Raw zip writer (#44)

Author: SimonCropp

readme.md

@@ -44,7 +44,7 @@ Example file formats that leverage System.IO.Packaging
 using var sourceStream = File.OpenRead(packagePath);
 await DeterministicPackage.ConvertAsync(sourceStream, targetStream);
 ```
-<sup><a href='/src/Tests/Tests.cs#L160-L165' title='Snippet source file'>snippet source</a> | <a href='#snippet-ConvertAsync' title='Start of snippet'>anchor</a></sup>
+<sup><a href='/src/Tests/Tests.cs#L174-L179' title='Snippet source file'>snippet source</a> | <a href='#snippet-ConvertAsync' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->
 
 
@@ -56,7 +56,7 @@ await DeterministicPackage.ConvertAsync(sourceStream, targetStream);
 using var sourceStream = File.OpenRead(packagePath);
 await DeterministicPackage.ConvertAsync(sourceStream, targetStream);
 ```
-<sup><a href='/src/Tests/Tests.cs#L160-L165' title='Snippet source file'>snippet source</a> | <a href='#snippet-ConvertAsync' title='Start of snippet'>anchor</a></sup>
+<sup><a href='/src/Tests/Tests.cs#L174-L179' title='Snippet source file'>snippet source</a> | <a href='#snippet-ConvertAsync' title='Start of snippet'>anchor</a></sup>
 <!-- endSnippet -->
 
 

src/DeterministicIoPackaging/DeterministicPackage_Convert.cs

@@ -20,22 +20,32 @@ public static async Task<MemoryStream> ConvertAsync(Stream source)
 
     public static void Convert(Stream source, Stream target)
     {
-        using var sourceArchive = ReadArchive(source);
-        using var targetArchive = CreateArchive(target);
-        foreach (var sourceEntry in sourceArchive.OrderedEntries())
+        var intermediate = new MemoryStream();
+        using (var sourceArchive = ReadArchive(source))
+        using (var targetArchive = CreateArchive(intermediate))
         {
-            DuplicateEntry(sourceEntry, targetArchive);
+            foreach (var sourceEntry in sourceArchive.OrderedEntries())
+            {
+                DuplicateEntry(sourceEntry, targetArchive);
+            }
         }
+
+        ZipStorer.RewriteAsStored(intermediate, target);
     }
 
     public static async Task ConvertAsync(Stream source, Stream target, Cancel token = default)
     {
-        using var sourceArchive = ReadArchive(source);
-        using var targetArchive = CreateArchive(target);
-        foreach (var sourceEntry in OrderedEntries(sourceArchive))
+        var intermediate = new MemoryStream();
+        using (var sourceArchive = ReadArchive(source))
+        using (var targetArchive = CreateArchive(intermediate))
         {
-            await DuplicateEntryAsync(sourceEntry, targetArchive, token);
+            foreach (var sourceEntry in OrderedEntries(sourceArchive))
+            {
+                await DuplicateEntryAsync(sourceEntry, targetArchive, token);
+            }
         }
+
+        ZipStorer.RewriteAsStored(intermediate, target);
     }
 
     private static IOrderedEnumerable<Entry> OrderedEntries(this Archive archive) =>

src/DeterministicIoPackaging/ZipStorer.cs

@@ -0,0 +1,112 @@
+using System.Text;
+
+namespace DeterministicIoPackaging;
+
+/// <summary>
+/// Rewrites a ZIP archive so every entry uses method 0 (Stored).
+/// This ensures byte-identical output across all .NET runtimes,
+/// since the built-in ZipArchive on net48 ignores CompressionLevel.NoCompression.
+/// </summary>
+static class ZipStorer
+{
+    public static void RewriteAsStored(MemoryStream source, Stream target)
+    {
+        source.Position = 0;
+        using var archive = new ZipArchive(source, ZipArchiveMode.Read, leaveOpen: true);
+
+        using var writer = new BinaryWriter(target, Encoding.UTF8, leaveOpen: true);
+        var entries = new List<(string name, byte[] data, uint crc, long headerOffset)>();
+
+        foreach (var entry in archive.Entries.OrderBy(_ => _.FullName, StringComparer.Ordinal))
+        {
+            var headerOffset = target.Position;
+
+            byte[] data;
+            using (var entryStream = entry.Open())
+            using (var ms = new MemoryStream())
+            {
+                entryStream.CopyTo(ms);
+                data = ms.ToArray();
+            }
+
+            var crc = Crc32(data);
+            var nameBytes = Encoding.UTF8.GetBytes(entry.FullName);
+
+            // Local file header
+            writer.Write(0x04034b50u);       // signature
+            writer.Write((ushort)20);        // version needed
+            writer.Write((ushort)0);         // general purpose flags
+            writer.Write((ushort)0);         // compression method: Stored
+            writer.Write(DosTime);           // last mod time
+            writer.Write(DosDate);           // last mod date
+            writer.Write(crc);               // crc-32
+            writer.Write((uint)data.Length);  // compressed size
+            writer.Write((uint)data.Length);  // uncompressed size
+            writer.Write((ushort)nameBytes.Length); // file name length
+            writer.Write((ushort)0);         // extra field length
+            writer.Write(nameBytes);
+            writer.Write(data);
+
+            entries.Add((entry.FullName, data, crc, headerOffset));
+        }
+
+        var centralStart = target.Position;
+
+        foreach (var (name, data, crc, headerOffset) in entries)
+        {
+            var nameBytes = Encoding.UTF8.GetBytes(name);
+
+            // Central directory header
+            writer.Write(0x02014b50u);       // signature
+            writer.Write((ushort)20);        // version made by
+            writer.Write((ushort)20);        // version needed
+            writer.Write((ushort)0);         // general purpose flags
+            writer.Write((ushort)0);         // compression method: Stored
+            writer.Write(DosTime);           // last mod time
+            writer.Write(DosDate);           // last mod date
+            writer.Write(crc);               // crc-32
+            writer.Write((uint)data.Length);  // compressed size
+            writer.Write((uint)data.Length);  // uncompressed size
+            writer.Write((ushort)nameBytes.Length); // file name length
+            writer.Write((ushort)0);         // extra field length
+            writer.Write((ushort)0);         // file comment length
+            writer.Write((ushort)0);         // disk number start
+            writer.Write((ushort)0);         // internal file attributes
+            writer.Write(0u);               // external file attributes
+            writer.Write((uint)headerOffset); // relative offset of local header
+            writer.Write(nameBytes);
+        }
+
+        var centralEnd = target.Position;
+        var centralSize = centralEnd - centralStart;
+
+        // End of central directory record
+        writer.Write(0x06054b50u);           // signature
+        writer.Write((ushort)0);             // disk number
+        writer.Write((ushort)0);             // disk with central directory
+        writer.Write((ushort)entries.Count);  // entries on this disk
+        writer.Write((ushort)entries.Count);  // total entries
+        writer.Write((uint)centralSize);      // central directory size
+        writer.Write((uint)centralStart);     // central directory offset
+        writer.Write((ushort)0);             // comment length
+    }
+
+    // DOS date/time for 2020-01-01 00:00:00
+    const ushort DosTime = 0;
+    const ushort DosDate = (40 << 9) | (1 << 5) | 1; // year=2020-1980=40, month=1, day=1
+
+    static uint Crc32(byte[] data)
+    {
+        var crc = 0xFFFFFFFFu;
+        foreach (var b in data)
+        {
+            crc ^= b;
+            for (var i = 0; i < 8; i++)
+            {
+                crc = (crc >> 1) ^ (0xEDB88320u & ~((crc & 1) - 1));
+            }
+        }
+
+        return ~crc;
+    }
+}