地圖後臺判斷一點是否在一個區域內

package com.eshangm.yymall.console.util;

import java.awt.geom.Point2D;
import java.util.List;

public class GeoUtils {
	/**
	 * 判斷點是否在多邊形內
	 * @param point 檢測點
	 * @param pts   多邊形的頂點
	 * @return      點在多邊形內返回true,不然返回false
	 */
	public static boolean IsPtInPoly(Point2D.Double point, List<Point2D.Double> pts){
	    
	    int N = pts.size();
	    boolean boundOrVertex = true; //若是點位於多邊形的頂點或邊上，也算作點在多邊形內，直接返回true
	    int intersectCount = 0;//cross points count of x 
	    double precision = 2e-10; //浮點類型計算時候與0比較時候的容差
	    Point2D.Double p1, p2;//neighbour bound vertices
	    Point2D.Double p = point; //當前點
	    
	    p1 = pts.get(0);//left vertex        
	    for(int i = 1; i <= N; ++i){//check all rays            
	        if(p.equals(p1)){
	            return boundOrVertex;//p is an vertex
	        }
	        
	        p2 = pts.get(i % N);//right vertex            
	        if(p.x < Math.min(p1.x, p2.x) || p.x > Math.max(p1.x, p2.x)){//ray is outside of our interests                
	            p1 = p2; 
	            continue;//next ray left point
	        }
	        
	        if(p.x > Math.min(p1.x, p2.x) && p.x < Math.max(p1.x, p2.x)){//ray is crossing over by the algorithm (common part of)
	            if(p.y <= Math.max(p1.y, p2.y)){//x is before of ray                    
	                if(p1.x == p2.x && p.y >= Math.min(p1.y, p2.y)){//overlies on a horizontal ray
	                    return boundOrVertex;
	                }
	                
	                if(p1.y == p2.y){//ray is vertical                        
	                    if(p1.y == p.y){//overlies on a vertical ray
	                        return boundOrVertex;
	                    }else{//before ray
	                        ++intersectCount;
	                    } 
	                }else{//cross point on the left side                        
	                    double xinters = (p.x - p1.x) * (p2.y - p1.y) / (p2.x - p1.x) + p1.y;//cross point of y                        
	                    if(Math.abs(p.y - xinters) < precision){//overlies on a ray
	                        return boundOrVertex;
	                    }
	                    
	                    if(p.y < xinters){//before ray
	                        ++intersectCount;
	                    } 
	                }
	            }
	        }else{//special case when ray is crossing through the vertex                
	            if(p.x == p2.x && p.y <= p2.y){//p crossing over p2                    
	                Point2D.Double p3 = pts.get((i+1) % N); //next vertex                    
	                if(p.x >= Math.min(p1.x, p3.x) && p.x <= Math.max(p1.x, p3.x)){//p.x lies between p1.x & p3.x
	                    ++intersectCount;
	                }else{
	                    intersectCount += 2;
	                }
	            }
	        }            
	        p1 = p2;//next ray left point
	    }
	    
	    if(intersectCount % 2 == 0){//偶數在多邊形外
	        return false;
	    } else { //奇數在多邊形內
	        return true;
	    }
	    
	}
}