Add Quaternion.Multiply(Vector3)

Functions as an equivalent to Unity's Quaternion*Vector3 operator

CHANGELOG.md

@@ -12,6 +12,7 @@
 - X10D: Added `Size.ToPoint()`
 - X10D: Added `Size.ToPointF()`
 - X10D: Added `Size.ToVector2()`
+- X10D: Added `Quaternion.Multiply(Vector3)` - this functions as an equivalent to Unity's `Quaternion * Vector3` operator
 - X10D: Added `Vector2.Deconstruct()`
 - X10D: Added `Vector2.ToPointF()`
 - X10D: Added `Vector2.ToSizeF()`

X10D/src/Numerics/QuaternionExtensions.cs

@@ -0,0 +1,41 @@
+using System.Numerics;
+
+namespace X10D.Numerics;
+
+/// <summary>
+///     Numeric-related extension methods for <see cref="Quaternion" />.
+/// </summary>
+public static class QuaternionExtensions
+{
+    /// <summary>
+    ///     Rotates the specified point with the specified rotation.
+    /// </summary>
+    /// <param name="rotation">The rotation.</param>
+    /// <param name="point">The point.</param>
+    /// <returns>The rotated point.</returns>
+    public static Vector3 Multiply(this in Quaternion rotation, in Vector3 point)
+    {
+        // the internet wrote it, I just handed it in.
+        // https://github.com/Unity-Technologies/UnityCsReference/blob/master/Runtime/Export/Math/Quaternion.cs
+        float x = rotation.X * 2.0f;
+        float y = rotation.Y * 2.0f;
+        float z = rotation.Z * 2.0f;
+        float xx = rotation.X * x;
+        float yy = rotation.Y * y;
+        float zz = rotation.Z * z;
+        float xy = rotation.X * y;
+        float xz = rotation.X * z;
+        float yz = rotation.Y * z;
+        float wx = rotation.W * x;
+        float wy = rotation.W * y;
+        float wz = rotation.W * z;
+
+        (float px, float py, float pz) = point;
+
+        return new Vector3(
+            (1.0f - (yy + zz)) * px + (xy - wz) * py + (xz + wy) * pz,
+            (xy + wz) * px + (1.0f - (xx + zz)) * py + (yz - wx) * pz,
+            (xz - wy) * px + (yz + wx) * py + (1.0f - (xx + yy)) * pz
+        );
+    }
+}