mosesdecoder/moses/SearchNormal.cpp

#include "Manager.h"
#include "Timer.h"
#include "SearchNormal.h"
#include "SentenceStats.h"

#include <boost/foreach.hpp>

using namespace std;

namespace Moses
{
/**
 * Organizing main function
 *
 * /param source input sentence
 * /param transOptColl collection of translation options to be used for this sentence
 */
SearchNormal::SearchNormal(Manager& manager, const InputType &source, const TranslationOptionCollection &transOptColl)
  :Search(manager)
  ,m_source(source)
  ,m_hypoStackColl(source.GetSize() + 1)
  ,interrupted_flag(0)
  ,m_transOptColl(transOptColl)
{
  VERBOSE(1, "Translating: " << m_source << endl);
  const StaticData &staticData = StaticData::Instance();

  // only if constraint decoding (having to match a specified output)
  // long sentenceID = source.GetTranslationId();

  // initialize the stacks: create data structure and set limits
  std::vector < HypothesisStackNormal >::iterator iterStack;
  for (size_t ind = 0 ; ind < m_hypoStackColl.size() ; ++ind) {
    HypothesisStackNormal *sourceHypoColl = new HypothesisStackNormal(m_manager);
    sourceHypoColl->SetMaxHypoStackSize(staticData.GetMaxHypoStackSize(),
                                        staticData.GetMinHypoStackDiversity());
    sourceHypoColl->SetBeamWidth(staticData.GetBeamWidth());

    m_hypoStackColl[ind] = sourceHypoColl;
  }
}

SearchNormal::~SearchNormal()
{
  RemoveAllInColl(m_hypoStackColl);
}

/**
 * Main decoder loop that translates a sentence by expanding
 * hypotheses stack by stack, until the end of the sentence.
 */
void SearchNormal::Decode()
{
  const StaticData &staticData = StaticData::Instance();
  SentenceStats &stats = m_manager.GetSentenceStats();

  // initial seed hypothesis: nothing translated, no words produced
  Hypothesis *hypo = Hypothesis::Create(m_manager,m_source, m_initialTransOpt);
  m_hypoStackColl[0]->AddPrune(hypo);

  // go through each stack
  std::vector < HypothesisStack* >::iterator iterStack;
  for (iterStack = m_hypoStackColl.begin() ; iterStack != m_hypoStackColl.end() ; ++iterStack) {
    // check if decoding ran out of time
    double _elapsed_time = GetUserTime();
    if (_elapsed_time > staticData.GetTimeoutThreshold()) {
      VERBOSE(1,"Decoding is out of time (" << _elapsed_time << "," << staticData.GetTimeoutThreshold() << ")" << std::endl);
      interrupted_flag = 1;
      return;
    }
    HypothesisStackNormal &sourceHypoColl = *static_cast<HypothesisStackNormal*>(*iterStack);

    // the stack is pruned before processing (lazy pruning):
    VERBOSE(3,"processing hypothesis from next stack");
    IFVERBOSE(2) {
      stats.StartTimeStack();
    }
    sourceHypoColl.PruneToSize(staticData.GetMaxHypoStackSize());
    VERBOSE(3,std::endl);
    sourceHypoColl.CleanupArcList();
    IFVERBOSE(2) {
      stats.StopTimeStack();
    }

    // go through each hypothesis on the stack and try to expand it
    HypothesisStackNormal::const_iterator iterHypo;
    for (iterHypo = sourceHypoColl.begin() ; iterHypo != sourceHypoColl.end() ; ++iterHypo) {
      Hypothesis &hypothesis = **iterHypo;
      ProcessOneHypothesis(hypothesis); // expand the hypothesis
    }
    // some logging
    IFVERBOSE(2) {
      OutputHypoStackSize();
    }

    // this stack is fully expanded;
    actual_hypoStack = &sourceHypoColl;

  }
  //OutputHypoStack();
}


/** Find all translation options to expand one hypothesis, trigger expansion
 * this is mostly a check for overlap with already covered words, and for
 * violation of reordering limits.
 * \param hypothesis hypothesis to be expanded upon
 */
void
SearchNormal::
ProcessOneHypothesis(const Hypothesis &hypothesis)
{
  // since we check for reordering limits, its good to have that limit handy
  int maxDistortion = StaticData::Instance().GetMaxDistortion();
  bool isWordLattice = StaticData::Instance().GetInputType() == WordLatticeInput;

  const WordsBitmap hypoBitmap = hypothesis.GetWordsBitmap();
  const size_t hypoFirstGapPos = hypoBitmap.GetFirstGapPos();
  size_t const sourceSize = m_source.GetSize();

  ReorderingConstraint const&
  ReoConstraint = m_source.GetReorderingConstraint();

  // no limit of reordering: only check for overlap
  if (maxDistortion < 0) {

    for (size_t startPos = hypoFirstGapPos ; startPos < sourceSize ; ++startPos) {
      TranslationOptionList const* tol;
      size_t endPos = startPos;
      for (tol = m_transOptColl.GetTranslationOptionList(startPos, endPos);
           tol && endPos < sourceSize;
           tol = m_transOptColl.GetTranslationOptionList(startPos, ++endPos)) {
        if (tol->size() == 0
            || hypoBitmap.Overlap(WordsRange(startPos, endPos))
            || !ReoConstraint.Check(hypoBitmap, startPos, endPos)) {
          continue;
        }

        //TODO: does this method include incompatible WordLattice hypotheses?
        ExpandAllHypotheses(hypothesis, startPos, endPos);
      }
    }
    return; // done with special case (no reordering limit)
  }

  // There are reordering limits. Make sure they are not violated.

  WordsRange prevRange = hypothesis.GetCurrSourceWordsRange();
  for (size_t startPos = hypoFirstGapPos ; startPos < sourceSize ; ++startPos) {

    // don't bother expanding phrases if the first position is already taken
    if(hypoBitmap.GetValue(startPos)) continue;

    size_t maxSize = sourceSize - startPos;
    size_t maxSizePhrase = StaticData::Instance().GetMaxPhraseLength();
    maxSize = (maxSize < maxSizePhrase) ? maxSize : maxSizePhrase;
    size_t closestLeft = hypoBitmap.GetEdgeToTheLeftOf(startPos);

    if (isWordLattice) {
      // first question: is there a path from the closest translated word to the left
      // of the hypothesized extension to the start of the hypothesized extension?
      // long version:
      // - is there anything to our left?
      // - is it farther left than where we're starting anyway?
      // - can we get to it?

      // closestLeft is exclusive: a value of 3 means 2 is covered, our
      // arc is currently ENDING at 3 and can start at 3 implicitly
      if (closestLeft != 0 && closestLeft != startPos
          && !m_source.CanIGetFromAToB(closestLeft, startPos))
        continue;

      if (prevRange.GetStartPos() != NOT_FOUND &&
          prevRange.GetStartPos() > startPos &&
          !m_source.CanIGetFromAToB(startPos, prevRange.GetStartPos()))
        continue;
    }

    WordsRange currentStartRange(startPos, startPos);
    if(m_source.ComputeDistortionDistance(prevRange, currentStartRange)
        > maxDistortion)
      continue;

    TranslationOptionList const* tol;
    size_t endPos = startPos;
    for (tol = m_transOptColl.GetTranslationOptionList(startPos, endPos);
         tol && endPos < sourceSize;
         tol = m_transOptColl.GetTranslationOptionList(startPos, ++endPos)) {
      WordsRange extRange(startPos, endPos);
      if (tol->size() == 0
          || hypoBitmap.Overlap(extRange)
          || !ReoConstraint.Check(hypoBitmap, startPos, endPos)
          || (isWordLattice && !m_source.IsCoveragePossible(extRange))) {
        continue;
      }

      // ask second question here: we already know we can get to our
      // starting point from the closest thing to the left. We now ask the
      // follow up: can we get from our end to the closest thing on the
      // right?
      //
      // long version: is anything to our right? is it farther
      // right than our (inclusive) end? can our end reach it?
      bool isLeftMostEdge = (hypoFirstGapPos == startPos);

      size_t closestRight = hypoBitmap.GetEdgeToTheRightOf(endPos);
      if (isWordLattice) {
        if (closestRight != endPos
            && ((closestRight + 1) < sourceSize)
            && !m_source.CanIGetFromAToB(endPos + 1, closestRight + 1)) {
          continue;
        }
      }

      if (isLeftMostEdge) {
        // any length extension is okay if starting at left-most edge
        ExpandAllHypotheses(hypothesis, startPos, endPos);
      } else { // starting somewhere other than left-most edge, use caution
        // the basic idea is this: we would like to translate a phrase
        // starting from a position further right than the left-most
        // open gap. The distortion penalty for the following phrase
        // will be computed relative to the ending position of the
        // current extension, so we ask now what its maximum value will
        // be (which will always be the value of the hypothesis starting
        // at the left-most edge).  If this value is less than the
        // distortion limit, we don't allow this extension to be made.
        WordsRange bestNextExtension(hypoFirstGapPos, hypoFirstGapPos);

        if (m_source.ComputeDistortionDistance(extRange, bestNextExtension)
            > maxDistortion) continue;

        // everything is fine, we're good to go
        ExpandAllHypotheses(hypothesis, startPos, endPos);
      }
    }
  }
}


/**
 * Expand a hypothesis given a list of translation options
 * \param hypothesis hypothesis to be expanded upon
 * \param startPos first word position of span covered
 * \param endPos last word position of span covered
 */

void
SearchNormal::
ExpandAllHypotheses(const Hypothesis &hypothesis, size_t startPos, size_t endPos)
{
  // early discarding: check if hypothesis is too bad to build
  // this idea is explained in (Moore&Quirk, MT Summit 2007)
  float expectedScore = 0.0f;
  if (StaticData::Instance().UseEarlyDiscarding()) {
    // expected score is based on score of current hypothesis
    expectedScore = hypothesis.GetScore();

    // add new future score estimate
    expectedScore +=
      m_transOptColl.GetFutureScore()
      .CalcFutureScore(hypothesis.GetWordsBitmap(), startPos, endPos);
  }

  // loop through all translation options
  const TranslationOptionList* tol
  = m_transOptColl.GetTranslationOptionList(startPos, endPos);
  if (!tol) return;
  TranslationOptionList::const_iterator iter;
  for (iter = tol->begin() ; iter != tol->end() ; ++iter) {
    ExpandHypothesis(hypothesis, **iter, expectedScore);
  }
}

/**
 * Expand one hypothesis with a translation option.
 * this involves initial creation, scoring and adding it to the proper stack
 * \param hypothesis hypothesis to be expanded upon
 * \param transOpt translation option (phrase translation)
 *        that is applied to create the new hypothesis
 * \param expectedScore base score for early discarding
 *        (base hypothesis score plus future score estimation)
 */
void SearchNormal::ExpandHypothesis(const Hypothesis &hypothesis, const TranslationOption &transOpt, float expectedScore)
{
  const StaticData &staticData = StaticData::Instance();
  SentenceStats &stats = m_manager.GetSentenceStats();

  Hypothesis *newHypo;
  if (! staticData.UseEarlyDiscarding()) {
    // simple build, no questions asked
    IFVERBOSE(2) {
      stats.StartTimeBuildHyp();
    }
    newHypo = hypothesis.CreateNext(transOpt);
    IFVERBOSE(2) {
      stats.StopTimeBuildHyp();
    }
    if (newHypo==NULL) return;
    newHypo->EvaluateWhenApplied(m_transOptColl.GetFutureScore());
  } else
    // early discarding: check if hypothesis is too bad to build
  {
    // worst possible score may have changed -> recompute
    size_t wordsTranslated = hypothesis.GetWordsBitmap().GetNumWordsCovered() + transOpt.GetSize();
    float allowedScore = m_hypoStackColl[wordsTranslated]->GetWorstScore();
    if (staticData.GetMinHypoStackDiversity()) {
      WordsBitmapID id = hypothesis.GetWordsBitmap().GetIDPlus(transOpt.GetStartPos(), transOpt.GetEndPos());
      float allowedScoreForBitmap = m_hypoStackColl[wordsTranslated]->GetWorstScoreForBitmap( id );
      allowedScore = std::min( allowedScore, allowedScoreForBitmap );
    }
    allowedScore += staticData.GetEarlyDiscardingThreshold();

    // add expected score of translation option
    expectedScore += transOpt.GetFutureScore();

    // check if transOpt score push it already below limit
    if (expectedScore < allowedScore) {
      IFVERBOSE(2) {
        stats.AddNotBuilt();
      }
      return;
    }

    // build the hypothesis without scoring
    IFVERBOSE(2) {
      stats.StartTimeBuildHyp();
    }
    newHypo = hypothesis.CreateNext(transOpt);
    if (newHypo==NULL) return;
    IFVERBOSE(2) {
      stats.StopTimeBuildHyp();
    }

    // ... and check if that is below the limit
    if (expectedScore < allowedScore) {
      IFVERBOSE(2) {
        stats.AddEarlyDiscarded();
      }
      FREEHYPO( newHypo );
      return;
    }

  }

  // logging for the curious
  IFVERBOSE(3) {
    newHypo->PrintHypothesis();
  }

  // add to hypothesis stack
  size_t wordsTranslated = newHypo->GetWordsBitmap().GetNumWordsCovered();
  IFVERBOSE(2) {
    stats.StartTimeStack();
  }
  m_hypoStackColl[wordsTranslated]->AddPrune(newHypo);
  IFVERBOSE(2) {
    stats.StopTimeStack();
  }
}

const std::vector < HypothesisStack* >& SearchNormal::GetHypothesisStacks() const
{
  return m_hypoStackColl;
}

/**
 * Find best hypothesis on the last stack.
 * This is the end point of the best translation, which can be traced back from here
 */
const Hypothesis *SearchNormal::GetBestHypothesis() const
{
  if (interrupted_flag == 0) {
    const HypothesisStackNormal &hypoColl = *static_cast<HypothesisStackNormal*>(m_hypoStackColl.back());
    return hypoColl.GetBestHypothesis();
  } else {
    const HypothesisStackNormal &hypoColl = *actual_hypoStack;
    return hypoColl.GetBestHypothesis();
  }
}

/**
 * Logging of hypothesis stack sizes
 */
void SearchNormal::OutputHypoStackSize()
{
  std::vector < HypothesisStack* >::const_iterator iterStack = m_hypoStackColl.begin();
  TRACE_ERR( "Stack sizes: " << (int)(*iterStack)->size());
  for (++iterStack; iterStack != m_hypoStackColl.end() ; ++iterStack) {
    TRACE_ERR( ", " << (int)(*iterStack)->size());
  }
  TRACE_ERR( endl);
}

void SearchNormal::OutputHypoStack()
{
  // all stacks
  int i = 0;
  vector < HypothesisStack* >::iterator iterStack;
  for (iterStack = m_hypoStackColl.begin() ; iterStack != m_hypoStackColl.end() ; ++iterStack) {
    HypothesisStackNormal &hypoColl = *static_cast<HypothesisStackNormal*>(*iterStack);
    TRACE_ERR( "Stack " << i++ << ": " << endl << hypoColl << endl);
  }
}

}