rand48.c

用于嵌入式Linux系统的标准C的库函数

/*
 * Copyright (c) 1993 Martin Birgmeier
 * All rights reserved.
 *
 * You may redistribute unmodified or modified versions of this source
 * code provided that the above copyright notice and this and the
 * following conditions are retained.
 *
 * This software is provided ``as is'', and comes with no warranties
 * of any kind. I shall in no event be liable for anything that happens
 * to anyone/anything when using this software.
 */

/*
FUNCTION
   <<rand48>>, <<drand48>>, <<erand48>>, <<lrand48>>, <<nrand48>>, <<mrand48>>, <<jrand48>>, <<srand48>>, <<seed48>>, <<lcong48>> ---pseudo random number generators and initialization routines

INDEX
       rand48
INDEX
       drand48
INDEX
       erand48
INDEX
       lrand48
INDEX
       nrand48
INDEX
       mrand48
INDEX
       jrand48
INDEX
       srand48
INDEX
       seed48
INDEX
       lcong48

ANSI_SYNOPSIS
       #include <stdlib.h>
       double drand48(void);
       double erand48(unsigned short <[xseed]>[3]);
       long lrand48(void);
       long nrand48(unsigned short <[xseed]>[3]);
       long mrand48(void);
       long jrand48(unsigned short <[xseed]>[3]);
       void srand48(long <[seed]>);
       unsigned short *seed48(unsigned short <[xseed]>[3]);
       void lcong48(unsigned short <[p]>[7]);

TRAD_SYNOPSIS
       #include <stdlib.h>
       double drand48();

       double erand48(<[xseed]>)
       unsigned short <[xseed]>[3];

       long lrand48();

       long nrand48(<[xseed]>)
       unsigned short <[xseed]>[3];

       long mrand48();

       long jrand48(<[xseed]>)
       unsigned short <[xseed]>[3];

       void srand48(<[seed]>)
       long <[seed]>;

       unsigned short *seed48(<[xseed]>)
       unsigned short <[xseed]>[3];

       void lcong48(<[p]>)
       unsigned short <[p]>[7];

DESCRIPTION
The <<rand48>> family of functions generates pseudo-random numbers
using a linear congruential algorithm working on integers 48 bits in size.
The particular formula employed is
r(n+1) = (a * r(n) + c) mod m
where the default values are
for the multiplicand a = 0xfdeece66d = 25214903917 and
the addend c = 0xb = 11. The modulo is always fixed at m = 2 ** 48.
r(n) is called the seed of the random number generator.

For all the six generator routines described next, the first
computational step is to perform a single iteration of the algorithm.

<<drand48>> and <<erand48>>
return values of type double. The full 48 bits of r(n+1) are
loaded into the mantissa of the returned value, with the exponent set
such that the values produced lie in the interval [0.0, 1.0].

<<lrand48>> and <<nrand48>>
return values of type long in the range
[0, 2**31-1]. The high-order (31) bits of
r(n+1) are loaded into the lower bits of the returned value, with
the topmost (sign) bit set to zero.

<<mrand48>> and <<jrand48>>
return values of type long in the range
[-2**31, 2**31-1]. The high-order (32) bits of
r(n+1) are loaded into the returned value.

<<drand48>>, <<lrand48>>, and <<mrand48>>
use an internal buffer to store r(n). For these functions
the initial value of r(0) = 0x1234abcd330e = 20017429951246.

On the other hand, <<erand48>>, <<nrand48>>, and <<jrand48>>
use a user-supplied buffer to store the seed r(n),
which consists of an array of 3 shorts, where the zeroth member
holds the least significant bits.

All functions share the same multiplicand and addend.

<<srand48>> is used to initialize the internal buffer r(n) of
<<drand48>>, <<lrand48>>, and <<mrand48>>
such that the 32 bits of the seed value are copied into the upper 32 bits
of r(n), with the lower 16 bits of r(n) arbitrarily being set to 0x330e.
Additionally, the constant multiplicand and addend of the algorithm are
reset to the default values given above.

<<seed48>> also initializes the internal buffer r(n) of
<<drand48>>, <<lrand48>>, and <<mrand48>>,
but here all 48 bits of the seed can be specified in an array of 3 shorts,
where the zeroth member specifies the lowest bits. Again,
the constant multiplicand and addend of the algorithm are
reset to the default values given above.
<<seed48>> returns a pointer to an array of 3 shorts which contains
the old seed.
This array is statically allocated, thus its contents are lost after
each new call to <<seed48>>.

Finally, <<lcong48>> allows full control over the multiplicand and
addend used in <<drand48>>, <<erand48>>, <<lrand48>>, <<nrand48>>,
<<mrand48>>, and <<jrand48>>,
and the seed used in <<drand48>>, <<lrand48>>, and <<mrand48>>.
An array of 7 shorts is passed as parameter; the first three shorts are
used to initialize the seed; the second three are used to initialize the
multiplicand; and the last short is used to initialize the addend.
It is thus not possible to use values greater than 0xffff as the addend.

Note that all three methods of seeding the random number generator
always also set the multiplicand and addend for any of the six
generator calls.

For a more powerful random number generator, see <<random>>.

PORTABILITY
SUS requires these functions.

No supporting OS subroutines are required.
*/

#include "rand48.h"

void
_DEFUN (__dorand48, (r, xseed),
       struct _reent *r _AND
       unsigned short xseed[3])
{
  unsigned long accu;
  unsigned short temp[2];

  _REENT_CHECK_RAND48(r);
  accu = (unsigned long) __rand48_mult[0] * (unsigned long) xseed[0] +
    (unsigned long) __rand48_add;
  temp[0] = (unsigned short) accu;     /* lower 16 bits */
  accu >>= sizeof(unsigned short) * 8;
  accu += (unsigned long) __rand48_mult[0] * (unsigned long) xseed[1] +
    (unsigned long) __rand48_mult[1] * (unsigned long) xseed[0];
  temp[1] = (unsigned short) accu;     /* middle 16 bits */
  accu >>= sizeof(unsigned short) * 8;
  accu += __rand48_mult[0] * xseed[2] + __rand48_mult[1] * xseed[1] + __rand48_mult[2] * xseed[0];
  xseed[0] = temp[0];
  xseed[1] = temp[1];
  xseed[2] = (unsigned short) accu;
}