engine: Control experimental PRNG option via game classification

This removes the use_prng Advanced Preferences option and replaces it
with a new possible value for randomness in the game classification
class, "biased". It also changes the enumeration label for it in the
GUI2 campaign dialog.

It also makes it so the synced RNG is automatically used with this
option as well as the existing deterministic randomness option ("save
random seed").

data/advanced_preferences.cfg

@@ -167,15 +167,5 @@
     default=no
 [/advanced_preference]
 
-[advanced_preference]
-    field=use_prng
-    name= _ "Use experimental PRNG combat"
-    description= _ "Enables more determinstic chance-to-hit calculations. This is an experimental feature designed to bring the observed hit/miss rate more in line with the displayed percentages.
-
-Note: this option only affects singleplayer, and the ‘Save random seed’ option must also be enabled when creating a game for this to work."
-    type=boolean
-    default=no
-[/advanced_preference]
-
 #ifdef __UNUSED__
 #endif

data/gui/window/campaign_dialog.cfg

@@ -323,10 +323,10 @@
 						label = _ "Predictable RNG"
 						details = "<span color='#777'>" + _ "Combat outcomes remain constant when reloading" + "</span>"
 					[/option]
-					#[option]
-					#	label = _ "Biased RNG"
-					#	details = "<span color='#777'>" + _ "Combat outcomes are more in line with displayed probabilities and unaffected by reloading" + "</span>"
-					#[/option]
+					[option]
+						label = _ "Biased RNG (experimental)"
+						details = "<span color='#777'>" + _ "Combat outcomes are more in line with displayed probabilities and unaffected by reloading" + "</span>"
+					[/option]
 				[/menu_button]
 			[/column]
 

src/actions/attack.cpp

@@ -25,6 +25,7 @@
 #include "ai/lua/aspect_advancements.hpp"
 #include "formula/callable_objects.hpp"
 #include "formula/formula.hpp"
+#include "game_classification.hpp"
 #include "game_config.hpp"
 #include "game_data.hpp"
 #include "game_events/pump.hpp"
@@ -861,7 +862,7 @@ attack::attack(const map_location& attacker,
 	, OOS_error_(false)
 
 	//new experimental prng mode.
-	, use_prng_(preferences::get("use_prng") == "yes" && randomness::generator->is_networked() == false)
+	, use_prng_(resources::classification->random_mode == "biased" && randomness::generator->is_networked() == false)
 {
 	if(use_prng_) {
 		std::cerr << "Using experimental PRNG for combat\n";

src/game_initialization/singleplayer.cpp

@@ -61,8 +61,16 @@ bool select_campaign(saved_game& state, jump_to_campaign_info jump_to_campaign)
 				return false;
 			}
 
-			if(dlg.get_rng_mode() != gui2::dialogs::campaign_selection::RNG_DEFAULT) {
-				random_mode = "deterministic";
+			switch(dlg.get_rng_mode()) {
+				case gui2::dialogs::campaign_selection::RNG_DEFAULT:
+					random_mode = "";
+					break;
+				case gui2::dialogs::campaign_selection::RNG_SAVE_SEED:
+					random_mode = "deterministic";
+					break;
+				case gui2::dialogs::campaign_selection::RNG_BIASED:
+					random_mode = "biased";
+					break;
 			}
 
 			difficulty = dlg.get_difficulty();

src/gui/dialogs/campaign_selection.cpp

@@ -465,7 +465,7 @@ void campaign_selection::post_show(window& window)
 	}
 
 
-	rng_mode_ = RNG_MODE(utils::clamp<unsigned>(find_widget<menu_button>(&window, "rng_menu", false).get_value(), RNG_DEFAULT, RNG_USE_PRNG));
+	rng_mode_ = RNG_MODE(utils::clamp<unsigned>(find_widget<menu_button>(&window, "rng_menu", false).get_value(), RNG_DEFAULT, RNG_BIASED));
 
 	preferences::set_modifications(engine_.active_mods(), false);
 }

src/gui/dialogs/campaign_selection.hpp

@@ -40,7 +40,7 @@ public:
 	{
 		RNG_DEFAULT,
 		RNG_SAVE_SEED,
-		RNG_USE_PRNG,
+		RNG_BIASED,
 	};
 
 	explicit campaign_selection(ng::create_engine& eng)

src/synced_context.cpp

@@ -233,7 +233,7 @@ void synced_context::send_user_choice()
 std::shared_ptr<randomness::rng> synced_context::get_rng_for_action()
 {
 	const std::string& mode = resources::classification->random_mode;
-	if(mode == "deterministic") {
+	if(mode == "deterministic" || mode == "biased") {
 		return std::make_shared<randomness::rng_deterministic>(resources::gamedata->rng());
 	} else {
 		return std::make_shared<randomness::synced_rng>(generate_random_seed);