--!strict

--[[
	Given a reactive object, updates all dependent reactive objects.
	Objects are only ever updated after all of their dependencies are updated,
	are only ever updated once, and won't be updated if their dependencies are
	unchanged.
]]

local PubTypes = require "../PubTypes"

type Set<T> = { [T]: any }
type Descendant = (PubTypes.Dependent & PubTypes.Dependency) | PubTypes.Dependent

-- Credit: https://blog.elttob.uk/2022/11/07/sets-efficient-topological-search.html
local function updateAll(root: PubTypes.Dependency)
	local counters: { [Descendant]: number } = {}
	local flags: { [Descendant]: boolean } = {}
	local queue: { Descendant } = {}
	local queueSize = 0
	local queuePos = 1

	for object in pairs(root.dependentSet) do
		queueSize += 1
		queue[queueSize] = object
		flags[object] = true
	end

	-- Pass 1: counting up
	while queuePos <= queueSize do
		local next = queue[queuePos]
		local counter = counters[next]
		if counter == nil then
			counters[next] = 1
		else
			counters[next] = counter + 1
		end
		if next.dependentSet ~= nil then
			for object in pairs(next.dependentSet) do
				queueSize += 1
				queue[queueSize] = object
			end
		end
		queuePos += 1
	end

	-- Pass 2: counting down + processing
	queuePos = 1
	while queuePos <= queueSize do
		local next = queue[queuePos]
		local counter = counters[next] - 1
		counters[next] = counter
		if
			counter == 0
			and flags[next]
			and next:update()
			and next.dependentSet ~= nil
		then
			for object in pairs(next.dependentSet) do
				flags[object] = true
			end
		end
		queuePos += 1
	end
end

return updateAll