1 files changed, 50 insertions, 46 deletions

diff --git a/src/mscorlib/src/System/ArraySegment.cs b/src/mscorlib/src/System/ArraySegment.cs
index bc39c2474f..b767e7bd77 100644
--- a/src/mscorlib/src/System/ArraySegment.cs
+++ b/src/mscorlib/src/System/ArraySegment.cs
@@ -16,6 +16,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Runtime.InteropServices;
+using System.Diagnostics;
 using System.Diagnostics.Contracts;
 
 namespace System
@@ -28,10 +29,10 @@ namespace System
     [Serializable]
     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)
         {
             if (array == null)
@@ -64,10 +65,10 @@ namespace System
         {
             get
             {
-                Contract.Assert(    (null == _array && 0 == _offset && 0 == _count)
+                Debug.Assert(    (null == _array && 0 == _offset && 0 == _count)
                                  || (null != _array && _offset >= 0 && _count >= 0 && _offset + _count <= _array.Length),
                                 "ArraySegment is invalid");
-                
+
                 return _array;
             }
         }
@@ -83,7 +84,7 @@ namespace System
                 // after reading each field out of an ArraySegment into their stack.
                 Contract.Ensures(Contract.Result<int>() >= 0);
 
-                Contract.Assert(    (null == _array && 0 == _offset && 0 == _count)
+                Debug.Assert(    (null == _array && 0 == _offset && 0 == _count)
                                  || (null != _array && _offset >= 0 && _count >= 0 && _offset + _count <= _array.Length),
                                 "ArraySegment is invalid");
 
@@ -102,19 +103,38 @@ namespace System
                 // after reading each field out of an ArraySegment into their stack.
                 Contract.Ensures(Contract.Result<int>() >= 0);
 
-                Contract.Assert(     (null == _array && 0 == _offset && 0 == _count)
+                Debug.Assert(     (null == _array && 0 == _offset && 0 == _count)
                                   || (null != _array && _offset >= 0 && _count >= 0 && _offset + _count <= _array.Length),
                                 "ArraySegment is invalid");
 
                 return _count;
             }
         }
-        
+
+        public Enumerator GetEnumerator()
+        {
+            if (_array == null)
+                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NullArray);
+            Contract.EndContractBlock();
+
+            return new Enumerator(this);
+        }
+
         public override int GetHashCode()
         {
-            return null == _array
-                        ? 0
-                        : _array.GetHashCode() ^ _offset ^ _count;
+            if (_array == null)
+            {
+                return 0;
+            }
+            
+            int hash = 5381;
+            hash = System.Numerics.Hashing.HashHelpers.Combine(hash, _offset);
+            hash = System.Numerics.Hashing.HashHelpers.Combine(hash, _count);
+            
+            // The array hash is expected to be an evenly-distributed mixture of bits,
+            // so rather than adding the cost of another rotation we just xor it.
+            hash ^= _array.GetHashCode();
+            return hash;
         }
 
         public override bool Equals(Object obj)
@@ -174,7 +194,7 @@ namespace System
 
             int index = System.Array.IndexOf<T>(_array, item, _offset, _count);
 
-            Contract.Assert(index == -1 || 
+            Debug.Assert(index == -1 ||
                             (index >= _offset && index < _offset + _count));
 
             return index >= 0 ? index - _offset : -1;
@@ -236,7 +256,7 @@ namespace System
 
             int index = System.Array.IndexOf<T>(_array, item, _offset, _count);
 
-            Contract.Assert(index == -1 ||
+            Debug.Assert(index == -1 ||
                             (index >= _offset && index < _offset + _count));
 
             return index >= 0;
@@ -259,46 +279,34 @@ namespace System
         #endregion
 
         #region IEnumerable<T>
-        IEnumerator<T> IEnumerable<T>.GetEnumerator()
-        {
-            if (_array == null)
-                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NullArray);
-            Contract.EndContractBlock();
 
-            return new ArraySegmentEnumerator(this);
-        }
+        IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
         #endregion
 
         #region IEnumerable
-        IEnumerator IEnumerable.GetEnumerator()
-        {
-            if (_array == null)
-                ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_NullArray);
-            Contract.EndContractBlock();
 
-            return new ArraySegmentEnumerator(this);
-        }
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
         #endregion
 
         [Serializable]
-        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)
             {
                 Contract.Requires(arraySegment.Array != null);
                 Contract.Requires(arraySegment.Offset >= 0);
                 Contract.Requires(arraySegment.Count >= 0);
                 Contract.Requires(arraySegment.Offset + arraySegment.Count <= arraySegment.Array.Length);
 
-                _array = arraySegment._array;
-                _start = arraySegment._offset;
-                _end = _start + arraySegment._count;
-                _current = _start - 1;
+                _array = arraySegment.Array;
+                _start = arraySegment.Offset;
+                _end = arraySegment.Offset + arraySegment.Count;
+                _current = arraySegment.Offset - 1;
             }
 
             public bool MoveNext()
@@ -315,19 +323,15 @@ namespace System
             {
                 get
                 {
-                    if (_current < _start) ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumNotStarted);
-                    if (_current >= _end) ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumEnded);
+                    if (_current < _start)
+                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumNotStarted();
+                    if (_current >= _end)
+                        ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumEnded();
                     return _array[_current];
                 }
             }
 
-            object IEnumerator.Current
-            {
-                get
-                {
-                    return Current;
-                }
-            }
+            object IEnumerator.Current => Current;
 
             void IEnumerator.Reset()
             {