AerState.h

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#pragma once
 
#ifndef _AER_STATE_H_
#define _AER_STATE_H_
 
#ifndef NO_QISKIT_AER
 
#ifdef INCLUDED_BY_FACTORY
 
#include <algorithm>
 
#include "QubitRegister.h"
#include "Simulator.h"
 
#include "QiskitAerState.h"
 
namespace Simulators {
// TODO: Maybe use the pimpl idiom
// https://en.cppreference.com/w/cpp/language/pimpl to hide the implementation
// for good but during development this should be good enough
namespace Private {
 
class IndividualSimulator;

class AerState : public ISimulator {
  friend class IndividualSimulator; 
public:
  AerState() {
    std::random_device rd;
 
    rng.seed(rd());
    Configure("method", "statevector");
  }

  void Initialize() override {
    SetMultithreading(enableMultithreading);
    if (simulationType == SimulationType::kMatrixProductState)
      Configure("mps_sample_measure_algorithm", useMPSMeasureNoCollapse
                                                    ? "mps_probabilities"
                                                    : "mps_apply_measure");
    state->initialize();
  }

  void InitializeState(size_t num_qubits,
                       std::vector<std::complex<double>> &amplitudes) override {
    Clear();
    state->initialize_statevector(num_qubits, amplitudes.data(), true);
  }

  /*
  void InitializeState(size_t num_qubits, std::vector<std::complex<double>,
  avoid_init_allocator<std::complex<double>>>& amplitudes) override
  {
          Clear();
          state->initialize_statevector(num_qubits, amplitudes.data(), true);
  }
  */

  void InitializeState(size_t num_qubits,
                       AER::Vector<std::complex<double>> &amplitudes) override {
    Clear();
    state->initialize_statevector(num_qubits, amplitudes.move_to_buffer(),
                                  false);
  }

  void InitializeState(size_t num_qubits,
                       Eigen::VectorXcd &amplitudes) override {
    Clear();
    state->initialize_statevector(num_qubits, amplitudes.data(), true);
  }

  void Reset() override {
    const auto numQubits = GetNumberOfQubits();
    Clear();
    AllocateQubits(numQubits);
    state->initialize();
  }

  void Configure(const char *key, const char *value) override {
    if (std::string("method") == key) {
      if (std::string("statevector") == value)
        simulationType = SimulationType::kStatevector;
      else if (std::string("matrix_product_state") == value)
        simulationType = SimulationType::kMatrixProductState;
      else if (std::string("stabilizer") == value)
        simulationType = SimulationType::kStabilizer;
      else if (std::string("tensor_network") == value)
        simulationType = SimulationType::kTensorNetwork;
      else
        simulationType = SimulationType::kOther;
    } else if (std::string("matrix_product_state_truncation_threshold") ==
               key) {
      singularValueThreshold = std::stod(value);
      if (singularValueThreshold > 0.)
        limitEntanglement = true;
      else
        limitEntanglement = false;
    } else if (std::string("matrix_product_state_max_bond_dimension") == key) {
      chi = std::stoi(value);
      if (chi > 0)
        limitSize = true;
      else
        limitSize = false;
    } else if (std::string("mps_sample_measure_algorithm") == key)
      useMPSMeasureNoCollapse = std::string("mps_probabilities") == value;
 
    state->configure(key, value);
  }

  std::string GetConfiguration(const char *key) const override {
    if (std::string("method") == key) {
      switch (simulationType) {
      case SimulationType::kStatevector:
        return "statevector";
      case SimulationType::kMatrixProductState:
        return "matrix_product_state";
      case SimulationType::kStabilizer:
        return "stabilizer";
      case SimulationType::kTensorNetwork:
        return "tensor_network";
      default:
        return "other";
      }
    } else if (std::string("matrix_product_state_truncation_threshold") ==
               key) {
      if (limitEntanglement && singularValueThreshold > 0.)
        return std::to_string(singularValueThreshold);
    } else if (std::string("matrix_product_state_max_bond_dimension") == key) {
      if (limitSize && limitSize > 0)
        return std::to_string(chi);
    } else if (std::string("mps_sample_measure_algorithm") == key) {
      return useMPSMeasureNoCollapse ? "mps_probabilities"
                                     : "mps_apply_measure";
    }
 
    return "";
  }

  size_t AllocateQubits(size_t num_qubits) override {
    const auto ids = state->allocate_qubits(num_qubits);
    return ids[0];
  }

  size_t GetNumberOfQubits() const override {
    if (state->is_initialized())
      return state->num_of_qubits();
 
    return 0;
  }

  void Clear() override {
    state->clear();
    SetMultithreading(enableMultithreading);
 
    if (simulationType == SimulationType::kMatrixProductState)
      Configure("mps_sample_measure_algorithm", useMPSMeasureNoCollapse
                                                    ? "mps_probabilities"
                                                    : "mps_apply_measure");
  }

  size_t Measure(const Types::qubits_vector &qubits) override {
    const size_t res = state->apply_measure(qubits);
 
    NotifyObservers(qubits);
 
    return res;
  }

  void ApplyReset(const Types::qubits_vector &qubits) override {
    state->apply_reset(qubits);
 
    NotifyObservers(qubits);
  }

  double Probability(Types::qubit_t outcome) override {
    return state->probability(outcome);
  }

  std::complex<double> Amplitude(Types::qubit_t outcome) override {
    return state->amplitude(outcome);
  }

  std::vector<double> AllProbabilities() override {
    return state->probabilities();
  }

  std::vector<double>
  Probabilities(const Types::qubits_vector &qubits) override {
    return state->probabilities(qubits);
  }

  std::unordered_map<Types::qubit_t, Types::qubit_t>
  SampleCounts(const Types::qubits_vector &qubits,
               size_t shots = 1000) override {
    if (qubits.empty() || shots == 0)
      return {};
 
    const std::unordered_map<Types::qubit_t, Types::qubit_t> res =
        state->sample_counts(qubits, shots);
 
    NotifyObservers(qubits);
 
    return res;
  }

  double ExpectationValue(const std::string &pauliStringOrig) override {
    if (pauliStringOrig.empty())
      return 1.0;
 
    std::string pauliString = pauliStringOrig;
    if (pauliString.size() > GetNumberOfQubits()) {
      for (size_t i = GetNumberOfQubits(); i < pauliString.size(); ++i) {
        const auto pauliOp = toupper(pauliString[i]);
        if (pauliOp != 'I' && pauliOp != 'Z')
          return 0.0;
      }
 
      pauliString.resize(GetNumberOfQubits());
    }
 
    AER::reg_t qubits;
    std::string pauli;
 
    pauli.reserve(pauliString.size());
    qubits.reserve(pauliString.size());
 
    for (size_t q = 0; q < pauliString.size(); ++q) {
      const char p = toupper(pauliString[q]);
      if (p == 'I')
        continue;
 
      pauli.push_back(p);
      qubits.push_back(q);
    }
 
    if (qubits.empty())
      return 1.0;
 
    // qiskit aer expects the pauli string in reverse order
    std::reverse(pauli.begin(), pauli.end());
 
    return state->expval_pauli(qubits, pauli);
  }

  SimulatorType GetType() const override { return SimulatorType::kQiskitAer; }

  SimulationType GetSimulationType() const override { return simulationType; }

  void Flush() override {
    state->flush_ops();
    // state->set_random_seed(); // avoid reusing the old seed
  }

  void SaveStateToInternalDestructive() override {
    savedAmplitudes = state->move_to_vector();
  }

  void RestoreInternalDestructiveSavedState() override {
    const size_t numQubits = static_cast<size_t>(log2(savedAmplitudes.size()));
    state->initialize_statevector(numQubits, savedAmplitudes.move_to_buffer(),
                                  false);
  }

  void SaveState() override {
    if (!state)
      return;
 
    const auto numQubits = GetNumberOfQubits();
 
    if (simulationType == SimulationType::kStatevector) {
      SaveStateToInternalDestructive();
      state->initialize_statevector(numQubits, savedAmplitudes.data(), true);
      return;
    }
 
    bool saved = false;
 
    if (state->is_initialized()) {
      AER::Operations::Op op;
 
      op.type = AER::Operations::OpType::save_state;
      op.name = "save_state";
      op.save_type = AER::Operations::DataSubType::single;
      op.string_params.push_back("s");
 
      for (size_t q = 0; q < numQubits; ++q)
        op.qubits.push_back(q);
 
      state->buffer_op(std::move(op));
      Flush();
 
      // get the state from the last result
      AER::ExperimentResult &last_result = state->last_result();
      // state should be in last_result.data
      if (last_result.status == AER::ExperimentResult::Status::completed) {
        savedState = std::move(last_result.data);
        saved = true;
      }
    } else {
      // try get the state from the last result
      AER::ExperimentResult &last_result_prev = state->last_result();
      // state should be in last_result.data
      if (last_result_prev.status == AER::ExperimentResult::Status::completed) {
        savedState = std::move(last_result_prev.data);
        saved = true;
      }
    }
 
    // this is a hack, for statevector and matrix product state if the last op
    // is executed, it can destroy the state! see also the workaround for
    // statevector for the stabilizer at least for now it doesn't seem to do
    // that
    // TODO: check everything!!!!
    if (saved && simulationType == SimulationType::kMatrixProductState)
      RestoreState();
  }

  void RestoreState() override {
    auto numQubits = GetNumberOfQubits();
 
    AER::Operations::Op op;
 
    switch (simulationType) {
    case SimulationType::kStatevector: {
      // op.type = AER::Operations::OpType::set_statevec;
      // op.name = "set_statevec";
 
      // const auto& vec = static_cast<AER::DataMap<AER::SingleData,
      // AER::Vector<complex_t>>>(savedState).value()["s"].value();
 
      // this is a hack until I figure it out
      Clear();
      numQubits = static_cast<size_t>(log2(savedAmplitudes.size()));
      state->initialize_statevector(numQubits, savedAmplitudes.data(), true);
 
      return;
    } break;
    case SimulationType::kMatrixProductState:
      op.type = AER::Operations::OpType::set_mps;
      op.name = "set_mps";
      op.mps = static_cast<AER::DataMap<AER::SingleData, AER::mps_container_t>>(
                   savedState)
                   .value()["s"]
                   .value();
 
      /*
      {
              auto& value = static_cast<AER::DataMap<AER::SingleData,
      AER::mps_container_t>>(savedState).value(); if (!value.empty())
              {
                      if (value.find("s") != value.end())
                              op.mps = value["s"].value();
                      else if (value.find("matrix_product_state") !=
      value.end()) op.mps = value["matrix_product_state"].value();
              }
      }
      */
 
      numQubits = op.mps.first.size();
      break;
    case SimulationType::kStabilizer:
      op.type = AER::Operations::OpType::set_stabilizer;
      op.name = "set_stabilizer";
      op.clifford =
          static_cast<AER::DataMap<AER::SingleData, json_t>>(savedState)
              .value()["s"]
              .value();
 
      /*
      {
              auto& value = static_cast<AER::DataMap<AER::SingleData,
      json_t>>(savedState).value(); if (!value.empty())
              {
                      if (value.find("s") != value.end())
                              op.clifford = value["s"].value();
                      else if (value.find("stabilizer") != value.end())
                              op.clifford = value["stabilizer"].value();
              }
      }
      */
 
      numQubits = op.clifford.num_qubits();
      break;
    case SimulationType::kTensorNetwork:
    default:
      throw std::runtime_error("AerState::RestoreState: not implemented yet "
                               "for this type of simulator.");
    }
 
    op.save_type = AER::Operations::DataSubType::single;
    op.string_params.push_back("s");
 
    for (size_t q = 0; q < numQubits; ++q)
      op.qubits.push_back(q);
 
    // WHY?
    if (!state->is_initialized()) {
      Clear();
      AllocateQubits(numQubits);
      state->initialize();
    }
 
    state->buffer_op(std::move(op));
    Flush();
  }

  std::complex<double> AmplitudeRaw(Types::qubit_t outcome) override {
    return savedAmplitudes[outcome];
  }

  void SetMultithreading(bool multithreading = true) override {
    enableMultithreading = multithreading;
    if (state && !state->is_initialized()) {
      const std::string nrThreads =
          std::to_string(enableMultithreading ? 0 : 1); // 0 means auto/all available, 1 limits to 1
      state->configure("max_parallel_threads", nrThreads);
      state->configure("parallel_state_update", nrThreads);
      const std::string threadsLimit = std::to_string(12); // set one less, multithreading is started if the value is bigger than this
      state->configure("statevector_parallel_threshold", threadsLimit);
    }
  }

  bool GetMultithreading() const override { return enableMultithreading; }

  bool IsQcsim() const override { return false; }

  Types::qubit_t MeasureNoCollapse() override {
    if (simulationType == SimulationType::kStatevector) {
      const double prob =
          1. - uniformZeroOne(rng); // this excludes 0 as probabiliy
      double accum = 0;
      Types::qubit_t state = 0;
      for (Types::qubit_t i = 0; i < savedAmplitudes.size(); ++i) {
        accum += std::norm(savedAmplitudes[i]);
        if (prob <= accum) {
          state = i;
          break;
        }
      }
 
      return state;
    }
 
    throw std::runtime_error(
        "AerState::MeasureNoCollapse: Invalid simulation type for measuring "
        "all the qubits without collapsing the state.");
 
    return 0;
  }
 
protected:
  SimulationType simulationType =
      SimulationType::kStatevector; 
  std::unique_ptr<QiskitAerState> state =
      std::make_unique<QiskitAerState>(); 
  AER::Vector<complex_t> savedAmplitudes; 
  bool limitSize = false;
  bool limitEntanglement = false;
  Eigen::Index chi = 10;              // if limitSize is true
  double singularValueThreshold = 0.; // if limitEntanglement is true
  bool enableMultithreading = true;   
  AER::Data savedState; 
  std::mt19937_64 rng;
  std::uniform_real_distribution<double> uniformZeroOne{0., 1.};
  bool useMPSMeasureNoCollapse =
      true; 
};
 
} // namespace Private
} // namespace Simulators
 
#endif
 
#endif
 
#endif // !_AER_STATE_H_