Fast Float Random Numbers

notes
a small and fast implementation for random float numbers in the range [-1,1], usable as
white noise oscillator.

compared to the naive usage of the rand() function it gives a speedup factor of 9-10.

compared to a direct implementation of the rand() function (visual studio implementation)
it still gives a speedup by a factor of 2-3.

apart from beeing faster it also provides more precision for the resulting floats since
its base values use full 32bit precision.
code
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// set the initial seed to whatever you like
static int RandSeed = 1;

// using rand() (16bit precision)
// takes about 110 seconds for 2 billion calls
float RandFloat1()
{
    return ((float)rand()/RAND_MAX) * 2.0f - 1.0f;
}

// direct implementation of rand() (16 bit precision)
// takes about 32 seconds for 2 billion calls
float RandFloat2()
{
    return ((float)(((RandSeed = RandSeed * 214013L + 2531011L) >> 16) & 0x7fff)/RAND_MAX) * 2.0f - 1.0f;
}

// fast rand float, using full 32bit precision
// takes about 12 seconds for 2 billion calls
float Fast_RandFloat()
{
    RandSeed *= 16807;
    return (float)RandSeed * 4.6566129e-010f;
}

Comments

There is no doubt that implementation 3 is fast, but the problem I had with it is that there's no obvious way to limit the amplitude of the generated signal.

So instead I tried implementation 2 and ran into a different problem. The code is written such that it assumes that RAND_MAX is equal to 0x7FFF, which was not true on my system (it was 0x7FFFFFFF). Fortunately, this was easy to fix. I simply removed the >> 16 and worked fine for me. My final implementation was:

return (float)(RandSeed = RandSeed * 214013L + 2531011L) / 0x7FFFFFFF * 2.0f * amp - amp;

where "amp" is the desired amplitude.
  • Date: 2009-12-29 22:53:23
  • By: earlevel [] earlevel [] com
It should be noted in the code that for method #3, you must initialize the seed to non-zero before using it.

I don't understand Judahmenter's comment about 3 not limiting the amplitude. As it stands it returns a value -1 to 1, so just multiply by your 'amp' value.
This turns into a handy 0-1 random number if you take off the sign bit:
(float)(RandSeed & 0x7FFFFFFF) * 4.6566129e-010f;