Akka.Persistence.Query: made Offset IComparable to itself (#3950)

* close #3947 - made Offset IComparable to itself

* forgot to mark second test as FACT

* approved API changes

src/core/Akka.API.Tests/CoreAPISpec.ApprovePersistenceQuery.approved.txt

@@ -47,10 +47,12 @@ namespace Akka.Persistence.Query
     public sealed class NoOffset : Akka.Persistence.Query.Offset
     {
         public static Akka.Persistence.Query.NoOffset Instance { get; }
+        public override int CompareTo(Akka.Persistence.Query.Offset other) { }
     }
-    public abstract class Offset
+    public abstract class Offset : System.IComparable<Akka.Persistence.Query.Offset>
     {
         protected Offset() { }
+        public abstract int CompareTo(Akka.Persistence.Query.Offset other);
         public static Akka.Persistence.Query.Offset NoOffset() { }
         public static Akka.Persistence.Query.Offset Sequence(long value) { }
     }
@@ -78,6 +80,7 @@ namespace Akka.Persistence.Query
         public Sequence(long value) { }
         public long Value { get; }
         public int CompareTo(Akka.Persistence.Query.Sequence other) { }
+        public override int CompareTo(Akka.Persistence.Query.Offset other) { }
         public override bool Equals(object obj) { }
         public override int GetHashCode() { }
     }

src/core/Akka.Persistence.Query.Tests/OffsetCompareSpecs.cs

@@ -0,0 +1,49 @@
+//-----------------------------------------------------------------------
+// <copyright file="OffsetCompareSpecs.cs" company="Akka.NET Project">
+//     Copyright (C) 2009-2019 Lightbend Inc. <http://www.lightbend.com>
+//     Copyright (C) 2013-2019 .NET Foundation <https://github.com/akkadotnet/akka.net>
+// </copyright>
+//-----------------------------------------------------------------------
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Xunit;
+
+namespace Akka.Persistence.Query.Tests
+{
+    public class OffsetCompareSpecs
+    {
+        [Theory]
+        [InlineData(new[]{ 1L, 3L, 4L, 5L, 6L })]
+        [InlineData(new[] { 6L, 2L, 1L, 5L, 3L })]
+        public void Offsets_should_compare_correctly(long[] seqNos)
+        {
+            var offsets = seqNos.Select(x => new Sequence(x)).Cast<Offset>();
+            var orderedSeqNos = seqNos.OrderBy(x => x).ToList();
+            var orderedOffset = new SortedSet<Offset>(offsets);
+
+            var i = 0;
+            foreach (var offset in orderedOffset.Cast<Sequence>())
+            {
+                offset.Value.Should().Be(orderedSeqNos[i]);
+                i++;
+            }
+        }
+
+        [Fact]
+        public void Offsets_of_different_types_should_throw_on_compare()
+        {
+            Offset seq = new Sequence(0L);
+
+            Action compare1 = () => seq.CompareTo(NoOffset.Instance);
+            Action compare2 = () => NoOffset.Instance.CompareTo(seq);
+
+            compare1.ShouldThrow<InvalidOperationException>();
+            compare2.ShouldThrow<InvalidOperationException>();
+        }
+    }
+}

src/core/Akka.Persistence.Query/Offset.cs

@@ -9,7 +9,13 @@
 
 namespace Akka.Persistence.Query
 {
-    public abstract class Offset
+    /// <summary>
+    /// Used in <see cref="IEventsByTagQuery"/> implementations to signal to Akka.Persistence.Query
+    /// where to begin and end event by tag queries.
+    ///
+    /// For concrete implementations, see <see cref="Sequence"/> and <see cref="NoOffset"/>.
+    /// </summary>
+    public abstract class Offset : IComparable<Offset>
     {
         /// <summary>
         /// Used when retrieving all events.
@@ -25,6 +31,12 @@ public abstract class Offset
         /// as the `offset` parameter in a subsequent query.
         /// </summary>
         public static Offset Sequence(long value) => new Sequence(value);
+
+        /// <summary>
+        /// Used to compare to other <see cref="Offset"/> implementations.
+        /// </summary>
+        /// <param name="other">The other offset to compare.</param>
+        public abstract int CompareTo(Offset other);
     }
 
     /// <summary>
@@ -59,6 +71,16 @@ public override bool Equals(object obj)
         }
 
         public override int GetHashCode() => Value.GetHashCode();
+
+        public override int CompareTo(Offset other)
+        {
+            if (other is Sequence seq)
+            {
+                return CompareTo(seq);
+            }
+
+            throw new InvalidOperationException($"Can't compare offset of type {GetType()} to offset of type {other.GetType()}");
+        }
     }
 
     /// <summary>
@@ -71,5 +93,15 @@ public sealed class NoOffset : Offset
         /// </summary>
         public static NoOffset Instance { get; } = new NoOffset();
         private NoOffset() { }
+
+        public override int CompareTo(Offset other)
+        {
+            if (other is NoOffset no)
+            {
+                return 0;
+            }
+
+            throw new InvalidOperationException($"Can't compare offset of type {GetType()} to offset of type {other.GetType()}");
+        }
     }
 }