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//******************************************************************************
//
// File: Random.c
//
// This C source file is copyright (C) 2008 by Alan Kaminsky. All rights
// reserved. For further information, contact the author, Alan Kaminsky, at
// ark@cs.rit.edu.
//
// This C source file is part of the Parallel Java Library ("PJ"). PJ is free
// software; you can redistribute it and/or modify it under the terms of the GNU
// General Public License as published by the Free Software Foundation; either
// version 3 of the License, or (at your option) any later version.
//
// PJ is distributed in the hope that it will be useful, but WITHOUT ANY
// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
// A PARTICULAR PURPOSE. See the GNU General Public License for more details.
//
// A copy of the GNU General Public License is provided in the file gpl.txt. You
// may also obtain a copy of the GNU General Public License on the World Wide
// Web at http://www.gnu.org/licenses/gpl.html.
//
//******************************************************************************
#include "Random.h"
/**
* Class DefaultRandom provides a default pseudorandom number generator (PRNG)
* designed for use in parallel scientific programming. To create an instance of
* class DefaultRandom, either use the <TT>DefaultRandom()</TT> constructor, or
* use the static <TT>getInstance(long)</TT> method in class {@linkplain
* Random}.
* <P>
* Class DefaultRandom generates random numbers by hashing successive counter
* values. The seed initializes the counter. The hash function is defined in W.
* Press et al., <I>Numerical Recipes: The Art of Scientific Computing, Third
* Edition</I> (Cambridge University Press, 2007), page 352. The hash function
* applied to the counter value <I>i</I> is:
* <P>
* <I>x</I> := 3935559000370003845 * <I>i</I> + 2691343689449507681 (mod 2<SUP>64</SUP>)
* <BR><I>x</I> := <I>x</I> xor (<I>x</I> right-shift 21)
* <BR><I>x</I> := <I>x</I> xor (<I>x</I> left-shift 37)
* <BR><I>x</I> := <I>x</I> xor (<I>x</I> right-shift 4)
* <BR><I>x</I> := 4768777513237032717 * <I>x</I> (mod 2<SUP>64</SUP>)
* <BR><I>x</I> := <I>x</I> xor (<I>x</I> left-shift 20)
* <BR><I>x</I> := <I>x</I> xor (<I>x</I> right-shift 41)
* <BR><I>x</I> := <I>x</I> xor (<I>x</I> left-shift 5)
* <BR>Return <I>x</I>
* <P>
* (The shift and arithmetic operations are all performed on unsigned 64-bit
* numbers.)
*
* @author Alan Kaminsky
* @version 11-Aug-2008
*/
#define D_2_POW_NEG_64 5.4210108624275221700e-20
// Hidden operations.
/**
* Return the hash of the given value.
*/
static unsigned long long int hash
(unsigned long long int x)
{
x = 3935559000370003845L * x + 2691343689449507681L;
x = x ^ (x >> 21);
x = x ^ (x << 37);
x = x ^ (x >> 4);
x = 4768777513237032717L * x;
x = x ^ (x << 20);
x = x ^ (x >> 41);
x = x ^ (x << 5);
return x;
}
/**
* Return the next 64-bit pseudorandom value in this PRNG's sequence.
*
* @return Pseudorandom value.
*/
static unsigned long long int next
(Random *this)
{
++ this->seed;
return hash (this->seed);
}
/**
* Return the 64-bit pseudorandom value the given number of positions ahead
* in this PRNG's sequence.
*
* @param skip Number of positions to skip, assumed to be > 0.
*
* @return Pseudorandom value.
*/
static unsigned long long int next2
(Random *this,
long long int skip)
{
this->seed += skip;
return hash (this->seed);
}
// Exported operations.
/**
* Set this PRNG's seed. Any seed value is allowed.
*
* @param seed Seed.
*/
void setSeed
(Random *this,
unsigned long long int seed)
{
this->seed = hash (seed);
}
/**
* Skip the given number of positions ahead in this PRNG's sequence. If
* <TT>skip</TT> <= 0, the <TT>skip()</TT> method does nothing.
*
* @param skip Number of positions to skip.
*/
void skip
(Random *this,
long long int skip)
{
if (skip > 0L) next2 (this, skip);
}
/**
* Return the double precision floating point value from the next
* pseudorandom value in this PRNG's sequence. The returned numbers have a
* uniform distribution in the range 0.0 (inclusive) to 1.0 (exclusive).
*
* @return Double value.
*/
double nextDouble
(Random *this)
{
// Next random number is in the range 0 .. 2^64 - 1.
// Divide by 2^64 yielding a number in the range 0.0 .. 1.0.
return (double) (next (this)) * D_2_POW_NEG_64;
}
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