Make seeded runs reproducible across the RNG families the simulation uses

code/asteroid/asteroid.cpp

@@ -40,6 +40,7 @@
 #include "ship/shipfx.h"
 #include "ship/shiphit.h"
 #include "stats/scoring.h"
+#include "utils/Random.h"
 #include "weapon/weapon.h"
 
 #include <algorithm>
@@ -1900,8 +1901,8 @@ static void asteroid_maybe_break_up(object *pasteroid_obj)
 								roids_to_create.push_back(split->asteroid_type);
 					}
 
-					std::random_device rd;
-					std::shuffle(roids_to_create.begin(), roids_to_create.end(), std::mt19937(rd()));
+					std::mt19937 shuffle_rng(util::Random::shuffle_seed());
+					std::shuffle(roids_to_create.begin(), roids_to_create.end(), shuffle_rng);
 
 					size_t total_roids = roids_to_create.size();
 					for (size_t i = 0; i < total_roids; i++) {

code/parse/sexp.cpp

@@ -15506,8 +15506,8 @@ void sexp_send_builtin_message(int n) {
 	}
 
 	if (shuffle) {
-		std::random_device rd;
-		std::shuffle(sender_names.begin(), sender_names.end(), std::mt19937(rd()));
+		std::mt19937 rng(util::Random::shuffle_seed());
+		std::shuffle(sender_names.begin(), sender_names.end(), rng);
 	}
 
 	auto subject = have_subject ? subject_entry->shipp() : nullptr;

code/ship/ship.cpp

@@ -11158,8 +11158,8 @@ static void ship_set_default_weapons(ship *shipp, ship_info *sip)
 		for (int fp = 0; fp < pm->gun_banks[i].num_slots; fp++) {
 			fpi.push_back(fp);
 		}
-		std::random_device rd;
-		std::shuffle(fpi.begin(), fpi.end(), std::mt19937(rd()));
+		std::mt19937 rng(util::Random::shuffle_seed());
+		std::shuffle(fpi.begin(), fpi.end(), rng);
 	}
 
 	swp->num_secondary_banks = sip->num_secondary_banks;
@@ -11745,8 +11745,8 @@ static void ship_model_change(int n, int ship_type)
 		for (int fp = 0; fp < pm->gun_banks[bank_i].num_slots; fp++) {
 			fpi.push_back(fp);
 		}
-		std::random_device rd;
-		std::shuffle(fpi.begin(), fpi.end(), std::mt19937(rd()));
+		std::mt19937 rng(util::Random::shuffle_seed());
+		std::shuffle(fpi.begin(), fpi.end(), rng);
 	}
 
 	model_delete_instance(sp->model_instance_num);
@@ -13496,8 +13496,8 @@ int ship_fire_primary(object * obj, int force, bool rollback_shot)
 					for (int fp = 0; fp < num_slots; fp++) {
 						fpi.push_back(fp);
 					}
-					std::random_device rd;
-					std::shuffle(fpi.begin(), fpi.end(), std::mt19937(rd()));
+					std::mt19937 rng(util::Random::shuffle_seed());
+					std::shuffle(fpi.begin(), fpi.end(), rng);
 				}
 
 				// The energy-consumption code executes even for ballistic primaries, because
@@ -13783,26 +13783,26 @@ int ship_fire_primary(object * obj, int force, bool rollback_shot)
 				switch (firing_pattern) {
 					case FiringPattern::RANDOM_EXHAUSTIVE: {
 						if (num_slots < (swp->primary_firepoint_next_to_fire_index[bank_to_fire] + point_count)) {
-							std::random_device rd;
-							std::shuffle(&swp->primary_firepoint_indices[bank_to_fire][0], &swp->primary_firepoint_indices[bank_to_fire][swp->primary_firepoint_next_to_fire_index[bank_to_fire]-1], std::mt19937(rd())); //NOLINT
+							std::mt19937 rng(util::Random::shuffle_seed());
+							std::shuffle(&swp->primary_firepoint_indices[bank_to_fire][0], &swp->primary_firepoint_indices[bank_to_fire][swp->primary_firepoint_next_to_fire_index[bank_to_fire]-1], rng); //NOLINT
 						} else if (swp->primary_firepoint_next_to_fire_index[bank_to_fire] < point_count) {
-							std::random_device rd;
-							std::shuffle(swp->primary_firepoint_indices[bank_to_fire].begin(), swp->primary_firepoint_indices[bank_to_fire].end(), std::mt19937(rd()));
+							std::mt19937 rng(util::Random::shuffle_seed());
+							std::shuffle(swp->primary_firepoint_indices[bank_to_fire].begin(), swp->primary_firepoint_indices[bank_to_fire].end(), rng);
 						}
 						break;
 					}
 					case FiringPattern::RANDOM_NONREPEATING: {
 						int shuffle_start = MIN(point_count, num_slots - point_count);
-						std::random_device rd;
+						std::mt19937 rng(util::Random::shuffle_seed());
 						auto middle_iterator = swp->primary_firepoint_indices[bank_to_fire].begin();
 						std::advance(middle_iterator, shuffle_start);
-						std::shuffle(middle_iterator, swp->primary_firepoint_indices[bank_to_fire].end(), std::mt19937(rd()));
+						std::shuffle(middle_iterator, swp->primary_firepoint_indices[bank_to_fire].end(), rng);
 						std::rotate(swp->primary_firepoint_indices[bank_to_fire].begin(), middle_iterator, swp->primary_firepoint_indices[bank_to_fire].end());
 						break;
 					}
 					case FiringPattern::RANDOM_REPEATING: {
-						std::random_device rd;
-						std::shuffle(swp->primary_firepoint_indices[bank_to_fire].begin(), swp->primary_firepoint_indices[bank_to_fire].end(), std::mt19937(rd()));
+						std::mt19937 rng(util::Random::shuffle_seed());
+						std::shuffle(swp->primary_firepoint_indices[bank_to_fire].begin(), swp->primary_firepoint_indices[bank_to_fire].end(), rng);
 						break;
 					}
 					default: {

code/utils/Random.cpp

@@ -7,6 +7,9 @@
 #include <cstdint>
 
 #include "globalincs/pstypes.h"
+#include "globalincs/systemvars.h"
+
+extern bool Cmdline_reuse_rng_seed;
 
 namespace util {
 
@@ -54,6 +57,23 @@ void Random::seed(unsigned int val)
 	SCP_rng.seed(val);
 }
 
+void Random::seed_ranges(unsigned int val)
+{
+	Assert(val > 0);
+	seeder.seed(val);
+}
+
+unsigned int Random::shuffle_seed()
+{
+	// multiplayer seeds the main RNG from the netgame and ignores -seed, so don't draw from it there
+	if (Cmdline_reuse_rng_seed && !(Game_mode & GM_MULTIPLAYER)) {
+		return static_cast<unsigned int>(next());
+	}
+
+	std::random_device rd;
+	return rd();
+}
+
 int Random::next()
 {
 	return SCP_rng.next();

code/utils/Random.h

@@ -10,6 +10,12 @@ class Random {
 
 	// seed(1) will result in RNG using its default seed
 	static void seed(unsigned int val);
+	// Seed independent RandomRange instances created after this call.
+	static void seed_ranges(unsigned int val);
+
+	// Seed for a one-off generator handed to std::shuffle. With -seed (outside multiplayer) it is drawn
+	// from the main RNG, so shuffles repeat from run to run; otherwise it comes from the OS, as before.
+	static unsigned int shuffle_seed();
 
 	// return a value in [0, MAX_VALUE]
 	static int next(); // uniformly between 0 and MAX_VALUE

freespace2/freespace.cpp

@@ -1882,8 +1882,15 @@ void game_init()
 
 	Game_current_mission_filename[0] = 0;
 
-	// seed the random number generator
-	Random::seed(static_cast<unsigned int>(time(nullptr)));
+	// Seed both game RNG families from the command line when reproducible runs
+	// are requested. RandomRange instances must receive their seed before the
+	// tables are parsed during game initialization.
+	if (Cmdline_reuse_rng_seed) {
+		Random::seed(Cmdline_rng_seed);
+		Random::seed_ranges(Cmdline_rng_seed);
+	} else {
+		Random::seed(static_cast<unsigned int>(time(nullptr)));
+	}
 
 	ImGui::CreateContext();