From 6782d81142929f96eb3ea871d1f8ef3eef69f65e Mon Sep 17 00:00:00 2001
From: frosch <frosch@openttd.org>
Date: Sat, 20 Sep 2008 10:53:08 +0000
Subject: [PATCH] (svn r14362) -Fix (r3403): Also connect multiheaded parts in
 free wagon chains.

---
 src/train_cmd.cpp | 52 +++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 44 insertions(+), 8 deletions(-)

diff --git a/src/train_cmd.cpp b/src/train_cmd.cpp
index c87e082769..18881c7195 100644
--- a/src/train_cmd.cpp
+++ b/src/train_cmd.cpp
@@ -4434,8 +4434,23 @@ void ConnectMultiheadedTrains()
 	}
 
 	FOR_ALL_VEHICLES(v) {
-		if (v->type == VEH_TRAIN && IsFrontEngine(v)) {
-			for (Vehicle *u = v; u != NULL; u = u->Next()) {
+		if (v->type == VEH_TRAIN) {
+			/* 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 = IsFrontEngine(v);
+
+			for (Vehicle *u = v; u != NULL; u = GetNextVehicle(u)) {
 				if (u->u.rail.other_multiheaded_part != NULL) continue; // we already linked this one
 
 				if (IsMultiheaded(u)) {
@@ -4445,14 +4460,35 @@ void ConnectMultiheadedTrains()
 						u->spritenum--;
 					}
 
+					/* Find a matching back part */
+					EngineID eid = u->engine_type;
 					Vehicle *w;
-					for (w = u->Next(); w != NULL && (w->engine_type != u->engine_type || w->u.rail.other_multiheaded_part != NULL); w = GetNextVehicle(w)) {}
-					if (w != NULL) {
-						/* we found a car to partner with this engine. Now we will make sure it face the right way */
-						if (IsTrainEngine(w)) {
-							ClearTrainEngine(w);
-							w->spritenum++;
+					if (sequential_matching) {
+						for (w = GetNextVehicle(u); w != NULL; w = GetNextVehicle(w)) {
+							if (w->engine_type != eid || w->u.rail.other_multiheaded_part != NULL || !IsMultiheaded(w)) continue;
+
+							/* we found a car to partner with this engine. Now we will make sure it face the right way */
+							if (IsTrainEngine(w)) {
+								ClearTrainEngine(w);
+								w->spritenum++;
+							}
+							break;
 						}
+					} else {
+						uint stack_pos = 0;
+						for (w = GetNextVehicle(u); w != NULL; w = GetNextVehicle(w)) {
+							if (w->engine_type != eid || w->u.rail.other_multiheaded_part != NULL || !IsMultiheaded(w)) continue;
+
+							if (IsTrainEngine(w)) {
+								stack_pos++;
+							} else {
+								if (stack_pos == 0) break;
+								stack_pos--;
+							}
+						}
+					}
+
+					if (w != NULL) {
 						w->u.rail.other_multiheaded_part = u;
 						u->u.rail.other_multiheaded_part = w;
 					} else {