Fix Batch regression for empty collection

This is a squashed merge of PR #743 that fixes #741.
morelinq · Dec 27, 2019 · b802173 · b802173
1 parent dc9542e
commit b802173
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diff --git a/MoreLinq.Test/BatchTest.cs b/MoreLinq.Test/BatchTest.cs
@@ -117,5 +117,15 @@ public void BatchReadOnlyCollectionSmallerThanSize(int oversize)
             reader.Read().AssertSequenceEqual(1, 2, 3, 4, 5);
             reader.ReadEnd();
         }
+
+        [TestCase(SourceKind.Sequence)]
+        [TestCase(SourceKind.BreakingList)]
+        [TestCase(SourceKind.BreakingReadOnlyList)]
+        [TestCase(SourceKind.BreakingCollection)]
+        public void BatchEmptySource(SourceKind kind)
+        {
+            var batches = Enumerable.Empty<int>().ToSourceKind(kind).Batch(100);
+            Assert.That(batches, Is.Empty);
+        }
     }
 }
diff --git a/MoreLinq/Batch.cs b/MoreLinq/Batch.cs
@@ -19,6 +19,7 @@ namespace MoreLinq
 {
     using System;
     using System.Collections.Generic;
+    using System.Linq;
 
     static partial class MoreEnumerable
     {
@@ -86,6 +87,10 @@ public static IEnumerable<TResult> Batch<TSource, TResult>(this IEnumerable<TSou
 
             switch (source)
             {
+                case ICollection<TSource> collection when collection.Count == 0:
+                {
+                    return Enumerable.Empty<TResult>();
+                }
                 case ICollection<TSource> collection when collection.Count <= size:
                 {
                     return _(); IEnumerable<TResult> _()
@@ -95,6 +100,10 @@ public static IEnumerable<TResult> Batch<TSource, TResult>(this IEnumerable<TSou
                         yield return resultSelector(bucket);
                     }
                 }
+                case IReadOnlyList<TSource> list when list.Count == 0:
+                {
+                    return Enumerable.Empty<TResult>();
+                }
                 case IReadOnlyList<TSource> list when list.Count <= size:
                 {
                     return _(); IEnumerable<TResult> _()