Imported Upstream version 4.6.0.125

Former-commit-id: a2155e9bd80020e49e72e86c44da02a8ac0e57a4

mcs/class/referencesource/System.ServiceModel.Web/System/ServiceModel/Dispatcher/NameValueCache.cs

@@ -0,0 +1,61 @@
+//------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.  All rights reserved.
+//------------------------------------------------------------
+using System;
+using System.Collections;
+using System.Collections.Concurrent;
+using System.Linq;
+using System.Text;
+using System.Threading;
+
+namespace System.ServiceModel.Dispatcher
+{
+    class NameValueCache<T>
+    {
+        // The NameValueCache implements a structure that uses a dictionary to map objects to
+        // indices of an array of cache entries.  This allows us to store the cache entries in 
+        // the order in which they were added to the cache, and yet still lookup any cache entry.
+        // The eviction policy of the cache is to evict the least-recently-added cache entry.  
+        // Using a pointer to the next available cache entry in the array, we can always be sure 
+        // that the given entry is the oldest entry. 
+        Hashtable cache;
+        string[] currentKeys;
+        int nextAvailableCacheIndex;
+        object cachelock;
+        internal const int maxNumberofEntriesInCache = 16;
+
+        public NameValueCache()
+            : this(maxNumberofEntriesInCache)
+        {
+        }
+
+        public NameValueCache(int maxCacheEntries)
+        {
+            cache = new Hashtable();
+            currentKeys = new string[maxCacheEntries];
+            cachelock = new object();
+        }
+
+        public T Lookup(string key)
+        {
+            return (T)cache[key];
+        }
+
+        public void AddOrUpdate(string key, T value)
+        {
+            lock (cache)
+            {
+                if (!cache.ContainsKey(key))
+                {
+                    if (!String.IsNullOrEmpty(currentKeys[nextAvailableCacheIndex]))
+                    {
+                        cache.Remove(currentKeys[nextAvailableCacheIndex]);
+                    }
+                    currentKeys[nextAvailableCacheIndex] = key;
+                    nextAvailableCacheIndex = ++nextAvailableCacheIndex % currentKeys.Length;
+                }
+                cache[key] = value;
+            }
+        }
+    }
+}