TwoElectronIntegralIndexes.h

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#ifndef SPARROW_TWOELECTRONINTEGRALINDEXES_H
#define SPARROW_TWOELECTRONINTEGRALINDEXES_H

#include <array>

namespace Scine {
namespace Sparrow {

namespace nddo {

class TwoElectronIntegralIndexes {
 public:
  using orb_index_t = int;
  using orbPair_index_t = int;
  using Array = std::array<std::array<orbPair_index_t, 9>, 9>;

  static orbPair_index_t getPairIndex(orb_index_t i1, orb_index_t i2) {
    return getIndex(i1, i2);
  }

  static bool pairIsInvalid(orb_index_t i1, orb_index_t i2) {
    return (getIndex(i1, i2) == invalidPair);
  }

 private:
  // Creates the index array if it was not yet done and returns the index
  static orbPair_index_t getIndex(orb_index_t i1, orb_index_t i2) {
    static Array pairIndexes = createUniqueIndexes();
    return pairIndexes[i1][i2];
  }

  // Array is created as exclusively invalidPair code except the valid pairs
  // where the casted orbital pair value is given
  static Array createUniqueIndexes();

  // Code for the invalid pair
  static constexpr orbPair_index_t invalidPair = 100;
};

} // namespace nddo

} // namespace Sparrow
} // namespace Scine
#endif // SPARROW_TWOELECTRONINTEGRALINDEXES_H