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c9c7f947b5
This came from a long discussion comparing PCG-* against xoroshiro*. Do not generate integers without bias because it doesn't affect us very much and it is easier to reimplement with modulo.
52 lines
1.1 KiB
C++
52 lines
1.1 KiB
C++
#include "prng.h"
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// From https://en.wikipedia.org/w/index.php?title=Permuted_congruential_generator&oldid=901497400#Example_code.
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//
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// > The generator recommended for most users is PCG-XSH-RR with 64-bit state
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// > and 32-bit output.
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#define NAME "pcg-xsh-rr"
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CPrng::CPrng() :
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m_Seeded(false)
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{
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}
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const char *CPrng::Description() const
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{
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if(!m_Seeded)
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{
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return NAME ":unseeded";
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}
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return m_aDescription;
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}
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static unsigned int RotateRight32(unsigned int x, int Shift)
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{
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return (x >> Shift) | (x << (-Shift & 31));
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}
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void CPrng::Seed(uint64 aSeed[2])
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{
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m_Seeded = true;
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str_format(m_aDescription, sizeof(m_aDescription), "%s:%016llx:%016llx", NAME, aSeed[0], aSeed[1]);
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m_Increment = (aSeed[1] << 1) | 1;
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m_State = aSeed[0] + m_Increment;
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RandomBits();
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}
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unsigned int CPrng::RandomBits()
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{
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dbg_assert(m_Seeded, "prng needs to be seeded before it can generate random numbers");
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uint64 x = m_State;
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unsigned int Count = x >> 59;
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static const uint64 MULTIPLIER = 6364136223846793005u;
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m_State = x * MULTIPLIER + m_Increment;
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x ^= x >> 18;
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return RotateRight32(x >> 27, Count);
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}
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