From 201af08f5f9fd7c93e08ced1a93539d649aa127f Mon Sep 17 00:00:00 2001
From: Matt Nadareski <mnadareski@outlook.com>
Date: Sat, 25 Apr 2026 21:45:53 -0400
Subject: [PATCH] Document alternative TGZ layout

---
 SabreTools.Data.Models/GZIP/Enums.cs | 26 ++++++++++++++++++++++++++
 SabreTools.Wrappers/GZip.cs          | 10 ++++++++--
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/SabreTools.Data.Models/GZIP/Enums.cs b/SabreTools.Data.Models/GZIP/Enums.cs
index b3c7048a..a48271c6 100644
--- a/SabreTools.Data.Models/GZIP/Enums.cs
+++ b/SabreTools.Data.Models/GZIP/Enums.cs
@@ -66,6 +66,32 @@ namespace SabreTools.Data.Models.GZIP
         RESERVED7 = 0x80,
     }
 
+    /// <summary>
+    /// TorrentGZ alternative header types
+    /// </summary>
+    public enum HeaderFileType
+    {
+        // Archive
+        Nothing = 0x00,
+        ZIP = 0x01,
+        GZ = 0x02,
+        SevenZip = 0x03,
+        RAR = 0x04,
+
+        // Specialty
+        CHD = 0x05,
+
+        // Copier header
+        A7800 = 0x06,
+        Lynx = 0x07,
+        FDS = 0x08,
+        NES = 0x09,
+        PCE = 0x0A,
+        PSID = 0x0B,
+        SNES = 0x0C,
+        SPC = 0x0D,
+    }
+
     /// <see href="https://www.ietf.org/rfc/rfc1952.txt"/>
     public enum OperatingSystem : byte
     {
diff --git a/SabreTools.Wrappers/GZip.cs b/SabreTools.Wrappers/GZip.cs
index 993c920b..83d40b7c 100644
--- a/SabreTools.Wrappers/GZip.cs
+++ b/SabreTools.Wrappers/GZip.cs
@@ -155,9 +155,15 @@ namespace SabreTools.Wrappers
                 // - 0x00-0x0F - MD5 hash of the internal file
                 // - 0x10-0x13 - CRC-32 checksum of the internal file
                 // - 0x14-0x1B - Little-endian file size of the internal file
-                if (Header.ExtraLength != 0x1C)
+                // The following fields are included for alternate headers:
+                // - 0x1C      - Alternative header type
+                // - 0x1D-0x2C - Alternative MD5 hash
+                // - 0x2D-0x40 - Alternative SHA-1 hash
+                // - 0x41-0x44 - Alternative CRC-32 checksum
+                // - 0x45-0x48 - Alternative little-endian uncompressed file size
+                if (Header.ExtraLength != 0x1C && Header.ExtraLength != 0x4D)
                     return false;
-                if (Header.ExtraFieldBytes is null || Header.ExtraFieldBytes.Length != 0x1C)
+                if (Header.ExtraFieldBytes is null || (Header.ExtraFieldBytes.Length != 0x1C && Header.ExtraFieldBytes.Length != 0x4D))
                     return false;
 
                 return true;