Fix biased output of randi_range

4 files changed, 26 insertions, 5 deletions

diff --git a/core/math/random_number_generator.h b/core/math/random_number_generator.h
index 416e52ea87..2e7941b345 100644
--- a/core/math/random_number_generator.h
+++ b/core/math/random_number_generator.h
@@ -58,12 +58,18 @@ public:
 	_FORCE_INLINE_ real_t randfn(real_t mean = 0.0, real_t deviation = 1.0) { return randbase.randfn(mean, deviation); }
 
 	_FORCE_INLINE_ int randi_range(int from, int to) {
-		unsigned int ret = randbase.rand();
-		if (to < from) {
-			return ret % (from - to + 1) + to;
-		} else {
-			return ret % (to - from + 1) + from;
+		int range;
+		int min;
+		if (to > from) {
+			range = to - from + 1;
+			min = from;
+		} else if (to < from) {
+			range = from - to + 1;
+			min = to;
+		} else { // from == to
+			return from;
 		}
+		return randbase.rand(range) + min;
 	}
 
 	RandomNumberGenerator() {}
diff --git a/core/math/random_pcg.h b/core/math/random_pcg.h
index 09b13ab74d..dfdae53eed 100644
--- a/core/math/random_pcg.h
+++ b/core/math/random_pcg.h
@@ -81,6 +81,10 @@ public:
 		current_seed = pcg.state;
 		return pcg32_random_r(&pcg);
 	}
+	_FORCE_INLINE_ uint32_t rand(uint32_t bounds) {
+		current_seed = pcg.state;
+		return pcg32_boundedrand_r(&pcg, bounds);
+	}
 
 	// 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,
diff --git a/thirdparty/misc/pcg.cpp b/thirdparty/misc/pcg.cpp
index c421e16f89..5f4bf40460 100644
--- a/thirdparty/misc/pcg.cpp
+++ b/thirdparty/misc/pcg.cpp
@@ -23,3 +23,13 @@ void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq)
     rng->state += initstate;
     pcg32_random_r(rng);
 }
+
+// Source from https://github.com/imneme/pcg-c-basic/blob/master/pcg_basic.c
+uint32_t pcg32_boundedrand_r(pcg32_random_t *rng, uint32_t bound) {
+	uint32_t threshold = -bound % bound;
+	for (;;) {
+		uint32_t r = pcg32_random_r(rng);
+		if (r >= threshold)
+			return r % bound;
+	}
+}
diff --git a/thirdparty/misc/pcg.h b/thirdparty/misc/pcg.h
index 6f42b3b094..0faab73e64 100644
--- a/thirdparty/misc/pcg.h
+++ b/thirdparty/misc/pcg.h
@@ -11,5 +11,6 @@
 typedef struct { uint64_t state;  uint64_t inc; } pcg32_random_t;
 uint32_t pcg32_random_r(pcg32_random_t* rng);
 void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq);
+uint32_t pcg32_boundedrand_r(pcg32_random_t* rng, uint32_t bound);
 
 #endif // RANDOM_H