From 456a7dfd588da6d5856ca54d8978e56db253bd64 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Wed, 1 Apr 2026 13:12:49 +0000
Subject: [PATCH] Revert default buffer to 80KB; add ZStandard
 minimumRewindBufferSize (DStreamInSize=131075)

Agent-Logs-Url: https://github.com/adamhathcock/sharpcompress/sessions/d5a696df-2906-4262-acf9-06cc174e91fc

Co-authored-by: adamhathcock <527620+adamhathcock@users.noreply.github.com>
---
 src/SharpCompress/Common/Constants.cs               | 10 ++++++----
 .../Compressors/ZStandard/ZstandardConstants.cs     | 13 +++++++++++++
 src/SharpCompress/Factories/TarWrapper.cs           |  6 +++++-
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/src/SharpCompress/Common/Constants.cs b/src/SharpCompress/Common/Constants.cs
index 9812501d..edd81e08 100644
--- a/src/SharpCompress/Common/Constants.cs
+++ b/src/SharpCompress/Common/Constants.cs
@@ -22,9 +22,11 @@ public static class Constants
     /// by rewinding and re-reading the same data.
     /// </para>
     /// <para>
-    /// <b>Default:</b> 163840 bytes (160KB) - sized to cover ZStandard's worst-case
-    /// first block on a tar archive (~131KB including frame header overhead).
-    /// ZStandard blocks can be up to 128KB, exceeding the previous 81KB default.
+    /// <b>Default:</b> 81920 bytes (80KB) — sufficient for most formats.
+    /// Formats that require larger buffers (e.g. BZip2, ZStandard) declare their
+    /// own minimum via <c>TarWrapper.MinimumRewindBufferSize</c>, and
+    /// <c>TarWrapper.MaximumRewindBufferSize</c> is used at stream construction
+    /// to ensure the correct capacity is allocated upfront.
     /// </para>
     /// <para>
     /// <b>Typical usage:</b> 500-1000 bytes for most archives
@@ -41,7 +43,7 @@ public static class Constants
     /// </list>
     /// </para>
     /// </remarks>
-    public static int RewindableBufferSize { get; set; } = 163840;
+    public static int RewindableBufferSize { get; set; } = 81920;
 
     public static CultureInfo DefaultCultureInfo { get; set; } = CultureInfo.InvariantCulture;
 }
diff --git a/src/SharpCompress/Compressors/ZStandard/ZstandardConstants.cs b/src/SharpCompress/Compressors/ZStandard/ZstandardConstants.cs
index ac4756d4..a43b2ef1 100644
--- a/src/SharpCompress/Compressors/ZStandard/ZstandardConstants.cs
+++ b/src/SharpCompress/Compressors/ZStandard/ZstandardConstants.cs
@@ -6,4 +6,17 @@ internal class ZstandardConstants
     /// Magic number found at start of ZStandard frame: 0xFD 0x2F 0xB5 0x28
     /// </summary>
     public const uint MAGIC = 0xFD2FB528;
+
+    /// <summary>
+    /// Maximum uncompressed size of a single ZStandard block: ZSTD_BLOCKSIZE_MAX = 128 KB.
+    /// </summary>
+    public const int BlockSizeMax = 1 << 17; // 131072 bytes
+
+    /// <summary>
+    /// Recommended input (compressed) buffer size for streaming decompression:
+    /// ZSTD_DStreamInSize = ZSTD_BLOCKSIZE_MAX + ZSTD_blockHeaderSize (3 bytes).
+    /// The ring buffer must be at least this large to hold the compressed bytes read
+    /// during format detection before the first rewind.
+    /// </summary>
+    public const int DStreamInSize = BlockSizeMax + 3;
 }
diff --git a/src/SharpCompress/Factories/TarWrapper.cs b/src/SharpCompress/Factories/TarWrapper.cs
index 013debaf..465cf836 100644
--- a/src/SharpCompress/Factories/TarWrapper.cs
+++ b/src/SharpCompress/Factories/TarWrapper.cs
@@ -88,7 +88,11 @@ public class TarWrapper(
             ZStandardStream.IsZStandardAsync,
             (stream) => new ZStandardStream(stream),
             (stream, _) => new ValueTask<Stream>(new ZStandardStream(stream)),
-            ["tar.zst", "tar.zstd", "tzst", "tzstd"]
+            ["tar.zst", "tar.zstd", "tzst", "tzstd"],
+            // ZStandard decompresses in blocks; the compressed size of the first block
+            // can be up to ZSTD_BLOCKSIZE_MAX + ZSTD_blockHeaderSize = 131075 bytes.
+            // The ring buffer must hold all compressed bytes read during format detection.
+            minimumRewindBufferSize: ZstandardConstants.DStreamInSize
         ),
         new(
             CompressionType.LZip,