Optimize PersistentGenericBag.EqualsSnapshot (#2399)

Author: bahusoid

src/NHibernate.Test/Async/GenericTest/BagGeneric/BagGenericFixture.cs

@@ -8,14 +8,16 @@
 //------------------------------------------------------------------------------
 
 
-using System;
 using System.Collections.Generic;
-using System.Text;
+using System.Linq;
+using NHibernate.Collection;
+using NHibernate.Util;
 using NUnit.Framework;
 
 namespace NHibernate.Test.GenericTest.BagGeneric
 {
 	using System.Threading.Tasks;
+	using System.Threading;
 	[TestFixture]
 	public class BagGenericFixtureAsync : TestCase
 	{
@@ -74,6 +76,51 @@ public async Task SimpleAsync()
 			s.Close();
 		}
 
+		[Test]
+		public async Task EqualsSnapshotAsync()
+		{
+			var a = new A {Name = "first generic type"};
+			var i0 = new B {Name = "1"};
+			var i4 = new B {Name = "4"};
+			a.Items = new List<B>
+			{
+				i0,
+				i0,
+				new B {Name = "2"},
+				new B {Name = "3"},
+				i4,
+				i4,
+			};
+			var lastIdx = a.Items.Count - 1;
+			using (var s = OpenSession())
+			{
+				await (s.SaveAsync(a));
+				await (s.FlushAsync());
+				var collection = (IPersistentCollection) a.Items;
+				var collectionPersister = Sfi.GetCollectionPersister(collection.Role);
+
+				a.Items[0] = i4;
+				Assert.Multiple(
+					async () =>
+					{
+						Assert.That(await (collection.EqualsSnapshotAsync(collectionPersister, CancellationToken.None)), Is.False, "modify first collection element");
+
+						a.Items[lastIdx] = i0;
+						Assert.That(await (collection.EqualsSnapshotAsync(collectionPersister, CancellationToken.None)), Is.True, "swap elements in collection");
+
+						a.Items[0] = i0;
+						a.Items[lastIdx] = i0;
+						Assert.That(await (collection.EqualsSnapshotAsync(collectionPersister, CancellationToken.None)), Is.False, "modify last collection element");
+
+						a.Items[lastIdx] = i4;
+						var reversed = a.Items.Reverse().ToArray();
+						a.Items.Clear();
+						ArrayHelper.AddAll(a.Items, reversed);
+						Assert.That(await (collection.EqualsSnapshotAsync(collectionPersister, CancellationToken.None)), Is.True, "reverse collection elements");
+					});
+			}
+		}
+
 		[Test]
 		public async Task CopyAsync()
 		{

src/NHibernate.Test/GenericTest/BagGeneric/BagGenericFixture.cs

@@ -1,6 +1,7 @@
-using System;
 using System.Collections.Generic;
-using System.Text;
+using System.Linq;
+using NHibernate.Collection;
+using NHibernate.Util;
 using NUnit.Framework;
 
 namespace NHibernate.Test.GenericTest.BagGeneric
@@ -63,6 +64,51 @@ public void Simple()
 			s.Close();
 		}
 
+		[Test]
+		public void EqualsSnapshot()
+		{
+			var a = new A {Name = "first generic type"};
+			var i0 = new B {Name = "1"};
+			var i4 = new B {Name = "4"};
+			a.Items = new List<B>
+			{
+				i0,
+				i0,
+				new B {Name = "2"},
+				new B {Name = "3"},
+				i4,
+				i4,
+			};
+			var lastIdx = a.Items.Count - 1;
+			using (var s = OpenSession())
+			{
+				s.Save(a);
+				s.Flush();
+				var collection = (IPersistentCollection) a.Items;
+				var collectionPersister = Sfi.GetCollectionPersister(collection.Role);
+
+				a.Items[0] = i4;
+				Assert.Multiple(
+					() =>
+					{
+						Assert.That(collection.EqualsSnapshot(collectionPersister), Is.False, "modify first collection element");
+
+						a.Items[lastIdx] = i0;
+						Assert.That(collection.EqualsSnapshot(collectionPersister), Is.True, "swap elements in collection");
+
+						a.Items[0] = i0;
+						a.Items[lastIdx] = i0;
+						Assert.That(collection.EqualsSnapshot(collectionPersister), Is.False, "modify last collection element");
+
+						a.Items[lastIdx] = i4;
+						var reversed = a.Items.Reverse().ToArray();
+						a.Items.Clear();
+						ArrayHelper.AddAll(a.Items, reversed);
+						Assert.That(collection.EqualsSnapshot(collectionPersister), Is.True, "reverse collection elements");
+					});
+			}
+		}
+
 		[Test]
 		public void Copy()
 		{

src/NHibernate/Collection/Generic/PersistentGenericBag.cs

@@ -295,12 +295,15 @@ public override bool EqualsSnapshot(ICollectionPersister persister)
 				return false;
 			}
 
-			foreach (var elt in _gbag)
+			for (var i = 0; i < _gbag.Count; i++)
 			{
-				if (CountOccurrences(elt, _gbag, elementType) != CountOccurrences(elt, sn, elementType))
-				{
+				if (elementType.IsSame(_gbag[i], sn[i]))
+					continue;
+
+				var elt = _gbag[i];
+				var countInSnapshot = CountOccurrences(elt, sn, elementType);
+				if (countInSnapshot == 0 || CountOccurrences(elt, _gbag, elementType) != countInSnapshot)
 					return false;
-				}
 			}
 
 			return true;