banan-os/kernel/kernel/Random.cpp

53 lines
1.1 KiB
C++

#include <kernel/CPUID.h>
#include <kernel/Debug.h>
#include <kernel/Random.h>
#include <kernel/Timer/Timer.h>
namespace Kernel
{
// Constants and algorithm from https://en.wikipedia.org/wiki/Permuted_congruential_generator
static uint64_t s_rand_seed = 0x4d595df4d0f33173;
static constexpr uint64_t s_rand_multiplier = 6364136223846793005;
static constexpr uint64_t s_rand_increment = 1442695040888963407;
void Random::initialize()
{
uint32_t ecx, edx;
CPUID::get_features(ecx, edx);
if (ecx & CPUID::ECX_RDRND)
{
asm volatile("rdrand %0" : "=a"(s_rand_seed));
dprintln("RNG seeded by RDRAND");
}
else
{
auto rt = SystemTimer::get().real_time();
s_rand_seed ^= rt.tv_sec ^ rt.tv_nsec;
dprintln("RNG seeded by real time");
}
}
uint32_t Random::get_u32()
{
auto rotr32 = [](uint32_t x, unsigned r) { return x >> r | x << (-r & 31); };
uint64_t x = s_rand_seed;
unsigned count = (unsigned)(x >> 59);
s_rand_seed = x * s_rand_multiplier + s_rand_increment;
x ^= x >> 18;
return rotr32(x >> 27, count) % UINT32_MAX;
}
uint64_t Random::get_u64()
{
return ((uint64_t)get_u32() << 32) | get_u32();
}
}