Re: [Libvisual-devel] randomness work. fast random double

[Vitaly V. B. <vit...@us...>]

On Sun, 03 Oct 2004 18:11:35 +0200
Dennis Smit <sy...@yo...> wrote:

> On Sun, 2004-10-03 at 18:48 +0300, Vitaly V. Bursov wrote:
> > > I've put some of the randomness request in CVS, not yet finished tho.
> > when you convert unsigned int to float you loose entropy...
> > I think it's ok to add double variant of functions...
> 
> Go ahead :)
OK :)

few bits of code....

==========================
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define val_a 1664525L
#define val_c 1013904223L
#define RMAX  4294967295U

static unsigned int _lv_randseed = 5415454;

double rnd()
{
#if 0
	union {
		unsigned int a[2];
		double d;
	} val;
	unsigned int t;

	/* EXECUTION TIME
	 * alu maths = 5 clocks
	 * fsub = 4 clocks
	 * TOTAL: 9 for double
	 * WARNING: numbers should be ieee754 compliant
	 */

	t = _lv_randseed = val_a * _lv_randseed + val_c;
	val.a[0]=(t<<20);
	val.a[1]=0x3ff00000U | (t>>12);

	return (*(double*)&val.d)-1.0;
#else
	unsigned int t;
	double x;

	/* EXECUTION TIME
	 * fild - load int to fpu = 4 clocks
	 * fdiv - normalize result = 16/20/24 clocks for signle/double/ext prec.
	 * TOTAL: 24 for double
	 */

	t = _lv_randseed = val_a * _lv_randseed + val_c;
	x = (double)t/RMAX;
	return x;
#endif
}

double x = 0.0;

int main()
{
	unsigned int i, t=1000000000;

	for (i=0;i<t;i++){
		x += rnd();
	}

}
==========================

the same idea can be used to generate floats.

should I make a patch?

-- 
Vitaly
GPG Key ID: F95A23B9