using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            double x1;
            double y1;
            double x3;
            double y3;
            double a;
            double b;
            double c;
            double d;
            double e;
            double f;
            double x;
            double y;
            double x2;
            double y2;
            Console.WriteLine("请输入x1 y1 x2 y2 x3 y3,其间以空格分隔，输入完成后按回车键进行计算");

            x1 = ReadToWhiteSpace(true);
            y1 = ReadToWhiteSpace(true);
            x2 = ReadToWhiteSpace(true);
            y2 = ReadToWhiteSpace(true);
            x3 = ReadToWhiteSpace(true);
            y3 = ReadToWhiteSpace(true);

            //三点一线或三个点有两个或三个点重合，则不能构成圆
            if (InvalidPoint(x1,y1,x2,y2,x3,y3))
            {
                Console.WriteLine("三点不构成圆!");

                Console.ReadKey();
                return;
            }

            a = 2*(x2 - x1);
            b = 2*(y2 - y1);
            c = x2*x2 + y2*y2 - x1*x1 - y1*y1;
            d = 2*(x3 - x2);
            e = 2*(y3 - y2);
            f = x3*x3 + y3*y3 - x2*x2 - y2*y2;
            x = (b*f - e*c)/(b*d - e*a);
            y = (d*c - a*f)/(b*d - e*a);

            Console.WriteLine(string.Format("圆心为({0},{1}),半径为{2}", x, y, Math.Sqrt((x - x1) * (x - x1) + (y - y1) * (y - y1))));

            Console.ReadKey();
            return ;
        }
        ///<summary>
        /// p1(x1,y1),p2(x2,y2),p(x,y)
        /// 判断p是否可以和p1,p2三点组成直线
        ///</summary>
        ///<returns></returns>
        private static bool InvalidPoint(double x1, double y1, double x2, double y2, double x, double y)
        {
             if (x1 == x2 && y1 == y2) //已知两点是重复的两点
                return true;
            else if (x1 == x2)//与X轴垂直，斜率无穷大
            {
                if (x == x1)
                    return true;
                else
                    return false;
            }
            else if (y1 == y2)//与X轴平行，斜率为0
            {
                if (y == y1)
                    return true;
                else
                    return false;
            }
            else
            {
                if (x == x1 && y != y1)
                    return false;
                else if (x == x1 && y == y1)
                    return true;
                else if (Convert.ToDouble(y1 - y2) / Convert.ToDouble(x1 - x2) == Convert.ToDouble(y1 - y) / Convert.ToDouble(x1 - x))
                {
                    return true;
                }
            }
            return false;
        }

        private static double ReadToWhiteSpace(bool skipleadingwhitespace)
        {
            string input = "";
            char nextchar;
            if (skipleadingwhitespace)
            {
                while (char.IsWhiteSpace(nextchar = (char)Console.Read()))
                {
                }
                input += nextchar;
            }
            while (!char.IsWhiteSpace(nextchar = (char)Console.Read()))
            {
                input += nextchar;
            }
            double result = 0;
            double.TryParse(input, out result);
            return result;
        }
    }
}