From 35351eaff1ae7145058dcd05bbb7d36642198527 Mon Sep 17 00:00:00 2001
From: Grisha Kotler <grisha@ravendb.net>
Date: Thu, 10 Oct 2024 22:34:56 +0300
Subject: [PATCH] RavenDB-22986 - improve the performance of getting the free
 pages count

---
 src/Voron/Impl/FreeSpace/FreeSpaceHandling.cs | 25 +++++++++++++++++++
 .../Impl/FreeSpace/IFreeSpaceHandling.cs      |  1 +
 .../Impl/FreeSpace/NoFreeSpaceHandling.cs     |  5 ++++
 src/Voron/Impl/FreeSpace/StreamBitArray.cs    | 13 ++++++++++
 4 files changed, 44 insertions(+)

diff --git a/src/Voron/Impl/FreeSpace/FreeSpaceHandling.cs b/src/Voron/Impl/FreeSpace/FreeSpaceHandling.cs
index d658b0b48b8b..ecae4358902d 100644
--- a/src/Voron/Impl/FreeSpace/FreeSpaceHandling.cs
+++ b/src/Voron/Impl/FreeSpace/FreeSpaceHandling.cs
@@ -349,6 +349,31 @@ public List<long> AllPages(LowLevelTransaction tx)
             }
         }
 
+        public int GetFreePagesCount(LowLevelTransaction tx)
+        {
+            var freeSpaceTree = GetFreeSpaceTree(tx);
+            if (freeSpaceTree.NumberOfEntries == 0)
+                return 0;
+
+            using (var it = freeSpaceTree.Iterate())
+            {
+                if (it.Seek(0) == false)
+                    return 0;
+
+                var count = 0;
+
+                do
+                {
+                    var stream = it.CreateReaderForCurrent();
+                    var current = new StreamBitArray(stream);
+                    count += current.GetNumberOfSetBits();
+
+                } while (it.MoveNext());
+
+                return count;
+            }
+        }
+
         public void FreePage(LowLevelTransaction tx, long pageNumber)
         {
             if (_guard.IsProcessingFixedSizeTree)
diff --git a/src/Voron/Impl/FreeSpace/IFreeSpaceHandling.cs b/src/Voron/Impl/FreeSpace/IFreeSpaceHandling.cs
index 3515b9e886e4..c7997b85c6cd 100644
--- a/src/Voron/Impl/FreeSpace/IFreeSpaceHandling.cs
+++ b/src/Voron/Impl/FreeSpace/IFreeSpaceHandling.cs
@@ -6,6 +6,7 @@ public interface IFreeSpaceHandling
     {
         long? TryAllocateFromFreeSpace(LowLevelTransaction tx, int num);
         List<long> AllPages(LowLevelTransaction tx);
+        int GetFreePagesCount(LowLevelTransaction txLowLevelTransaction);
         void FreePage(LowLevelTransaction tx, long pageNumber);
         long GetFreePagesOverhead(LowLevelTransaction tx);
         IEnumerable<long> GetFreePagesOverheadPages(LowLevelTransaction tx);
diff --git a/src/Voron/Impl/FreeSpace/NoFreeSpaceHandling.cs b/src/Voron/Impl/FreeSpace/NoFreeSpaceHandling.cs
index 0564f9b31ec2..dd27d0078760 100644
--- a/src/Voron/Impl/FreeSpace/NoFreeSpaceHandling.cs
+++ b/src/Voron/Impl/FreeSpace/NoFreeSpaceHandling.cs
@@ -14,6 +14,11 @@ public List<long> AllPages(LowLevelTransaction tx)
             return new List<long>();
         }
 
+        public int GetFreePagesCount(LowLevelTransaction txLowLevelTransaction)
+        {
+            return 0;
+        }
+
         public void FreePage(LowLevelTransaction tx, long pageNumber)
         {
             
diff --git a/src/Voron/Impl/FreeSpace/StreamBitArray.cs b/src/Voron/Impl/FreeSpace/StreamBitArray.cs
index 258bdb983dec..da10ba61a95c 100644
--- a/src/Voron/Impl/FreeSpace/StreamBitArray.cs
+++ b/src/Voron/Impl/FreeSpace/StreamBitArray.cs
@@ -35,6 +35,7 @@
 using System;
 using System.Diagnostics;
 using System.IO;
+using System.Numerics;
 using Sparrow.Server;
 
 namespace Voron.Impl.FreeSpace
@@ -110,6 +111,18 @@ public bool Get(int index)
             return (_inner[index >> 5] & (1 << (index & 31))) != 0;
         }
 
+        public int GetNumberOfSetBits()
+        {
+            var count = 0;
+
+            for (int i = 0; i < _inner.Length; i++)
+            {
+                count += BitOperations.PopCount((uint)_inner[i]);
+            }
+
+            return count;
+        }
+
         public void Set(int index, bool value)
         {
             if (value)