# Random Number Generator

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### #1Nils Pipenbrinck

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Posted 21 March 2008 - 10:03 PM

Hi folks.

As a little exercise in SSE2 programming I wrote a little random number generator based on a linear feedback shift register of 62 bit length.

It is a bit slower than the standard library rand-function and useless for cryptographical uses, but it has a much larger sequence of random numbers.

The sequence repeats at 4.61169E+18 numbers.

Here it is the code - in case someone ever needs such a thing. It's written for MSVC.

#include <emmintrin.h>

static __m128i LFSR;

void InitRandom (int Seed)
{
LFSR = _mm_cvtsi32_si128 (Seed);
}

int GetRandom (int NumBits)
{
__m128i seed = LFSR;
__m128i one  = _mm_cvtsi32_si128(1);
int i;

for (i=0; i<NumBits; i++)
{

// generate xor of adjecting bits
__m128i temp = _mm_xor_si128(seed, _mm_srli_epi64(seed,1));

// generate xor of feedback bits 5,6 and 62,61
__m128i NewBit = _mm_xor_si128( _mm_srli_epi64(temp,5),
_mm_srli_epi64(temp,61));

NewBit = _mm_and_si128 (NewBit, one);

// Shift & insert new result bit:
seed = _mm_or_si128 (NewBit, _mm_add_epi64 (seed,seed));
}

// Write back seed...
LFSR = seed;

// generate mask of NumBit ones.
mask = _mm_srli_epi64 (_mm_cmpeq_epi8(seed, seed), 64-NumBits);

// return random number:
}



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