--[[
	Intended for use in tests.

	Similar to await(), but instead of yielding if the promise is unresolved,
	_unwrap will throw. This indicates an assumption that a promise has
	resolved.
]]
function Promise.prototype:_unwrap()
	if self._status == Promise.Status.Started then
		error("Promise has not resolved or rejected.", 2)
	end

	local success = self._status == Promise.Status.Resolved

	return success, unpack(self._values, 1, self._valuesLength)
end

function Promise.prototype:_resolve(...)
	if self._status ~= Promise.Status.Started then
		if Promise.is((...)) then
			(...):_consumerCancelled(self)
		end
		return
	end

	-- If the resolved value was a Promise, we chain onto it!
	if Promise.is((...)) then
		-- Without this warning, arguments sometimes mysteriously disappear
		if select("#", ...) > 1 then
			local message = string.format(
				"When returning a Promise from andThen, extra arguments are " ..
				"discarded! See:\n\n%s",
				self._source
			)
			warn(message)
		end

		local chainedPromise = ...

		local promise = chainedPromise:andThen(
			function(...)
				self:_resolve(...)
			end,
			function(...)
				local maybeRuntimeError = chainedPromise._values[1]

				-- Backwards compatibility < v2
				if chainedPromise._error then
					maybeRuntimeError = Error.new({
						error = chainedPromise._error,
						kind = Error.Kind.ExecutionError,
						context = "[No stack trace available as this Promise originated from an older version of the Promise library (< v2)]",
					})
				end

				if Error.isKind(maybeRuntimeError, Error.Kind.ExecutionError) then
					return self:_reject(maybeRuntimeError:extend({
						error = "This Promise was chained to a Promise that errored.",
						trace = "",
						context = string.format(
							"The Promise at:\n\n%s\n...Rejected because it was chained to the following Promise, which encountered an error:\n",
							self._source
						),
					}))
				end

				self:_reject(...)
			end
		)

		if promise._status == Promise.Status.Cancelled then
			self:cancel()
		elseif promise._status == Promise.Status.Started then
			-- Adopt ourselves into promise for cancellation propagation.
			self._parent = promise
			promise._consumers[self] = true
		end

		return
	end

	self._status = Promise.Status.Resolved
	self._valuesLength, self._values = pack(...)

	-- We assume that these callbacks will not throw errors.
	for _, callback in ipairs(self._queuedResolve) do
		coroutine.wrap(callback)(...)
	end

	self:_finalize()
end

function Promise.prototype:_reject(...)
	if self._status ~= Promise.Status.Started then
		return
	end

	self._status = Promise.Status.Rejected
	self._valuesLength, self._values = pack(...)

	-- If there are any rejection handlers, call those!
	if not isEmpty(self._queuedReject) then
		-- We assume that these callbacks will not throw errors.
		for _, callback in ipairs(self._queuedReject) do
			coroutine.wrap(callback)(...)
		end
	else
		-- At this point, no one was able to observe the error.
		-- An error handler might still be attached if the error occurred
		-- synchronously. We'll wait one tick, and if there are still no
		-- observers, then we should put a message in the console.

		local err = tostring((...))

		coroutine.wrap(function()
			Promise._timeEvent:Wait()

			-- Someone observed the error, hooray!
			if not self._unhandledRejection then
				return
			end

			-- Build a reasonable message
			local message = string.format(
				"Unhandled Promise rejection:\n\n%s\n\n%s",
				err,
				self._source
			)

			if Promise.TEST then
				-- Don't spam output when we're running tests.
				return
			end

			warn(message)
		end)()
	end

	self:_finalize()
end