Add "very reduced" mode to the vehicle breakdowns setting

src/lang/extra/english.txt

@@ -2026,3 +2026,5 @@ STR_ERROR_TUNNEL_DISALLOWED_ROAD                                :{WHITE}Tunnels
 STR_REFIT_SHIP_PART_DROPDOWN_TOOLTIP                            :{BLACK}Select which part of this ship to refit
 STR_REFIT_WHOLE_SHIP                                            :Whole ship
 STR_REFIT_SHIP_PART                                             :Part {NUM}
+
+STR_VERY_REDUCED                                                :Very reduced

src/table/settings/settings.ini

@@ -164,6 +164,14 @@ static const SettingDescEnumEntry _viewport_map_modes[] = {
 { 0, STR_NULL }
 };
 
+static const SettingDescEnumEntry _vehicle_breakdown_modes[] = {
+{ 0,               STR_DISASTER_NONE },
+{ 64,              STR_VERY_REDUCED },
+{ 1,               STR_DISASTER_REDUCED },
+{ 2,               STR_DISASTER_NORMAL },
+{ 0, STR_NULL }
+};
+
 /* Some settings do not need to be synchronised when playing in multiplayer.
  * These include for example the GUI settings and will not be saved with the
  * savegame.
@@ -365,18 +373,14 @@ def      = 0
 min      = 0
 max      = 2
 
-[SDT_VAR]
+[SDT_ENUM]
 var      = difficulty.vehicle_breakdowns
 type     = SLE_UINT8
 from     = SLV_97
-flags    = SF_GUI_DROPDOWN
 def      = 1
-min      = 0
-max      = 2
-interval = 1
+enumlist = _vehicle_breakdown_modes
 str      = STR_CONFIG_SETTING_VEHICLE_BREAKDOWNS
 strhelp  = STR_CONFIG_SETTING_VEHICLE_BREAKDOWNS_HELPTEXT
-strval   = STR_DISASTER_NONE
 cat      = SC_BASIC
 
 [SDT_VAR]

src/vehicle.cpp

@@ -2175,6 +2175,9 @@ void CheckVehicleBreakdown(Vehicle *v)
 	}
 	/**
 	 * Chance is (1 - reliability) * breakdown_setting * breakdown_chance / 10.
+	 * breakdown_setting is scaled by 2 to support a value of 1/2 (setting value 64).
+	 * Chance is (1 - reliability) * breakdown_scaling_x2 * breakdown_chance / 20.
+	 *
 	 * At 90% reliabilty, normal setting (2) and average breakdown_chance (128),
 	 * a vehicle will break down (on average) every 100 days.
 	 * This *should* mean that vehicles break down about as often as (or a little less than) they used to.
@@ -2182,7 +2185,8 @@ void CheckVehicleBreakdown(Vehicle *v)
 	 * their impact will be significantly less.
 	 */
 	uint32 r1 = Random();
-	if ((uint32) (0xffff - v->reliability) * _settings_game.difficulty.vehicle_breakdowns * chance > GB(r1, 0, 24) * 10) {
+	uint32 breakdown_scaling_x2 = (_settings_game.difficulty.vehicle_breakdowns == 64) ? 1 : (_settings_game.difficulty.vehicle_breakdowns * 2);
+	if ((uint32) (0xffff - v->reliability) * breakdown_scaling_x2 * chance > GB(r1, 0, 24) * 10 * 2) {
 		uint32 r2 = Random();
 		v->breakdown_ctr = GB(r1, 24, 6) + 0xF;
 		if (v->type == VEH_TRAIN) SetBit(Train::From(v)->First()->flags, VRF_CONSIST_BREAKDOWN);