RNG.cpp

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#include <../../nrnconf.h>
// This may look like C code, but it is really -*- C++ -*-
/* 
Copyright (C) 1989 Free Software Foundation

This file is part of the GNU C++ Library.  This library is free
software; you can redistribute it and/or modify it under the terms of
the GNU Library General Public License as published by the Free
Software Foundation; either version 2 of the License, or (at your
option) any later version.  This library 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 Library General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with this library; if not, write to the Free Software
Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#ifdef __GNUG__
#pragma implementation
#endif
#include <assert.h>
#include <RNG.h>

// These two static fields get initialized by RNG::RNG().
PrivateRNGSingleType RNG::singleMantissa;
PrivateRNGDoubleType RNG::doubleMantissa;

//
//  The scale constant is 2^-31. It is used to scale a 31 bit
//  long to a double.
//

//static const double randomDoubleScaleConstant = 4.656612873077392578125e-10;
//static const float  randomFloatScaleConstant = 4.656612873077392578125e-10;

static char initialized = 0;

RNG::~RNG(){}

RNG::RNG()
{
  if (!initialized)
  {

    assert (sizeof(double) == 2 * sizeof(uint32_t)); 

    //
    //  The following is a hack that I attribute to
    //  Andres Nowatzyk at CMU. The intent of the loop
    //  is to form the smallest number 0 <= x < 1.0,
    //  which is then used as a mask for two longwords.
    //  this gives us a fast way way to produce double
    //  precision numbers from longwords.
    //
    //  I know that this works for IEEE and VAX floating
    //  point representations.
    //
    //  A further complication is that gnu C will blow
    //  the following loop, unless compiled with -ffloat-store,
    //  because it uses extended representations for some of
    //  of the comparisons. Thus, we have the following hack.
    //  If we could specify #pragma optimize, we wouldn't need this.
    //

    PrivateRNGDoubleType t;
    PrivateRNGSingleType s;

#if _IEEE == 1 || defined(linux)
    
    t.d = 1.5;
    if ( t.u[1] == 0 ) {        // sun word order?
        t.u[0] = 0x3fffffff;
        t.u[1] = 0xffffffff;
    }
    else {
        t.u[0] = 0xffffffff;    // encore word order?
        t.u[1] = 0x3fffffff;
    }

    s.u = 0x3fffffff;
#else
    volatile double x = 1.0; // volatile needed when fp hardware used,
                             // and has greater precision than memory doubles
    double y = 0.5;
    do {                // find largest fp-number < 2.0
        t.d = x;
        x += y;
        y *= 0.5;
    } while (x != t.d && x < 2.0);

    volatile float xx = 1.0; // volatile needed when fp hardware used,
                             // and has greater precision than memory floats
    float yy = 0.5;
    do {                // find largest fp-number < 2.0
        s.s = xx;
        xx += yy;
        yy *= 0.5;
    } while (xx != s.s && xx < 2.0);
#endif
    // set doubleMantissa to 1 for each doubleMantissa bit
    doubleMantissa.d = 1.0;
    doubleMantissa.u[0] ^= t.u[0];
    doubleMantissa.u[1] ^= t.u[1];

    // set singleMantissa to 1 for each singleMantissa bit
    singleMantissa.s = 1.0;
    singleMantissa.u ^= s.u;

    initialized = 1;
    }
}

float RNG::asFloat()
{
    PrivateRNGSingleType result;
    result.s = 1.0;
    result.u |= (asLong() & singleMantissa.u);
    result.s -= 1.0;
    assert( result.s < 1.0 && result.s >= 0);
    return( result.s );
}
    
double RNG::asDouble()
{
    PrivateRNGDoubleType result;
    result.d = 1.0;
    result.u[0] |= (asLong() & doubleMantissa.u[0]);
    result.u[1] |= (asLong() & doubleMantissa.u[1]);
    result.d -= 1.0;
    assert( result.d < 1.0 && result.d >= 0);
    return( result.d );
}