Add Reset() to the Zlib-based Encoders/Decoders

src/libraries/Common/src/Interop/Interop.zlib.cs

@@ -23,6 +23,9 @@ internal static unsafe partial ZLibNative.ErrorCode DeflateInit2_(
         [LibraryImport(Libraries.CompressionNative, EntryPoint = "CompressionNative_DeflateEnd")]
         internal static unsafe partial ZLibNative.ErrorCode DeflateEnd(ZLibNative.ZStream* stream);
 
+        [LibraryImport(Libraries.CompressionNative, EntryPoint = "CompressionNative_DeflateReset")]
+        internal static unsafe partial ZLibNative.ErrorCode DeflateReset(ZLibNative.ZStream* stream);
+
         [LibraryImport(Libraries.CompressionNative, EntryPoint = "CompressionNative_InflateInit2_")]
         internal static unsafe partial ZLibNative.ErrorCode InflateInit2_(ZLibNative.ZStream* stream, int windowBits);
 

src/libraries/Common/src/System/IO/Compression/ZLibNative.cs

@@ -333,6 +333,28 @@ public unsafe ErrorCode Deflate(FlushCode flush)
                 }
             }
 
+            public unsafe ErrorCode DeflateReset()
+            {
+                bool refAdded = false;
+                try
+                {
+                    DangerousAddRef(ref refAdded);
+                    EnsureState(State.InitializedForDeflate);
+
+                    fixed (ZStream* stream = &_zStream)
+                    {
+                        return Interop.ZLib.DeflateReset(stream);
+                    }
+                }
+                finally
+                {
+                    if (refAdded)
+                    {
+                        DangerousRelease();
+                    }
+                }
+            }
+
             private unsafe ErrorCode DeflateEnd()
             {
                 EnsureState(State.InitializedForDeflate);

src/libraries/Common/tests/System/IO/Compression/EncoderDecoderTestBase.cs

@@ -557,6 +557,94 @@ public void Reset_AllowsReuseForMultipleDecompressions(bool useDictionary)
             }
         }
 
+        [Fact]
+        public void Reset_OnFreshInstance_IsNoOp()
+        {
+            if (!SupportsReset)
+                return;
+
+            byte[] input = CreateTestData();
+
+            using EncoderAdapter encoder = CreateEncoder(ValidQuality, ValidWindowLog);
+            encoder.Reset();
+            encoder.Reset();
+
+            byte[] compressed = new byte[GetMaxCompressedLength(input.Length)];
+            OperationStatus compressStatus = encoder.Compress(input, compressed, out int consumed, out int compressedLength, isFinalBlock: true);
+            Assert.Equal(OperationStatus.Done, compressStatus);
+            Assert.Equal(input.Length, consumed);
+
+            using DecoderAdapter decoder = CreateDecoder();
+            decoder.Reset();
+            decoder.Reset();
+
+            byte[] output = new byte[input.Length];
+            OperationStatus decompressStatus = decoder.Decompress(compressed.AsSpan(0, compressedLength), output, out _, out int written);
+            Assert.Equal(OperationStatus.Done, decompressStatus);
+            Assert.Equal(input.Length, written);
+            Assert.Equal(input, output);
+        }
+
+        [Fact]
+        public void Reset_AfterUnfinishedCompression_ProducesIndependentStream()
+        {
+            if (!SupportsReset)
+                return;
+
+            byte[] input = CreateTestData();
+
+            using EncoderAdapter encoder = CreateEncoder(ValidQuality, ValidWindowLog);
+
+            // Start compressing without finalizing, leaving the encoder mid-operation.
+            byte[] scratch = new byte[GetMaxCompressedLength(input.Length)];
+            encoder.Compress(input, scratch, out _, out _, isFinalBlock: false);
+
+            // Reset must discard the in-progress state so the next operation is independent.
+            encoder.Reset();
+
+            byte[] compressed = new byte[GetMaxCompressedLength(input.Length)];
+            OperationStatus status = encoder.Compress(input, compressed, out int consumed, out int compressedLength, isFinalBlock: true);
+            Assert.Equal(OperationStatus.Done, status);
+            Assert.Equal(input.Length, consumed);
+
+            // The produced stream must be a valid, complete stream that round-trips.
+            byte[] output = new byte[input.Length];
+            using DecoderAdapter decoder = CreateDecoder();
+            OperationStatus decompressStatus = decoder.Decompress(compressed.AsSpan(0, compressedLength), output, out _, out int written);
+            Assert.Equal(OperationStatus.Done, decompressStatus);
+            Assert.Equal(input.Length, written);
+            Assert.Equal(input, output);
+        }
+
+        [Fact]
+        public void Reset_AfterPartialDecompression_ProducesIndependentResult()
+        {
+            if (!SupportsReset)
+                return;
+
+            byte[] input = CreateTestData();
+            byte[] compressed = new byte[GetMaxCompressedLength(input.Length)];
+            Assert.True(TryCompress(input, compressed, out int compressedLength));
+            compressed = compressed.AsSpan(0, compressedLength).ToArray();
+
+            using DecoderAdapter decoder = CreateDecoder();
+
+            // Decompress into a buffer too small to hold the full result, leaving the decoder mid-stream.
+            byte[] tooSmall = new byte[input.Length / 2];
+            OperationStatus partialStatus = decoder.Decompress(compressed, tooSmall, out _, out _);
+            Assert.Equal(OperationStatus.DestinationTooSmall, partialStatus);
+
+            // Reset must discard the partial state so the same stream can be decoded from scratch.
+            decoder.Reset();
+
+            byte[] output = new byte[input.Length];
+            OperationStatus status = decoder.Decompress(compressed, output, out int consumed, out int written);
+            Assert.Equal(OperationStatus.Done, status);
+            Assert.Equal(compressed.Length, consumed);
+            Assert.Equal(input.Length, written);
+            Assert.Equal(input, output);
+        }
+
         [Theory]
         [MemberData(nameof(GetRoundTripTestData))]
         public void RoundTrip_SuccessfullyCompressesAndDecompresses(int quality, bool useDictionary, bool staticEncode, bool staticDecode)

src/libraries/System.IO.Compression/ref/System.IO.Compression.cs

@@ -23,6 +23,7 @@ public sealed partial class DeflateDecoder : System.IDisposable
         public DeflateDecoder() { }
         public System.Buffers.OperationStatus Decompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesConsumed, out int bytesWritten) { throw null; }
         public void Dispose() { }
+        public void Reset() { }
         public static bool TryDecompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten) { throw null; }
     }
     public sealed partial class DeflateEncoder : System.IDisposable
@@ -35,6 +36,7 @@ public DeflateEncoder(System.IO.Compression.ZLibCompressionOptions options) { }
         public void Dispose() { }
         public System.Buffers.OperationStatus Flush(System.Span<byte> destination, out int bytesWritten) { throw null; }
         public static long GetMaxCompressedLength(long inputLength) { throw null; }
+        public void Reset() { }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten) { throw null; }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten, int quality) { throw null; }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten, int quality, int windowLog2) { throw null; }
@@ -80,6 +82,7 @@ public sealed partial class GZipDecoder : System.IDisposable
         public GZipDecoder() { }
         public System.Buffers.OperationStatus Decompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesConsumed, out int bytesWritten) { throw null; }
         public void Dispose() { }
+        public void Reset() { }
         public static bool TryDecompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten) { throw null; }
     }
     public sealed partial class GZipEncoder : System.IDisposable
@@ -92,6 +95,7 @@ public GZipEncoder(System.IO.Compression.ZLibCompressionOptions options) { }
         public void Dispose() { }
         public System.Buffers.OperationStatus Flush(System.Span<byte> destination, out int bytesWritten) { throw null; }
         public static long GetMaxCompressedLength(long inputLength) { throw null; }
+        public void Reset() { }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten) { throw null; }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten, int quality) { throw null; }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten, int quality, int windowLog2) { throw null; }
@@ -227,6 +231,7 @@ public sealed partial class ZLibDecoder : System.IDisposable
         public ZLibDecoder() { }
         public System.Buffers.OperationStatus Decompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesConsumed, out int bytesWritten) { throw null; }
         public void Dispose() { }
+        public void Reset() { }
         public static bool TryDecompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten) { throw null; }
     }
     public sealed partial class ZLibEncoder : System.IDisposable
@@ -239,6 +244,7 @@ public ZLibEncoder(System.IO.Compression.ZLibCompressionOptions options) { }
         public void Dispose() { }
         public System.Buffers.OperationStatus Flush(System.Span<byte> destination, out int bytesWritten) { throw null; }
         public static long GetMaxCompressedLength(long inputLength) { throw null; }
+        public void Reset() { }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten) { throw null; }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten, int quality) { throw null; }
         public static bool TryCompress(System.ReadOnlySpan<byte> source, System.Span<byte> destination, out int bytesWritten, int quality, int windowLog2) { throw null; }

src/libraries/System.IO.Compression/src/System/IO/Compression/DeflateDecoder.cs

@@ -13,6 +13,7 @@ namespace System.IO.Compression
     public sealed class DeflateDecoder : IDisposable
     {
         private ZLibNative.ZLibStreamHandle? _state;
+        private readonly int _windowBits;
         private bool _disposed;
         private bool _finished;
 
@@ -27,6 +28,7 @@ public DeflateDecoder()
 
         internal DeflateDecoder(int windowBits)
         {
+            _windowBits = windowBits;
             _state = ZLibNative.ZLibStreamHandle.CreateForInflate(windowBits);
         }
 
@@ -53,6 +55,7 @@ private void EnsureNotDisposed()
         /// <param name="bytesConsumed">When this method returns, the total number of bytes that were read from <paramref name="source"/>.</param>
         /// <param name="bytesWritten">When this method returns, the total number of bytes that were written to <paramref name="destination"/>.</param>
         /// <returns>One of the enumeration values that describes the status with which the span-based operation finished.</returns>
+        /// <exception cref="ObjectDisposedException">The decoder has been disposed.</exception>
         public OperationStatus Decompress(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesConsumed, out int bytesWritten)
         {
             EnsureNotDisposed();
@@ -106,6 +109,22 @@ public OperationStatus Decompress(ReadOnlySpan<byte> source, Span<byte> destinat
             }
         }
 
+        /// <summary>
+        /// Resets the decoder to its initial state so the same instance can be reused for a new, independent decompression operation.
+        /// </summary>
+        /// <remarks>
+        /// After this method returns, any sliding-window history from a previous decompression is discarded.
+        /// </remarks>
+        /// <exception cref="ObjectDisposedException">The decoder has been disposed.</exception>
+        public void Reset()
+        {
+            EnsureNotDisposed();
+            Debug.Assert(_state is not null);
+
+            _state.InflateReset2_(_windowBits);
+            _finished = false;
+        }
+
         /// <summary>
         /// Tries to decompress a source byte span into a destination span.
         /// </summary>

src/libraries/System.IO.Compression/src/System/IO/Compression/DeflateEncoder.cs

@@ -296,6 +296,22 @@ public OperationStatus Flush(Span<byte> destination, out int bytesWritten)
             }
         }
 
+        /// <summary>
+        /// Resets the encoder to its initial state so the same instance can be reused for a new, independent compression operation.
+        /// </summary>
+        /// <remarks>
+        /// The encoder keeps the compression quality and window size it was created with. Any pending output or unflushed input from a previous, unfinished compression is discarded.
+        /// </remarks>
+        /// <exception cref="ObjectDisposedException">The encoder has been disposed.</exception>
+        public void Reset()
+        {
+            EnsureNotDisposed();
+            Debug.Assert(_state is not null);
+
+            _state.DeflateReset();
+            _finished = false;
+        }
+
         /// <summary>
         /// Tries to compress a source byte span into a destination span using the default quality.
         /// </summary>

src/libraries/System.IO.Compression/src/System/IO/Compression/GZipDecoder.cs

@@ -50,6 +50,19 @@ public OperationStatus Decompress(ReadOnlySpan<byte> source, Span<byte> destinat
             return _deflateDecoder.Decompress(source, destination, out bytesConsumed, out bytesWritten);
         }
 
+        /// <summary>
+        /// Resets the decoder to its initial state so the same instance can be reused for a new, independent decompression operation.
+        /// </summary>
+        /// <remarks>
+        /// After this method returns, any sliding-window history from a previous decompression is discarded.
+        /// </remarks>
+        /// <exception cref="ObjectDisposedException">The decoder has been disposed.</exception>
+        public void Reset()
+        {
+            EnsureNotDisposed();
+            _deflateDecoder.Reset();
+        }
+
         /// <summary>
         /// Tries to decompress a source byte span into a destination span.
         /// </summary>

src/libraries/System.IO.Compression/src/System/IO/Compression/GZipEncoder.cs

@@ -119,6 +119,19 @@ public OperationStatus Flush(Span<byte> destination, out int bytesWritten)
             return _deflateEncoder.Flush(destination, out bytesWritten);
         }
 
+        /// <summary>
+        /// Resets the encoder to its initial state so the same instance can be reused for a new, independent compression operation.
+        /// </summary>
+        /// <remarks>
+        /// The encoder keeps the compression quality and window size it was created with. Any pending output or unflushed input from a previous, unfinished compression is discarded.
+        /// </remarks>
+        /// <exception cref="ObjectDisposedException">The encoder has been disposed.</exception>
+        public void Reset()
+        {
+            EnsureNotDisposed();
+            _deflateEncoder.Reset();
+        }
+
         /// <summary>
         /// Tries to compress a source byte span into a destination span using the default quality.
         /// </summary>

src/libraries/System.IO.Compression/src/System/IO/Compression/ZLibDecoder.cs

@@ -44,12 +44,26 @@ private void EnsureNotDisposed()
         /// <param name="bytesConsumed">When this method returns, the total number of bytes that were read from <paramref name="source"/>.</param>
         /// <param name="bytesWritten">When this method returns, the total number of bytes that were written to <paramref name="destination"/>.</param>
         /// <returns>One of the enumeration values that describes the status with which the span-based operation finished.</returns>
+        /// <exception cref="ObjectDisposedException">The decoder has been disposed.</exception>
         public OperationStatus Decompress(ReadOnlySpan<byte> source, Span<byte> destination, out int bytesConsumed, out int bytesWritten)
         {
             EnsureNotDisposed();
             return _deflateDecoder.Decompress(source, destination, out bytesConsumed, out bytesWritten);
         }
 
+        /// <summary>
+        /// Resets the decoder to its initial state so the same instance can be reused for a new, independent decompression operation.
+        /// </summary>
+        /// <remarks>
+        /// After this method returns, any sliding-window history from a previous decompression is discarded.
+        /// </remarks>
+        /// <exception cref="ObjectDisposedException">The decoder has been disposed.</exception>
+        public void Reset()
+        {
+            EnsureNotDisposed();
+            _deflateDecoder.Reset();
+        }
+
         /// <summary>
         /// Tries to decompress a source byte span into a destination span.
         /// </summary>

src/libraries/System.IO.Compression/src/System/IO/Compression/ZLibEncoder.cs

@@ -111,6 +111,19 @@ public OperationStatus Flush(Span<byte> destination, out int bytesWritten)
             return _deflateEncoder.Flush(destination, out bytesWritten);
         }
 
+        /// <summary>
+        /// Resets the encoder to its initial state so the same instance can be reused for a new, independent compression operation.
+        /// </summary>
+        /// <remarks>
+        /// The encoder keeps the compression quality and window size it was created with. Any pending output or unflushed input from a previous, unfinished compression is discarded.
+        /// </remarks>
+        /// <exception cref="ObjectDisposedException">The encoder has been disposed.</exception>
+        public void Reset()
+        {
+            EnsureNotDisposed();
+            _deflateEncoder.Reset();
+        }
+
         /// <summary>
         /// Tries to compress a source byte span into a destination span using the default quality.
         /// </summary>

src/libraries/System.IO.Compression/tests/DeflateEncoderDecoderTests.cs

@@ -24,7 +24,7 @@ public override OperationStatus Flush(Span<byte> destination, out int bytesWritt
                 _encoder.Flush(destination, out bytesWritten);
 
             public override void Dispose() => _encoder.Dispose();
-            public override void Reset() => throw new NotSupportedException();
+            public override void Reset() => _encoder.Reset();
         }
 
         public class DeflateDecoderAdapter : DecoderAdapter
@@ -40,7 +40,7 @@ public override OperationStatus Decompress(ReadOnlySpan<byte> source, Span<byte>
                 _decoder.Decompress(source, destination, out bytesConsumed, out bytesWritten);
 
             public override void Dispose() => _decoder.Dispose();
-            public override void Reset() => throw new NotSupportedException();
+            public override void Reset() => _decoder.Reset();
         }
 
         protected override EncoderAdapter CreateEncoder() =>

src/libraries/System.IO.Compression/tests/GZipEncoderDecoderTests.cs

@@ -24,7 +24,7 @@ public override OperationStatus Flush(Span<byte> destination, out int bytesWritt
                 _encoder.Flush(destination, out bytesWritten);
 
             public override void Dispose() => _encoder.Dispose();
-            public override void Reset() => throw new NotSupportedException();
+            public override void Reset() => _encoder.Reset();
         }
 
         public class GZipDecoderAdapter : DecoderAdapter
@@ -40,7 +40,7 @@ public override OperationStatus Decompress(ReadOnlySpan<byte> source, Span<byte>
                 _decoder.Decompress(source, destination, out bytesConsumed, out bytesWritten);
 
             public override void Dispose() => _decoder.Dispose();
-            public override void Reset() => throw new NotSupportedException();
+            public override void Reset() => _decoder.Reset();
         }
 
         protected override EncoderAdapter CreateEncoder() =>

src/libraries/System.IO.Compression/tests/ZLibEncoderDecoderTestBase.cs

@@ -9,7 +9,7 @@ namespace System.IO.Compression
     public abstract class ZLibEncoderDecoderTestBase : EncoderDecoderTestBase
     {
         protected override bool SupportsDictionaries => false;
-        protected override bool SupportsReset => false;
+        protected override bool SupportsReset => true;
 
         protected override string WindowLogParamName => "windowLog2";
         protected override string InputLengthParamName => "inputLength";