Add identity method to reduce JIT-ing

This is a squashed merge of PR ##880.

Co-authored-by: Atif Aziz <[email protected]>

MoreLinq/Batch.cs

@@ -50,7 +50,7 @@ static partial class MoreEnumerable
 
         public static IEnumerable<IEnumerable<TSource>> Batch<TSource>(this IEnumerable<TSource> source, int size)
         {
-            return Batch(source, size, x => x);
+            return Batch(source, size, IdFn);
         }
 
         /// <summary>

MoreLinq/GroupAdjacent.cs

@@ -87,7 +87,7 @@ public static IEnumerable<IGrouping<TKey, TSource>> GroupAdjacent<TSource, TKey>
             if (source == null) throw new ArgumentNullException(nameof(source));
             if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));
 
-            return GroupAdjacent(source, keySelector, e => e, comparer);
+            return GroupAdjacent(source, keySelector, IdFn, comparer);
         }
 
         /// <summary>
@@ -208,7 +208,7 @@ public static IEnumerable<TResult> GroupAdjacent<TSource, TKey, TResult>(
             // This should be removed once the target framework is bumped to something that supports covariance
             TResult ResultSelectorWrapper(TKey key, IList<TSource> group) => resultSelector(key, group);
 
-            return GroupAdjacentImpl(source, keySelector, i => i, ResultSelectorWrapper,
+            return GroupAdjacentImpl(source, keySelector, IdFn, ResultSelectorWrapper,
                                      EqualityComparer<TKey>.Default);
         }
 
@@ -253,7 +253,7 @@ public static IEnumerable<TResult> GroupAdjacent<TSource, TKey, TResult>(
 
             // This should be removed once the target framework is bumped to something that supports covariance
             TResult ResultSelectorWrapper(TKey key, IList<TSource> group) => resultSelector(key, group);
-            return GroupAdjacentImpl(source, keySelector, i => i, ResultSelectorWrapper,
+            return GroupAdjacentImpl(source, keySelector, IdFn, ResultSelectorWrapper,
                                      comparer ?? EqualityComparer<TKey>.Default);
         }
 

MoreLinq/IdFn.cs

@@ -0,0 +1,24 @@
+#region License and Terms
+// MoreLINQ - Extensions to LINQ to Objects
+// Copyright (c) 2022 Turning Code, LLC. All rights reserved.
+//
+// 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.
+#endregion
+
+namespace MoreLinq
+{
+    static partial class MoreEnumerable
+    {
+        static T IdFn<T>(T x) => x;
+    }
+}

MoreLinq/OrderedMerge.cs

@@ -70,7 +70,7 @@ public static IEnumerable<T> OrderedMerge<T>(
             IEnumerable<T> second,
             IComparer<T>? comparer)
         {
-            return OrderedMerge(first, second, e => e, f => f, s => s, (a, _) => a, comparer);
+            return OrderedMerge(first, second, IdFn, IdFn, IdFn, (a, _) => a, comparer);
         }
 
         /// <summary>
@@ -98,7 +98,7 @@ public static IEnumerable<T> OrderedMerge<T, TKey>(
             IEnumerable<T> second,
             Func<T, TKey> keySelector)
         {
-            return OrderedMerge(first, second, keySelector, a => a, b => b, (a, _) => a, null);
+            return OrderedMerge(first, second, keySelector, IdFn, IdFn, (a, _) => a, null);
         }
 
         /// <summary>

MoreLinq/Rank.cs

@@ -32,7 +32,7 @@ public static partial class MoreEnumerable
 
         public static IEnumerable<int> Rank<TSource>(this IEnumerable<TSource> source)
         {
-            return source.RankBy(x => x);
+            return source.RankBy(IdFn);
         }
 
         /// <summary>
@@ -45,7 +45,7 @@ public static IEnumerable<int> Rank<TSource>(this IEnumerable<TSource> source)
 
         public static IEnumerable<int> Rank<TSource>(this IEnumerable<TSource> source, IComparer<TSource> comparer)
         {
-            return source.RankBy(x => x, comparer);
+            return source.RankBy(IdFn, comparer);
         }
 
         /// <summary>

MoreLinq/Split.cs

@@ -50,7 +50,7 @@ public static IEnumerable<IEnumerable<TSource>> Split<TSource>(this IEnumerable<
         public static IEnumerable<IEnumerable<TSource>> Split<TSource>(this IEnumerable<TSource> source,
             TSource separator, int count)
         {
-            return Split(source, separator, count, s => s);
+            return Split(source, separator, count, IdFn);
         }
 
         /// <summary>
@@ -129,7 +129,7 @@ public static IEnumerable<IEnumerable<TSource>> Split<TSource>(this IEnumerable<
         public static IEnumerable<IEnumerable<TSource>> Split<TSource>(this IEnumerable<TSource> source,
             TSource separator, IEqualityComparer<TSource>? comparer, int count)
         {
-            return Split(source, separator, comparer, count, s => s);
+            return Split(source, separator, comparer, count, IdFn);
         }
 
         /// <summary>
@@ -216,7 +216,7 @@ public static IEnumerable<IEnumerable<TSource>> Split<TSource>(this IEnumerable<
         public static IEnumerable<IEnumerable<TSource>> Split<TSource>(this IEnumerable<TSource> source,
             Func<TSource, bool> separatorFunc, int count)
         {
-            return Split(source, separatorFunc, count, s => s);
+            return Split(source, separatorFunc, count, IdFn);
         }
 
         /// <summary>