loolwsd: combine single rows only

Combining as much as possible is too aggressive and
seems to harm performance where large areas are
invalidated during editing (f.e. inserting text
at the begining of a paragraph).

With this patch we only combine a single row at a
time. This is reasonable since rows almost always are
invalidated when a line is edited. Larger blocks
are typically less important.

This is experimental and might get reverted if proves
to harm more than help.

Change-Id: I6e4420e07a031805c1e2729b3f32de580ae4806e
Reviewed-on: https://gerrit.libreoffice.org/29289
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>
Tested-by: Ashod Nakashian <ashnakash@gmail.com>

loolwsd/MessageQueue.cpp

@@ -201,43 +201,32 @@ MessageQueue::Payload TileQueue::get_impl()
     tiles.emplace_back(TileDesc::parse(msg));
 
     // Combine as many tiles as possible with the top one.
-    bool added;
-    do
+    for (size_t i = 0; i < _queue.size(); )
     {
-        added = false;
-        for (size_t i = 0; i < _queue.size(); )
+        auto& it = _queue[i];
+        msg = std::string(it.data(), it.size());
+        if (msg.compare(0, 5, "tile ") != 0 ||
+            msg.find("id=") != std::string::npos)
         {
-            auto& it = _queue[i];
-            msg = std::string(it.data(), it.size());
-            if (msg.compare(0, 5, "tile ") != 0 ||
-                msg.find("id=") != std::string::npos)
-            {
-                // Don't combine non-tiles or tiles with id.
-                ++i;
-                continue;
-            }
+            // Don't combine non-tiles or tiles with id.
+            ++i;
+            continue;
+        }
 
-            auto tile2 = TileDesc::parse(msg);
-            Log::trace() << "combining candidate: " << msg << Log::end;
+        auto tile2 = TileDesc::parse(msg);
+        Log::trace() << "combining candidate: " << msg << Log::end;
 
-            // Check if adjacent tiles.
-            bool found = false;
-            for (auto& tile : tiles)
-            {
-                if (tile.isAdjacent(tile2))
-                {
-                    tiles.emplace_back(tile2);
-                    _queue.erase(_queue.begin() + i);
-                    found = true;
-                    added = true;
-                    break;
-                }
-            }
-
-            i += !found;
+        // Check if it's on the same row.
+        if (tiles[0].onSameRow(tile2))
+        {
+            tiles.emplace_back(tile2);
+            _queue.erase(_queue.begin() + i);
+        }
+        else
+        {
+            ++i;
         }
     }
-    while (added);
 
     Log::trace() << "Combined " << tiles.size() << " tiles, leaving " << _queue.size() << " in queue." << Log::end;
 

loolwsd/TileDesc.hpp

@@ -90,6 +90,21 @@ public:
         return intersects(other);
     }
 
+    bool onSameRow(const TileDesc& other) const
+    {
+        if (other.getPart() != getPart() ||
+            other.getWidth() != getWidth() ||
+            other.getHeight() != getHeight() ||
+            other.getTileWidth() != getTileWidth() ||
+            other.getTileHeight() != getTileHeight())
+        {
+            return false;
+        }
+
+        return other.getTilePosY() + other.getTileHeight() >= getTilePosY() &&
+               other.getTilePosY() <= getTilePosY() + getTileHeight();
+    }
+
     /// Serialize this instance into a string.
     /// Optionally prepend a prefix.
     std::string serialize(const std::string& prefix = "") const