test(gfx): unit tests of the smoke of a vehicle taken off the tracks (#90)

The spawn count of the smoke source of a vehicle moves out of
smoke_source::update() into smoke::vehicle_spawn_count() (vehicle/smoke.h),
a free function on the mover state, which joins the core library so the
unit tests can call it the way the source does. The rates are the same as
before, the steady rate of the source still counted in before the share of
the engine.

Tests: tests/unit/test_smoke.cpp. A vehicle taken off the tracks spawns no
smoke and drops the pending particles, with the engine starting, idling or
under load; a road vehicle back from the pool puffs again; the engine adds
to the steady rate; the pending count stays within the room of the source.
Two of the four fail without the fix.

Checked again on td.scn with the SM42 driven into a portal: the smoke
source keeps puffing on the run and drops to 0 particles once the vehicle
is taken off the tracks.

tests/unit/test_smoke.cpp

@@ -0,0 +1,85 @@
+#include "stdafx.h"
+#include "doctest.h"
+
+#include "vehicle.h"
+
+#include "vehicle/smoke.h"
+
+// the smoke of a vehicle's engine: how many particles the smoke source of the vehicle's model spawns in an update
+
+namespace {
+
+// a source of 20 particles per second, with room for 100
+smoke::spawn_rate const source{0.f, 20.f, 1.f, 100.f};
+double const update{0.1}; // s
+
+// a diesel at work, its engine in the state of one of the cases below
+test::vehicle diesel() {
+	test::vehicle vehicle{test::definition::load("wagon")};
+	vehicle->MainCtrlPosNo = 1;
+	vehicle->DElist[1].RPM = 1500.0;
+	vehicle->enrot = 300.0;
+	vehicle->EnginePower = 400.0;
+	return vehicle;
+}
+
+} // namespace
+
+TEST_CASE("smoke: a vehicle taken off the tracks puffs no more (#90)") {
+	auto vehicle{diesel()};
+	SUBCASE("engine starting") { vehicle->dizel_spinup = true; }
+	SUBCASE("engine idling") { vehicle->DirAbsolute = 0; }
+	SUBCASE("engine under load") {
+		vehicle->DirAbsolute = 1;
+		vehicle->Im = 400.0;
+	}
+	// on the tracks the engine puffs
+	REQUIRE(smoke::vehicle_spawn_count(vehicle.mover(), true, source, update) >= 1.f);
+
+	// gone through a portal, the pending particles are dropped too
+	auto pending{source};
+	pending.pending = 5.f;
+	CHECK(smoke::vehicle_spawn_count(vehicle.mover(), false, pending, update) == 0.f);
+}
+
+TEST_CASE("smoke: a road vehicle back on the road from the pool puffs again") {
+	auto vehicle{diesel()};
+	// waiting in the pool, the count stays at zero update after update
+	auto rate{source};
+	for (auto i{0}; i < 10; ++i) {
+		rate.pending = smoke::vehicle_spawn_count(vehicle.mover(), false, rate, update);
+	}
+	REQUIRE(rate.pending == 0.f);
+
+	CHECK(smoke::vehicle_spawn_count(vehicle.mover(), true, rate, update) >= 1.f);
+}
+
+TEST_CASE("smoke: the engine adds to the steady rate of the source") {
+	auto vehicle{diesel()};
+	vehicle->DirAbsolute = 0;
+	auto const steady{source.rate * update}; // the rate of a source of the scenery
+
+	vehicle->EnginePower = 0.0;
+	CHECK(smoke::vehicle_spawn_count(vehicle.mover(), true, source, update) == doctest::Approx(steady));
+
+	vehicle->EnginePower = 400.0;
+	auto const working{smoke::vehicle_spawn_count(vehicle.mover(), true, source, update)};
+	CHECK(working > steady);
+
+	// the faster the starting engine turns, the more it puffs
+	vehicle->dizel_spinup = true;
+	vehicle->enrot = 100.0;
+	auto const slow{smoke::vehicle_spawn_count(vehicle.mover(), true, source, update)};
+	vehicle->enrot = 400.0;
+	CHECK(smoke::vehicle_spawn_count(vehicle.mover(), true, source, update) > slow);
+}
+
+TEST_CASE("smoke: the pending particles never outgrow the room of the source") {
+	auto vehicle{diesel()};
+	vehicle->dizel_spinup = true;
+	vehicle->enrot = 100000.0;
+	auto rate{source};
+	rate.pending = 99.f;
+
+	CHECK(smoke::vehicle_spawn_count(vehicle.mover(), true, rate, 10.0) == rate.limit);
+}