extends GutTest
## The client numbers its input frames a fixed "lead" ahead of the server's
## tick, and the server accepts a window around its own tick. These two numbers
## have to be chosen together.
##
## They were not, and the result was the worst kind of bug: the server silently
## dropped every input from a client whose lead had drifted into the gap between
## "server stops accepting" and "client notices", the client never corrected
## because by its own reckoning nothing was wrong, and the server coasted on a
## stale input forever. It needed two independent clocks to drift apart, so it
## only appeared after a few minutes -- and then never went away.

var world: SimWorld
const PEER := 5


func before_each() -> void:
	world = SimWorld.new(1)
	world.add_player(PEER, "tester")
	world.tick = 1000


func _queue_at_lead(lead: int) -> void:
	var frames: Array[InputFrame] = [
		InputFrame.make(world.tick + lead, Vector2.RIGHT, 0.0, 0)]
	world.queue_input(PEER, frames)


## The invariant that was violated. The client corrects itself at
## INPUT_LEAD_MAX; the server must keep accepting well past that, or there is a
## band where the server refuses input the client still thinks is fine.
func test_the_server_accepts_every_lead_the_client_tolerates() -> void:
	assert_gt(SimConfig.INPUT_MAX_LEAD, SimConfig.INPUT_LEAD_MAX,
		"the server's acceptance window must be wider than the client's " +
		"correction band, or drift lands in a silent dead zone")


func test_inputs_across_the_whole_client_band_are_accepted() -> void:
	for lead in range(SimConfig.INPUT_LEAD_MIN, SimConfig.INPUT_LEAD_MAX + 1):
		world.players[PEER].input_queue.clear()
		world.players[PEER].last_input_tick = 0
		_queue_at_lead(lead)
		assert_eq(world.players[PEER].input_queue.size(), 1,
			"lead %d is inside the band the client will not correct, so the " % lead +
			"server has to accept it")


func test_absurd_lead_is_still_rejected() -> void:
	_queue_at_lead(SimConfig.INPUT_MAX_LEAD + 1)
	assert_eq(world.players[PEER].input_queue.size(), 0,
		"the window is wider, not gone -- a client claiming a far-future tick " +
		"still buys nothing")


## What the drift actually looked like from the server's side: nothing arrives,
## so it coasts on the last input it saw. That is correct behaviour for a brief
## hiccup and catastrophic as a permanent state, which is why the client now
## has a stall detector rather than relying on its lead estimate alone.
func test_a_starved_server_coasts_then_stops() -> void:
	var p: SimPlayer = world.players[PEER]
	_queue_at_lead(SimConfig.INPUT_TARGET_LEAD)
	world.step()
	var moved_once: Vector2 = p.pos
	assert_ne(moved_once, Vector2.ZERO, "setup: the input should have moved us")
	for _i in SimConfig.INPUT_MAX_AGE + 5:
		world.step()
	var coasted: Vector2 = p.pos
	for _i in 60:
		world.step()
	assert_eq(p.pos, coasted, "coasting has to end, or a silent client drifts forever")