Add town growth setting, rate depends on proportion of transported cargo

src/lang/english.txt

@@ -1727,6 +1727,8 @@ STR_CONFIG_SETTING_ZOOM_LVL_OUT_64X                             :64x
 STR_CONFIG_SETTING_ZOOM_LVL_OUT_128X                            :128x
 STR_CONFIG_SETTING_TOWN_GROWTH                                  :Town growth speed: {STRING2}
 STR_CONFIG_SETTING_TOWN_GROWTH_HELPTEXT                         :Speed of town growth
+STR_CONFIG_SETTING_TOWN_GROWTH_CARGO_TRANSPORTED                :Town growth speed depends on transported cargo: {STRING2}
+STR_CONFIG_SETTING_TOWN_GROWTH_CARGO_TRANSPORTED_HELPTEXT       :Percentage of town growth speed which depends on proportion of town cargo transported in the last month
 STR_CONFIG_SETTING_TOWN_GROWTH_EXTREME_SLOW                     :Extremely slow
 STR_CONFIG_SETTING_TOWN_GROWTH_VERY_SLOW                        :Very slow
 STR_CONFIG_SETTING_TOWN_GROWTH_NONE                             :None

src/settings_gui.cpp

@@ -1771,6 +1771,7 @@ static SettingsContainer &GetSettingsTree()
 			SettingsPage *towns = environment->Add(new SettingsPage(STR_CONFIG_SETTING_ENVIRONMENT_TOWNS));
 			{
 				towns->Add(new SettingEntry("economy.town_growth_rate"));
+				towns->Add(new SettingEntry("economy.town_growth_cargo_transported"));
 				towns->Add(new SettingEntry("economy.allow_town_roads"));
 				towns->Add(new SettingEntry("economy.allow_town_level_crossings"));
 				towns->Add(new SettingEntry("economy.found_town"));

src/settings_type.h

@@ -541,6 +541,7 @@ struct EconomySettings {
 	bool   mod_road_rebuild;                 ///< roadworks remove unnecessary RoadBits
 	bool   multiple_industry_per_town;       ///< allow many industries of the same type per town
 	int8   town_growth_rate;                 ///< town growth rate
+	uint8  town_growth_cargo_transported;    ///< percentage of town growth rate which depends on proportion of transported cargo in the last month
 	uint8  larger_towns;                     ///< the number of cities to build. These start off larger and grow twice as fast
 	uint8  initial_city_size;                ///< multiplier for the initial size of the cities compared to towns
 	TownLayoutByte town_layout;              ///< select town layout, @see TownLayout

src/table/settings.ini

@@ -1751,6 +1751,20 @@ strhelp  = STR_CONFIG_SETTING_TOWN_GROWTH_HELPTEXT
 strval   = STR_CONFIG_SETTING_TOWN_GROWTH_EXTREME_SLOW
 orderproc = OrderTownGrowthRate
 
+[SDT_VAR]
+base     = GameSettings
+var      = economy.town_growth_cargo_transported
+type     = SLE_UINT8
+def      = 0
+min      = 0
+max      = 100
+interval = 10
+str      = STR_CONFIG_SETTING_TOWN_GROWTH_CARGO_TRANSPORTED
+strhelp  = STR_CONFIG_SETTING_TOWN_GROWTH_CARGO_TRANSPORTED_HELPTEXT
+strval   = STR_CONFIG_SETTING_PERCENTAGE
+cat      = SC_EXPERT
+patxname = ""town_growth.economy.town_growth_cargo_transported""
+
 [SDT_VAR]
 base     = GameSettings
 var      = economy.larger_towns

src/town_cmd.cpp

@@ -3405,6 +3405,19 @@ static void UpdateTownGrowRate(Town *t)
 	}
 	if (t->larger_town) m /= 2;
 
+	if (_settings_game.economy.town_growth_cargo_transported > 0) {
+		uint32 inverse_m = UINT32_MAX / m;
+		auto calculate_cargo_ratio_fix15 = [](const TransportedCargoStat<uint32> &stat) -> uint32 {
+			return stat.old_max ? ((uint64) (stat.old_act << 15)) / stat.old_max : 1 << 15;
+		};
+		uint32 cargo_ratio_fix16 = calculate_cargo_ratio_fix15(t->supplied[CT_PASSENGERS]) + calculate_cargo_ratio_fix15(t->supplied[CT_MAIL]);
+		uint32 cargo_dependant_part = (((uint64) cargo_ratio_fix16) * ((uint64) inverse_m) * _settings_game.economy.town_growth_cargo_transported) >> 16;
+		uint32 non_cargo_dependant_part = ((uint64) inverse_m) * (100 - _settings_game.economy.town_growth_cargo_transported);
+		uint32 total = (cargo_dependant_part + non_cargo_dependant_part);
+		if (total == 0) return;
+		m = ((uint64) UINT32_MAX * 100) / total;
+	}
+
 	t->growth_rate = m / (t->cache.num_houses / 50 + 1);
 	t->grow_counter = min(t->growth_rate, t->grow_counter);