feat(Animation): add InOutElastic

Dalamud/Interface/Animation/EasingFunctions/InOutElastic.cs

@@ -0,0 +1,35 @@
+using System;
+
+namespace Dalamud.Interface.Animation.EasingFunctions
+{
+    /// <summary>
+    /// Class providing an "InOutCirc" easing animation.
+    /// </summary>
+    public class InOutElastic : Easing
+    {
+        private static readonly double Constant = (2 * Math.PI) / 4.5;
+        
+        /// <summary>
+        /// Initializes a new instance of the <see cref="InOutElastic"/> class.
+        /// </summary>
+        /// <param name="duration">The duration of the animation.</param>
+        public InOutElastic(TimeSpan duration)
+            : base(duration)
+        {
+            // ignored
+        }
+
+        /// <inheritdoc/>
+        public override void Update()
+        {
+            var p = this.Progress;
+            this.Value = p == 0
+                        ? 0
+                        : p == 1
+                            ? 1
+                            : p < 0.5
+                                ? -(Math.Pow(2, (20 * p) - 10) * Math.Sin(((20 * p) - 11.125) * Constant)) / 2
+                                : (Math.Pow(2, (-20 * p) + 10) * Math.Sin(((20 * p) - 11.125) * Constant) / 2) + 1;
+        }
+    }
+}