refactor: added RcStackArray4, RcStackArray8

- https://github.com/ikpil/DotRecast/issues/41

src/DotRecast.Core/Collections/RcStackArray4.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace DotRecast.Core.Collections
+{
+    public struct RcStackArray4<T> where T : struct
+    {
+        public static readonly RcStackArray4<T> Empty = new RcStackArray4<T>();
+
+        private const int Size = 4;
+        public int Length => Size;
+        
+        public T V0;
+        public T V1;
+        public T V2;
+        public T V3;
+        
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void ThrowExceptionIfIndexOutOfRange(int index)
+        {
+            if (0 > index || index >= Size)
+            {
+                throw new IndexOutOfRangeException($"{index}");
+            }
+        }
+
+        public T this[int index]
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                ThrowExceptionIfIndexOutOfRange(index);
+
+                return index switch
+                {
+                    0 => V0,
+                    1 => V1,
+                    2 => V2,
+                    3 => V3,
+                    _ => throw new IndexOutOfRangeException($"{index}")
+                };
+            }
+
+            set
+            {
+                ThrowExceptionIfIndexOutOfRange(index);
+
+                switch (index)
+                {
+                    case 0: V0 = value; break;
+                    case 1: V1 = value; break;
+                    case 2: V2 = value; break;
+                    case 3: V3 = value; break;
+                }
+            }
+        }
+    }
+}

src/DotRecast.Core/Collections/RcStackArray8.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace DotRecast.Core.Collections
+{
+    public struct RcStackArray8<T>
+    {
+        public static readonly RcStackArray8<T> Empty = new RcStackArray8<T>();
+
+        private const int Size = 8;
+        public int Length => Size;
+        
+        public T V0;
+        public T V1;
+        public T V2;
+        public T V3;
+        public T V4;
+        public T V5;
+        public T V6;
+        public T V7;
+        
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void ThrowExceptionIfIndexOutOfRange(int index)
+        {
+            if (0 > index || index >= Size)
+            {
+                throw new IndexOutOfRangeException($"{index}");
+            }
+        }
+
+        public T this[int index]
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                ThrowExceptionIfIndexOutOfRange(index);
+
+                return index switch
+                {
+                    0 => V0,
+                    1 => V1,
+                    2 => V2,
+                    3 => V3,
+                    4 => V4,
+                    5 => V5,
+                    6 => V6,
+                    7 => V7,
+                    _ => throw new IndexOutOfRangeException($"{index}")
+                };
+            }
+
+            set
+            {
+                ThrowExceptionIfIndexOutOfRange(index);
+
+                switch (index)
+                {
+                    case 0: V0 = value; break;
+                    case 1: V1 = value; break;
+                    case 2: V2 = value; break;
+                    case 3: V3 = value; break;
+                    case 4: V4 = value; break;
+                    case 5: V5 = value; break;
+                    case 6: V6 = value; break;
+                    case 7: V7 = value; break;
+                }
+            }
+        }
+    }
+}

src/DotRecast.Core/RcRand.cs

@@ -20,5 +20,10 @@ namespace DotRecast.Core
         {
             return (float)_r.NextDouble();
         }
+
+        public int NextInt32()
+        {
+            return _r.Next();
+        }
     }
 }

test/DotRecast.Core.Test/RcStackArrayTest.cs

@@ -0,0 +1,53 @@
+using System.Collections.Generic;
+using DotRecast.Core.Collections;
+using NUnit.Framework;
+
+namespace DotRecast.Core.Test;
+
+[Parallelizable]
+public class RcStackArrayTest
+{
+    [Test]
+    public void TestRcStackArray()
+    {
+        var rand = new RcRand();
+
+        // excepted values
+        var list = new List<int>();
+        for (int i = 0; i < 1024; ++i)
+        {
+            list.Add(rand.NextInt32());
+        }
+
+        {
+            RcStackArray4<int> array = RcStackArray4<int>.Empty;
+            for (int i = 0; i < array.Length; ++i)
+            {
+                array[i] = list[i];
+            }
+
+            for (int i = 0; i < array.Length; ++i)
+            {
+                Assert.That(array[i], Is.EqualTo(list[i]));
+            }
+        }
+
+        {
+            RcStackArray8<int> array = RcStackArray8<int>.Empty;
+            for (int i = 0; i < array.Length; ++i)
+            {
+                array[i] = list[i];
+            }
+
+            for (int i = 0; i < array.Length; ++i)
+            {
+                Assert.That(array[i], Is.EqualTo(list[i]));
+            }
+        }
+    }
+
+    public void Test<T>(T a)
+    {
+        T s = a;
+    }
+}