fastrand.c

著名操作系统Plan 9的第三版的部分核心源代码。现在很难找到了。Plan 9是bell实验室开发的Unix后继者。

#include	<u.h>
#include	<libc.h>

/*
 *	algorithm by
 *	D. P. Mitchell & J. A. Reeds
 *
 *	adapted to stretch the rate of a true random
 *	number generator by
 *	D. L. Presotto
 */
#define	LEN	607
#define	TAP	273
#define	MASK	0x7fffffffL
#define	A	48271
#define	M	2147483647
#define	Q	44488
#define	R	3399
#define	FREQ	4096

static	ulong	rng_vec[LEN];
static	ulong*	rng_tap = rng_vec;
static	ulong*	rng_feed = 0;

static void
seedrand(void)
{
	long lo, hi, x, seed;
	int i;

	rng_tap = rng_vec;
	rng_feed = rng_vec+LEN-TAP;
	seed = truerand()%M;
	if(seed < 0)
		seed += M;
	if(seed == 0)
		seed = 89482311;
	x = seed;
	/*
	 *	Initialize by x[n+1] = 48271 * x[n] mod (2**31 - 1)
	 */
	for(i = -20; i < LEN; i++) {
		hi = x / Q;
		lo = x % Q;
		x = A*lo - R*hi;
		if(x < 0)
			x += M;
		if(i >= 0)
			rng_vec[i] = x;
		if((x%FREQ) == 0)
			x ^= truerand();
	}
}

ulong
fastrand(void)
{
	ulong x;

	rng_tap--;
	if(rng_tap < rng_vec) {
		if(rng_feed == 0) {
			seedrand();
			rng_tap--;
		}
 		rng_tap += LEN;
	}
	rng_feed--;
	if(rng_feed < rng_vec)
		rng_feed += LEN;
	x = *rng_feed + *rng_tap;
	if((x%FREQ) == 0)
		rng_vec[x%LEN] ^= truerand();
	*rng_feed = x;
	return x;
}