feat: Circle2D.IntersectWith(LineSegment2D)

Author: f-frhs

src/Spatial.Tests/Euclidean/Circle2DTests.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using MathNet.Spatial.Euclidean;
 using NUnit.Framework;
 
@@ -69,9 +69,23 @@ public void CircleIntersectWithLine2D(string sc, double radius, string sps, stri
             var actual = circle.IntersectWith(line);
             Assert.That(actual.Length, Is.EqualTo(2));
 
-            var expected = new [] { Point2D.Parse(esp0), Point2D.Parse(esp1) };
+            var expected = new[] { Point2D.Parse(esp0), Point2D.Parse(esp1) };
             AssertGeometry.AreEqual(actual[0], expected[0]);
             AssertGeometry.AreEqual(actual[1], expected[1]);
         }
+
+        [TestCase("0,0", 1.41421356 /*=sqrt(2)*/, "0,0", "+1,+1", "+1,+1")]
+        [TestCase("0,0", 1, "0,0", "+1,0", "+1,0")]
+        [TestCase("0,0", 1, "0,0", "0,+1", "0,+1")]
+        public void CircleIntersectWithLineSegment2D(string sCenter, double radius, string sStart, string sEnd, string sExpected)
+        {
+            var circle = new Circle2D(Point2D.Parse(sCenter), radius);
+            var segment = new LineSegment2D(Point2D.Parse(sStart), Point2D.Parse(sEnd));
+            var actual = circle.IntersectWith(segment);
+            Assert.That(actual.Length, Is.EqualTo(1));
+
+            var expected = Point2D.Parse(sExpected);
+            AssertGeometry.AreEqual(actual[0], expected);
+        }
     }
 }

src/Spatial/Euclidean/Circle2D.cs

@@ -1,4 +1,4 @@
-using MathNet.Spatial.Internals;
+using MathNet.Spatial.Internals;
 using System;
 using System.Diagnostics.Contracts;
 using System.Linq;
@@ -177,20 +177,20 @@ private double[] findParameterTs(Line2D line)
             return ts;
         }
 
-            if (discriminant.IsNearlyEqualTo(0, 1e-6))
-            {
-                var t = -b / (2 * a);
-                return new[] { Point2D.OfVector((s + t * d).ToVector()) };
-            }
-
-
-            var t1 = (-b - Math.Sqrt(b * b - 4 * a * c)) / (2 * a);
-            var t2 = (-b + Math.Sqrt(b * b - 4 * a * c)) / (2 * a);
-            var ts = new double[] { t1, t2 };
-            var result = ts.Select(t => Point2D.OfVector((s + t * d).ToVector())).ToArray();
+        /// <summary>
+        /// Returns intersection a point2D array if this circle and the given line have the intersections
+        /// </summary>
+        /// <param name="line">the given line-segment</param>
+        /// <returns>intersections as a Point2D Array, depending on the count.</returns>
+        public Point2D[] IntersectWith(LineSegment2D line)
+        {
+            var ts = findParameterTs(line.ToLine2D())
+                .Where(t => 0 <= t && t <= line.Length);
+            var result = ts.Select(t => line.StartPoint + t * line.Direction).ToArray();
             return result;
         }
 
+
         /// <summary>
         /// Returns a value to indicate if a pair of circles are equal
         /// </summary>

src/Spatial/Euclidean/LineSegment2D.cs

@@ -236,5 +236,9 @@ void IXmlSerializable.WriteXml(XmlWriter writer)
             writer.WriteElement("StartPoint", StartPoint);
             writer.WriteElement("EndPoint", EndPoint);
         }
+
+        /// <summary>convert this to Line2D </summary>
+        /// <returns>converted Line2D object</returns>
+        public Line2D ToLine2D() => new Line2D(StartPoint, EndPoint);
     }
 }