random.c

newos is new operation system

 * Initialize the random number generator based on the given seed.  If the
 * type is the trivial no-state-information type, just remember the seed.
 * Otherwise, initializes state[] based on the given "seed" via a linear
 * congruential generator.  Then, the pointers are set to known locations
 * that are exactly rand_sep places apart.  Lastly, it cycles the state
 * information a given number of times to get rid of any initial dependencies
 * introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 * for default usage relies on values produced by this routine.
 */
void
srandom(unsigned long x)
{
	long i;

	if (rand_type == TYPE_0) {
		state[0] = x;
	} else {
		state[0] = x;
		for (i = 1; i < rand_deg; i++) {
			state[i] = good_rand(state[i - 1]);
		}
		fptr = &state[rand_sep];
		rptr = &state[0];
		for (i = 0; i < 10 * rand_deg; i++) {
			(void)random();
		}
	}
}

/*
 * srandomdev:
 *
 * Many programs choose the seed value in a totally predictable manner.
 * This often causes problems.  We seed the generator using the much more
 * secure urandom(4) interface.  Note that this particular seeding
 * procedure can generate states which are impossible to reproduce by
 * calling srandom() with any value, since the succeeding terms in the
 * state buffer are no longer derived from the LC algorithm applied to
 * a fixed seed.
 */
#if 0
/* not yet supported by NewOS */
void
srandomdev()
{
	int fd, done;
	size_t len;

	if (rand_type == TYPE_0)
		len = sizeof state[0];
	else
		len = rand_deg * sizeof state[0];

	done = 0;
	fd = _open("/dev/urandom", O_RDONLY, 0);
	if (fd >= 0) {
		if (_read(fd, (void *) state, len) == (ssize_t) len)
			done = 1;
		_close(fd);
	}

	if (!done) {
		struct timeval tv;
		unsigned long junk;

		gettimeofday(&tv, NULL);
		srandom(getpid() ^ tv.tv_sec ^ tv.tv_usec ^ junk);
		return;
	}

	if (rand_type != TYPE_0) {
		fptr = &state[rand_sep];
		rptr = &state[0];
	}
}

/*
 * initstate:
 *
 * Initialize the state information in the given array of n bytes for future
 * random number generation.  Based on the number of bytes we are given, and
 * the break values for the different R.N.G.'s, we choose the best (largest)
 * one we can and set things up for it.  srandom() is then called to
 * initialize the state information.
 *
 * Note that on return from srandom(), we set state[-1] to be the type
 * multiplexed with the current value of the rear pointer; this is so
 * successive calls to initstate() won't lose this information and will be
 * able to restart with setstate().
 *
 * Note: the first thing we do is save the current state, if any, just like
 * setstate() so that it doesn't matter when initstate is called.
 *
 * Returns a pointer to the old state.
 *
 * Note: The Sparc platform requires that arg_state begin on a long
 * word boundary; otherwise a bus error will occur. Even so, lint will
 * complain about mis-alignment, but you should disregard these messages.
 */
char *
initstate(seed, arg_state, n)
	unsigned long seed;		/* seed for R.N.G. */
	char *arg_state;		/* pointer to state array */
	long n;				/* # bytes of state info */
{
	register char *ostate = (char *)(&state[-1]);
	register long *long_arg_state = (long *) arg_state;

	if (rand_type == TYPE_0)
		state[-1] = rand_type;
	else
		state[-1] = MAX_TYPES * (rptr - state) + rand_type;
	if (n < BREAK_0) {
		(void)fprintf(stderr,
		    "random: not enough state (%ld bytes); ignored.\n", n);
		return(0);
	}
	if (n < BREAK_1) {
		rand_type = TYPE_0;
		rand_deg = DEG_0;
		rand_sep = SEP_0;
	} else if (n < BREAK_2) {
		rand_type = TYPE_1;
		rand_deg = DEG_1;
		rand_sep = SEP_1;
	} else if (n < BREAK_3) {
		rand_type = TYPE_2;
		rand_deg = DEG_2;
		rand_sep = SEP_2;
	} else if (n < BREAK_4) {
		rand_type = TYPE_3;
		rand_deg = DEG_3;
		rand_sep = SEP_3;
	} else {
		rand_type = TYPE_4;
		rand_deg = DEG_4;
		rand_sep = SEP_4;
	}
	state = (long *) (long_arg_state + 1); /* first location */
	end_ptr = &state[rand_deg];	/* must set end_ptr before srandom */
	srandom(seed);
	if (rand_type == TYPE_0)
		long_arg_state[0] = rand_type;
	else
		long_arg_state[0] = MAX_TYPES * (rptr - state) + rand_type;
	return(ostate);
}

/*
 * setstate:
 *
 * Restore the state from the given state array.
 *
 * Note: it is important that we also remember the locations of the pointers
 * in the current state information, and restore the locations of the pointers
 * from the old state information.  This is done by multiplexing the pointer
 * location into the zeroeth word of the state information.
 *
 * Note that due to the order in which things are done, it is OK to call
 * setstate() with the same state as the current state.
 *
 * Returns a pointer to the old state information.
 *
 * Note: The Sparc platform requires that arg_state begin on a long
 * word boundary; otherwise a bus error will occur. Even so, lint will
 * complain about mis-alignment, but you should disregard these messages.
 */
char *
setstate(arg_state)
	char *arg_state;		/* pointer to state array */
{
	register long *new_state = (long *) arg_state;
	register long type = new_state[0] % MAX_TYPES;
	register long rear = new_state[0] / MAX_TYPES;
	char *ostate = (char *)(&state[-1]);

	if (rand_type == TYPE_0)
		state[-1] = rand_type;
	else
		state[-1] = MAX_TYPES * (rptr - state) + rand_type;
	switch(type) {
	case TYPE_0:
	case TYPE_1:
	case TYPE_2:
	case TYPE_3:
	case TYPE_4:
		rand_type = type;
		rand_deg = degrees[type];
		rand_sep = seps[type];
		break;
	default:
		(void)fprintf(stderr,
		    "random: state info corrupted; not changed.\n");
	}
	state = (long *) (new_state + 1);
	if (rand_type != TYPE_0) {
		rptr = &state[rear];
		fptr = &state[(rear + rand_sep) % rand_deg];
	}
	end_ptr = &state[rand_deg];		/* set end_ptr too */
	return(ostate);
}
#endif

/*
 * random:
 *
 * If we are using the trivial TYPE_0 R.N.G., just do the old linear
 * congruential bit.  Otherwise, we do our fancy trinomial stuff, which is
 * the same in all the other cases due to all the global variables that have
 * been set up.  The basic operation is to add the number at the rear pointer
 * into the one at the front pointer.  Then both pointers are advanced to
 * the next location cyclically in the table.  The value returned is the sum
 * generated, reduced to 31 bits by throwing away the "least random" low bit.
 *
 * Note: the code takes advantage of the fact that both the front and
 * rear pointers can't wrap on the same call by not testing the rear
 * pointer if the front one has wrapped.
 *
 * Returns a 31-bit random number.
 */
long
random(void)
{
	long i;
	long *f;
	long *r;

	if (rand_type == TYPE_0) {
		i = state[0];
		state[0] = i = (good_rand(i)) & 0x7fffffff;
	} else {
		/*
		 * Use local variables rather than static variables for speed.
		 */
		f = fptr; r = rptr;
		*f += *r;
		i = (*f >> 1) & 0x7fffffff;	/* chucking least random bit */
		if (++f >= end_ptr) {
			f = state;
			++r;
		}
		else if (++r >= end_ptr) {
			r = state;
		}

		fptr = f; rptr = r;
	}
	return(i);
}