Try merging free image lists if failed to allocate one

src/drawing/Image.cpp

@@ -72,21 +72,57 @@ static bool AllocatedListRemove(uint32 baseImageId, uint32 count)
 }
 #endif
 
-static uint32 AllocateImageList(uint32 count)
+static uint32 GetNumFreeImagesRemaining()
 {
-    Guard::Assert(count != 0, GUARD_LINE);
+    return MAX_IMAGES - _allocatedImageCount;
+}
 
-    if (!_initialised)
-    {
-        _initialised = true;
-        _freeLists.clear();
-        _freeLists.push_back({ BASE_IMAGE_ID, MAX_IMAGES });
+static void InitialiseImageList()
+{
+    Guard::Assert(!_initialised, GUARD_LINE);
+
+    _freeLists.clear();
+    _freeLists.push_back({ BASE_IMAGE_ID, MAX_IMAGES });
 #ifdef DEBUG
-        _allocatedLists.clear();
+    _allocatedLists.clear();
 #endif
-        _allocatedImageCount = 0;
-    }
+    _allocatedImageCount = 0;
+    _initialised = true;
+}
 
+/**
+ * Merges all the free lists into one, a process of defragmentation.
+ */
+static void MergeFreeLists()
+{
+    _freeLists.sort(
+        [](const ImageList &a, const ImageList &b) -> bool
+        {
+            return a.BaseId < b.BaseId;
+        });
+    for (auto it = _freeLists.begin(); it != _freeLists.end(); it++)
+    {
+        bool mergeHappened;
+        do
+        {
+            mergeHappened = false;
+            auto nextIt = std::next(it);
+            if (nextIt != _freeLists.end())
+            {
+                if (it->BaseId + it->Count == nextIt->BaseId)
+                {
+                    // Merge next list into this list
+                    it->Count += nextIt->Count;
+                    _freeLists.erase(nextIt);
+                    mergeHappened = true;
+                }
+            }
+        } while (mergeHappened);
+    }
+}
+
+static uint32 TryAllocateImageList(uint32 count)
+{
     for (auto it = _freeLists.begin(); it != _freeLists.end(); it++)
     {
         ImageList imageList = *it;
@@ -110,6 +146,30 @@ static uint32 AllocateImageList(uint32 count)
     return INVALID_IMAGE_ID;
 }
 
+static uint32 AllocateImageList(uint32 count)
+{
+    Guard::Assert(count != 0, GUARD_LINE);
+
+    if (!_initialised)
+    {
+        InitialiseImageList();
+    }
+
+    uint32 baseImageId = INVALID_IMAGE_ID;
+    uint32 freeImagesRemaining = GetNumFreeImagesRemaining();
+    if (freeImagesRemaining >= count)
+    {
+        baseImageId = TryAllocateImageList(count);
+        if (baseImageId == INVALID_IMAGE_ID)
+        {
+            // Defragment and try again
+            MergeFreeLists();
+            baseImageId = TryAllocateImageList(count);
+        }
+    }
+    return baseImageId;
+}
+
 static void FreeImageList(uint32 baseImageId, uint32 count)
 {
     Guard::Assert(_initialised, GUARD_LINE);