packed_index.hpp

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// Copyright (c) 2013-2023 Anton Kozhevnikov, Thomas Schulthess
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/** \file packed_index.hpp
 *
 *  \brief Packed index for symmetric matrices.
 */
 
#ifndef __PACKED_INDEX_HPP__
#define __PACKED_INDEX_HPP__
 
namespace sirius {
 
/// Pack two indices into one for symmetric matrices.
inline int packed_index(int i__, int j__)
{
    /* suppose we have a symmetric matrix: M_{ij} = M_{ji}
           j
       +-------+
       | + + + |
      i|   + + |   -> idx = j * (j + 1) / 2 + i  for  i <= j
       |     + |
       +-------+
 
       i, j are row and column indices 
    */
 
    if (i__ > j__) {
        std::swap(i__, j__);
    }
    return j__ * (j__ + 1) / 2 + i__;
}
 
}
 
#endif