Replace an O(n) lookup in LINQ query parsing by an O(1) one

The HQL generators registry already uses dictionaries to resolve
generators for methods found in LINQ expression. But some generators
do not declare statically their supported methods and instead have
to been queried with a loop on them for support of methods encountered
in the LINQ query.

The result of this lookup can be cached for avoiding calling this loop
again on the next query using the method.
If nhibernate#1868 is merged, this change will compensate for nhibernate#1868 causing up to
three such lookup by method.

Author: fredericDelaporte

src/NHibernate/Linq/Functions/DefaultLinqToHqlGeneratorsRegistry.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;
 
@@ -10,6 +11,9 @@ public class DefaultLinqToHqlGeneratorsRegistry : ILinqToHqlGeneratorsRegistry
 		private readonly Dictionary<MemberInfo, IHqlGeneratorForProperty> registeredProperties = new Dictionary<MemberInfo, IHqlGeneratorForProperty>();
 		private readonly List<IRuntimeMethodHqlGenerator> runtimeMethodHqlGenerators = new List<IRuntimeMethodHqlGenerator>();
 
+		private readonly ConcurrentDictionary<MethodInfo, IHqlGeneratorForMethod> _cachedRuntimeMethodHqlGenerators =
+			new ConcurrentDictionary<MethodInfo, IHqlGeneratorForMethod>();
+
 		public DefaultLinqToHqlGeneratorsRegistry()
 		{
 			RegisterGenerator(new StandardLinqExtensionMethodGenerator());
@@ -69,14 +73,15 @@ public DefaultLinqToHqlGeneratorsRegistry()
 
 		protected bool GetRuntimeMethodGenerator(MethodInfo method, out IHqlGeneratorForMethod methodGenerator)
 		{
-			methodGenerator = null;
-
-			foreach (var typeGenerator in runtimeMethodHqlGenerators.Where(typeGenerator => typeGenerator.SupportsMethod(method)))
-			{
-				methodGenerator = typeGenerator.GetMethodGenerator(method);
-				return true;
-			}
-			return false;
+			methodGenerator = _cachedRuntimeMethodHqlGenerators.GetOrAdd(
+				method,
+				m =>
+					runtimeMethodHqlGenerators
+						.Where(g => g.SupportsMethod(m))
+						.Select(g => g.GetMethodGenerator(m))
+						.FirstOrDefault());
+
+			return methodGenerator != null;
 		}
 
 		public virtual bool TryGetGenerator(MethodInfo method, out IHqlGeneratorForMethod generator)
@@ -112,6 +117,7 @@ public virtual void RegisterGenerator(MemberInfo property, IHqlGeneratorForPrope
 		public void RegisterGenerator(IRuntimeMethodHqlGenerator generator)
 		{
 			runtimeMethodHqlGenerators.Add(generator);
+			_cachedRuntimeMethodHqlGenerators.Clear();
 		}
 	}
 }