using System;

namespace gist.yifeitao.com
{
    /// <summary>
    /// 二维坐标
    /// </summary>
    public struct Ponit2D
    {
        public double X;
        public double Y;

        public Ponit2D(double x, double y)
        {
            X = x;
            Y = y;
        }

        public static Ponit2D operator -(Ponit2D left, Ponit2D right)
        {
            return new Ponit2D(left.X - right.X, left.Y - right.Y);
        }

        public override string ToString()
        {
            return X + "," + Y;
        }
        /// <summary>
        /// 二维向量叉乘
        /// </summary>
        /// <param name="left">(x1,y1,0)</param>
        /// <param name="right">(x2,y2,0)</param>
        /// <returns>(0,0,z)</returns>
        public static double Cross(Ponit2D left, Ponit2D right)
        {
            return left.X*right.Y - left.Y*right.X;
        }
        /// <summary>
        /// 测试点S、T是否在线段AB的同侧
        /// </summary>
        /// <param name="pA"></param>
        /// <param name="pB"></param>
        /// <param name="pT"></param>
        /// <param name="pS"></param>
        /// <returns></returns>
        public static bool SameSideTest(Ponit2D pA, Ponit2D pB, Ponit2D pT, Ponit2D pS)
        {
            // ReSharper disable once InconsistentNaming
            Ponit2D vectorAB = pB - pA;
            // ReSharper disable once InconsistentNaming
            Ponit2D vectorAP = pT - pA;
            // ReSharper disable once InconsistentNaming
            Ponit2D vectorAQ = pS - pA;
            //判断条件，两个叉乘的符号相同，此处判定较为严格，靠近线段时认定为方向相同
            double r = Cross(vectorAB, vectorAP) * Cross(vectorAB, vectorAQ);
            return r >= 0;
        }
        /// <summary>
        /// 测试点T是否在三角形ABC内部
        /// </summary>
        /// <param name="pA"></param>
        /// <param name="pB"></param>
        /// <param name="pC"></param>
        /// <param name="pT"></param>
        /// <returns></returns>
        public static bool TriangleTest(Ponit2D pA, Ponit2D pB, Ponit2D pC, Ponit2D pT)
        {
            bool r = SameSideTest(pA, pB, pC, pT)
                && SameSideTest(pB, pC, pA, pT)
                && SameSideTest(pC, pA, pB, pT);
            return r;
        }
        /// <summary>
        /// 测试点T是否在凸四边形ABCD内部
        /// </summary>
        /// <param name="pA"></param>
        /// <param name="pB"></param>
        /// <param name="pC"></param>
        /// <param name="pD"></param>
        /// <param name="pT"></param>
        /// <returns></returns>
        public static bool QuadrilateralTest(Ponit2D pA, Ponit2D pB, Ponit2D pC, Ponit2D pD, Ponit2D pT)
        {
            //求重心
            Ponit2D pV = new Ponit2D((pA.X + pB.X + pC.X + pD.X) / 4, (pA.Y + pB.Y + pC.Y + pD.Y) / 4);
            //分割为四个三角形测试
            return TriangleTest(pA, pB, pV, pT)
                   || TriangleTest(pB, pC, pV, pT)
                   || TriangleTest(pC, pD, pV, pT)
                   || TriangleTest(pD, pA, pV, pT);
        }
        /// <summary>
        /// 测试四边形ABCD是否为凸
        /// </summary>
        /// <param name="pA"></param>
        /// <param name="pB"></param>
        /// <param name="pC"></param>
        /// <param name="pD"></param>
        /// <returns></returns>
        public static bool BulgeTest(Ponit2D pA, Ponit2D pB, Ponit2D pC, Ponit2D pD)
        {
            return !TriangleTest(pA, pB, pC, pD)
                   && !TriangleTest(pD, pB, pC, pA)
                   && !TriangleTest(pA, pD, pC, pB)
                   && !TriangleTest(pA, pB, pD, pC);
        }
    }
}