random.cpp

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//
// Random.cpp
//
// $Id: //poco/1.2/Foundation/src/Random.cpp#2 $
//
// Library: Foundation
// Package: Crypt
// Module:  Random
//
// Definition of class Random.
//
// Copyright (c) 2004-2006, Applied Informatics Software Engineering GmbH.
// and Contributors.
//
// Permission is hereby granted, free of charge, to any person or organization
// obtaining a copy of the software and accompanying documentation covered by
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// 
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//
//
// Based on the FreeBSD random number generator.
// src/lib/libc/stdlib/random.c,v 1.13 2000/01/27 23:06:49 jasone Exp
//
// Copyright (c) 1983, 1993
// The Regents of the University of California.  All rights reserved.
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#include "Poco/Random.h"
#include "Poco/RandomStream.h"
#include "time.h"


/*
 * random.c:
 *
 * An improved random number generation package.  In addition to the standard
 * rand()/srand() like interface, this package also has a special state info
 * interface.  The initstate() routine is called with a seed, an array of
 * bytes, and a count of how many bytes are being passed in; this array is
 * then initialized to contain information for random number generation with
 * that much state information.  Good sizes for the amount of state
 * information are 32, 64, 128, and 256 bytes.  The state can be switched by
 * calling the setstate() routine with the same array as was initiallized
 * with initstate().  By default, the package runs with 128 bytes of state
 * information and generates far better random numbers than a linear
 * congruential generator.  If the amount of state information is less than
 * 32 bytes, a simple linear congruential R.N.G. is used.
 *
 * Internally, the state information is treated as an array of longs; the
 * zeroeth element of the array is the type of R.N.G. being used (small
 * integer); the remainder of the array is the state information for the
 * R.N.G.  Thus, 32 bytes of state information will give 7 longs worth of
 * state information, which will allow a degree seven polynomial.  (Note:
 * the zeroeth word of state information also has some other information
 * stored in it -- see setstate() for details).
 *
 * The random number generation technique is a linear feedback shift register
 * approach, employing trinomials (since there are fewer terms to sum up that
 * way).  In this approach, the least significant bit of all the numbers in
 * the state table will act as a linear feedback shift register, and will
 * have period 2^deg - 1 (where deg is the degree of the polynomial being
 * used, assuming that the polynomial is irreducible and primitive).  The
 * higher order bits will have longer periods, since their values are also
 * influenced by pseudo-random carries out of the lower bits.  The total
 * period of the generator is approximately deg*(2**deg - 1); thus doubling
 * the amount of state information has a vast influence on the period of the
 * generator.  Note: the deg*(2**deg - 1) is an approximation only good for
 * large deg, when the period of the shift register is the dominant factor.
 * With deg equal to seven, the period is actually much longer than the
 * 7*(2**7 - 1) predicted by this formula.
 *
 * Modified 28 December 1994 by Jacob S. Rosenberg.
 * The following changes have been made:
 * All references to the type u_int have been changed to unsigned long.
 * All references to type int have been changed to type long.  Other
 * cleanups have been made as well.  A warning for both initstate and
 * setstate has been inserted to the effect that on Sparc platforms
 * the 'arg_state' variable must be forced to begin on word boundaries.
 * This can be easily done by casting a long integer array to char *.
 * The overall logic has been left STRICTLY alone.  This software was
 * tested on both a VAX and Sun SpacsStation with exactly the same
 * results.  The new version and the original give IDENTICAL results.
 * The new version is somewhat faster than the original.  As the
 * documentation says:  "By default, the package runs with 128 bytes of
 * state information and generates far better random numbers than a linear
 * congruential generator.  If the amount of state information is less than
 * 32 bytes, a simple linear congruential R.N.G. is used."  For a buffer of
 * 128 bytes, this new version runs about 19 percent faster and for a 16
 * byte buffer it is about 5 percent faster.
 */


/*
 * For each of the currently supported random number generators, we have a
 * break value on the amount of state information (you need at least this
 * many bytes of state info to support this random number generator), a degree
 * for the polynomial (actually a trinomial) that the R.N.G. is based on, and
 * the separation between the two lower order coefficients of the trinomial.
 */
#define	TYPE_0		0		/* linear congruential */
#define	BREAK_0		8
#define	DEG_0		0
#define	SEP_0		0

#define	TYPE_1		1		/* x**7 + x**3 + 1 */
#define	BREAK_1		32
#define	DEG_1		7
#define	SEP_1		3

#define	TYPE_2		2		/* x**15 + x + 1 */
#define	BREAK_2		64
#define	DEG_2		15
#define	SEP_2		1

#define	TYPE_3		3		/* x**31 + x**3 + 1 */
#define	BREAK_3		128
#define	DEG_3		31
#define	SEP_3		3

#define	TYPE_4		4		/* x**63 + x + 1 */
#define	BREAK_4		256
#define	DEG_4		63
#define	SEP_4		1


namespace Poco {


Random::Random(int stateSize)
{
	poco_assert (BREAK_0 <= stateSize && stateSize <= BREAK_4);

	_pBuffer = new char[stateSize];
	initState((UInt32) time(NULL), _pBuffer, stateSize);
}


Random::~Random()
{
	delete [] _pBuffer;
}


/*
 * Compute x = (7^5 * x) mod (2^31 - 1)
 * wihout overflowing 31 bits:
 *      (2^31 - 1) = 127773 * (7^5) + 2836
 * From "Random number generators: good ones are hard to find",
 * Park and Miller, Communications of the ACM, vol. 31, no. 10,
 * October 1988, p. 1195.
 */
inline UInt32 Random::goodRand(Int32 x)
{
	Int32 hi, lo;

	if (x == 0) x = 123459876;
	hi = x / 127773;
	lo = x % 127773;
	x = 16807 * lo - 2836 * hi;
	if (x < 0) x += 0x7FFFFFFF;

	return x;
}


/*
 * 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 Random::seed(UInt32 x)
{
	int i, lim;

	_state[0] = x;
	if (_randType == TYPE_0)
		lim = NSHUFF;
	else 
	{
		for (i = 1; i < _randDeg; i++)
			_state[i] = goodRand(_state[i - 1]);
		_fptr = &_state[_randSep];
		_rptr = &_state[0];
		lim = 10 * _randDeg;
	}
	for (i = 0; i < lim; i++)
		next();
}


/*
 * Many programs choose the seed value in a totally predictable manner.
 * This often causes problems.  We seed the generator using the much more
 * secure random(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.
 */
void Random::seed()
{
	std::streamsize len;

	if (_randType == TYPE_0)
		len = sizeof _state[0];
	else
		len = _randDeg * sizeof _state[0];

	RandomInputStream rstr;
	rstr.read((char*) _state, len);
}


/*
 * 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.
 *
 * word boundary; otherwise a bus error will occur. Even so, lint will
 * complain about mis-alignment, but you should disregard these messages.
 */
void Random::initState(UInt32 s, char* argState, Int32 n)
{
	UInt32* intArgState = (UInt32*) argState;

	if (n < BREAK_0) 
	{
		poco_bugcheck_msg("not enough state");
		return;
	}
	if (n < BREAK_1) 
	{
		_randType = TYPE_0;
		_randDeg  = DEG_0;
		_randSep  = SEP_0;
	} 
	else if (n < BREAK_2) 
	{
		_randType = TYPE_1;
		_randDeg  = DEG_1;
		_randSep  = SEP_1;
	} 
	else if (n < BREAK_3) 
	{
		_randType = TYPE_2;
		_randDeg  = DEG_2;
		_randSep  = SEP_2;
	} 
	else if (n < BREAK_4) 
	{
		_randType = TYPE_3;
		_randDeg  = DEG_3;
		_randSep  = SEP_3;
	} 
	else 
	{
		_randType = TYPE_4;
		_randDeg = DEG_4;
		_randSep = SEP_4;
	}
	_state  = intArgState + 1; /* first location */
	_endPtr = &_state[_randDeg];	/* must set end_ptr before seed */
	seed(s);
	if (_randType == TYPE_0)
		intArgState[0] = _randType;
	else
		intArgState[0] = MAX_TYPES * (int) (_rptr - _state) + _randType;
}


/*
 * Next:
 *
 * 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.
 */
UInt32 Random::next()
{
	UInt32 i;
	UInt32 *f, *r;

	if (_randType == TYPE_0) 
	{
		i = _state[0];
		_state[0] = i = goodRand(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 >= _endPtr) {
			f = _state;
			++r;
		}
		else if (++r >= _endPtr) {
			r = _state;
		}

		_fptr = f; _rptr = r;
	}
	return i;
}


} // namespace Poco