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Add RandomNumberGenerator::rand_weighted method
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joaoh82 committed Feb 29, 2024
1 parent bb6b06c commit 88df5ea
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Showing 5 changed files with 48 additions and 0 deletions.
1 change: 1 addition & 0 deletions core/math/random_number_generator.cpp
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Expand Up @@ -42,6 +42,7 @@ void RandomNumberGenerator::_bind_methods() {
ClassDB::bind_method(D_METHOD("randfn", "mean", "deviation"), &RandomNumberGenerator::randfn, DEFVAL(0.0), DEFVAL(1.0));
ClassDB::bind_method(D_METHOD("randf_range", "from", "to"), &RandomNumberGenerator::randf_range);
ClassDB::bind_method(D_METHOD("randi_range", "from", "to"), &RandomNumberGenerator::randi_range);
ClassDB::bind_method(D_METHOD("rand_weighted", "weights"), &RandomNumberGenerator::rand_weighted);
ClassDB::bind_method(D_METHOD("randomize"), &RandomNumberGenerator::randomize);

ADD_PROPERTY(PropertyInfo(Variant::INT, "seed"), "set_seed", "get_seed");
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2 changes: 2 additions & 0 deletions core/math/random_number_generator.h
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Expand Up @@ -57,6 +57,8 @@ class RandomNumberGenerator : public RefCounted {
_FORCE_INLINE_ real_t randfn(real_t p_mean = 0.0, real_t p_deviation = 1.0) { return randbase.randfn(p_mean, p_deviation); }
_FORCE_INLINE_ int randi_range(int p_from, int p_to) { return randbase.random(p_from, p_to); }

_FORCE_INLINE_ int rand_weighted(const Vector<float> &p_weights) { return randbase.rand_weighted(p_weights); }

RandomNumberGenerator() { randbase.randomize(); }
};

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21 changes: 21 additions & 0 deletions core/math/random_pcg.cpp
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Expand Up @@ -31,6 +31,7 @@
#include "random_pcg.h"

#include "core/os/os.h"
#include "core/templates/vector.h"

RandomPCG::RandomPCG(uint64_t p_seed, uint64_t p_inc) :
pcg(),
Expand All @@ -42,6 +43,26 @@ void RandomPCG::randomize() {
seed(((uint64_t)OS::get_singleton()->get_unix_time() + OS::get_singleton()->get_ticks_usec()) * pcg.state + PCG_DEFAULT_INC_64);
}

int RandomPCG::rand_weighted(const Vector<float> &p_weights) {
ERR_FAIL_COND_V_MSG(p_weights.is_empty(), -1, "Weights array is empty.");
int64_t weights_size = p_weights.size();
const float *weights = p_weights.ptr();
float weights_sum = 0.0;
for (int64_t i = 0; i < weights_size; ++i) {
weights_sum += weights[i];
}

float remaining_distance = Math::randf() * weights_sum;
for (int64_t i = 0; i < weights_size; ++i) {
remaining_distance -= weights[i];
if (remaining_distance < 0) {
return i;
}
}

return -1;
}

double RandomPCG::random(double p_from, double p_to) {
return randd() * (p_to - p_from) + p_from;
}
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5 changes: 5 additions & 0 deletions core/math/random_pcg.h
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Expand Up @@ -59,6 +59,9 @@ static int __bsr_clz32(uint32_t x) {
#define LDEXPF(s, e) ldexp(s, e)
#endif

template <class T>
class Vector;

class RandomPCG {
pcg32_random_t pcg;
uint64_t current_seed = 0; // The seed the current generator state started from.
Expand Down Expand Up @@ -87,6 +90,8 @@ class RandomPCG {
return pcg32_boundedrand_r(&pcg, bounds);
}

int rand_weighted(const Vector<float> &p_weights);

// Obtaining floating point numbers in [0, 1] range with "good enough" uniformity.
// These functions sample the output of rand() as the fraction part of an infinite binary number,
// with some tricks applied to reduce ops and branching:
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19 changes: 19 additions & 0 deletions doc/classes/RandomNumberGenerator.xml
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Expand Up @@ -17,6 +17,25 @@
<link title="Random number generation">$DOCS_URL/tutorials/math/random_number_generation.html</link>
</tutorials>
<methods>
<method name="rand_weighted">
<return type="int" />
<param index="0" name="weights" type="PackedFloat32Array" />
<description>
Returns a random index with non-uniform weights. Prints an error and returns [code]-1[/code] if the array is empty.
[codeblocks]
[gdscript]
var rnd = RandomNumberGenerator.new()

var my_array = ["one", "two", "three, "four"]
var weights = PackedFloat32Array([0.5, 1, 1, 2])

# Prints one of the four elements in `my_array`.
# It is more likely to print "four", and less likely to print "two".
print(my_array[rng.rand_weighted(weights)])
[/gdscript]
[/codeblocks]
</description>
</method>
<method name="randf">
<return type="float" />
<description>
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