cosine_search.hpp

Go to the documentation of this file.

#ifndef MLPACK_METHODS_CF_COSINE_SEARCH_HPP
#define MLPACK_METHODS_CF_COSINE_SEARCH_HPP

#include <mlpack/prereqs.hpp>
#include <mlpack/methods/neighbor_search/neighbor_search.hpp>

namespace mlpack {
namespace cf {

class CosineSearch
{
 public:
  CosineSearch(const arma::mat& referenceSet)
  {
    // Normalize all vectors to unit length.
    arma::mat normalizedSet = arma::normalise(referenceSet, 2, 0);

    neighborSearch.Train(std::move(normalizedSet));
  }

  void Search(const arma::mat& query, const size_t k,
              arma::Mat<size_t>& neighbors, arma::mat& similarities)
  {
    // Normalize query vectors to unit length.
    arma::mat normalizedQuery = arma::normalise(query, 2, 0);

    neighborSearch.Search(normalizedQuery, k, neighbors, similarities);

    // Resulting similarities from Search() are Euclidean distance.
    // For unit vectors a and b, cos(a, b) = 1 - dis(a, b) ^ 2 / 2,
    // where dis(a, b) is Euclidean distance.
    // Furthermore, we restrict the range of similarity to be [0, 1]:
    // similarities = (cos(a,b) + 1) / 2.0. As a result we have the following
    // formula.
    similarities = 1 - arma::pow(similarities, 2) / 4.0;
  }

 private:
  neighbor::KNN neighborSearch;
};

} // namespace cf
} // namespace mlpack

#endif