diff options
| -rw-r--r-- | core/math/random_pcg.cpp | 8 | ||||
| -rw-r--r-- | core/math/random_pcg.h | 62 |
2 files changed, 62 insertions, 8 deletions
diff --git a/core/math/random_pcg.cpp b/core/math/random_pcg.cpp index 45467b32b2..b82b24ac4d 100644 --- a/core/math/random_pcg.cpp +++ b/core/math/random_pcg.cpp @@ -44,13 +44,9 @@ void RandomPCG::randomize() { } double RandomPCG::random(double p_from, double p_to) { - unsigned int r = rand(); - double ret = (double)r / (double)RANDOM_MAX; - return (ret) * (p_to - p_from) + p_from; + return randd() * (p_to - p_from) + p_from; } float RandomPCG::random(float p_from, float p_to) { - unsigned int r = rand(); - float ret = (float)r / (float)RANDOM_MAX; - return (ret) * (p_to - p_from) + p_from; + return randf() * (p_to - p_from) + p_from; } diff --git a/core/math/random_pcg.h b/core/math/random_pcg.h index 230eb9a11b..e48b1b9551 100644 --- a/core/math/random_pcg.h +++ b/core/math/random_pcg.h @@ -35,6 +35,28 @@ #include "thirdparty/misc/pcg.h" +#if defined(__GNUC__) || (_llvm_has_builtin(__builtin_clz)) +#define CLZ32(x) __builtin_clz(x) +#elif defined(_MSC_VER) +#include "intrin.h" +static int __bsr_clz32(uint32_t x) { + unsigned long index; + _BitScanReverse(&index, x); + return 31 - index; +} +#define CLZ32(x) __bsr_clz32(x) +#else +#endif + +#if defined(__GNUC__) || (_llvm_has_builtin(__builtin_ldexp) && _llvm_has_builtin(__builtin_ldexpf)) +#define LDEXP(s, e) __builtin_ldexp(s, e) +#define LDEXPF(s, e) __builtin_ldexpf(s, e) +#else +#include "math.h" +#define LDEXP(s, e) ldexp(s, e) +#define LDEXPF(s, e) ldexp(s, e) +#endif + class RandomPCG { pcg32_random_t pcg; uint64_t current_seed; // seed with this to get the same state @@ -58,8 +80,44 @@ public: current_seed = pcg.state; return pcg32_random_r(&pcg); } - _FORCE_INLINE_ double randd() { return (double)rand() / (double)RANDOM_MAX; } - _FORCE_INLINE_ float randf() { return (float)rand() / (float)RANDOM_MAX; } + + // 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: + // 1. Instead of shifting to the first 1 and connecting random bits, we simply set the MSB and LSB to 1. + // Provided that the RNG is actually uniform bit by bit, this should have the exact same effect. + // 2. In order to compensate for exponent info loss, we count zeros from another random number, + // and just add that to the initial offset. + // This has the same probability as counting and shifting an actual bit stream: 2^-n for n zeroes. + // For all numbers above 2^-96 (2^-64 for floats), the functions should be uniform. + // However, all numbers below that threshold are floored to 0. + // The thresholds are chosen to minimize rand() calls while keeping the numbers within a totally subjective quality standard. + // If clz or ldexp isn't available, fall back to bit truncation for performance, sacrificing uniformity. + _FORCE_INLINE_ double randd() { +#if defined(CLZ32) + uint32_t proto_exp_offset = rand(); + if (unlikely(proto_exp_offset == 0)) { + return 0; + } + uint64_t significand = (((uint64_t)rand()) << 32) | rand() | 0x8000000000000001U; + return LDEXP((double)significand, -64 - CLZ32(proto_exp_offset)); +#else +#pragma message("RandomPCG::randd - intrinsic clz is not available, falling back to bit truncation") + return (double)(((((uint64_t)rand()) << 32) | rand()) & 0x1FFFFFFFFFFFFFU) / (double)0x1FFFFFFFFFFFFFU; +#endif + } + _FORCE_INLINE_ float randf() { +#if defined(CLZ32) + uint32_t proto_exp_offset = rand(); + if (unlikely(proto_exp_offset == 0)) { + return 0; + } + return LDEXPF((float)(rand() | 0x80000001), -32 - CLZ32(proto_exp_offset)); +#else +#pragma message("RandomPCG::randf - intrinsic clz is not available, falling back to bit truncation") + return (float)(rand() & 0xFFFFFF) / (float)0xFFFFFF; +#endif + } double random(double p_from, double p_to); float random(float p_from, float p_to); |