DilogarithmVariable.cc

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// -*- C++ -*-

// PLearn (A C++ Machine Learning Library)
// Copyright (C) 1998 Pascal Vincent
// Copyright (C) 1999-2002 Pascal Vincent, Yoshua Bengio, Rejean Ducharme and University of Montreal
// Copyright (C) 2001-2002 Nicolas Chapados, Ichiro Takeuchi, Jean-Sebastien Senecal
// Copyright (C) 2002 Xiangdong Wang, Christian Dorion

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/* *******************************************************      
   * $Id: DilogarithmVariable.cc,v 1.6 2004/04/27 16:02:26 morinf Exp $
   * This file is part of the PLearn library.
   ******************************************************* */

#include "DilogarithmVariable.h"

namespace PLearn {
using namespace std;


PLEARN_IMPLEMENT_OBJECT(DilogarithmVariable, 
                        "This Var computes the dilogarithm function", 
                        "The dilogarithm function is useful to compute the primitive of the softplus.\n"
                        "  dilogarithm(x) = sum_{k=1}^\\infty x^k/k^2\n"
                        "so dilogarithm'(x) = -(1/x)log(1-x), i.e. e^x dilogarithm'(-e^x)=log(1+e^x)=softplus(x)\n"
                        "and primitive(softplus)(x) = -dilogarithm(-e^x)\n");

DilogarithmVariable::DilogarithmVariable(Variable* input) 
  : inherited(input, input->length(), input->width())
{}


void DilogarithmVariable::recomputeSize(int& l, int& w) const
{
    if (input) {
        l = input->length();
        w = input->width();
    } else
        l = w = 0;
}

void DilogarithmVariable::fprop()
{
  for(int i=0; i<nelems(); i++)
  {
    valuedata[i] = dilogarithm(input->valuedata[i]);
#ifdef BOUNDCHECK
    if (is_missing(valuedata[i]))
        PLWARNING("dilogarithm returned NaN");
#endif
  }
}


void DilogarithmVariable::bprop()
{
  for(int i=0; i<nelems(); i++)
  {
    real xi = input->valuedata[i];
    if (xi!=0)
      input->gradientdata[i] -= gradientdata[i] * log1p(-xi)/xi;
  }
}


void DilogarithmVariable::symbolicBprop()
{
  PLERROR("DilogarithmVariable::symbolicBprop() not implemented");
}

} // end of namespace PLearn

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