Add ability to restore RandomNumberGenerator state

core/math/random_number_generator.cpp

@@ -36,6 +36,9 @@ void RandomNumberGenerator::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_seed", "seed"), &RandomNumberGenerator::set_seed);
 	ClassDB::bind_method(D_METHOD("get_seed"), &RandomNumberGenerator::get_seed);
 
+	ClassDB::bind_method(D_METHOD("set_state", "state"), &RandomNumberGenerator::set_state);
+	ClassDB::bind_method(D_METHOD("get_state"), &RandomNumberGenerator::get_state);
+
 	ClassDB::bind_method(D_METHOD("randi"), &RandomNumberGenerator::randi);
 	ClassDB::bind_method(D_METHOD("randf"), &RandomNumberGenerator::randf);
 	ClassDB::bind_method(D_METHOD("randfn", "mean", "deviation"), &RandomNumberGenerator::randfn, DEFVAL(0.0), DEFVAL(1.0));
@@ -44,6 +47,8 @@ void RandomNumberGenerator::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("randomize"), &RandomNumberGenerator::randomize);
 
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "seed"), "set_seed", "get_seed");
-	// Default value is non-deterministic, override it for doc generation purposes.
+	ADD_PROPERTY(PropertyInfo(Variant::INT, "state"), "set_state", "get_state");
+	// Default values are non-deterministic, override for doc generation purposes.
 	ADD_PROPERTY_DEFAULT("seed", 0);
+	ADD_PROPERTY_DEFAULT("state", 0);
 }

core/math/random_number_generator.h

@@ -47,6 +47,10 @@ class RandomNumberGenerator : public Reference {
 
 	_FORCE_INLINE_ uint64_t get_seed() { return randbase.get_seed(); }
 
+	_FORCE_INLINE_ void set_state(uint64_t p_state) { randbase.set_state(p_state); }
+
+	_FORCE_INLINE_ uint64_t get_state() const { return randbase.get_state(); }
+
 	_FORCE_INLINE_ void randomize() { randbase.randomize(); }
 
 	_FORCE_INLINE_ uint32_t randi() { return randbase.rand(); }

core/math/random_pcg.h

@@ -61,7 +61,7 @@ static int __bsr_clz32(uint32_t x) {
 
 class RandomPCG {
 	pcg32_random_t pcg;
-	uint64_t current_seed; // seed with this to get the same state
+	uint64_t current_seed; // The seed the current generator state started from.
 	uint64_t current_inc;
 
 public:
@@ -76,6 +76,9 @@ class RandomPCG {
 	}
 	_FORCE_INLINE_ uint64_t get_seed() { return current_seed; }
 
+	_FORCE_INLINE_ void set_state(uint64_t p_state) { pcg.state = p_state; }
+	_FORCE_INLINE_ uint64_t get_state() const { return pcg.state; }
+
 	void randomize();
 	_FORCE_INLINE_ uint32_t rand() {
 		current_seed = pcg.state;

doc/classes/RandomNumberGenerator.xml

@@ -13,6 +13,7 @@
 		    rng.randomize()
 		    var my_random_number = rng.randf_range(-10.0, 10.0)
 		[/codeblock]
+		[b]Note:[/b] The default values of [member seed] and [member state] properties are pseudo-random, and changes when calling [method randomize]. The [code]0[/code] value documented here is a placeholder, and not the actual default seed.
 	</description>
 	<tutorials>
 	</tutorials>
@@ -74,9 +75,27 @@
 	</methods>
 	<members>
 		<member name="seed" type="int" setter="set_seed" getter="get_seed" default="0">
-			The seed used by the random number generator. A given seed will give a reproducible sequence of pseudo-random numbers.
+			Initializes the random number generator state based on the given seed value. A given seed will give a reproducible sequence of pseudo-random numbers.
 			[b]Note:[/b] The RNG does not have an avalanche effect, and can output similar random streams given similar seeds. Consider using a hash function to improve your seed quality if they're sourced externally.
-			[b]Note:[/b] The default value of this property is pseudo-random, and changes when calling [method randomize]. The [code]0[/code] value documented here is a placeholder, and not the actual default seed.
+			[b]Note:[/b] Setting this property produces a side effect of changing the internal [member state], so make sure to initialize the seed [i]before[/i] modifying the [member state]:
+			[codeblock]
+			var rng = RandomNumberGenerator.new()
+			rng.seed = hash("Godot")
+			rng.state = 100 # Restore to some previously saved state.
+			[/codeblock]
+			[b]Warning:[/b] the getter of this property returns the previous [member state], and not the initial seed value, which is going to be fixed in Godot 4.0.
+		</member>
+		<member name="state" type="int" setter="set_state" getter="get_state" default="0">
+			The current state of the random number generator. Save and restore this property to restore the generator to a previous state:
+			[codeblock]
+			var rng = RandomNumberGenerator.new()
+			print(rng.randf())
+			var saved_state = rng.state # Store current state.
+			print(rng.randf()) # Advance internal state.
+			rng.state = saved_state # Restore the state.
+			print(rng.randf()) # Prints the same value as in previous.
+			[/codeblock]
+			[b]Note:[/b] Do not set state to arbitrary values, since the random number generator requires the state to have certain qualities to behave properly. It should only be set to values that came from the state property itself. To initialize the random number generator with arbitrary input, use [member seed] instead.
 		</member>
 	</members>
 	<constants>