matchOneToOne.h

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//
// Utility function for matching elements of one vector to another
// using best matching distance. All pairwise distances are calculated
// and then sorted; the best match corresponds to the smallest
// distance. Both matched elements are removed from the pool,
// then the best match is found among the remaining elements, etc.
//
// I. Volobouev
// April 2008

#ifndef RecoJets_FFTJetAlgorithms_matchOneToOne_h
#define RecoJets_FFTJetAlgorithms_matchOneToOne_h

#include <vector>
#include <algorithm>

namespace fftjetcms {
  namespace Private {
    struct matchOneToOne_MatchInfo {
      double distance;
      unsigned i1;
      unsigned i2;

      inline bool operator<(const matchOneToOne_MatchInfo& r) const { return distance < r.distance; }

      inline bool operator>(const matchOneToOne_MatchInfo& r) const { return distance > r.distance; }
    };
  }  // namespace Private

  //
  // (*matchFrom1To2)[idx] will be set to the index of the element in v2
  // which corresponds to the element at index "idx" in v1. If no match
  // to the element at index "idx" in v1 is found, (*matchFrom1To2)[idx]
  // is set to -1. All non-negative (*matchFrom1To2)[idx] values will be
  // unique. The function returns the total number of matches made.
  //
  template <class T1, class T2, class DistanceCalculator>
  unsigned matchOneToOne(const std::vector<T1>& v1,
                         const std::vector<T2>& v2,
                         const DistanceCalculator& calc,
                         std::vector<int>* matchFrom1To2,
                         const double maxMatchingDistance = 1.0e300) {
    unsigned nused = 0;
    matchFrom1To2->clear();

    const unsigned n1 = v1.size();
    if (n1) {
      matchFrom1To2->reserve(n1);
      for (unsigned i1 = 0; i1 < n1; ++i1)
        matchFrom1To2->push_back(-1);

      const unsigned n2 = v2.size();
      if (n2) {
        const unsigned nmatches = n1 * n2;
        std::vector<Private::matchOneToOne_MatchInfo> distanceTable(nmatches);
        std::vector<int> taken2(n2);

        for (unsigned i2 = 0; i2 < n2; ++i2)
          taken2[i2] = 0;

        Private::matchOneToOne_MatchInfo* m;
        for (unsigned i1 = 0; i1 < n1; ++i1)
          for (unsigned i2 = 0; i2 < n2; ++i2) {
            m = &distanceTable[i1 * n2 + i2];
            m->distance = calc(v1[i1], v2[i2]);
            m->i1 = i1;
            m->i2 = i2;
          }

        std::sort(distanceTable.begin(), distanceTable.end());
        for (unsigned i = 0; i < nmatches && nused < n1 && nused < n2; ++i) {
          m = &distanceTable[i];
          if (m->distance > maxMatchingDistance)
            break;
          if ((*matchFrom1To2)[m->i1] < 0 && !taken2[m->i2]) {
            (*matchFrom1To2)[m->i1] = static_cast<int>(m->i2);
            taken2[m->i2] = 1;
            ++nused;
          }
        }
      }
    }

    return nused;
  }
}  // namespace fftjetcms

#endif  // RecoJets_FFTJetAlgorithms_matchOneToOne_h