pspectrum_string_kernel_impl.hpp

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#ifndef MLPACK_CORE_KERNELS_PSPECTRUM_STRING_KERNEL_IMPL_HPP
#define MLPACK_CORE_KERNELS_PSPECTRUM_STRING_KERNEL_IMPL_HPP

// In case it has not been included yet.
#include "pspectrum_string_kernel.hpp"

namespace mlpack {
namespace kernel {

template<typename VecType>
double PSpectrumStringKernel::Evaluate(const VecType& a,
                                       const VecType& b) const
{
  // Get the map of substrings for the two strings we are interested in.
  const std::map<std::string, int>& aMap = counts[a[0]][a[1]];
  const std::map<std::string, int>& bMap = counts[b[0]][b[1]];

  double eval = 0;

  // Loop through the two maps (which, when iterated through, are sorted
  // alphabetically).
  std::map<std::string, int>::const_iterator aIt = aMap.begin();
  std::map<std::string, int>::const_iterator bIt = bMap.begin();

  while ((aIt != aMap.end()) && (bIt != bMap.end()))
  {
    // Compare alphabetically (this is how std::map is ordered).
    int result = (*aIt).first.compare((*bIt).first);

    if (result == 0) // The same substring.
    {
      eval += ((*aIt).second * (*bIt).second);

      // Now increment both.
      ++aIt;
      ++bIt;
    }
    else if (result > 0)
    {
      // aIt is "ahead" of bIt (alphabetically); so increment bIt to "catch up".
      ++bIt;
    }
    else
    {
      // bIt is "ahead" of aIt (alphabetically); so increment aIt to "catch up".
      ++aIt;
    }
  }

  return eval;
}
} // namespace kernel
} // namespace mlpack

#endif