2013/Libraries/Red/Util/Promise.luau

local RunService = game:GetService "RunService"

local Spawn = require "./Spawn"

local Promise = {}
Promise.__index = Promise

function Promise.Promise(Callback: (
	Resolve: (...any) -> (),
	Reject: (...any) -> ()
) -> ())
	local self = setmetatable({}, Promise)

	self.Status = "Pending"
	self.OnResolve = {} :: { (...any) -> () }
	self.OnReject = {} :: { (...any) -> () }

	self.Value = {} :: { any }

	self.Thread = nil :: thread?
	self.Thread = coroutine.create(function()
		local ok, err = ypcall(Callback, function(...)
			self:_Resolve(...)
		end, function(...)
			self:_Reject(...)
		end)

		if not ok then
			self:_Reject(err)
		end
	end)

	coroutine.resume(self.Thread :: thread)

	return self
end

Promise.new = Promise.Promise :: (
	Callback: (Resolve: (...any) -> (), Reject: (...any) -> ()) -> ()
) -> Promise

function Promise.Resolve(...: any): Promise
	local self = setmetatable({}, Promise)

	self.Status = "Resolved"
	self.OnResolve = {} :: { (...any) -> () }
	self.OnReject = {} :: { (...any) -> () }
	self.Value = { ... } :: { any }
	self.Thread = nil :: thread?

	return self
end

function Promise.Reject(...: any): Promise
	local self = setmetatable({}, Promise)

	self.Status = "Rejected"
	self.OnResolve = {} :: { (...any) -> () }
	self.OnReject = {} :: { (...any) -> () }
	self.Value = { ... } :: { any }
	self.Thread = nil :: thread?

	return self
end

function Promise._Resolve(self: Promise, ...: any)
	assert(
		self.Status == "Pending",
		"Cannot resolve a promise that is not pending."
	)

	self.Status = "Resolved"
	self.Value = { ... }

	for _, Callback in ipairs(self.OnResolve) do
		Spawn(Callback, ...)
	end

	-- task.defer(task.cancel, self.Thread :: thread)
	coroutine.resume(coroutine.create(function()
		coroutine.yield(self.Thread :: thread)
	end))
end

function Promise._Reject(self: Promise, ...: any)
	assert(
		self.Status == "Pending",
		"Cannot reject a promise that is not pending."
	)

	self.Status = "Rejected"
	self.Value = { ... }

	for _, Callback in ipairs(self.OnReject) do
		Spawn(Callback, ...)
	end

	-- task.defer(task.cancel, self.Thread :: thread)
	coroutine.resume(coroutine.create(function()
		coroutine.yield(self.Thread :: thread)
	end))
end

function Promise.Then(
	self: Promise,
	OnResolve: ((...any) -> ...any)?,
	OnReject: ((...any) -> ...any)?
): Promise
	return Promise.Promise(function(Resolve, Reject)
		local function PromiseResolutionProcedure(
			Value: Promise | any,
			...: any
		)
			if type(Value) == "table" and getmetatable(Value) == Promise then
				if Value.Status == "Pending" then
					table.insert(Value.OnResolve, Resolve)
					table.insert(Value.OnReject, Reject)
				elseif Value.Status == "Resolved" then
					Resolve(Value.Value)
				elseif Value.Status == "Rejected" then
					Reject(Value.Value)
				end
			else
				Resolve(Value, ...)
			end
		end

		if self.Status == "Pending" then
			if OnResolve then
				table.insert(self.OnResolve, function(...)
					PromiseResolutionProcedure(OnResolve(...))
				end)
			else
				table.insert(self.OnResolve, PromiseResolutionProcedure)
			end

			if OnReject then
				table.insert(self.OnReject, function(...)
					PromiseResolutionProcedure(OnReject(...))
				end)
			else
				table.insert(self.OnReject, Reject)
			end
		elseif self.Status == "Resolved" then
			if OnResolve then
				PromiseResolutionProcedure(OnResolve(unpack(self.Value)))
			else
				Resolve(unpack(self.Value))
			end
		elseif self.Status == "Rejected" then
			if OnReject then
				PromiseResolutionProcedure(OnReject(unpack(self.Value)))
			else
				Reject(unpack(self.Value))
			end
		end
	end)
end

function Promise.Catch(self: Promise, OnReject: (...any) -> ())
	return self:Then(nil, OnReject)
end

function Promise.Finally(self: Promise, Finally: () -> ())
	return self:Then(function(...)
		Finally()
		return ...
	end, function(Error)
		Finally()
		error(Error)
	end)
end

function Promise.Await(self: Promise): ...any
	if self.Status == "Resolved" then
		return unpack(self.Value)
	elseif self.Status == "Rejected" then
		return error(unpack(self.Value))
	end

	local c = 0
	repeat -- bruh
		RunService.Stepped:wait()
		c += 1
	until self.Status ~= "Pending" or c > 500 -- random

	local Current = coroutine.running()

	local function Resume()
		coroutine.resume(Current)
	end

	table.insert(self.OnResolve, Resume)
	table.insert(self.OnReject, Resume)

	coroutine.yield()

	if self.Status == "Resolved" then
		return unpack(self.Value)
	end
	return error(unpack(self.Value))
end

export type Promise = typeof(Promise.Promise(...))

return Promise