1 files changed, 173 insertions, 1 deletions
diff --git a/src/Jellyfin.Extensions/StreamExtensions.cs b/src/Jellyfin.Extensions/StreamExtensions.cs
index 0cfac384e3..36361c58e8 100644
--- a/src/Jellyfin.Extensions/StreamExtensions.cs
+++ b/src/Jellyfin.Extensions/StreamExtensions.cs
@@ -1,17 +1,22 @@
+using System;
+using System.Buffers;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Threading;
+using System.Threading.Tasks;
 
 namespace Jellyfin.Extensions
 {
     /// <summary>
-    /// Class BaseExtensions.
+    /// Extension methods for the <see cref="Stream"/> class.
     /// </summary>
     public static class StreamExtensions
     {
+        private const int StreamComparisonBufferSize = 81920;
+
         /// <summary>
         /// Reads all lines in the <see cref="Stream" />.
         /// </summary>
@@ -60,5 +65,172 @@ namespace Jellyfin.Extensions
                 yield return line;
             }
         }
+
+        /// <summary>
+        /// Determines whether a stream is identical to a file on disk.
+        /// </summary>
+        /// <param name="stream">The stream to compare.</param>
+        /// <param name="path">The file path to compare against.</param>
+        /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+        /// <returns>True if the stream and file are identical; otherwise false.</returns>
+        /// <exception cref="ArgumentException"><paramref name="stream"/> does not support seeking.</exception>
+        /// <remarks>
+        /// The entire stream is compared against the file from the beginning (the position is reset to 0 on entry)
+        /// and restored to its original value after the call.
+        /// </remarks>
+        public static async Task<bool> IsFileIdenticalAsync(this Stream stream, string path, CancellationToken cancellationToken = default)
+        {
+            ArgumentNullException.ThrowIfNull(stream);
+            ArgumentException.ThrowIfNullOrEmpty(path);
+
+            if (!stream.CanSeek)
+            {
+                throw new ArgumentException("Stream must support seeking.", nameof(stream));
+            }
+
+            var originalPosition = stream.Position;
+            try
+            {
+                stream.Position = 0;
+
+                var existingFileStream = new FileStream(
+                    path,
+                    FileMode.Open,
+                    FileAccess.Read,
+                    FileShare.Read,
+                    bufferSize: StreamComparisonBufferSize,
+                    FileOptions.Asynchronous | FileOptions.SequentialScan);
+                await using (existingFileStream.ConfigureAwait(false))
+                {
+                    return await stream.IsStreamIdenticalAsync(existingFileStream, cancellationToken).ConfigureAwait(false);
+                }
+            }
+            finally
+            {
+                stream.Position = originalPosition;
+            }
+        }
+
+        /// <summary>
+        /// Determines whether two streams are identical.
+        /// </summary>
+        /// <param name="a">The first stream to compare.</param>
+        /// <param name="b">The second stream to compare.</param>
+        /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
+        /// <returns>True if the streams are identical; otherwise false.</returns>
+        /// <remarks>
+        /// Seekable streams are compared from the beginning (their position is reset to 0 on entry).
+        /// Non-seekable streams are compared from their current read position. Stream positions are not
+        /// restored after the call.
+        /// </remarks>
+        public static async Task<bool> IsStreamIdenticalAsync(this Stream a, Stream b, CancellationToken cancellationToken = default)
+        {
+            ArgumentNullException.ThrowIfNull(a);
+            ArgumentNullException.ThrowIfNull(b);
+
+            if (ReferenceEquals(a, b))
+            {
+                return true;
+            }
+
+            if (a.CanSeek is var aCanSeek && aCanSeek)
+            {
+                a.Position = 0;
+            }
+
+            if (b.CanSeek is var bCanSeek && bCanSeek)
+            {
+                b.Position = 0;
+            }
+
+            if (aCanSeek && bCanSeek && b.Length != a.Length)
+            {
+                return false;
+            }
+
+            // MemoryStreams only unlock a fast path if their underlying buffer is exposed via TryGetBuffer.
+            var segmentA = a is MemoryStream streamA && streamA.TryGetBuffer(out var bufA) ? bufA : default;
+            var segmentB = b is MemoryStream streamB && streamB.TryGetBuffer(out var bufB) ? bufB : default;
+
+            // Fast path A: both streams expose buffers, compare segments directly
+            if (segmentA.Array is not null && segmentB.Array is not null)
+            {
+                return segmentA.AsSpan().SequenceEqual(segmentB.AsSpan());
+            }
+
+            if (segmentB.Array is not null) // && segmentA.Array is null guaranteed by previous check
+            {
+                // swap so that segmentA is the non-null one, compared to b we need only one fast path B
+                (segmentA, b) = (segmentB, a);
+            }
+
+            if (segmentA.Array is not null) // either a was non-null, or b was non-null and was swapped there
+            {
+                // Fast path B: only one stream exposed a buffer, compare against the other chunk-by-chunk
+                var bufferB = ArrayPool<byte>.Shared.Rent(StreamComparisonBufferSize);
+                try
+                {
+                    var memoryB = bufferB.AsMemory();
+                    int offset = 0;
+                    int bytesRead;
+                    while ((bytesRead = await b.ReadAtLeastAsync(memoryB, memoryB.Length, throwOnEndOfStream: false, cancellationToken).ConfigureAwait(false)) > 0)
+                    {
+                        if (offset + bytesRead > segmentA.Count || !segmentA.AsSpan(offset, bytesRead).SequenceEqual(memoryB.Span[..bytesRead]))
+                        {
+                            return false;
+                        }
+
+                        offset += bytesRead;
+                    }
+
+                    return offset == segmentA.Count;
+                }
+                finally
+                {
+                    ArrayPool<byte>.Shared.Return(bufferB);
+                }
+            }
+            else
+            {
+                var bufferA = ArrayPool<byte>.Shared.Rent(StreamComparisonBufferSize);
+                var bufferB = ArrayPool<byte>.Shared.Rent(StreamComparisonBufferSize);
+                try
+                {
+                    var memoryA = bufferA.AsMemory();
+                    var memoryB = bufferB.AsMemory();
+                    while (true)
+                    {
+                        cancellationToken.ThrowIfCancellationRequested();
+
+                        var taskA = a.ReadAtLeastAsync(memoryA, memoryA.Length, throwOnEndOfStream: false, cancellationToken).AsTask();
+                        var taskB = b.ReadAtLeastAsync(memoryB, memoryB.Length, throwOnEndOfStream: false, cancellationToken).AsTask();
+                        await Task.WhenAll(taskA, taskB).ConfigureAwait(false);
+
+                        var bytesReadA = await taskA.ConfigureAwait(false);
+                        var bytesReadB = await taskB.ConfigureAwait(false);
+
+                        if (bytesReadA != bytesReadB)
+                        {
+                            return false;
+                        }
+
+                        if (bytesReadA == 0)
+                        {
+                            return true;
+                        }
+
+                        if (!memoryA.Span[..bytesReadA].SequenceEqual(memoryB.Span[..bytesReadB]))
+                        {
+                            return false;
+                        }
+                    }
+                }
+                finally
+                {
+                    ArrayPool<byte>.Shared.Return(bufferA);
+                    ArrayPool<byte>.Shared.Return(bufferB);
+                }
+            }
+        }
     }
 }