package com.liuzhen.chapterThree;

public class ConvexHull {
    //蠻力法解決凸包問題，返回點集合中凸多邊形的點集合
    public static Point[] getConvexPoint(Point[] A){
        Point[] result = new Point[A.length];
        int len = 0;  //用於計算最終返回結果中是凸包中點的個數
        for(int i = 0;i < A.length;i++){
            for(int j = 0;j < A.length;j++){
                
 
if(j == i)     //除去選中作為確定直線的第一個點
                    continue;
                
                int[] judge = new int[A.length];   //存放點到直線距離所使用判斷公式的結果
                
                for(int k = 0;k < A.length;k++){
                    int a = A[j].getY() - A[i].getY();
                    int b = A[i].getX() - A[j].getX();
                    
 
int c = (A[i].getX())*(A[j].getY()) - (A[i].getY())*(A[j].getX());

                    judge[k] = a*(A[k].getX()) + b*(A[k].getY()) - c;  //根據公式計算具體判斷結果
                }
                
                if(JudgeArray(judge)){  // 如果點均在直線的一邊,則相應的A[i]是凸包中的點
                    result[len++] = A[i];
                    
 
break;
                }    
            }
        }
        Point[] result1 = new Point[len];
        for(int m = 0;m < len;m++)
            result1[m] = result[m];
        return result1;
    }
    
    //判斷陣列中元素是否全部大於等於0或者小於等於0，如果是則返回true，否則返回false
    public static boolean JudgeArray(int[] Array){
        boolean judge = false;
        int len1 = 0, len2 = 0;
        
        for(int i = 0;i < Array.length;i++){
            if(Array[i] >= 0)
                len1++;
        }
        for(int j = 0;j < Array.length;j++){
            if(Array[j] <= 0)
                len2++;
        }
        
        if(len1 == Array.length || len2 == Array.length)
            judge = true;
        return judge;
    }
    
    public static void main(String[] args){
        Point[] A = new Point[8];
        A[0] = new Point(1,0);
        A[1] = new Point(0,1);
        A[2] = new Point(0,-1);
        A[3] = new Point(-1,0);
        A[4] = new Point(2,0);
        A[5] = new Point(0,2);
        A[6] = new Point(0,-2);
        A[7] = new Point(-2,0);
        
        Point[] result = getConvexPoint(A);
        System.out.println("集合A中滿足凸包的點集為：");
        for(int i = 0;i < result.length;i++)
            System.out.println("("+result[i].getX()+","+result[i].getY()+")");
    }
}