Consolidate LINQ's internal IIListProvider/IPartition into base Iterator class

src/libraries/System.Linq/src/System.Linq.csproj

@@ -9,6 +9,7 @@
   <PropertyGroup>
     <TargetPlatformIdentifier>$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)'))</TargetPlatformIdentifier>
     <OptimizeForSize Condition="'$(TargetPlatformIdentifier)' == 'browser' or '$(TargetPlatformIdentifier)' == 'android' or '$(TargetPlatformIdentifier)' == 'ios' or '$(TargetPlatformIdentifier)' == 'tvos'">true</OptimizeForSize>
+    <DefineConstants Condition="'$(OptimizeForSize)' == 'true'">$(DefineConstants);OPTIMIZE_FOR_SIZE</DefineConstants>
   </PropertyGroup>
 
   <ItemGroup Condition="'$(OptimizeForSize)' == true">
@@ -23,15 +24,16 @@
     <Compile Include="System\Linq\DefaultIfEmpty.SpeedOpt.cs" />
     <Compile Include="System\Linq\Distinct.SpeedOpt.cs" />
     <Compile Include="System\Linq\Grouping.SpeedOpt.cs" />
+    <Compile Include="System\Linq\Iterator.SpeedOpt.cs" />
     <Compile Include="System\Linq\Lookup.SpeedOpt.cs" />
     <Compile Include="System\Linq\OrderedEnumerable.SpeedOpt.cs" />
-    <Compile Include="System\Linq\Partition.SpeedOpt.cs" />
     <Compile Include="System\Linq\Range.SpeedOpt.cs" />
     <Compile Include="System\Linq\Repeat.SpeedOpt.cs" />
     <Compile Include="System\Linq\Reverse.SpeedOpt.cs" />
     <Compile Include="System\Linq\Select.SpeedOpt.cs" />
     <Compile Include="System\Linq\SelectMany.SpeedOpt.cs" />
     <Compile Include="System\Linq\Skip.SpeedOpt.cs" />
+    <Compile Include="System\Linq\SkipTake.SpeedOpt.cs" />
     <Compile Include="System\Linq\Take.SpeedOpt.cs" />
     <Compile Include="System\Linq\Union.SpeedOpt.cs" />
     <Compile Include="System\Linq\Where.SpeedOpt.cs" />
@@ -61,8 +63,6 @@
     <Compile Include="System\Linq\Index.cs" />
     <Compile Include="System\Linq\Intersect.cs" />
     <Compile Include="System\Linq\Iterator.cs" />
-    <Compile Include="System\Linq\IIListProvider.cs" />
-    <Compile Include="System\Linq\IPartition.cs" />
     <Compile Include="System\Linq\Join.cs" />
     <Compile Include="System\Linq\Last.cs" />
     <Compile Include="System\Linq\Lookup.cs" />

src/libraries/System.Linq/src/System/Linq/AppendPrepend.SpeedOpt.cs

@@ -8,15 +8,6 @@ namespace System.Linq
 {
     public static partial class Enumerable
     {
-        private abstract partial class AppendPrependIterator<TSource> : IIListProvider<TSource>
-        {
-            public abstract TSource[] ToArray();
-
-            public abstract List<TSource> ToList();
-
-            public abstract int GetCount(bool onlyIfCheap);
-        }
-
         private sealed partial class AppendPrepend1Iterator<TSource>
         {
             private TSource[] LazyToArray()
@@ -130,14 +121,63 @@ public override List<TSource> ToList()
 
             public override int GetCount(bool onlyIfCheap)
             {
-                if (_source is IIListProvider<TSource> listProv)
+                if (_source is Iterator<TSource> iterator)
                 {
-                    int count = listProv.GetCount(onlyIfCheap);
+                    int count = iterator.GetCount(onlyIfCheap);
                     return count == -1 ? -1 : count + 1;
                 }
 
                 return !onlyIfCheap || _source is ICollection<TSource> ? _source.Count() + 1 : -1;
             }
+
+            public override TSource? TryGetFirst(out bool found)
+            {
+                if (_appending)
+                {
+                    TSource? first = _source.TryGetFirst(out found);
+                    if (found)
+                    {
+                        return first;
+                    }
+                }
+
+                found = true;
+                return _item;
+            }
+
+            public override TSource? TryGetLast(out bool found)
+            {
+                if (!_appending)
+                {
+                    TSource? last = _source.TryGetLast(out found);
+                    if (found)
+                    {
+                        return last;
+                    }
+                }
+
+                found = true;
+                return _item;
+            }
+
+            public override TSource? TryGetElementAt(int index, out bool found)
+            {
+                if (!_appending)
+                {
+                    if (index == 0)
+                    {
+                        found = true;
+                        return _item;
+                    }
+
+                    index--;
+                    return
+                        _source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
+                        TryGetElementAtNonIterator(_source, index, out found);
+                }
+
+                return base.TryGetElementAt(index, out found);
+            }
         }
 
         private sealed partial class AppendPrependN<TSource>
@@ -232,9 +272,9 @@ public override List<TSource> ToList()
 
             public override int GetCount(bool onlyIfCheap)
             {
-                if (_source is IIListProvider<TSource> listProv)
+                if (_source is Iterator<TSource> iterator)
                 {
-                    int count = listProv.GetCount(onlyIfCheap);
+                    int count = iterator.GetCount(onlyIfCheap);
                     return count == -1 ? -1 : count + _appendCount + _prependCount;
                 }
 

src/libraries/System.Linq/src/System/Linq/Cast.SpeedOpt.cs

@@ -8,11 +8,11 @@ namespace System.Linq
 {
     public static partial class Enumerable
     {
-        private sealed partial class CastICollectionIterator<TResult> : IPartition<TResult>
+        private sealed partial class CastICollectionIterator<TResult>
         {
-            public int GetCount(bool onlyIfCheap) => _source.Count;
+            public override int GetCount(bool onlyIfCheap) => _source.Count;
 
-            public TResult[] ToArray()
+            public override TResult[] ToArray()
             {
                 TResult[] array = new TResult[_source.Count];
 
@@ -25,7 +25,7 @@ public TResult[] ToArray()
                 return array;
             }
 
-            public List<TResult> ToList()
+            public override List<TResult> ToList()
             {
                 List<TResult> list = new(_source.Count);
 
@@ -37,7 +37,7 @@ public List<TResult> ToList()
                 return list;
             }
 
-            public TResult? TryGetElementAt(int index, out bool found)
+            public override TResult? TryGetElementAt(int index, out bool found)
             {
                 if (index >= 0)
                 {
@@ -65,7 +65,7 @@ public List<TResult> ToList()
                 return default;
             }
 
-            public TResult? TryGetFirst(out bool found)
+            public override TResult? TryGetFirst(out bool found)
             {
                 IEnumerator e = _source.GetEnumerator();
                 try
@@ -85,7 +85,7 @@ public List<TResult> ToList()
                 return default;
             }
 
-            public TResult? TryGetLast(out bool found)
+            public override TResult? TryGetLast(out bool found)
             {
                 IEnumerator e = _source.GetEnumerator();
                 try
@@ -110,13 +110,6 @@ public List<TResult> ToList()
                     (e as IDisposable)?.Dispose();
                 }
             }
-
-            public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> selector) =>
-                new SelectIPartitionIterator<TResult, TResult2>(this, selector);
-
-            public IPartition<TResult>? Skip(int count) => new EnumerablePartition<TResult>(this, count, -1);
-
-            public IPartition<TResult>? Take(int count) => new EnumerablePartition<TResult>(this, 0, count - 1);
         }
     }
 }

src/libraries/System.Linq/src/System/Linq/Concat.SpeedOpt.cs

@@ -342,13 +342,9 @@ private TSource[] PreallocatingToArray()
             }
         }
 
-        private abstract partial class ConcatIterator<TSource> : IPartition<TSource>
+        private abstract partial class ConcatIterator<TSource>
         {
-            public abstract int GetCount(bool onlyIfCheap);
-
-            public abstract TSource[] ToArray();
-
-            public List<TSource> ToList()
+            public override List<TSource> ToList()
             {
                 int count = GetCount(onlyIfCheap: true);
                 var list = count != -1 ? new List<TSource>(count) : new List<TSource>();
@@ -367,16 +363,6 @@ public List<TSource> ToList()
                 return list;
             }
 
-            public abstract TSource? TryGetElementAt(int index, out bool found);
-
-            public abstract TSource? TryGetFirst(out bool found);
-
-            public abstract TSource? TryGetLast(out bool found);
-
-            public IPartition<TSource>? Skip(int count) => new EnumerablePartition<TSource>(this, count, -1);
-
-            public IPartition<TSource>? Take(int count) => new EnumerablePartition<TSource>(this, 0, count - 1);
-
         }
     }
 }

src/libraries/System.Linq/src/System/Linq/Count.cs

@@ -20,10 +20,12 @@ public static int Count<TSource>(this IEnumerable<TSource> source)
                 return collectionoft.Count;
             }
 
-            if (source is IIListProvider<TSource> listProv)
+#if !OPTIMIZE_FOR_SIZE
+            if (source is Iterator<TSource> iterator)
             {
-                return listProv.GetCount(onlyIfCheap: false);
+                return iterator.GetCount(onlyIfCheap: false);
             }
+#endif
 
             if (source is ICollection collection)
             {
@@ -105,15 +107,17 @@ public static bool TryGetNonEnumeratedCount<TSource>(this IEnumerable<TSource> s
                 return true;
             }
 
-            if (source is IIListProvider<TSource> listProv)
+#if !OPTIMIZE_FOR_SIZE
+            if (source is Iterator<TSource> iterator)
             {
-                int c = listProv.GetCount(onlyIfCheap: true);
+                int c = iterator.GetCount(onlyIfCheap: true);
                 if (c >= 0)
                 {
                     count = c;
                     return true;
                 }
             }
+#endif
 
             if (source is ICollection collection)
             {

src/libraries/System.Linq/src/System/Linq/DefaultIfEmpty.SpeedOpt.cs

@@ -8,15 +8,15 @@ namespace System.Linq
 {
     public static partial class Enumerable
     {
-        private sealed partial class DefaultIfEmptyIterator<TSource> : IIListProvider<TSource>
+        private sealed partial class DefaultIfEmptyIterator<TSource>
         {
-            public TSource[] ToArray()
+            public override TSource[] ToArray()
             {
                 TSource[] array = _source.ToArray();
                 return array.Length == 0 ? [_default] : array;
             }
 
-            public List<TSource> ToList()
+            public override List<TSource> ToList()
             {
                 List<TSource> list = _source.ToList();
                 if (list.Count == 0)
@@ -27,7 +27,7 @@ public List<TSource> ToList()
                 return list;
             }
 
-            public int GetCount(bool onlyIfCheap)
+            public override int GetCount(bool onlyIfCheap)
             {
                 int count;
                 if (!onlyIfCheap || _source is ICollection<TSource> || _source is ICollection)
@@ -36,11 +36,51 @@ public int GetCount(bool onlyIfCheap)
                 }
                 else
                 {
-                    count = _source is IIListProvider<TSource> listProv ? listProv.GetCount(onlyIfCheap: true) : -1;
+                    count = _source is Iterator<TSource> iterator ? iterator.GetCount(onlyIfCheap: true) : -1;
                 }
 
                 return count == 0 ? 1 : count;
             }
+
+            public override TSource? TryGetFirst(out bool found)
+            {
+                TSource? first = _source.TryGetFirst(out found);
+                if (found)
+                {
+                    return first;
+                }
+
+                found = true;
+                return _default;
+            }
+
+            public override TSource? TryGetLast(out bool found)
+            {
+                TSource? last = _source.TryGetLast(out found);
+                if (found)
+                {
+                    return last;
+                }
+
+                found = true;
+                return _default;
+            }
+
+            public override TSource? TryGetElementAt(int index, out bool found)
+            {
+                TSource? item = _source.TryGetElementAt(index, out found);
+                if (found)
+                {
+                    return item;
+                }
+
+                if (index == 0)
+                {
+                    found = true;
+                }
+
+                return _default;
+            }
         }
     }
 }

src/libraries/System.Linq/src/System/Linq/Distinct.SpeedOpt.cs

@@ -7,13 +7,15 @@ namespace System.Linq
 {
     public static partial class Enumerable
     {
-        private sealed partial class DistinctIterator<TSource> : IIListProvider<TSource>
+        private sealed partial class DistinctIterator<TSource>
         {
-            public TSource[] ToArray() => HashSetToArray(new HashSet<TSource>(_source, _comparer));
+            public override TSource[] ToArray() => HashSetToArray(new HashSet<TSource>(_source, _comparer));
 
-            public List<TSource> ToList() => new List<TSource>(new HashSet<TSource>(_source, _comparer));
+            public override List<TSource> ToList() => new List<TSource>(new HashSet<TSource>(_source, _comparer));
 
-            public int GetCount(bool onlyIfCheap) => onlyIfCheap ? -1 : new HashSet<TSource>(_source, _comparer).Count;
+            public override int GetCount(bool onlyIfCheap) => onlyIfCheap ? -1 : new HashSet<TSource>(_source, _comparer).Count;
+
+            public override TSource? TryGetFirst(out bool found) => _source.TryGetFirst(out found);
         }
     }
 }