java计算多边形面积 多边形外扩算法java

地图围栏坐标内陷外廓，目前搜索到很多Swift和c++的没有java的，所以我觉得翻译一下
参考：折线平行线的计算方法

不废话直接上代码：

public class Point2D {
    public double x=0.0;
    public double y=0.0;
    Point2D(double x,double y){
        this.x = x;
        this.y = y;
    }

    @Override
    public String toString() {
        return "Point2D{" +
                "x=" + x +
                ", y=" + y +
                '}';
    }
}

public class PolygonOutlineUtil {

    //+
    public static Point2D plus(Point2D left,Point2D right){
        return new Point2D(left.x+right.x, left.y+right.y);
    }
    //-
    public static Point2D reduce(Point2D left,Point2D right) {
        return new Point2D(left.x-right.x, left.y-right.y);
    }
    //value*
    public static Double rideDouble(Point2D left, Point2D right) {
        return left.x*right.x + left.y*right.y;
    }
    //*
    public static Point2D ride(Point2D left, Double value){
        return new Point2D(left.x*value, left.y*value);
    }

    // 自定义的向量差乘运算符号 **
    public static Double operator (Point2D left, Point2D right) {
        return left.x*right.y - left.y*right.x;
    }
    public static List<Point2D> pList = new ArrayList<>(); // 原始顶点坐标， 在initPList函数当中初始化赋值
    public static List<Point2D> dpList = new ArrayList<>();// 边向量dpList［i＋1］－ dpLIst［i］ 在 initDPList函数当中计算后赋值
    public static List<Point2D> ndpList = new ArrayList<>(); // 单位化的边向量， 在initNDPList函数当中计算后肤质，实际使用的时候，完全可以用dpList来保存他们
    public static List<Point2D> newList = new ArrayList<>();  // 新的折线顶点，在compute函数当中，赋值


    // 初始化顶点队列
    public static void initPList(String str){
         String[] split = str.split(";");
        for(String point:split){
            String[] split1 = point.split(",");
            pList.add(new Point2D(Double.valueOf(split1[0]), Double.valueOf(split1[1])));
        }
    }


    // 初始化dpList  两顶点间向量差
    public static void initDPList(){
        System.out.println("计算两顶点间向量差:dpList");
        for (int index=0; index<pList.size();index++){
            int i= index==pList.size()-1 ? 0: index+1;
            dpList.add(reduce(pList.get(index==pList.size()-1 ? 0: index+1),pList.get(index)));
        }
    }



    // 初始化ndpList，单位化两顶点向量差
    public static void initNDPList(){
        System.out.println("开始计算单位化两顶点向量差:ndpList");
        for(int index =0; index<dpList.size();index++) {
            ndpList.add(ride(dpList.get(index),( 1.0 / Math.sqrt(rideDouble(dpList.get(index),dpList.get(index))))));
        }
    }

    // 计算新顶点， 注意参数为负是向内收缩， 为正是向外扩张
    public static void computeLine(Double dist){
        System.out.println("开始计算新顶点");
        int count = pList.size();
        for (int index = 0 ; index<count; index++) {
            int startIndex = index==0 ? count-1 : index-1;
            int endIndex   = index;
            Double sina = operator(ndpList.get(startIndex),ndpList.get(endIndex));
            Double length = dist / sina;
            Point2D vector = reduce(ndpList.get(endIndex),ndpList.get(startIndex));
            newList.add(plus(pList.get(index),ride(vector,length)));
        }
    }

    // 整个算法的调用顺序
    public static void main(String[] args) {
        String str = "30.280196656341005,101.70201860368253;31.17443221616588,102.44943223893644;31.612013574947458,105.5988347902894;30.606585065678747,105.95238883048297";
        initPList(str);
        initDPList();//0.005
        initNDPList();
        computeLine(0.01D);//正数为外廓，负数为内陷
        newList.forEach(e->{
			System.out.println("{lat: "+ e.x +", lng: "+e.y+"},");
        });
    }

}