From 041d4e22a38ca27dd08900906c69f62879bcc700 Mon Sep 17 00:00:00 2001
From: Ahmed Yasin Koculu <koculu@gmail.com>
Date: Mon, 8 Jun 2026 01:47:19 +0200
Subject: [PATCH] Fix first multipart merge disk segment detection

---
 .../FixedSizeKeyAndValueTests.cs              | 34 ++++++++++++++++++-
 src/ZoneTree/Core/ZoneTree.Merge.cs           | 14 +++++---
 2 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/src/ZoneTree.UnitTests/FixedSizeKeyAndValueTests.cs b/src/ZoneTree.UnitTests/FixedSizeKeyAndValueTests.cs
index ad3bf35..2d7115b 100644
--- a/src/ZoneTree.UnitTests/FixedSizeKeyAndValueTests.cs
+++ b/src/ZoneTree.UnitTests/FixedSizeKeyAndValueTests.cs
@@ -34,6 +34,38 @@ public sealed class FixedSizeKeyAndValueTests
         data.Maintenance.Drop();
     }
 
+    [Test]
+    public void FirstMultiPartMergeDoesNotTreatReadOnlySegmentAsDisk()
+    {
+        var dataPath = "data/FirstMultiPartMergeDoesNotTreatReadOnlySegmentAsDisk";
+        if (Directory.Exists(dataPath))
+            Directory.Delete(dataPath, true);
+
+        using var data = new ZoneTreeFactory<int, int>()
+            .DisableDeletion()
+            .SetMutableSegmentMaxItemCount(100)
+            .SetDataDirectory(dataPath)
+            .SetWriteAheadLogDirectory(dataPath)
+            .ConfigureDiskSegmentOptions(options =>
+            {
+                options.DiskSegmentMode = DiskSegmentMode.MultiPartDiskSegment;
+                options.MinimumRecordCount = 2;
+                options.MaximumRecordCount = 3;
+            })
+            .OpenOrCreate();
+
+        for (var i = 0; i < 4; ++i)
+        {
+            data.Upsert(i, i);
+        }
+
+        data.Maintenance.MoveMutableSegmentForward();
+        data.Maintenance.StartMergeOperation().Join();
+
+        Assert.That(data.Maintenance.DiskSegment.GetPartCount(), Is.EqualTo(2));
+        data.Maintenance.Drop();
+    }
+
     [Test]
     public void IntStringTreeTest()
     {
@@ -357,4 +389,4 @@ public sealed class FixedSizeKeyAndValueTests
         ReloadIntIntTreeTestHelper(dataPath, true);
 
     }
-}
\ No newline at end of file
+}
diff --git a/src/ZoneTree/Core/ZoneTree.Merge.cs b/src/ZoneTree/Core/ZoneTree.Merge.cs
index 06ef651..740283e 100644
--- a/src/ZoneTree/Core/ZoneTree.Merge.cs
+++ b/src/ZoneTree/Core/ZoneTree.Merge.cs
@@ -138,7 +138,8 @@ public sealed partial class ZoneTree<TKey, TValue> : IZoneTree<TKey, TValue>, IZ
 
         var mergingSegments = new List<ISeekableIterator<TKey, TValue>>();
         mergingSegments.AddRange(readOnlySegmentsArray);
-        if (oldDiskSegment is not NullDiskSegment<TKey, TValue>)
+        var hasDiskSegment = oldDiskSegment is not NullDiskSegment<TKey, TValue>;
+        if (hasDiskSegment)
             mergingSegments.Add(oldDiskSegment.GetSeekableIterator());
 
         if (IsCancelMergeRequested)
@@ -154,7 +155,7 @@ public sealed partial class ZoneTree<TKey, TValue> : IZoneTree<TKey, TValue>, IZ
             Options.DiskSegmentOptions.DiskSegmentMode == DiskSegmentMode.MultiPartDiskSegment;
 
         var len = mergingSegments.Count;
-        var diskSegmentIndex = len - 1;
+        var diskSegmentIndex = hasDiskSegment ? len - 1 : -1;
 
         using IDiskSegmentCreator<TKey, TValue> diskSegmentCreator =
             enableMultiPartDiskSegment ?
@@ -199,9 +200,12 @@ public sealed partial class ZoneTree<TKey, TValue> : IZoneTree<TKey, TValue>, IZ
         var hasPrev = false;
         TKey prevKey = default;
 
-        var firstKeysOfEveryPart = oldDiskSegment.GetFirstKeysOfEveryPart();
-        var lastKeysOfEveryPart = oldDiskSegment.GetLastKeysOfEveryPart();
-        var lastValuesOfEveryPart = oldDiskSegment.GetLastValuesOfEveryPart();
+        var lastKeysOfEveryPart = hasDiskSegment ?
+            oldDiskSegment.GetLastKeysOfEveryPart() :
+            Array.Empty<TKey>();
+        var lastValuesOfEveryPart = hasDiskSegment ?
+            oldDiskSegment.GetLastValuesOfEveryPart() :
+            Array.Empty<TValue>();
         var diskSegmentMinimumRecordCount = Options.DiskSegmentOptions.MinimumRecordCount;
 
         var dropCount = 0;