Fixed concatenation of unbounded number of streams

Author: sakno

src/DotNext.IO/IO/SparseStream.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
@@ -207,9 +208,20 @@ protected override ReadOnlySpan<Stream> Streams
         => MemoryMarshal.CreateReadOnlySpan(in Unsafe.As<T, Stream>(ref Unsafe.AsRef(in streams)), streams.Length);
 }
 
-internal sealed class UnboundedSparseStream(ReadOnlySpan<Stream> streams, bool leaveOpen) : SparseStream(leaveOpen)
+internal sealed class UnboundedSparseStream : SparseStream
 {
-    private MemoryOwner<Stream> streams = streams.Copy();
+    private MemoryOwner<Stream> streams;
+
+    internal UnboundedSparseStream(Stream stream, ReadOnlySpan<Stream> streams, bool leaveOpen)
+        : base(leaveOpen)
+    {
+        Debug.Assert(streams.Length < int.MaxValue);
+
+        this.streams = Memory.AllocateExactly<Stream>(streams.Length + 1);
+        var output = this.streams.Span;
+        output[0] = stream;
+        streams.CopyTo(output.Slice(1));
+    }
 
     protected override ReadOnlySpan<Stream> Streams => streams.Span;
 

src/DotNext.IO/IO/StreamExtensions.cs

@@ -31,7 +31,10 @@ private static Stream Combine(Stream stream, ReadOnlySpan<Stream> others, bool l
             [var s1, var s2, var s3, var s4] => new SparseStream<(Stream, Stream, Stream, Stream, Stream)>((stream, s1, s2, s3, s4), leaveOpen),
             [var s1, var s2, var s3, var s4, var s5] => new SparseStream<(Stream, Stream, Stream, Stream, Stream, Stream)>((stream, s1, s2, s3, s4,
                 s5), leaveOpen),
-            _ => new UnboundedSparseStream(others, leaveOpen),
+            [var s1, var s2, var s3, var s4, var s5, var s6] => new SparseStream<(Stream, Stream, Stream, Stream, Stream, Stream, Stream)>((stream, s1, s2, s3, s4,
+                s5, s6), leaveOpen),
+            { Length: int.MaxValue } => throw new InsufficientMemoryException(),
+            _ => new UnboundedSparseStream(stream, others, leaveOpen),
         };
 
     /// <summary>

src/DotNext.Tests/IO/StreamExtensionsTests.cs

@@ -200,6 +200,36 @@ public static void ReadBytesFromCombinedStream()
         Equal(-1, combined.ReadByte());
     }
 
+    [Theory]
+    [InlineData(2)]
+    [InlineData(3)]
+    [InlineData(4)]
+    [InlineData(5)]
+    [InlineData(6)]
+    [InlineData(7)]
+    [InlineData(8)]
+    [InlineData(9)]
+    [InlineData(10)]
+    public static void CombineManyStreams(byte streamCount)
+    {
+        using var stream = GetStreams(streamCount).Combine(leaveOpen: false);
+        var actual = new byte[streamCount];
+        stream.ReadExactly(actual);
+        var expected = Set.Range<byte, EnclosedEndpoint<byte>, DisclosedEndpoint<byte>>(0, streamCount);
+        Equal(expected, actual);
+        
+        static IEnumerable<Stream> GetStreams(byte streamCount)
+        {
+            for (var i = 0; i < streamCount; i++)
+            {
+                var ms = new MemoryStream();
+                ms.WriteByte((byte)i);
+                ms.Seek(0L, SeekOrigin.Begin);
+                yield return ms;
+            }
+        }
+    }
+
     [Fact]
     public static async Task UnsupportedMethodsOfSparseStream()
     {