CombinationsIterator.h

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/*
  Copyright (C) 2002-2026 CERN for the benefit of the ATLAS collaboration
*/
 
#ifndef TRIG_HLTJETHYPO_COMBINATIONSITERATOR_H
#define TRIG_HLTJETHYPO_COMBINATIONSITERATOR_H
 
#include "CombinationsGenerator.h"
#include "TrigHLTJetHypo/TrigHLTJetHypoUtils/HypoJetDefs.h"
 
#include <vector>
#include <iterator>
#include <iosfwd>
 
/*
 * CombinatonsIterator is a forward iterator. It iterates over
 * combinations (n choose k) gnerated by a CombinationsGenerator instance.
 * of a HypoJetVector.
 *
 * Dereferencing the iterator returns a HypoJetVector corresponding
 * to the current combination.
 *
 * CombinationsIterator can generates the
 * end of iteration CombinationsIterator
 */
 
class CombinationsIterator {
 
 public:
  using iterator_category = std::forward_iterator_tag;
  using difference_type = std::ptrdiff_t;
  using value_type = HypoJetVector;
  using reference = HypoJetVector&;
  using pointer = HypoJetVector*;
 
 
  friend std::ostream& operator << (std::ostream&, const CombinationsIterator&);
  CombinationsIterator(std::size_t k,
               const HypoJetVector& input_vals,
               bool end=false);
 
  CombinationsIterator() {}
 
 
  reference operator*() {return m_vals;}
 
 
  pointer operator->() {return &m_vals;}
 
  // pre-increment
  CombinationsIterator& operator++() {
    m_end = m_gen.bump();
    auto indices = m_gen.get();
 
    // overwrite m_vals according to the combinations indicies
    std::transform(indices.cbegin(),
           indices.cend(),
           m_vals.begin(),
           [iv = this->m_input_vals](const auto& ind) {
             return iv.at(ind);
           });
    
    return *this;
  }
 
  // post-increment
  CombinationsIterator operator++(int) {
    CombinationsIterator tmp = *this;
    ++(*this);
    return tmp;
  }
 
 
  // create an end of iteration marker from a CombinationsIterator
  CombinationsIterator endIter() const;
 
  // enable an equality test with an end iterator
  friend bool operator==(const CombinationsIterator& a,
             const CombinationsIterator& b) {
  return a.m_end == b.m_end and
    a.m_k == b.m_k and
    a.m_input_vals == b.m_input_vals;
    
  
  }
 
 
  // enable an inequality test with an end iterator
  friend bool operator!=(const CombinationsIterator& a,
             const CombinationsIterator& b) {
    return !(a==b);
  }
 
 
 private:
  CombinationsGenerator m_gen{0,0};
  std::size_t m_k{0};
 
  HypoJetVector m_input_vals;
  HypoJetVector m_vals;
 
  bool m_end{false};
};
 
std::ostream& operator << (std::ostream& os, const CombinationsIterator& iter);
 
 
#endif