// $Id$

/***********************************************************************
Moses - factored phrase-based language decoder
Copyright (C) 2006 University of Edinburgh

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 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
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
***********************************************************************/

#ifndef moses_LanguageModel_h
#define moses_LanguageModel_h

#include <string>
#include <cstddef>
#include "../FeatureFunction.h"

namespace Moses
{

class FactorCollection;
class Factor;
class Phrase;

//! Abstract base class which represent a language model on a contiguous phrase
class LanguageModel : public StatefulFeatureFunction {
protected:
  LanguageModel();

  // This can't be in the constructor for virual function dispatch reasons

  bool m_enableOOVFeature;
  
public:
  virtual ~LanguageModel();

  // Make another feature without copying the underlying model data.  
  virtual LanguageModel *Duplicate() const = 0;

  bool OOVFeatureEnabled() const {
    return m_enableOOVFeature;
  }

  float GetWeight() const;
  float GetOOVWeight() const;

  std::string GetScoreProducerWeightShortName(unsigned) const {
    return "lm";
  }

  virtual void InitializeBeforeSentenceProcessing() {}

  virtual void CleanUpAfterSentenceProcessing() {}

  virtual const FFState* EmptyHypothesisState(const InputType &input) const = 0;

  /* whether this LM can be used on a particular phrase.
   * Should return false if phrase size = 0 or factor types required don't exists
   */
  virtual bool Useable(const Phrase &phrase) const = 0;

  /* calc total unweighted LM score of this phrase and return score via arguments.
   * Return scores should always be in natural log, regardless of representation with LM implementation.
   * Uses GetValue() of inherited class.
   * Useable() should be called beforehand on the phrase
   * \param fullScore scores of all unigram, bigram... of contiguous n-gram of the phrase
   * \param ngramScore score of only n-gram of order m_nGramOrder
   * \param oovCount number of LM OOVs
   */
  virtual void CalcScore(const Phrase &phrase, float &fullScore, float &ngramScore, std::size_t &oovCount) const = 0;
};

}

#endif