Fix scheduler not respecting priority

lib/Loop.lua

@@ -18,14 +18,6 @@ local function systemName(system: System)
 	return debug.info(fn, "s") .. "->" .. debug.info(fn, "n")
 end
 
-local function systemPriority(system: System)
-	if type(system) == "table" then
-		return system.priority or 0
-	end
-
-	return 0
-end
-
 --[=[
 	@class Loop
 
@@ -222,9 +214,46 @@ function Loop:replaceSystem(old: System, new: System)
 end
 
 local function orderSystemsByDependencies(unscheduledSystems: { System })
-	local sortedUnscheduledSystems = table.clone(unscheduledSystems)
+	local systemPriorityMap = {}
+	local visiting = "v"
+
+	local function systemPriority(system: System)
+		local priority = systemPriorityMap[system]
+
+		if not priority then
+			priority = 0
+
+			systemPriorityMap[system] = visiting
+
+			if type(system) == "table" then
+				if system.after then
+					for _, dependency in system.after do
+						if systemPriorityMap[dependency] ~= visiting then
+							priority = math.max(priority, systemPriority(dependency) + 1)
+						else
+							local errorStatement = {
+								`Cyclic dependency detected: System '{systemName(system)}' is set to execute after System '{systemName(
+									dependency
+								)}', and vice versa. This creates a loop that prevents the systems from being able to execute in a valid order.`,
+								"To resolve this issue, reconsider the dependencies between these systems. One possible solution is to update the 'after' field from one of the systems.",
+							}
+							error(table.concat(errorStatement, "\n"))
+						end
+					end
+				elseif system.priority then
+					priority = system.priority
+				end
+			end
+
+			systemPriorityMap[system] = priority
+		end
 
-	table.sort(sortedUnscheduledSystems, function(a, b)
+		return priority
+	end
+
+	local scheduledSystems = table.clone(unscheduledSystems)
+
+	table.sort(scheduledSystems, function(a, b)
 		local priorityA = systemPriority(a)
 		local priorityB = systemPriority(b)
 
@@ -242,39 +271,6 @@ local function orderSystemsByDependencies(unscheduledSystems: { System })
 		return priorityA < priorityB
 	end)
 
-	local scheduledSystemsSet = {}
-	local scheduledSystems = {}
-
-	local visited, explored = 1, 2
-
-	local function scheduleSystem(system)
-		scheduledSystemsSet[system] = visited
-
-		if type(system) == "table" and system.after then
-			for _, dependency in system.after do
-				if scheduledSystemsSet[dependency] == nil then
-					scheduleSystem(dependency)
-				elseif scheduledSystemsSet[dependency] == visited then
-					error(
-						`Unable to schedule systems due to cyclic dependency between: \n{systemName(system)} \nAND \n{systemName(
-							dependency
-						)}`
-					)
-				end
-			end
-		end
-
-		scheduledSystemsSet[system] = explored
-
-		table.insert(scheduledSystems, system)
-	end
-
-	for _, system in sortedUnscheduledSystems do
-		if scheduledSystemsSet[system] == nil then
-			scheduleSystem(system)
-		end
-	end
-
 	return scheduledSystems
 end
 

lib/Loop.spec.lua

@@ -164,6 +164,48 @@ return function()
 			connection.default:Disconnect()
 		end)
 
+		it("should call systems in order with dependent system after priority system", function()
+			local loop = Loop.new()
+
+			local order = {}
+			local systemB = {
+				system = function()
+					table.insert(order, "b")
+				end,
+			}
+			local systemC = {
+				system = function()
+					table.insert(order, "c")
+				end,
+				priority = 1000,
+			}
+			local systemA = {
+				system = function()
+					table.insert(order, "a")
+				end,
+				after = { systemC },
+			}
+
+			loop:scheduleSystems({
+				systemB,
+				systemC,
+				systemA,
+			})
+
+			local connection = loop:begin({ default = bindable.Event })
+
+			expect(#order).to.equal(0)
+
+			bindable:Fire()
+
+			expect(#order).to.equal(3)
+			expect(order[1]).to.equal("b")
+			expect(order[2]).to.equal("c")
+			expect(order[3]).to.equal("a")
+
+			connection.default:Disconnect()
+		end)
+
 		it("should not schedule systems more than once", function()
 			local loop = Loop.new()