Hydrate by best match

plugin/src/Reconciler/hydrate.lua

@@ -4,6 +4,16 @@
 ]]
 
 local invariant = require(script.Parent.Parent.invariant)
+local countMatchingProperties = require(script.Parent.countMatchingProperties)
+
+-- When several existing children share a Name and ClassName we disambiguate
+-- them by scoring how well each one's properties match the virtual instance.
+-- That scoring is far more expensive than a Name/ClassName check, so we only do
+-- it for reasonably-sized groups. Larger groups (e.g. a folder with thousands of
+-- identically-named parts) fall back to the original order-based matching, which
+-- bounds the added work to roughly MAX_CANDIDATES_TO_SCORE^2 property reads per
+-- group regardless of how large the group is.
+local MAX_CANDIDATES_TO_SCORE = 32
 
 local function hydrate(instanceMap, virtualInstances, rootId, rootInstance)
 	local virtualInstance = virtualInstances[rootId]
@@ -16,34 +26,106 @@ local function hydrate(instanceMap, virtualInstances, rootId, rootInstance)
 
 	local existingChildren = rootInstance:GetChildren()
 
-	-- For each existing child, we'll track whether it's been paired with an
-	-- instance that the Rojo server knows about.
-	local isExistingChildVisited = {}
-	for i = 1, #existingChildren do
-		isExistingChildVisited[i] = false
+	-- Group existing children by Name then ClassName so each virtual child can
+	-- find its candidate matches without scanning every sibling. This is what
+	-- keeps hydration fast for parents with thousands of children. Nesting the
+	-- two tables (rather than a combined key) keeps the Name and ClassName checks
+	-- exact, with no way for one to bleed into the other.
+	local buckets = {}
+	for _, childInstance in existingChildren do
+		-- We guard accessing Name and ClassName in order to avoid tripping over
+		-- children of DataModel that Rojo won't have permissions to access at all.
+		local accessSuccess, name, className = pcall(function()
+			return childInstance.Name, childInstance.ClassName
+		end)
+		if not accessSuccess then
+			continue
+		end
+
+		local bucketsByClassName = buckets[name]
+		if bucketsByClassName == nil then
+			bucketsByClassName = {}
+			buckets[name] = bucketsByClassName
+		end
+
+		local bucket = bucketsByClassName[className]
+		if bucket == nil then
+			bucket = { cursor = 1, instances = {} }
+			bucketsByClassName[className] = bucket
+		end
+
+		table.insert(bucket.instances, childInstance)
 	end
 
+	-- Tracks which existing children have already been paired, so one instance
+	-- isn't matched to two different virtual instances.
+	local visited = {}
+
 	for _, childId in ipairs(virtualInstance.Children) do
 		local virtualChild = virtualInstances[childId]
 
-		for childIndex, childInstance in existingChildren do
-			if not isExistingChildVisited[childIndex] then
-				-- We guard accessing Name and ClassName in order to avoid
-				-- tripping over children of DataModel that Rojo won't have
-				-- permissions to access at all.
-				local accessSuccess, name, className = pcall(function()
-					return childInstance.Name, childInstance.ClassName
-				end)
+		local bucketsByClassName = buckets[virtualChild.Name]
+		local bucket = bucketsByClassName and bucketsByClassName[virtualChild.ClassName]
+		if bucket == nil then
+			continue
+		end
 
-				-- This rule is very conservative and could be loosened in the
-				-- future, or more heuristics could be introduced.
-				if accessSuccess and name == virtualChild.Name and className == virtualChild.ClassName then
-					isExistingChildVisited[childIndex] = true
-					hydrate(instanceMap, virtualInstances, childId, childInstance)
-					break
+		local instances = bucket.instances
+
+		-- Advance past any leading children that have already been paired. The
+		-- cursor makes order-based matching amortized O(1) per child even for
+		-- very large groups, rather than rescanning the visited prefix.
+		while bucket.cursor <= #instances and visited[instances[bucket.cursor]] do
+			bucket.cursor += 1
+		end
+		if bucket.cursor > #instances then
+			continue
+		end
+
+		-- The cursor points at the earliest unvisited child, so the slots from
+		-- here to the end bound how many candidates remain. Visited children
+		-- after the cursor (gaps) only appear once a group is small enough to be
+		-- scored -- the order-based path below always takes the earliest, which
+		-- keeps the visited region a contiguous prefix. So whenever this count
+		-- exceeds the cap it is exact, and we can pick the earliest match without
+		-- collecting anything.
+		local remaining = #instances - bucket.cursor + 1
+
+		local match
+		if remaining > MAX_CANDIDATES_TO_SCORE then
+			-- Too many to score affordably; take the earliest in child order,
+			-- reproducing the original Name + ClassName behavior.
+			match = instances[bucket.cursor]
+		else
+			-- Collect the (at most `remaining`) unvisited candidates.
+			local candidates = {}
+			for index = bucket.cursor, #instances do
+				local childInstance = instances[index]
+				if not visited[childInstance] then
+					table.insert(candidates, childInstance)
+				end
+			end
+
+			if #candidates == 1 then
+				-- Only one candidate, so there's nothing to disambiguate.
+				match = candidates[1]
+			else
+				-- Break the tie by choosing the candidate whose properties best
+				-- match the virtual instance, falling back to the earliest in
+				-- child order when scores are equal.
+				local bestScore = -1
+				for _, childInstance in candidates do
+					local score = countMatchingProperties(childInstance, virtualChild, instanceMap)
+					if score > bestScore then
+						bestScore = score
+						match = childInstance
+					end
 				end
 			end
 		end
+
+		visited[match] = true
+		hydrate(instanceMap, virtualInstances, childId, match)
 	end
 end