Add simple raycast method (#2937)

Robust.Shared.Maths/MathHelper.cs

@@ -12,6 +12,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 
@@ -615,6 +616,72 @@ namespace Robust.Shared.Maths
 
         #endregion InterpolateCubic
 
+        #region Intersections
+
+        // MIT License
+
+        // Copyright (c) 2019 Erin Catto
+
+        // Permission is hereby granted, free of charge, to any person obtaining a copy
+        // of this software and associated documentation files (the "Software"), to deal
+        // in the Software without restriction, including without limitation the rights
+        // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+        // copies of the Software, and to permit persons to whom the Software is
+        // furnished to do so, subject to the following conditions:
+
+        // The above copyright notice and this permission notice shall be included in all
+        // copies or substantial portions of the Software.
+
+        // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+        // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+        // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+        // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+        // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+        // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+        // SOFTWARE.
+
+        /// <summary>
+        /// Gets the intersection between a line and a circle.
+        /// Essentially a reduced raycast.
+        /// </summary>
+        /// <returns></returns>
+        public static bool TryGetIntersecting(Vector2 start, Vector2 end, float radius, [NotNullWhen(true)] out Vector2? point)
+        {
+            var maxFraction = (end - start).Length;
+            float b = Vector2.Dot(start, start) - radius * radius;
+
+            // Solve quadratic equation.
+            var r = end - start;
+            float c =  Vector2.Dot(start, r);
+            float rr = Vector2.Dot(r, r);
+            float sigma = c * c - rr * b;
+
+            // Check for negative discriminant and short segment.
+            if (sigma < 0.0f || rr < float.Epsilon)
+            {
+                point = null;
+                return false;
+            }
+
+            // Find the point of intersection of the line with the circle.
+            float a = -(c + MathF.Sqrt(sigma));
+
+            // Is the intersection point on the segment?
+            if (0.0f <= a && a <= maxFraction * rr)
+            {
+                a /= rr;
+                var lineToEnd = end - start;
+                // a is a fraction so need to work out the distance along the line we need to be.
+                point = start + lineToEnd * a;
+                return true;
+            }
+
+            point = null;
+            return false;
+        }
+
+        #endregion
+
         #endregion Public Members
     }
 }