Imported Upstream version 3.6.0

Former-commit-id: da6be194a6b1221998fc28233f2503bd61dd9d14

external/aspnetwebstack/src/HashCodeCombiner.cs

@@ -0,0 +1,53 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. See License.txt in the project root for license information.
+
+using System.Collections;
+
+namespace Microsoft.Internal.Web.Utils
+{
+    internal class HashCodeCombiner
+    {
+        private long _combinedHash64 = 0x1505L;
+
+        public int CombinedHash
+        {
+            get { return _combinedHash64.GetHashCode(); }
+        }
+
+        public HashCodeCombiner Add(IEnumerable e)
+        {
+            if (e == null)
+            {
+                Add(0);
+            }
+            else
+            {
+                int count = 0;
+                foreach (object o in e)
+                {
+                    Add(o);
+                    count++;
+                }
+                Add(count);
+            }
+            return this;
+        }
+
+        public HashCodeCombiner Add(int i)
+        {
+            _combinedHash64 = ((_combinedHash64 << 5) + _combinedHash64) ^ i;
+            return this;
+        }
+
+        public HashCodeCombiner Add(object o)
+        {
+            int hashCode = (o != null) ? o.GetHashCode() : 0;
+            Add(hashCode);
+            return this;
+        }
+
+        public static HashCodeCombiner Start()
+        {
+            return new HashCodeCombiner();
+        }
+    }
+}