Program Listing for File qaoa_features.hpp

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#ifndef QAOA_EXTRA_FEATURES_HPP
#define QAOA_EXTRA_FEATURES_HPP

#include <vector>

#include "qureg.hpp"

namespace qaoa
{

// This function uses the IQS vector not as a quantum state but as a vector of size
// 2^num_qubits. This function initializes such vector to represent the diagonal of
// a classical Hamiltonian. Specifically, the cost function is that of the MaxCut problem
// based on the graph instance passed in input via its adjacency matrix.
// The graph has undirected edges of uniform weigth.
//
// It returns the max number of edges that can be cut.

  template<typename Type>
  int InitializeVectorAsMaxCutCostFunction(QubitRegister<Type> & diag,
                                           std::vector<int> & adjacency);

  template<typename Type>
  typename QubitRegister<Type>::BaseType
  InitializeVectorAsWeightedMaxCutCostFunction(QubitRegister<Type> & diag,
      std::vector<typename QubitRegister<Type>::BaseType> & adjacency);

// Implement the QAOA operation:
//     |psi> <== exp(-i C gamma) |psi>
// for classical Hamiltonian C.
// The diagonal of C is provided as a (not-normalized) IQS vector.

  template<typename Type>
  void ImplementQaoaLayerBasedOnCostFunction(QubitRegister<Type> & psi,
                                             QubitRegister<Type> & diag,
                                             typename QubitRegister<Type>::BaseType gamma);

// Return the expectation value of the cost function.

  template<typename Type>
  typename QubitRegister<Type>::BaseType
  GetExpectationValueFromCostFunction(const QubitRegister<Type> & psi,
                                      const QubitRegister<Type> & diag);

// Return the expectation value of the (cost function)^2.

  template<typename Type>
  typename QubitRegister<Type>::BaseType
  GetExpectationValueSquaredFromCostFunction(const QubitRegister<Type> & psi,
                                             const QubitRegister<Type> & diag);

// Return the histogram with the probability of measuring a graph coloring
// associated to a specific value of the cost function.

  template<typename Type>
  std::vector<typename QubitRegister<Type>::BaseType>
  GetHistogramFromCostFunction(const QubitRegister<Type> & psi,
                               const QubitRegister<Type> & diag, int max_value);

// Return the histogram with the probability of measuring a graph coloring
// associated to a specific value of the cost function for a weighted graph with cutvals rounded down.

  template<typename Type>
  std::vector<typename QubitRegister<Type>::BaseType>
  GetHistogramFromCostFunctionWithWeightsRounded(const QubitRegister<Type> & psi,
                                                 const QubitRegister<Type> & diag,
                                                 double max_value);

// Return the histogram with the probability of measuring a graph coloring
// associated to a specific value of the cost function for a weighted graph binned
// to a specified bin_width.

  template<typename Type>
  std::vector<typename QubitRegister<Type>::BaseType>
  GetHistogramFromCostFunctionWithWeightsBinned(const QubitRegister<Type> & psi,
                                                const QubitRegister<Type> & diag,
                                                double max_value, double bin_width);


}       // namespace 'qaoa'

#endif      // end of guards QAOA_EXTRA_FEATURES_HPP