jecs/demo/src/ServerScriptService/systems/replication.luau

local ReplicatedStorage = game:GetService("ReplicatedStorage")
local types = require("../../ReplicatedStorage/types")
local ct = require("../../ReplicatedStorage/components")
local jecs = require(ReplicatedStorage.ecs)
local remotes = require("../../ReplicatedStorage/remotes")
local components = ct :: {[string]: jecs.Entity }

return function(world: ty.World)

	--- integration test

 --    for _ = 1, 10 do
 --    	local e = world:entity()
 --     	world:set(e, ct.TestA, true)
 --    end

    local storages = {} :: { [jecs.Entity]: {[jecs.Entity]: any }}
    local networked_components = {}
    local networked_pairs = {}

    for component in world:each(ct.Networked) do
    	local name = world:get(component, jecs.Name) :: string
		if components[name] == nil then
			continue
		end

        storages[component] = {}

        table.insert(networked_components, component)
    end

	for relation in world:each(ct.NetworkedPair) do
		local name = world:get(relation, jecs.Name) :: string
		if not components[name] then
			continue
		end
		table.insert(networked_pairs, relation)
	end

	for _, component in networked_components do
		local name = world:get(component, jecs.Name) :: string
		if not components[name] then
			error(`Networked Component (%id{component}%name{name})`)
		end
		local is_tag = jecs.is_tag(world, component)
		local storage = storages[component]
		if is_tag then
		    world:added(component, function(entity)
		        storage[entity] = true
		    end)
		else
		    world:added(component, function(entity, _, value)
		        storage[entity] = value
		    end)
		    world:changed(component, function(entity, _, value)
		        storage[entity] = value
		    end)
		end

        world:removed(component, function(entity)
            storage[entity] = "jecs.Remove"
		end)
	end

	for _, relation in networked_pairs do
		world:added(relation, function(entity, id, value)
	        local is_tag = jecs.is_tag(world, id)
	        local storage = storages[id]
	        if not storage then
	            storage = {}
	            storages[id] = storage
	        end
	        if is_tag then
	            storage[entity] = true
	        else
	            storage[entity] = value
	        end
	    end)

	    world:changed(relation, function(entity, id, value)
	        local is_tag = jecs.is_tag(world, id)
	        if is_tag then
	            return
	        end

	        local storage = storages[id]
	        if not storage then
	            storage = {}
	            storages[id] = storage
	        end

	        storage[entity] = value
	    end)

	    world:removed(relation, function(entity, id)
	        local storage = storages[id]
	        if not storage then
	            storage = {}
	            storages[id] = storage
	        end

	        storage[entity] = "jecs.Remove"
	    end)
	end

    local players_added = collect(Players.PlayerAdded)

    return function()
        local snapshot_lazy: ty.Snapshot
        local set_ids_lazy: { jecs.Entity }

		for player in players_added do
			if not snapshot_lazy then
				snapshot_lazy, set_ids_lazy =  {}, {}

				for component, storage in storages do
					local set_values = {}
					local set_n = 0

					local q = world:query(component)
					local is_tag = jecs.is_tag(world, component)
					for _, archetype in q:archetypes() do
						local entities = archetype.entities
						local entities_len = #entities
						table.move(entities, 1, entities_len, set_n + 1, set_ids_lazy)
						if is_tag then
							set_values = table.create(entities_len, true)
						else
							local column = archetype.columns[archetype.records[component]]
							table.move(column, 1, entities_len, set_n + 1, set_values)
						end

						set_n += entities_len
					end

					local set = table.move(set_ids_lazy, 1, set_n, 1, {})

					snapshot_lazy[tostring(component)] = {
					    set = if set_n > 0 then set else nil,
					    values = if set_n > 0 then set_values else nil,
					}
				end
			end

			remotes.replication:FireClient(player, snapshot_lazy)
		end

		local snapshot = {} :: ty.Snapshot

		local set_ids = {}
		local removed_ids = {}

		for component, storage in storages do
		    local set_values = {} :: { any }
		    local set_n = 0
		    local removed_n = 0
		    for e, v in storage do
		        if v ~= "jecs.Remove" then
		            set_n += 1
		            set_ids[set_n] = e
		            set_values[set_n] = v or true
		        elseif not world:contains(e) then
		            removed_n += 1
		            removed_ids[removed_n] = e
		        end
		    end

		    table.clear(storage)

            local dirty = false

            if set_n > 0 or removed_n > 0 then
            	dirty = true
            end

            if dirty then
            	local removed = table.move(removed_ids, 1, removed_n, 1, {}) :: { jecs.Entity }
             	local set = table.move(set_ids, 1, set_n, 1, {}) :: { jecs.Entity }
	            snapshot[tostring(component)] = {
                    set = if set_n > 0 then set else nil,
                    values = if set_n > 0 then set_values else nil,
                    removed = if removed_n > 0 then removed else nil
                }
	        end
        end
        if next(snapshot) ~= nil then
        	remotes.replication:FireAllClients(snapshot)
        end
    end
end