//-----------------------------------------------------------------------------

//-----------------------------------------------------------------------------

// ZipfVariableImpl

class ZipfVariableImpl : public RandomVariableBase { 

public:

  /**

   * \param n the number of possible items

   * \param alpha the alpha parameter

   */

  ZipfVariableImpl (long n, double alpha);

  /**

   * \A zipf variable with N=1 and alpha=0

   */

  ZipfVariableImpl ();

  /**

   * \return A random value from this distribution

   */

  virtual double GetValue ();

  virtual RandomVariableBase* Copy(void) const;

private:

  long m_n;

  double m_alpha;

  double m_c; //the normalization constant

};

RandomVariableBase* ZipfVariableImpl::Copy () const

{

  return new ZipfVariableImpl (m_n, m_alpha);

}

ZipfVariableImpl::ZipfVariableImpl ()

    :m_n(1), m_alpha(0), m_c(1)

{

}

ZipfVariableImpl::ZipfVariableImpl (long n, double alpha)

    :m_n(n), m_alpha(alpha), m_c(0)

{

  //calculate the normalization constant c

  for(int i=1;i<=n;i++)

    m_c+=(1.0/pow((double)i,alpha));

  m_c=1.0/m_c;

}

double

ZipfVariableImpl::GetValue ()

{

  if(!m_generator)

  {

    m_generator = new RngStream();

  }

  double u = m_generator->RandU01();

  double sum_prob=0,zipf_value=0;

  for(int i=1;i<=m_n;i++)

  {

    sum_prob+=m_c/pow((double)i,m_alpha);

    if(sum_prob>u)

    {

      zipf_value=i;

      break;

    }

  }

  return zipf_value;

}

ZipfVariable::ZipfVariable ()

  : RandomVariable (ZipfVariableImpl ())

{}

ZipfVariable::ZipfVariable (long n, double alpha)

  : RandomVariable (ZipfVariableImpl (n, alpha))

{}