Fixed XZ block CRC64 computation and added notes on XZ format specifics

docs/FORMATS.md

@@ -48,9 +48,15 @@
   2. Skip through the decompressed data to reach each file's starting position
   
   **Recommendation**: For best performance with 7Zip archives, use synchronous extraction methods (`MoveToNextEntry()` and `WriteEntryToDirectory()`) when possible. Use async methods only when you need to avoid blocking the thread (e.g., in UI applications or async-only contexts).
-  
+
   **Technical Details**: 7Zip archives group files into "folders" (compression units), where all files in a folder share one continuous LZMA-compressed stream. The LZMA decoder maintains internal state (dictionary window, decoder positions) that assumes sequential, non-interruptible processing. Async operations can yield control during awaits, which would corrupt this shared state. To avoid this, async extraction creates a fresh decoder stream for each file.
 
+### XZ Format Notes
+
+- XZ is a container format around LZMA2-compressed blocks, not just raw LZMA/LZMA2 data.
+- XZ streams can include per-block integrity checks selected by the stream header: CRC32, CRC64/XZ, SHA-256, or none. SharpCompress validates these checks while reading XZ blocks.
+- Raw LZMA/LZMA2 decoding does not provide the same container-level CRC validation; it only validates what the decoder format itself can detect, such as malformed compressed data or invalid end markers.
+
 ## Compression Streams
 
 For those who want to directly compress/decompress bits. The single file formats are represented here as well. However, BZip2, LZip and XZ have no metadata (GZip has a little) so using them without something like a Tar file makes little sense.

src/SharpCompress/Compressors/Xz/Crc64.cs

@@ -6,10 +6,13 @@ namespace SharpCompress.Compressors.Xz;
 public static class Crc64
 {
     public const ulong DefaultSeed = 0x0;
+    internal const ulong XZ_SEED = 0xffffffffffffffff;
 
     internal static ulong[]? Table;
+    private static ulong[]? _xzTable;
 
     public const ulong Iso3309Polynomial = 0xD800000000000000;
+    private const ulong XZ_POLYNOMIAL = 0xC96C5795D7870F42;
 
     public static ulong Compute(byte[] buffer) => Compute(DefaultSeed, buffer);
 
@@ -20,6 +23,15 @@ public static class Crc64
         return CalculateHash(seed, Table, buffer);
     }
 
+    public static ulong ComputeXz(byte[] buffer) => ~UpdateXz(XZ_SEED, buffer);
+
+    public static ulong UpdateXz(ulong seed, ReadOnlySpan<byte> buffer)
+    {
+        _xzTable ??= CreateTable(XZ_POLYNOMIAL);
+
+        return CalculateHash(seed, _xzTable, buffer);
+    }
+
     public static ulong CalculateHash(ulong seed, ulong[] table, ReadOnlySpan<byte> buffer)
     {
         var crc = seed;

src/SharpCompress/Compressors/Xz/XZBlock.cs

@@ -21,7 +21,7 @@ public sealed partial class XZBlock : XZReadOnlyStream
     private readonly CheckType _checkType;
     private readonly int _checkSize;
     private uint _crc32 = Crc32.DefaultSeed;
-    private ulong _crc64 = Crc64.DefaultSeed;
+    private ulong _crc64 = Crc64.XZ_SEED;
     private readonly SHA256? _sha256;
     private bool _streamConnected;
     private int _numFilters;
@@ -125,8 +125,7 @@ public sealed partial class XZBlock : XZReadOnlyStream
                 _crc32 = Crc32.Update(_crc32, bytes);
                 break;
             case CheckType.CRC64:
-                Crc64.Table ??= Crc64.CreateTable(Crc64.Iso3309Polynomial);
-                _crc64 = Crc64.CalculateHash(_crc64, Crc64.Table, bytes);
+                _crc64 = Crc64.UpdateXz(_crc64, bytes);
                 break;
             case CheckType.SHA256:
                 _sha256.NotNull().TransformBlock(buffer, offset, count, null, 0);

tests/SharpCompress.Test/Xz/Crc64Tests.cs

@@ -27,4 +27,12 @@ public class Crc64Tests
 
         Assert.Equal((ulong)0x416B4150508661EE, actual);
     }
+
+    [Fact]
+    public void XzCheckString()
+    {
+        var actual = Crc64.ComputeXz(Encoding.ASCII.GetBytes("123456789"));
+
+        Assert.Equal(0x995DC9BBDF1939FAUL, actual);
+    }
 }

tests/SharpCompress.Test/Xz/XZBlockAsyncTests.cs

@@ -85,7 +85,7 @@ public class XzBlockAsyncTests : XzTestsBase
     {
         using var xzBlock = new XZBlock(CompressedStream, CheckType.CRC64, 8);
         using var sr = new StreamReader(xzBlock);
-        Assert.Equal(await sr.ReadToEndAsync().ConfigureAwait(false), Original);
+        Assert.Equal(Original, await sr.ReadToEndAsync().ConfigureAwait(false));
     }
 
     [Fact]
@@ -101,8 +101,8 @@ public class XzBlockAsyncTests : XzTestsBase
     [Fact]
     public async ValueTask SkipsPaddingWhenPresentAsync()
     {
-        // CompressedIndexedStream's first block has 1-byte padding.
-        using var xzBlock = new XZBlock(CompressedIndexedStream, CheckType.CRC64, 8);
+        // CompressedIndexedStream uses CRC32 checks.
+        using var xzBlock = new XZBlock(CompressedIndexedStream, CheckType.CRC32, 4);
         using var sr = new StreamReader(xzBlock);
         await sr.ReadToEndAsync().ConfigureAwait(false);
         Assert.Equal(0L, CompressedIndexedStream.Position % 4L);

tests/SharpCompress.Test/Xz/XZBlockTests.cs

@@ -72,7 +72,7 @@ public class XzBlockTests : XzTestsBase
     {
         var xzBlock = new XZBlock(CompressedStream, CheckType.CRC64, 8);
         using var sr = new StreamReader(xzBlock);
-        Assert.Equal(sr.ReadToEnd(), Original);
+        Assert.Equal(Original, sr.ReadToEnd());
     }
 
     [Fact]
@@ -88,8 +88,8 @@ public class XzBlockTests : XzTestsBase
     [Fact]
     public void SkipsPaddingWhenPresent()
     {
-        // CompressedIndexedStream's first block has 1-byte padding.
-        using var xzBlock = new XZBlock(CompressedIndexedStream, CheckType.CRC64, 8);
+        // CompressedIndexedStream uses CRC32 checks.
+        using var xzBlock = new XZBlock(CompressedIndexedStream, CheckType.CRC32, 4);
         using var sr = new StreamReader(xzBlock);
         sr.ReadToEnd();
         Assert.Equal(0L, CompressedIndexedStream.Position % 4L);