drand48.c

<B>Digital的Unix操作系统VAX 4.2源码</B>

/*	@(#)drand48.c	2.2	*/
/*LINTLIBRARY*/
/*
 *	drand48, etc. pseudo-random number generator
 *	This implementation assumes unsigned short integers of at least
 *	16 bits, long integers of at least 32 bits, and ignores
 *	overflows on adding or multiplying two unsigned integers.
 *	Two's-complement representation is assumed in a few places.
 *	Some extra masking is done if unsigneds are exactly 16 bits
 *	or longs are exactly 32 bits, but so what?
 *	An assembly-language implementation would run significantly faster.
 */
#ifndef HAVEFP
#define HAVEFP 1
#endif
#define N	16
#define MASK	((unsigned)(1 << (N - 1)) + (1 << (N - 1)) - 1)
#define LOW(x)	((unsigned)(x) & MASK)
#define HIGH(x)	LOW((x) >> N)
#define MUL(x, y, z)	{ long l = (long)(x) * (long)(y); \
		(z)[0] = LOW(l); (z)[1] = HIGH(l); }
#define CARRY(x, y)	((long)(x) + (long)(y) > MASK)
#define ADDEQU(x, y, z)	(z = CARRY(x, (y)), x = LOW(x + (y)))
#define X0	0x330E
#define X1	0xABCD
#define X2	0x1234
#define A0	0xE66D
#define A1	0xDEEC
#define A2	0x5
#define C	0xB
#define SET3(x, x0, x1, x2)	((x)[0] = (x0), (x)[1] = (x1), (x)[2] = (x2))
#define SETLOW(x, y, n) SET3(x, LOW((y)[n]), LOW((y)[(n)+1]), LOW((y)[(n)+2]))
#define SEED(x0, x1, x2) (SET3(x, x0, x1, x2), SET3(a, A0, A1, A2), c = C)
#define REST(v)	for (i = 0; i < 3; i++) { xsubi[i] = x[i]; x[i] = temp[i]; } \
		return (v);
#define NEST(TYPE, f, F)	TYPE f(xsubi) register unsigned short *xsubi; { \
	register int i; register TYPE v; unsigned temp[3]; \
	for (i = 0; i < 3; i++) { temp[i] = x[i]; x[i] = LOW(xsubi[i]); }  \
	v = F(); REST(v); }
#define HI_BIT	(1L << (2 * N - 1))

static unsigned x[3] = { X0, X1, X2 }, a[3] = { A0, A1, A2 }, c = C;
static unsigned short lastx[3];
static void next();

#if HAVEFP
double
drand48()
{
#if pdp11
	static double two16m; /* old pdp11 cc can't compile an expression */
	two16m = 1.0 / (1L << N); /* in "double" initializer! */
#else
	static double two16m = 1.0 / (1L << N);
#endif

	next();
	return (two16m * (two16m * (two16m * x[0] + x[1]) + x[2]));
}

NEST(double, erand48, drand48);

#else

long
irand48(m)
/* Treat x[i] as a 48-bit fraction, and multiply it by the 16-bit
 * multiplier m.  Return integer part as result.
 */
register unsigned short m;
{
	unsigned r[4], p[2], carry0 = 0;

	next();
	MUL(m, x[0], &r[0]);
	MUL(m, x[2], &r[2]);
	MUL(m, x[1], p);
	if (CARRY(r[1], p[0]))
		ADDEQU(r[2], 1, carry0);
	return (r[3] + carry0 + CARRY(r[2], p[1]));
}

long
krand48(xsubi, m)
/* same as irand48, except user provides storage in xsubi[] */
register unsigned short *xsubi;
unsigned short m;
{
	register int i;
	register long iv;
	unsigned temp[3];

	for (i = 0; i < 3; i++) {
		temp[i] = x[i];
		x[i] = xsubi[i];
	}
	iv = irand48(m);
	REST(iv);
}
#endif

long
lrand48()
{
	next();
	return (((long)x[2] << (N - 1)) + (x[1] >> 1));
}

long
mrand48()
{
	register long l;

	next();
	/* sign-extend in case length of a long > 32 bits
						(as on Honeywell) */
	return ((l = ((long)x[2] << N) + x[1]) & HI_BIT ? l | -HI_BIT : l);
}

static void
next()
{
	unsigned p[2], q[2], r[2], carry0, carry1;

	MUL(a[0], x[0], p);
	ADDEQU(p[0], c, carry0);
	ADDEQU(p[1], carry0, carry1);
	MUL(a[0], x[1], q);
	ADDEQU(p[1], q[0], carry0);
	MUL(a[1], x[0], r);
	x[2] = LOW(carry0 + carry1 + CARRY(p[1], r[0]) + q[1] + r[1] +
		a[0] * x[2] + a[1] * x[1] + a[2] * x[0]);
	x[1] = LOW(p[1] + r[0]);
	x[0] = LOW(p[0]);
}

void
srand48(seedval)
long seedval;
{
	SEED(X0, LOW(seedval), HIGH(seedval));
}

unsigned short *
seed48(seed16v)
unsigned short seed16v[3];
{
	SETLOW(lastx, x, 0);
	SEED(LOW(seed16v[0]), LOW(seed16v[1]), LOW(seed16v[2]));
	return (lastx);
}

void
lcong48(param)
unsigned short param[7];
{
	SETLOW(x, param, 0);
	SETLOW(a, param, 3);
	c = LOW(param[6]);
}

NEST(long, nrand48, lrand48);

NEST(long, jrand48, mrand48);

#ifdef DRIVER
/*
	This should print the sequences of integers in Tables 2
		and 1 of the TM:
	1623, 3442, 1447, 1829, 1305, ...
	657EB7255101, D72A0C966378, 5A743C062A23, ...
 */
#include <stdio.h>

main()
{
	int i;

	for (i = 0; i < 80; i++) {
		printf("%4d ", (int)(4096 * drand48()));
		printf("%.4X%.4X%.4X\n", x[2], x[1], x[0]);
	}
}
#endif