Fluid spread refactor (#11908)

Co-authored-by: metalgearsloth <31366439+metalgearsloth@users.noreply.github.com>
Fix undefined

Content.Tests/Shared/DirectionRandomizerTest.cs

@@ -0,0 +1,60 @@
+using System;
+using System.Collections.Generic;
+using Content.Shared;
+using NUnit.Framework;
+using Robust.Shared.Maths;
+using Robust.UnitTesting;
+
+namespace Content.Tests.Shared;
+
+[TestFixture]
+public sealed class DirectionRandomizerTest : RobustUnitTest
+{
+    [Test]
+    [TestCase(new[]
+    {
+        Direction.East,
+        Direction.NorthEast,
+        Direction.West,
+        Direction.NorthWest,
+        Direction.South,
+        Direction.SouthWest,
+        Direction.North,
+        Direction.SouthEast,
+    })]
+    [TestCase(new[]
+    {
+        Direction.East,
+        Direction.West,
+        Direction.South,
+        Direction.North,
+    })]
+    [TestCase(new[]
+    {
+        Direction.East,
+        Direction.West,
+    })]
+    public void TestRandomization(Direction[] x)
+    {
+        var set = new HashSet<Direction>(x);
+        var randomizer = new Span<Direction>(x);
+        randomizer.Shuffle();
+        foreach (var direction in randomizer)
+        {
+            if (set.Contains(direction))
+            {
+                set.Remove(direction);
+            }
+            else
+            {
+                // Asserts no double direction
+                Assert.Fail("Post randomization the enumerator had repeated direction");
+            }
+        }
+        // Because of above foreach this asserts
+        // rand[1,2,3] - [1,2,3] == {}
+        // i.e. randomized set minus original set is empty
+        Assert.IsTrue(set.Count == 0, "Each element must appear once ");
+
+    }
+}