language: C# (mono-2.8)
date: 155 days 12 hours ago
link:
visibility: private 
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using System;
using System.Drawing;
 
public class Test
{
        public static void Main()
        {
        Console.WriteLine(LinesIntersect("0,0-2,8:8,0-0,20")); // No intersect
        
        Console.WriteLine(LinesIntersect("0,10-2,0:10,0-0,5")); // Intersect
        
        Console.WriteLine(LinesIntersect("0,0-0,10:2,0-2,10")); // Parallel, vertical
        Console.WriteLine(LinesIntersect("0,0-5,5:2,0-7,5")); // Parallel, diagonal
        
        Console.WriteLine(LinesIntersect("0,0-5,5:2,2-7,7")); // Collinear, overlap
        Console.WriteLine(LinesIntersect("0,0-5,5:7,7-10,10")); // Collinear, no overlap
        
        }
    
    // Utility wrapper around LinesIntersect, expecting a string in the form:
    // A-B:C-D
    // Where A, B, C and D are cartesian coordinates in the form:
    // XX.XX,YY.YY
    static bool LinesIntersect(string s)
    {
        PointF A, B, C, D;
        
        string[] split = s.Split(',', '-', ':');
        
        A = new PointF(float.Parse(split[0]), float.Parse(split[1]));
        B = new PointF(float.Parse(split[2]), float.Parse(split[3]));
        C = new PointF(float.Parse(split[4]), float.Parse(split[5]));
        D = new PointF(float.Parse(split[6]), float.Parse(split[7]));
        
        return LinesIntersect(A, B, C, D);
    }
    
    // Determines if the lines AB and CD intersect.
        static bool LinesIntersect(PointF A, PointF B, PointF C, PointF D)
        {
                PointF CmP = new PointF(C.X - A.X, C.Y - A.Y);
                PointF r = new PointF(B.X - A.X, B.Y - A.Y);
                PointF s = new PointF(D.X - C.X, D.Y - C.Y);
 
                float CmPxr = CmP.X * r.Y - CmP.Y * r.X;
                float CmPxs = CmP.X * s.Y - CmP.Y * s.X;
                float rxs = r.X * s.Y - r.Y * s.X;
 
                if (CmPxr == 0f)
                {
                        // Lines are collinear, and so intersect if they have any overlap
 
                        return ((C.X - A.X < 0f) != (C.X - B.X < 0f))
                                || ((C.Y - A.Y < 0f) != (C.Y - B.Y < 0f));
                }
 
                if (rxs == 0f)
                        return false; // Lines are parallel.
 
                float rxsr = 1f / rxs;
                float t = CmPxs * rxsr;
                float u = CmPxr * rxsr;
 
                return (t >= 0f) && (t <= 1f) && (u >= 0f) && (u <= 1f);
        }
}