TGMert.cpp

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//Copyright (c) 2011, University of Cambridge
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#include "TGMert.h"
#include "function-weight.h"
#include "fast-shortest-distance.h"
#include "mapping-shortest-path.h"

void const ComputeShortestPath (const fst::VectorFst<FunctionArc>& fst,
                                double x, MertLine& line) {
  fst::VectorFst < fst::ArcTpl<fst::TropicalWeightTpl<double> > > pathFst;
  mertfst::SingleShortestPath (fst, &pathFst, x);
  TopSort (&pathFst);
  for (fst::StateIterator
       < fst::VectorFst<fst::ArcTpl<fst::TropicalWeightTpl<double> > >
       > siter (pathFst); !siter.Done(); siter.Next() ) {
    for (fst::ArcIterator
         < fst::VectorFst<fst::ArcTpl<fst::TropicalWeightTpl<double> > >
         > aiter (pathFst, siter.Value() ); !aiter.Done(); aiter.Next() ) {
      line.t.push_back (aiter.Value().olabel);
      line.score += aiter.Value().weight.Value();
    }
  }
}

MertList ComputeFromFunctionArc (const fst::VectorFst<FunctionArc>& fst) {
  FunctionWeight sd = mertfst::ShortestDistance (fst);
  // Need to handle first case.
  const MertList& mlconst = sd.Value();
  // Hacky but am really getting fed up with all constness
  // and don't want to keep on copying this list
  MertList& ml = const_cast<MertList&> (mlconst);
  if (ml.size() == 1) {
    ComputeShortestPath (fst, 0, ml.front() );
  } else {
    double x = (++ml.begin() )->x;
    ComputeShortestPath (fst, x - 1, ml.front() );
    MertIter prev = ++ml.begin();
    for (MertIter next =
           ++ (++ml.begin() ); next != sd.Value().end(); ++next) {
      // Sometimes can end up with a very large gamma interval in the region of 1e16. When this happens the
      // midpoint is also huge. Any shortest path computations get corrupted for that interval. Therefore use this
      // little hack to protect ourselves from it. The value of 1000 is a magic number and has no significance
      // other than it empirically seems OK.
      double midpoint;
      if ( (next->x - prev->x) / 2 > 1000) {
        if (abs (prev->x) < abs (next->x) ) {
          midpoint = prev->x + 1000;
        } else {
          midpoint = next->x - 1000;
        }
      } else {
        //Do normal midpoint calc
        midpoint = (prev->x + next->x) / 2.0;
      }
      ComputeShortestPath (fst, midpoint, *prev);
      ++prev;
    }
    ComputeShortestPath (fst, prev->x + 1, ml.back() );
  }
  return ml;
}