha * (a * a - b * b) / (b * b);
    var A = 1 + uSq / 16384 * (4096 + uSq * (-768 + uSq * (320 - 175 * uSq)));
    var B = uSq / 1024 * (256 + uSq * (-128 + uSq * (74 - 47 * uSq)));
 
    var sigma = s / (b * A), sigmaP = 2 * Math.PI;
    while (Math.abs(sigma - sigmaP) > 1e-12) {
      var cos2SigmaM = Math.cos(2 * sigma1 + sigma);
      var sinSigma = Math.sin(sigma);
      var cosSigma = Math.cos(sigma);
      var deltaSigma = B * sinSigma * (cos2SigmaM + B / 4 * (cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM) -
        B / 6 * cos2SigmaM * (-3 + 4 * sinSigma * sinSigma) * (-3 + 4 * cos2SigmaM * cos2SigmaM)));
      sigmaP = sigma;
      sigma = s / (b * A) + deltaSigma;
    }
 
    var tmp = sinU1 * sinSigma - cosU1 * cosSigma * cosAlpha1;
    var lat2 = Math.atan2(sinU1 * cosSigma + cosU1 * sinSigma * cosAlpha1,
      (1 - f) * Math.sqrt(sinAlpha * sinAlpha + tmp * tmp));
    var lambda = Math.atan2(sinSigma * sinAlpha1, cosU1 * cosSigma - sinU1 * sinSigma * cosAlpha1);
    var C = f / 16 * cosSqAlpha * (4 + f * (4 - 3 * cosSqAlpha));
    var L = lambda - (1 - C) * f * sinAlpha *
      (sigma + C * sinSigma * (cos2SigmaM + C * cosSigma * (-1 + 2 * cos2SigmaM * cos2SigmaM)));
    var revAz = Math.atan2(sinAlpha, -tmp);  // final bearing
    var lng_destina = lng1 * 1 + deg(L);
    
    var lnglat_destination = {lng: lng_destina, lat: deg(lat2)};
 
    return lnglat_destination;
  }

// 根据经纬度计算两点的距离
function getDistanceBy2Point( startPoint,  endPoint){
  var radLat1 = rad(startPoint.lat);
  var radLat2 = rad(endPoint.lat);
  var a = rad(startPoint.lat) - rad(endPoint.lat);
  var b = rad(startPoint.lng) - rad(endPoint.lng);
  var s = 2 * Math.asin(Math.sqrt(Math.pow(Math.sin(a/2),2) +
  Math.cos(radLat1)*Math.cos(radLat2)*Math.pow(Math.sin(b/2),2)));
  s = s * 6378137.0 ;// EARTH_RADIUS;
  s = Math.round(s * 10000) / 10000;
  return s; //return的距离单位为m
}

// 简单海拔计算
function getAltSimply(startPoint, endPoint, subDist, totalDist){
  return subDist==0?startPoint.alt:startPoint.alt+(endPoint.alt-startPoint.alt) * subDist/totalDist;
}

// 根据经纬度获取方位角
function getLineAngle( startPoint,  endPoint){
    var mathPI = 3.1415926535897931;
    var tmpValue = 0;
    var latStart = rad(startPoint.lat);//Y
    var lngStart = rad(startPoint.lng);//X
    var latEnd = rad(endPoint.lat);
    var lngEnd = rad(endPoint.lng);
    if (startPoint.lng == endPoint.lng || startPoint.lat == endPoint.lat)
    {
        if (startPoint.lng == endPoint.lng)
        {
            /// 经度相同
            return endPoint.lat >= startPoint.lat?0:180;
        }
        else
        {
            /// 纬度相同
            return endPoint.lng >= startPoint.lng?90:270;
        }
    }

    var tmpValue = Math.sin(latStart) * Math.sin(latEnd) + Math.cos(latStart) * Math.cos(latEnd) * Math.cos(lngEnd - lngStart);
    var tmpValue = Math.sqrt(1 - tmpValue * tmpValue);
    var tmpValue = Math.cos(latEnd) * Math.sin(lngEnd - lngStart) / tmpValue;
    var resultAngle = Math.abs(Math.asin(tmpValue) * 180 / mathPI);

    if (endPoint.lng > startPoint.lng)
    {
        if (endPoint.lat >= startPoint.lat)
        {
            return resultAngle; /// 第一象限 
        }
        else
        {
            return 180 - resultAngle;  /// 第二象限 
        }
    }
    else
    {
        if (endPoint.lat >= startPoint.lat)
        {
            return 360 - resultAngle; /// 第四象限
        }
        else
        {
            return 180 + resultAngle; /// 第三象限
        }
    }
}