/**
 *  Converts all trains to the new subtype format introduced in savegame 16.2
 *  It also links multiheaded engines or make them forget they are multiheaded if no suitable partner is found
 */
void ConvertOldMultiheadToNew()
{
	Train *t;
	FOR_ALL_TRAINS(t) SetBit(t->subtype, 7); // indicates that it's the old format and needs to be converted in the next loop

	FOR_ALL_TRAINS(t) {
		if (HasBit(t->subtype, 7) && ((t->subtype & ~0x80) == 0 || (t->subtype & ~0x80) == 4)) {
			for (Train *u = t; u != NULL; u = u->Next()) {
				const RailVehicleInfo *rvi = RailVehInfo(u->engine_type);

				ClrBit(u->subtype, 7);
				switch (u->subtype) {
					case 0: // TS_Front_Engine
						if (rvi->railveh_type == RAILVEH_MULTIHEAD) u->SetMultiheaded();
						u->SetFrontEngine();
						u->SetEngine();
						break;

					case 1: // TS_Artic_Part
						u->subtype = 0;
						u->SetArticulatedPart();
						break;

					case 2: // TS_Not_First
						u->subtype = 0;
						if (rvi->railveh_type == RAILVEH_WAGON) {
							/* normal wagon */
							u->SetWagon();
							break;
						}
						if (rvi->railveh_type == RAILVEH_MULTIHEAD && rvi->image_index == u->spritenum - 1) {
							/* rear end of a multiheaded engine */
							u->SetMultiheaded();
							break;
						}
						if (rvi->railveh_type == RAILVEH_MULTIHEAD) u->SetMultiheaded();
						u->SetEngine();
						break;

					case 4: // TS_Free_Car
						u->subtype = 0;
						u->SetWagon();
						u->SetFreeWagon();
						break;
					default: NOT_REACHED();
				}
			}
		}
	}
}
/*
 * Link front and rear multiheaded engines to each other
 * This is done when loading a savegame
 */
void ConnectMultiheadedTrains()
{
	Train *v;

	FOR_ALL_TRAINS(v) {
		v->other_multiheaded_part = NULL;
	}

	FOR_ALL_TRAINS(v) {
		if (v->IsFrontEngine() || v->IsFreeWagon()) {
			/* Two ways to associate multiheaded parts to each other:
			 * sequential-matching: Trains shall be arranged to look like <..>..<..>..<..>..
			 * bracket-matching:    Free vehicle chains shall be arranged to look like ..<..<..>..<..>..>..
			 *
			 * Note: Old savegames might contain chains which do not comply with these rules, e.g.
			 *   - the front and read parts have invalid orders
			 *   - different engine types might be combined
			 *   - there might be different amounts of front and rear parts.
			 *
			 * Note: The multiheaded parts need to be matched exactly like they are matched on the server, else desyncs will occur.
			 *   This is why two matching strategies are needed.
			 */

			bool sequential_matching = v->IsFrontEngine();

			for (Train *u = v; u != NULL; u = u->GetNextVehicle()) {
				if (u->other_multiheaded_part != NULL) continue; // we already linked this one

				if (u->IsMultiheaded()) {
					if (!u->IsEngine()) {
						/* we got a rear car without a front car. We will convert it to a front one */
						u->SetEngine();
						u->spritenum--;
					}

					/* Find a matching back part */
					EngineID eid = u->engine_type;
					Train *w;
					if (sequential_matching) {
						for (w = u->GetNextVehicle(); w != NULL; w = w->GetNextVehicle()) {
							if (w->engine_type != eid || w->other_multiheaded_part != NULL || !w->IsMultiheaded()) continue;

							/* we found a car to partner with this engine. Now we will make sure it face the right way */
							if (w->IsEngine()) {
								w->ClearEngine();
								w->spritenum++;
							}
							break;
						}
					} else {
						uint stack_pos = 0;
						for (w = u->GetNextVehicle(); w != NULL; w = w->GetNextVehicle()) {
							if (w->engine_type != eid || w->other_multiheaded_part != NULL || !w->IsMultiheaded()) continue;

							if (w->IsEngine()) {
								stack_pos++;
							} else {
								if (stack_pos == 0) break;
								stack_pos--;
							}
						}
					}

					if (w != NULL) {
						w->other_multiheaded_part = u;
						u->other_multiheaded_part = w;
					} else {
						/* we got a front car and no rear cars. We will fake this one for forget that it should have been multiheaded */
						u->ClearMultiheaded();
					}
				}
			}
		}
	}
}