From 020b640465b01617b74f7f91ba90569434257d6f Mon Sep 17 00:00:00 2001
From: Jeremy Powell <jeremy@visionaid.com>
Date: Thu, 5 Dec 2024 23:00:46 +1300
Subject: [PATCH] Handle range lock sector

---
 OpenMcdf/Fat.cs           | 13 +++++++++++++
 OpenMcdf/FatEnumerator.cs |  2 +-
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/OpenMcdf/Fat.cs b/OpenMcdf/Fat.cs
index 1450920..9b392b8 100644
--- a/OpenMcdf/Fat.cs
+++ b/OpenMcdf/Fat.cs
@@ -10,7 +10,11 @@ namespace OpenMcdf;
 /// </summary>
 internal sealed class Fat : ContextBase, IEnumerable<FatEntry>, IDisposable
 {
+    const uint RangeLockSectorOffset = 0x7FFFFF00;
+    const uint RangeLockSectorId = RangeLockSectorOffset / (uint)(1 << Header.SectorShiftV4);
+
     private readonly FatSectorEnumerator fatSectorEnumerator;
+    private readonly Func<uint, bool> isFree;
     private readonly byte[] cachedSectorBuffer;
     Sector cachedSector = Sector.EndOfChain;
     private bool isDirty;
@@ -20,6 +24,13 @@ public Fat(RootContextSite rootContextSite)
     {
         fatSectorEnumerator = new(rootContextSite);
         cachedSectorBuffer = new byte[Context.SectorSize];
+
+        if (Context.Version == Version.V3)
+            isFree = value => value == SectorType.Free;
+        else if (Context.Version == Version.V4)
+            isFree = value => value == SectorType.Free && value != RangeLockSectorId;
+        else
+            throw new NotSupportedException($"Unsupported major version: {Context.Version}.");
     }
 
     public void Dispose()
@@ -29,6 +40,8 @@ public void Dispose()
         fatSectorEnumerator.Dispose();
     }
 
+    public bool IsFree(uint key) => isFree(key);
+
     public uint this[uint key]
     {
         get
diff --git a/OpenMcdf/FatEnumerator.cs b/OpenMcdf/FatEnumerator.cs
index cdc429f..10e29e4 100644
--- a/OpenMcdf/FatEnumerator.cs
+++ b/OpenMcdf/FatEnumerator.cs
@@ -77,7 +77,7 @@ public bool MoveNextFreeEntry()
     {
         while (MoveNext())
         {
-            if (value == SectorType.Free)
+            if (fat.IsFree(value))
                 return true;
         }