Merge pull request #860 from polyadic/memoize-optimizations

Specialize Memoize for `IList` and `ICollection`

Author: tautropfli

Funcky.Async.Test/Extensions/AsyncEnumerableExtensions/MemoizeTest.cs

@@ -80,12 +80,30 @@ public async Task DisposingAMemoizedBufferDoesNotDisposeOriginalBuffer()
     }
 
     [Fact]
-    public async Task MemoizingAMemoizedBufferTwiceReturnsTheOriginalObject()
+    public async Task DisposingAMemoizedBorrowedBufferDoesNotDisposeOriginalBorrowedBuffer()
     {
-        var source = AsyncEnumerateOnce.Create(Enumerable.Empty<int>());
-        await using var memoized = source.Memoize();
-        await using var memoizedBuffer = memoized.Memoize();
-        await using var memoizedBuffer2 = memoizedBuffer.Memoize();
-        Assert.Same(memoizedBuffer, memoizedBuffer2);
+        var source = AsyncEnumerateOnce.Create<int>([]);
+        await using var firstMemoization = source.Memoize();
+        await using var borrowedBuffer = firstMemoization.Memoize();
+
+        await using (borrowedBuffer.Memoize())
+        {
+        }
+
+        await borrowedBuffer.ForEachAsync(NoOperation<int>);
+    }
+
+    /// <summary>This test disallows "re-borrowing" i.e. creating a fresh BorrowedBuffer over the original buffer.</summary>
+    [Fact]
+    public async Task UsagesOfSecondBorrowThrowAfterFirstBorrowIsDisposed()
+    {
+        var source = AsyncEnumerateOnce.Create<int>([]);
+        await using var firstMemoization = source.Memoize();
+        await using var firstBorrow = firstMemoization.Memoize();
+        await using var secondBorrow = firstBorrow.Memoize();
+#pragma warning disable IDISP017
+        await firstBorrow.DisposeAsync();
+#pragma warning restore IDISP017
+        await Assert.ThrowsAsync<ObjectDisposedException>(async () => await secondBorrow.ForEachAsync(NoOperation<int>));
     }
 }

Funcky.Async/Extensions/AsyncEnumerableExtensions/Memoize.cs

@@ -17,7 +17,7 @@ public static IAsyncBuffer<TSource> Memoize<TSource>(this IAsyncEnumerable<TSour
             : MemoizedAsyncBuffer.Create(source);
 
     private static IAsyncBuffer<TSource> Borrow<TSource>(IAsyncBuffer<TSource> buffer)
-        => buffer as BorrowedAsyncBuffer<TSource> ?? new BorrowedAsyncBuffer<TSource>(buffer);
+        => new BorrowedAsyncBuffer<TSource>(buffer);
 
     private static class MemoizedAsyncBuffer
     {

Funcky.Test/Extensions/EnumerableExtensions/MemoizeTest.cs

@@ -28,7 +28,7 @@ public void TheUnderlyingEnumerableIsOnlyEnumeratedOnce()
     [Fact]
     public void MemoizingAnEmptyListIsEmpty()
     {
-        var empty = Enumerable.Empty<string>();
+        var empty = Enumerable.Empty<string>().PreventLinqOptimizations();
         using var memoized = empty.Memoize();
 
         Assert.Empty(memoized);
@@ -80,12 +80,46 @@ public void DisposingAMemoizedBufferDoesNotDisposeOriginalBuffer()
     }
 
     [Fact]
-    public void MemoizingAMemoizedBufferTwiceReturnsTheOriginalObject()
+    public void DisposingAMemoizedBorrowedBufferDoesNotDisposeOriginalBorrowedBuffer()
     {
         var source = EnumerateOnce.Create<int>([]);
+        using var firstMemoization = source.Memoize();
+        using var borrowedBuffer = firstMemoization.Memoize();
+
+        using (borrowedBuffer.Memoize())
+        {
+        }
+
+        borrowedBuffer.ForEach(NoOperation);
+    }
+
+    /// <summary>This test disallows "re-borrowing" i.e. creating a fresh BorrowedBuffer over the original buffer.</summary>
+    [Fact]
+    public void UsagesOfSecondBorrowThrowAfterFirstBorrowIsDisposed()
+    {
+        var source = EnumerateOnce.Create<int>([]);
+        using var firstMemoization = source.Memoize();
+        using var firstBorrow = firstMemoization.Memoize();
+        using var secondBorrow = firstBorrow.Memoize();
+#pragma warning disable IDISP017
+        firstBorrow.Dispose();
+#pragma warning restore IDISP017
+        Assert.Throws<ObjectDisposedException>(() => secondBorrow.ForEach(NoOperation));
+    }
+
+    [Fact]
+    public void MemoizingAListReturnsAnObjectImplementingIList()
+    {
+        var source = new List<int> { 10, 20, 30 };
+        using var memoized = source.Memoize();
+        Assert.IsType<IList<int>>(memoized, exactMatch: false);
+    }
+
+    [Fact]
+    public void MemoizingACollectionReturnsAnObjectImplementingICollection()
+    {
+        var source = new HashSet<int> { 10, 20, 30 };
         using var memoized = source.Memoize();
-        using var memoizedBuffer = memoized.Memoize();
-        using var memoizedBuffer2 = memoizedBuffer.Memoize();
-        Assert.Same(memoizedBuffer, memoizedBuffer2);
+        Assert.IsType<ICollection<int>>(memoized, exactMatch: false);
     }
 }

Funcky/Buffers/CollectionBuffer.cs

@@ -0,0 +1,84 @@
+using System.Collections;
+using System.Diagnostics.CodeAnalysis;
+
+namespace Funcky.Buffers;
+
+internal static class CollectionBuffer
+{
+    public static CollectionBuffer<T> Create<T>(ICollection<T> list) => new(list);
+}
+
+[SuppressMessage("IDisposableAnalyzers.Correctness", "IDISP025:Class with no virtual dispose method should be sealed", Justification = "Dispose doesn't do anything except flag object as disposed")]
+internal class CollectionBuffer<T>(ICollection<T> source) : IBuffer<T>, ICollection<T>
+{
+    private bool _disposed;
+
+    public int Count
+    {
+        get
+        {
+            ThrowIfDisposed();
+            return source.Count;
+        }
+    }
+
+    public bool IsReadOnly
+    {
+        get
+        {
+            ThrowIfDisposed();
+            return source.IsReadOnly;
+        }
+    }
+
+    public void Dispose()
+    {
+        _disposed = true;
+    }
+
+    public IEnumerator<T> GetEnumerator()
+    {
+        ThrowIfDisposed();
+        return source.GetEnumerator();
+    }
+
+    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+    public void Add(T item)
+    {
+        ThrowIfDisposed();
+        source.Add(item);
+    }
+
+    public void Clear()
+    {
+        ThrowIfDisposed();
+        source.Clear();
+    }
+
+    public bool Contains(T item)
+    {
+        ThrowIfDisposed();
+        return source.Contains(item);
+    }
+
+    public void CopyTo(T[] array, int arrayIndex)
+    {
+        ThrowIfDisposed();
+        source.CopyTo(array, arrayIndex);
+    }
+
+    public bool Remove(T item)
+    {
+        ThrowIfDisposed();
+        return source.Remove(item);
+    }
+
+    protected void ThrowIfDisposed()
+    {
+        if (_disposed)
+        {
+            throw new ObjectDisposedException(nameof(ListBuffer<T>));
+        }
+    }
+}

Funcky/Buffers/ListBuffer.cs

@@ -0,0 +1,42 @@
+namespace Funcky.Buffers;
+
+internal static class ListBuffer
+{
+    public static ListBuffer<T> Create<T>(IList<T> list) => new(list);
+}
+
+internal sealed class ListBuffer<T>(IList<T> source) : CollectionBuffer<T>(source), IList<T>
+{
+    public T this[int index]
+    {
+        get
+        {
+            ThrowIfDisposed();
+            return source[index];
+        }
+
+        set
+        {
+            ThrowIfDisposed();
+            source[index] = value;
+        }
+    }
+
+    public int IndexOf(T item)
+    {
+        ThrowIfDisposed();
+        return source.IndexOf(item);
+    }
+
+    public void Insert(int index, T item)
+    {
+        ThrowIfDisposed();
+        source.Insert(index, item);
+    }
+
+    public void RemoveAt(int index)
+    {
+        ThrowIfDisposed();
+        source.RemoveAt(index);
+    }
+}

Funcky/Extensions/EnumerableExtensions/Memoize.cs

@@ -1,6 +1,5 @@
-#pragma warning disable SA1010 // StyleCop support for collection expressions is missing
 using System.Collections;
-using System.Diagnostics.CodeAnalysis;
+using Funcky.Buffers;
 
 namespace Funcky.Extensions;
 
@@ -15,13 +14,16 @@ public static partial class EnumerableExtensions
     /// <returns>A lazy buffer of the underlying sequence.</returns>
     [Pure]
     public static IBuffer<TSource> Memoize<TSource>(this IEnumerable<TSource> source)
-        => source is IBuffer<TSource> buffer
-            ? Borrow(buffer)
-            : MemoizedBuffer.Create(source);
+        => source switch
+        {
+            IBuffer<TSource> buffer => Borrow(buffer),
+            IList<TSource> list => ListBuffer.Create(list),
+            ICollection<TSource> list => CollectionBuffer.Create(list),
+            _ => MemoizedBuffer.Create(source),
+        };
 
-    [SuppressMessage("IDisposableAnalyzers", "IDISP015: Member should not return created and cached instance.", Justification = "False positive.")]
     private static IBuffer<TSource> Borrow<TSource>(IBuffer<TSource> buffer)
-        => buffer as BorrowedBuffer<TSource> ?? new BorrowedBuffer<TSource>(buffer);
+        => new BorrowedBuffer<TSource>(buffer);
 
     private static class MemoizedBuffer
     {