Add: filter for "engines only" in build train window (#8733)

src/build_vehicle_gui.cpp

@@ -86,8 +86,9 @@ static const NWidgetPart _nested_build_vehicle_widgets[] = {
 };
 
 /** Special cargo filter criteria */
-static const CargoID CF_ANY  = CT_NO_REFIT; ///< Show all vehicles independent of carried cargo (i.e. no filtering)
-static const CargoID CF_NONE = CT_INVALID;  ///< Show only vehicles which do not carry cargo (e.g. train engines)
+static const CargoID CF_ANY     = CT_NO_REFIT;   ///< Show all vehicles independent of carried cargo (i.e. no filtering)
+static const CargoID CF_NONE    = CT_INVALID;    ///< Show only vehicles which do not carry cargo (e.g. train engines)
+static const CargoID CF_ENGINES = CT_AUTO_REFIT; ///< Show only engines (for rail vehicles only)
 
 bool _engine_sort_direction; ///< \c false = descending, \c true = ascending.
 byte _engine_sort_last_criteria[]       = {0, 0, 0, 0};                 ///< Last set sort criteria, for each vehicle type.
@@ -531,16 +532,21 @@ const StringID _engine_sort_listing[][12] = {{
 	INVALID_STRING_ID
 }};
 
-/** Cargo filter functions */
-static bool CDECL CargoFilter(const EngineID *eid, const CargoID cid)
+/** Filters vehicles by cargo and engine (in case of rail vehicle). */
+static bool CDECL CargoAndEngineFilter(const EngineID *eid, const CargoID cid)
 {
-	if (cid == CF_ANY) return true;
-	CargoTypes refit_mask = GetUnionOfArticulatedRefitMasks(*eid, true) & _standard_cargo_mask;
-	return (cid == CF_NONE ? refit_mask == 0 : HasBit(refit_mask, cid));
+	if (cid == CF_ANY) {
+		return true;
+	} else if (cid == CF_ENGINES) {
+		return Engine::Get(*eid)->GetPower() != 0;
+	} else {
+		CargoTypes refit_mask = GetUnionOfArticulatedRefitMasks(*eid, true) & _standard_cargo_mask;
+		return (cid == CF_NONE ? refit_mask == 0 : HasBit(refit_mask, cid));
+	}
 }
 
 static GUIEngineList::FilterFunction * const _filter_funcs[] = {
-	&CargoFilter,
+	&CargoAndEngineFilter,
 };
 
 static int DrawCargoCapacityInfo(int left, int right, int y, EngineID engine, TestedEngineDetails &te)
@@ -1046,8 +1052,8 @@ struct BuildVehicleWindow : Window {
 	EngineID sel_engine;                        ///< Currently selected engine, or #INVALID_ENGINE
 	EngineID rename_engine;                     ///< Engine being renamed.
 	GUIEngineList eng_list;
-	CargoID cargo_filter[NUM_CARGO + 2];        ///< Available cargo filters; CargoID or CF_ANY or CF_NONE
-	StringID cargo_filter_texts[NUM_CARGO + 3]; ///< Texts for filter_cargo, terminated by INVALID_STRING_ID
+	CargoID cargo_filter[NUM_CARGO + 3];        ///< Available cargo filters; CargoID or CF_ANY or CF_NONE or CF_ENGINES
+	StringID cargo_filter_texts[NUM_CARGO + 4]; ///< Texts for filter_cargo, terminated by INVALID_STRING_ID
 	byte cargo_filter_criteria;                 ///< Selected cargo filter
 	int details_height;                         ///< Minimal needed height of the details panels (found so far).
 	Scrollbar *vscroll;
@@ -1165,9 +1171,15 @@ struct BuildVehicleWindow : Window {
 		this->cargo_filter_texts[filter_items] = STR_PURCHASE_INFO_ALL_TYPES;
 		filter_items++;
 
-		/* Add item for vehicles not carrying anything, e.g. train engines.
-		 * This could also be useful for eyecandy vehicles of other types, but is likely too confusing for joe, */
+		/* Specific filters for trains. */
 		if (this->vehicle_type == VEH_TRAIN) {
+			/* Add item for locomotives only in case of trains. */
+			this->cargo_filter[filter_items] = CF_ENGINES;
+			this->cargo_filter_texts[filter_items] = STR_PURCHASE_INFO_ENGINES_ONLY;
+			filter_items++;
+
+			/* Add item for vehicles not carrying anything, e.g. train engines.
+			 * This could also be useful for eyecandy vehicles of other types, but is likely too confusing for joe, */
 			this->cargo_filter[filter_items] = CF_NONE;
 			this->cargo_filter_texts[filter_items] = STR_PURCHASE_INFO_NONE;
 			filter_items++;
@@ -1254,7 +1266,7 @@ struct BuildVehicleWindow : Window {
 	bool FilterSingleEngine(EngineID eid)
 	{
 		CargoID filter_type = this->cargo_filter[this->cargo_filter_criteria];
-		return (filter_type == CF_ANY || CargoFilter(&eid, filter_type));
+		return CargoAndEngineFilter(&eid, filter_type);
 	}
 
 	/* Figure out what train EngineIDs to put in the list */

src/lang/english.txt

@@ -3566,6 +3566,7 @@ STR_PURCHASE_INFO_PWAGPOWER_PWAGWEIGHT                          :{BLACK}Powered
 STR_PURCHASE_INFO_REFITTABLE_TO                                 :{BLACK}Refittable to: {GOLD}{STRING2}
 STR_PURCHASE_INFO_ALL_TYPES                                     :All cargo types
 STR_PURCHASE_INFO_NONE                                          :None
+STR_PURCHASE_INFO_ENGINES_ONLY                                  :Engines only
 STR_PURCHASE_INFO_ALL_BUT                                       :All but {CARGO_LIST}
 STR_PURCHASE_INFO_MAX_TE                                        :{BLACK}Max. Tractive Effort: {GOLD}{FORCE}
 STR_PURCHASE_INFO_AIRCRAFT_RANGE                                :{BLACK}Range: {GOLD}{COMMA} tiles