/**
 * 判断坐标是否在中国境外
 */
function outOfChina(lng, lat) {
  return (
    lng < 72.004 || lng > 137.8347 ||
    lat < 0.8293 || lat > 55.8271
  );
}

/**
 * 纬度转换
 */
function transformLat(x, y) {
  const PI = 3.1415926535897932384626;
  let ret = -100.0 + 2.0 * x + 3.0 * y + 0.2 * y * y + 0.1 * x * y + 0.2 * Math.sqrt(Math.abs(x));
  ret += (20.0 * Math.sin(6.0 * x * PI) + 20.0 * Math.sin(2.0 * x * PI)) * 2.0 / 3.0;
  ret += (20.0 * Math.sin(y * PI) + 40.0 * Math.sin(y / 3.0 * PI)) * 2.0 / 3.0;
  ret += (160.0 * Math.sin(y / 12.0 * PI) + 320 * Math.sin(y * PI / 30.0)) * 2.0 / 3.0;
  return ret;
}

/**
 * 经度转换
 */
function transformLng(x, y) {
  const PI = 3.1415926535897932384626;
  let ret = 300.0 + x + 2.0 * y + 0.1 * x * x + 0.1 * x * y + 0.1 * Math.sqrt(Math.abs(x));
  ret += (20.0 * Math.sin(6.0 * x * PI) + 20.0 * Math.sin(2.0 * x * PI)) * 2.0 / 3.0;
  ret += (20.0 * Math.sin(x * PI) + 40.0 * Math.sin(x / 3.0 * PI)) * 2.0 / 3.0;
  ret += (150.0 * Math.sin(x / 12.0 * PI) + 300.0 * Math.sin(x / 30.0 * PI)) * 2.0 / 3.0;
  return ret;
}

/**
 * WGS84 转 GCJ-02（火星坐标系）
 */
export const wgs84ToGcj02 = function(lng, lat){
  const PI = 3.1415926535897932384626;
  const a = 6378245.0;  // 长半轴
  const ee = 0.00669342162296594323; // 扁率

  // 不在中国境内的坐标不转换
  if (outOfChina(lng, lat)) {
    return [lng, lat];
  }

  let dlat = transformLat(lng - 105.0, lat - 35.0);
  let dlng = transformLng(lng - 105.0, lat - 35.0);
  const radlat = lat / 180.0 * PI;
  let magic = Math.sin(radlat);
  magic = 1 - ee * magic * magic;
  const sqrtmagic = Math.sqrt(magic);
  
  dlat = (dlat * 180.0) / ((a * (1 - ee)) / (magic * sqrtmagic) * PI);
  dlng = (dlng * 180.0) / (a / sqrtmagic * Math.cos(radlat) * PI);
  
  const mglat = lat + dlat;
  const mglng = lng + dlng;
  
  return [mglng, mglat];
}

export const getDistance = function(lat1, lng1, lat2, lng2) {
  const R = 6371000;
  const dLat = (lat2 - lat1) * Math.PI / 180;
  const dLng = (lng2 - lng1) * Math.PI / 180;
  const a =
    0.5 - Math.cos(dLat)/2 + 
    Math.cos(lat1 * Math.PI / 180) * 
    Math.cos(lat2 * Math.PI / 180) * 
    (1 - Math.cos(dLng))/2;
  
  return R * 2 * Math.asin(Math.sqrt(a));
}