CSHARP-700: Improvements to CombGuidGenerator and its unit tests.

Author: rstam

MongoDB.Bson/Serialization/IdGenerators/CombGuidGenerator.cs

@@ -51,34 +51,51 @@ public static CombGuidGenerator Instance
         /// <returns>An Id.</returns>
         public object GenerateId(object container, object document)
         {
+            var guid = Guid.NewGuid();
+            var timestamp = DateTime.UtcNow;
+            return NewCombGuid(guid, timestamp);
+        }
+
+        /// <summary>
+        /// Tests whether an Id is empty.
+        /// </summary>
+        /// <param name="id">The Id.</param>
+        /// <returns>True if the Id is empty.</returns>
+        public bool IsEmpty(object id)
+        {
+            return id == null || (Guid)id == Guid.Empty;
+        }
+
+        /// <summary>
+        /// Create a new CombGuid from a given Guid and timestamp.
+        /// </summary>
+        /// <param name="guid">The base Guid.</param>
+        /// <param name="timestamp">The timestamp.</param>
+        /// <returns>A new CombGuid created by combining the base Guid with the timestamp.</returns>
+        public Guid NewCombGuid(Guid guid, DateTime timestamp)
+        {
+            // note: Guids generated by CombGuidGenerator are only considered ascending by SQL Server which compares Guids in an unusual way
+            // to generate Guids considered ascending by MongoDB use the AscendingGuidGenerator
+
             var baseDate = new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc);
-            var now = DateTime.UtcNow;
-            var days = (ushort)(now - baseDate).TotalDays;
-            var milliseconds = (int)now.TimeOfDay.TotalMilliseconds;
+            var days = (ushort)(timestamp - baseDate).Days;
+            var timeTicks = (int)(timestamp.TimeOfDay.Ticks * 300 / TimeSpan.TicksPerSecond); // convert from .NET resolution to SQL Server resolution
 
-            // replace last 6 bytes of a new Guid with 2 bytes from days and 4 bytes from milliseconds
+            // replace last 6 bytes of a new Guid with 2 bytes from days and 4 bytes from time of day
             // see: The Cost of GUIDs as Primary Keys by Jimmy Nilson
             // at: http://www.informit.com/articles/article.aspx?p=25862&seqNum=7
 
-            var bytes = Guid.NewGuid().ToByteArray();
+            var bytes = guid.ToByteArray();
+
             Array.Copy(BitConverter.GetBytes(days), 0, bytes, 10, 2);
-            Array.Copy(BitConverter.GetBytes(milliseconds), 0, bytes, 12, 4);
+            Array.Copy(BitConverter.GetBytes(timeTicks), 0, bytes, 12, 4);
             if (BitConverter.IsLittleEndian)
             {
                 Array.Reverse(bytes, 10, 2);
                 Array.Reverse(bytes, 12, 4);
             }
-            return new Guid(bytes);
-        }
 
-        /// <summary>
-        /// Tests whether an Id is empty.
-        /// </summary>
-        /// <param name="id">The Id.</param>
-        /// <returns>True if the Id is empty.</returns>
-        public bool IsEmpty(object id)
-        {
-            return id == null || (Guid)id == Guid.Empty;
+            return new Guid(bytes);
         }
     }
 }

MongoDB.BsonUnitTests/MongoDB.BsonUnitTests.csproj

@@ -118,6 +118,7 @@
     <Compile Include="Serialization\Conventions\ReadWriteMemberFinderConventionsTests.cs" />
     <Compile Include="Serialization\Conventions\StringObjectIdGeneratorConventionsTests.cs" />
     <Compile Include="Serialization\IdGenerators\AscendingGuidGeneratorTests.cs" />
+    <Compile Include="Serialization\IdGenerators\CombGuidGeneratorTests.cs" />
     <Compile Include="Serialization\LegacyBsonClassMapTests.cs" />
     <Compile Include="Serialization\Serializers\ExtraElementsTests.cs" />
     <Compile Include="Serialization\Options\RepresentationSerializationOptionsTests.cs" />

MongoDB.BsonUnitTests/Serialization/IdGenerators/CombGuidGeneratorTests.cs

@@ -0,0 +1,61 @@
+/* Copyright 2010-2013 10gen Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System;
+using System.Linq;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.IdGenerators;
+using NUnit.Framework;
+
+namespace MongoDB.BsonUnitTests.Serialization
+{
+    [TestFixture]
+    public class CombGuidGeneratorTests
+    {
+        private CombGuidGenerator _generator = new CombGuidGenerator();
+
+        [Test]
+        public void TestNewCombGuid()
+        {
+            var guid = Guid.NewGuid();
+            var timestamp = new DateTime(2013, 4, 2, 0, 0, 0, 500, DateTimeKind.Utc); // half a second past midnight
+            var combGuid = _generator.NewCombGuid(guid, timestamp);
+
+            var expectedDays = (short)(timestamp.Date - new DateTime(1900, 1, 1, 0, 0, 0, DateTimeKind.Utc)).Days;
+            var expectedTimeTicks = 150; // half a second in SQL Server resolution
+
+            var bytes = combGuid.ToByteArray();
+            if (BitConverter.IsLittleEndian)
+            {
+                Array.Reverse(bytes, 10, 2);
+                Array.Reverse(bytes, 12, 4);
+            }
+            var days = BitConverter.ToInt16(bytes, 10);
+            var timeTicks = BitConverter.ToInt32(bytes, 12);
+
+            Assert.IsTrue(guid.ToByteArray().Take(10).SequenceEqual(bytes.Take(10))); // first 10 bytes are from the base Guid
+            Assert.AreEqual(expectedDays, days);
+            Assert.AreEqual(expectedTimeTicks, timeTicks);
+        }
+
+        [Test]
+        public void TestIsEmpty()
+        {
+            Assert.IsTrue(_generator.IsEmpty(null));
+            Assert.IsTrue(_generator.IsEmpty(Guid.Empty));
+            Assert.IsFalse(_generator.IsEmpty(Guid.NewGuid()));
+        }
+    }
+}