From 06dd0eb96dcb903b8807430abedea19f1e83d246 Mon Sep 17 00:00:00 2001
From: Alexander Radchenko <radchenkosasha@gmail.com>
Date: Mon, 12 Dec 2016 23:42:44 +0700
Subject: [PATCH] Make it easier to iterate through an ArraySegment

---
 .../src/System/ArraySegment.cs                | 57 ++++++++-----------
 1 file changed, 24 insertions(+), 33 deletions(-)

diff --git a/src/System.Private.CoreLib/src/System/ArraySegment.cs b/src/System.Private.CoreLib/src/System/ArraySegment.cs
index c8a20726551..84f66f20d1b 100644
--- a/src/System.Private.CoreLib/src/System/ArraySegment.cs
+++ b/src/System.Private.CoreLib/src/System/ArraySegment.cs
@@ -28,9 +28,9 @@ namespace System
 
     public struct ArraySegment<T> : IList<T>, IReadOnlyList<T>
     {
-        private T[] _array;
-        private int _offset;
-        private int _count;
+        private readonly T[] _array;
+        private readonly int _offset;
+        private readonly int _count;
 
         public ArraySegment(T[] array)
         {
@@ -110,6 +110,15 @@ public int Count
             }
         }
 
+        public Enumerator GetEnumerator()
+        {
+            if (_array == null)
+                throw new InvalidOperationException(SR.InvalidOperation_NullArray);
+            Contract.EndContractBlock();
+
+            return new Enumerator(this);
+        }
+
         public override int GetHashCode()
         {
             if (_array == null)
@@ -266,35 +275,21 @@ bool ICollection<T>.Remove(T item)
         #endregion
 
         #region IEnumerable<T>
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-        {
-            if (_array == null)
-                throw new InvalidOperationException(SR.InvalidOperation_NullArray);
-            Contract.EndContractBlock();
-
-            return new ArraySegmentEnumerator(this);
-        }
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
         #endregion
 
         #region IEnumerable
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            if (_array == null)
-                throw new InvalidOperationException(SR.InvalidOperation_NullArray);
-            Contract.EndContractBlock();
-
-            return new ArraySegmentEnumerator(this);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
         #endregion
 
-        private sealed class ArraySegmentEnumerator : IEnumerator<T>
+        public struct Enumerator : IEnumerator<T>
         {
-            private T[] _array;
-            private int _start;
-            private int _end;
+            private readonly T[] _array;
+            private readonly int _start;
+            private readonly int _end; // cache Offset + Count, since it's a little slow
             private int _current;
 
-            internal ArraySegmentEnumerator(ArraySegment<T> arraySegment)
+            internal Enumerator(ArraySegment<T> arraySegment)
             {
                 Debug.Assert(arraySegment.Array != null);
                 Debug.Assert(arraySegment.Offset >= 0);
@@ -321,19 +316,15 @@ public T Current
             {
                 get
                 {
-                    if (_current < _start) throw new InvalidOperationException(SR.InvalidOperation_EnumNotStarted);
-                    if (_current >= _end) throw new InvalidOperationException(SR.InvalidOperation_EnumEnded);
+                    if (_current < _start)
+                        throw new InvalidOperationException(SR.InvalidOperation_EnumNotStarted);
+                    if (_current >= _end)
+                        throw new InvalidOperationException(SR.InvalidOperation_EnumEnded);
                     return _array[_current];
                 }
             }
 
-            object IEnumerator.Current
-            {
-                get
-                {
-                    return Current;
-                }
-            }
+            object IEnumerator.Current => Current;
 
             void IEnumerator.Reset()
             {