[Array] Add benchmark on visitor.

Author: reegeek

Documents/BenchmarksResults/VisitorOnArray.md

@@ -0,0 +1,23 @@
+## VisitorOnArray
+
+### Source
+[VisitorOnArray.cs](../../src/StructLinq.Benchmark/VisitorOnArray.cs)
+
+### Results:
+``` ini
+
+BenchmarkDotNet=v0.13.0, OS=Windows 10.0.19043.1081 (21H1/May2021Update)
+Intel Core i7-8750H CPU 2.20GHz (Coffee Lake), 1 CPU, 12 logical and 6 physical cores
+.NET SDK=5.0.301
+  [Host]     : .NET 5.0.7 (5.0.721.25508), X64 RyuJIT
+  DefaultJob : .NET 5.0.7 (5.0.721.25508), X64 RyuJIT
+
+
+```
+|               Method |     Mean |     Error |    StdDev | Gen 0 | Gen 1 | Gen 2 | Allocated |
+|--------------------- |---------:|----------:|----------:|------:|------:|------:|----------:|
+|                  Sum | 2.704 μs | 0.0385 μs | 0.0301 μs |     - |     - |     - |         - |
+|              Visitor | 1.870 μs | 0.0027 μs | 0.0023 μs |     - |     - |     - |         - |
+|        VisitorOnTake | 1.502 μs | 0.0037 μs | 0.0032 μs |     - |     - |     - |         - |
+|        VisitorOnSkip | 1.838 μs | 0.0066 μs | 0.0062 μs |     - |     - |     - |         - |
+| VisitorOnSkipAndTake | 1.467 μs | 0.0046 μs | 0.0040 μs |     - |     - |     - |         - |

src/StructLinq.Benchmark/VisitorOnArray.cs

@@ -0,0 +1,74 @@
+using System.Linq;
+using System.Runtime.CompilerServices;
+using BenchmarkDotNet.Attributes;
+
+namespace StructLinq.Benchmark
+{
+    [MemoryDiagnoser]
+    public class VisitorOnArray
+    {
+        private const int Count = 1_000;
+        private readonly int[] array;
+        
+        public VisitorOnArray()
+        {
+            array = Enumerable.Range(0, Count).ToArray();
+        }
+
+        [Benchmark]
+        public int Sum()
+        {
+            return array.ToStructEnumerable().Sum(x=>x);
+        }
+
+        [Benchmark]
+        public int Visitor()
+        {
+            var visitor = new SumVisitor(0);
+            array.ToStructEnumerable().Visit(ref visitor);
+            return visitor.sum;
+        }
+
+        [Benchmark]
+        public int VisitorOnTake()
+        {
+            var visitor = new SumVisitor(0);
+            array.ToStructEnumerable().Take(900, x=>x).Visit(ref visitor);
+            return visitor.sum;
+        }
+
+        [Benchmark]
+        public int VisitorOnSkip()
+        {
+            var visitor = new SumVisitor(0);
+            array.ToStructEnumerable().Skip(100, x=>x).Visit(ref visitor);
+            return visitor.sum;
+        }
+
+        [Benchmark]
+        public int VisitorOnSkipAndTake()
+        {
+            var visitor = new SumVisitor(0);
+            array.ToStructEnumerable().Take(900, x=>x).Skip(100, x => x).Visit(ref visitor);
+            return visitor.sum;
+        }
+
+
+        private struct SumVisitor : IVisitor<int>
+        {
+            public int sum;
+
+            public SumVisitor(int sum)
+            {
+                this.sum = sum;
+            }
+
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            public bool Visit(int input)
+            {
+                sum += input;
+                return true;
+            }
+        }
+    }
+}

src/StructLinq/Array/ArrayEnumerable.cs

@@ -1,4 +1,5 @@
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using StructLinq.Utils;
 
 namespace StructLinq.Array
@@ -20,7 +21,7 @@ public ArrayEnumerable(T[] array, int start, int length)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public readonly ArrayStructEnumerator<T> GetEnumerator()
         {
-            return new ArrayStructEnumerator<T>(array, start, Count);
+            return new(array, start, Count);
         }
 
         public readonly int Count
@@ -34,7 +35,7 @@ public void Slice(uint start, uint? length)
         {
             checked
             {
-                this.start = (int)start + this.start;
+                this.start = MathHelpers.Min((int)start + this.start, this.length);
                 if (length.HasValue)
                     this.length = MathHelpers.Min((int)length.Value + this.start, this.length);
             }
@@ -47,14 +48,14 @@ public readonly object Clone()
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public T Get(int i)
+        public readonly T Get(int i)
         {
             return array[start + i];
         }
 
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public VisitStatus Visit<TVisitor>(ref TVisitor visitor)
+        public readonly VisitStatus Visit<TVisitor>(ref TVisitor visitor)
             where TVisitor : IVisitor<T>
         {
             unchecked