Add self-intersection check

Intersection.cs

@@ -0,0 +1,47 @@
+using System.Collections.Generic;
+
+namespace RiverTrace
+{
+    class Intersection
+    {
+        private static bool LinesIntersect(Vector a, Vector b, Vector c, Vector d)
+        {
+            // Source:
+            //   http://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect#comment19165734_565282
+
+            Vector CmP = new Vector(c.X - a.X, c.Y - a.Y);
+            Vector r = new Vector(b.X - a.X, b.Y - a.Y);
+            Vector s = new Vector(d.X - c.X, d.Y - c.Y);
+
+            double CmPxr = CmP.X * r.Y - CmP.Y * r.X;
+            double CmPxs = CmP.X * s.Y - CmP.Y * s.X;
+            double rxs = r.X * s.Y - r.Y * s.X;
+
+            if (CmPxr == 0.0)
+            {
+                return ((c.X - a.X < 0.0) != (c.X - b.X < 0.0)) ||
+                    ((c.Y - a.Y < 0.0) != (c.Y - b.Y < 0.0));
+            }
+
+            if (rxs == 0.0)
+                return false;
+
+            double rxsr = 1.0 / rxs;
+            double t = CmPxs * rxsr;
+            double u = CmPxr * rxsr;
+
+            return (t >= 0.0) && (t <= 1.0) && (u >= 0.0) && (u <= 1.0);
+        }
+
+        public static bool Check(List<Vector> way, Vector point)
+        {
+            if (way.Count < 3)
+                return true;
+            Vector lastPoint = way[way.Count - 1];
+            for (int i = 0; i < way.Count - 2; i++)
+                if (LinesIntersect(way[i], way[i + 1], lastPoint, point))
+                    return false;
+            return true;
+        }
+    }
+}

RiverTrace.csproj

@@ -57,6 +57,7 @@
     <Compile Include="Bing.cs" />
     <Compile Include="Color.cs" />
     <Compile Include="Config.cs" />
+    <Compile Include="Intersection.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="Projection.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />

Tracer.cs

@@ -95,21 +95,21 @@ public Tracer()
 
             tileMap = new TileMap(Config.Data.zoom);
 
-            var way = new List<Vector>();
-
             Vector p1 = new Vector();
             Vector p2 = new Vector();
             Projection.DegToPix(Config.Data.lat1, Config.Data.lon1, Config.Data.zoom, out p1.X, out p1.Y);
             Projection.DegToPix(Config.Data.lat2, Config.Data.lon2, Config.Data.zoom, out p2.X, out p2.Y);
 
-            way.Add(p1);
             Vector lastDirection = p2 - p1;
             lastDirection.Normalize();
 
             CalcSampleDimensions(p1, lastDirection);
 
             int pixelRange = (int)(scanRadius * 2) + 1;
 
+            var way = new List<Vector>();
+            way.Add(p1);
+
             List<SimpleBitmap> samples = new List<SimpleBitmap>();
 
             Vector lastPoint = p1;
@@ -182,16 +182,20 @@ public Tracer()
                     Config.Data.angleStep - Config.Data.angleRange);
                 lastDirection = lastDirection.Rotated(bestAngle);
                 lastPoint += lastDirection * scanRadius * Config.Data.advanceRate;
+
+                if (!Intersection.Check(way, lastPoint))
+                    break;
+
                 way.Add(lastPoint);
             }
-            sw.Stop();
-
-            way.Reverse();
             if (Config.Data.simplificationStrength > 0.0)
             {
                 way = Simplify.DouglasPeuckerReduction(way,
                     riverWidthPx * Config.Data.simplificationStrength);
             }
+            way.Reverse();
+            sw.Stop();
+
             WriteOsm(way);
 
             for (int i = 0; i < 25; i++)