QCSimState.h

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#pragma once
 
#ifndef _QCSIMSTATE_H_
#define _QCSIMSTATE_H_
 
#ifdef INCLUDED_BY_FACTORY
 
#include <algorithm>
 
#include "Simulator.h"
 
#include "Clifford.h"
#include "MPSSimulator.h"
#include "QubitRegister.h"
 
#include "../TensorNetworks/ForestContractor.h"
#include "../TensorNetworks/TensorNetwork.h"
 
#include "../Utils/Alias.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 QCSimState : public ISimulator {
public:
  QCSimState() : rng(std::random_device{}()), uniformZeroOne(0, 1) {}

  void Initialize() override {
    if (nrQubits != 0) {
      if (simulationType == SimulationType::kMatrixProductState) {
        mpsSimulator =
            std::make_unique<QC::TensorNetworks::MPSSimulator>(nrQubits);
        if (limitEntanglement && singularValueThreshold > 0.)
          mpsSimulator->setLimitEntanglement(singularValueThreshold);
        if (limitSize && chi > 0)
          mpsSimulator->setLimitBondDimension(chi);
      } else if (simulationType == SimulationType::kStabilizer)
        cliffordSimulator =
            std::make_unique<QC::Clifford::StabilizerSimulator>(nrQubits);
      else if (simulationType == SimulationType::kTensorNetwork) {
        tensorNetwork =
            std::make_unique<TensorNetworks::TensorNetwork>(nrQubits);
        // for now the only used contractor is the forest one, but we'll use
        // more in the future
        const auto tensorContractor =
            std::make_shared<TensorNetworks::ForestContractor>();
        tensorNetwork->SetContractor(tensorContractor);
      } else
        state = std::make_unique<QC::QubitRegister<>>(nrQubits);
 
      SetMultithreading(enableMultithreading);
    }
  }

  void InitializeState(size_t num_qubits,
                       std::vector<std::complex<double>> &amplitudes) override {
    if (num_qubits == 0)
      return;
    Clear();
    nrQubits = num_qubits;
    Initialize();
    if (simulationType != SimulationType::kStatevector)
      throw std::runtime_error("QCSimState::InitializeState: Invalid "
                               "simulation type for initializing the state.");
 
    Eigen::VectorXcd amplitudesEigen(
        Eigen::Map<Eigen::VectorXcd, Eigen::Unaligned>(amplitudes.data(),
                                                       amplitudes.size()));
    state->setRegisterStorageFastNoNormalize(amplitudesEigen);
  }

  /*
  void InitializeState(size_t num_qubits, std::vector<std::complex<double>,
  avoid_init_allocator<std::complex<double>>>& amplitudes) override
  {
          Clear();
          nrQubits = num_qubits;
          Initialize();
          Eigen::VectorXcd amplitudesEigen(Eigen::Map<Eigen::VectorXcd,
  Eigen::Unaligned>(amplitudes.data(), amplitudes.size()));
          state->setRegisterStorageFastNoNormalize(amplitudesEigen);
  }
  */

#ifndef NO_QISKIT_AER
  void InitializeState(size_t num_qubits,
                       AER::Vector<std::complex<double>> &amplitudes) override {
    if (num_qubits == 0)
      return;
    Clear();
    nrQubits = num_qubits;
    Initialize();
    if (simulationType != SimulationType::kStatevector)
      throw std::runtime_error("QCSimState::InitializeState: Invalid "
                               "simulation type for initializing the state.");
 
    Eigen::VectorXcd amplitudesEigen(
        Eigen::Map<Eigen::VectorXcd, Eigen::Unaligned>(amplitudes.data(),
                                                       amplitudes.size()));
    state->setRegisterStorageFastNoNormalize(amplitudesEigen);
  }
#endif

  void InitializeState(size_t num_qubits,
                       Eigen::VectorXcd &amplitudes) override {
    if (num_qubits == 0)
      return;
    Clear();
    nrQubits = num_qubits;
    Initialize();
 
    if (simulationType != SimulationType::kStatevector)
      throw std::runtime_error("QCSimState::InitializeState: Invalid "
                               "simulation type for initializing the state.");
 
    state = std::make_unique<QC::QubitRegister<>>(nrQubits, amplitudes);
    state->SetMultithreading(enableMultithreading);
  }

  void Reset() override {
    if (mpsSimulator)
      mpsSimulator->Clear();
    else if (cliffordSimulator)
      cliffordSimulator->Reset();
    else if (tensorNetwork)
      tensorNetwork->Clear();
    else if (state)
      state->Reset();
  }

  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 if (std::string("matrix_product_state_truncation_threshold") ==
               key) {
      singularValueThreshold = std::stod(value);
      if (singularValueThreshold > 0.) {
        limitEntanglement = true;
        if (mpsSimulator)
          mpsSimulator->setLimitEntanglement(singularValueThreshold);
      } else
        limitEntanglement = false;
    } else if (std::string("matrix_product_state_max_bond_dimension") == key) {
      chi = std::stoi(value);
      if (chi > 0) {
        limitSize = true;
        if (mpsSimulator)
          mpsSimulator->setLimitBondDimension(chi);
      } else
        limitSize = false;
    } else if (std::string("mps_sample_measure_algorithm") == key)
      useMPSMeasureNoCollapse = std::string("mps_probabilities") == 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 {
    if ((simulationType == SimulationType::kStatevector && state) ||
        (simulationType == SimulationType::kMatrixProductState &&
         mpsSimulator) ||
        (simulationType == SimulationType::kStabilizer && cliffordSimulator) ||
        (simulationType == SimulationType::kTensorNetwork && tensorNetwork))
      return 0;
 
    const size_t oldNrQubits = nrQubits;
    nrQubits += num_qubits;
    return oldNrQubits;
  }

  size_t GetNumberOfQubits() const override { return nrQubits; }

  void Clear() override {
    state = nullptr;
    mpsSimulator = nullptr;
    cliffordSimulator = nullptr;
    tensorNetwork = nullptr;
    nrQubits = 0;
  }

  size_t Measure(const Types::qubits_vector &qubits) override {
    // TODO: this is inefficient, maybe implement it better in qcsim
    // for now it has the possibility of measuring a qubits interval, but not a
    // list of qubits
 
    size_t res = 0;
    size_t mask = 1ULL;
 
    DontNotify();
    if (simulationType == SimulationType::kStatevector) {
      for (size_t qubit : qubits) {
        if (state->MeasureQubit(static_cast<unsigned int>(qubit)))
          res |= mask;
        mask <<= 1;
      }
    } else if (simulationType == SimulationType::kStabilizer) {
      for (size_t qubit : qubits) {
        if (cliffordSimulator->MeasureQubit(static_cast<unsigned int>(qubit)))
          res |= mask;
        mask <<= 1;
      }
    } else if (simulationType == SimulationType::kTensorNetwork) {
      for (size_t qubit : qubits) {
        if (tensorNetwork->Measure(static_cast<unsigned int>(qubit)))
          res |= mask;
        mask <<= 1;
      }
    } else {
      /*
      for (size_t qubit : qubits)
      {
              if (mpsSimulator->MeasureQubit(static_cast<unsigned int>(qubit)))
                      res |= mask;
              mask <<= 1;
      }
      */
      const std::set<Eigen::Index> qubitsSet(qubits.begin(), qubits.end());
      auto measured = mpsSimulator->MeasureQubits(qubitsSet);
      for (Types::qubit_t qubit : qubits) {
        if (measured[qubit])
          res |= mask;
        mask <<= 1;
      }
    }
    Notify();
 
    NotifyObservers(qubits);
 
    return res;
  }

  void ApplyReset(const Types::qubits_vector &qubits) override {
    QC::Gates::PauliXGate xGate;
 
    DontNotify();
    if (simulationType == SimulationType::kStatevector) {
      for (size_t qubit : qubits)
        if (state->MeasureQubit(static_cast<unsigned int>(qubit)))
          state->ApplyGate(xGate, static_cast<unsigned int>(qubit));
    } else if (simulationType == SimulationType::kStabilizer) {
      for (size_t qubit : qubits)
        if (cliffordSimulator->MeasureQubit(static_cast<unsigned int>(qubit)))
          cliffordSimulator->ApplyX(static_cast<unsigned int>(qubit));
    } else if (simulationType == SimulationType::kTensorNetwork) {
      for (size_t qubit : qubits)
        if (tensorNetwork->Measure(static_cast<unsigned int>(qubit)))
          tensorNetwork->AddGate(xGate, static_cast<unsigned int>(qubit));
    } else {
      for (size_t qubit : qubits)
        if (mpsSimulator->MeasureQubit(static_cast<unsigned int>(qubit)))
          mpsSimulator->ApplyGate(xGate, static_cast<unsigned int>(qubit));
    }
    Notify();
 
    NotifyObservers(qubits);
  }

  double Probability(Types::qubit_t outcome) override {
    if (simulationType == SimulationType::kMatrixProductState)
      return mpsSimulator->getBasisStateProbability(
          static_cast<unsigned int>(outcome));
    else if (simulationType == SimulationType::kStabilizer)
      return cliffordSimulator->getBasisStateProbability(
          static_cast<unsigned int>(outcome));
    else if (simulationType == SimulationType::kTensorNetwork)
      return tensorNetwork->getBasisStateProbability(outcome);
 
    return state->getBasisStateProbability(static_cast<unsigned int>(outcome));
  }

  std::complex<double> Amplitude(Types::qubit_t outcome) override {
    if (simulationType == SimulationType::kMatrixProductState)
      return mpsSimulator->getBasisStateAmplitude(
          static_cast<unsigned int>(outcome));
    else if (simulationType == SimulationType::kStabilizer)
      throw std::runtime_error(
          "QCSimState::Amplitude: Invalid simulation type for obtaining the "
          "amplitude of the specified outcome.");
    else if (simulationType == SimulationType::kTensorNetwork)
      throw std::runtime_error("QCSimState::Amplitude: Not supported for the "
                               "tensor network simulator.");
 
    return state->getBasisStateAmplitude(static_cast<unsigned int>(outcome));
  }

  std::vector<double> AllProbabilities() override {
    // TODO: In principle this could be done, but why? It should be costly.
    if (simulationType == SimulationType::kTensorNetwork)
      throw std::runtime_error("QCSimState::AllProbabilities: Invalid "
                               "simulation type for obtaining probabilities.");
    else if (simulationType == SimulationType::kStabilizer)
      return cliffordSimulator->AllProbabilities();
 
    const Eigen::VectorXcd probs =
        simulationType == SimulationType::kMatrixProductState
            ? mpsSimulator->getRegisterStorage().cwiseAbs2()
            : state->getRegisterStorage().cwiseAbs2();
 
    std::vector<double> result(probs.size());
 
    for (int i = 0; i < probs.size(); ++i)
      result[i] = probs[i].real();
 
    return result;
  }

  std::vector<double>
  Probabilities(const Types::qubits_vector &qubits) override {
    if (simulationType == SimulationType::kStabilizer)
      throw std::runtime_error("QCSimState::Probabilities: Invalid simulation "
                               "type for obtaining probabilities.");
    else if (simulationType == SimulationType::kTensorNetwork) {
      // TODO: Implement this!!!
      throw std::runtime_error("QCSimState::Probabilities: Not implemented yet "
                               "for the tensor network simulator.");
    }
 
    std::vector<double> result(qubits.size());
 
    if (simulationType == SimulationType::kMatrixProductState) {
      for (int i = 0; i < static_cast<int>(qubits.size()); ++i)
        result[i] = mpsSimulator->getBasisStateProbability(qubits[i]);
    } else {
      const Eigen::VectorXcd &reg = state->getRegisterStorage();
 
      for (int i = 0; i < static_cast<int>(qubits.size()); ++i)
        result[i] = std::norm(reg[qubits[i]]);
    }
 
    return result;
  }

  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 {};
 
    // TODO: this is inefficient, maybe implement it better in qcsim
    // for now it has the possibility of measuring a qubits interval, but not a
    // list of qubits
    std::unordered_map<Types::qubit_t, Types::qubit_t> result;
 
    DontNotify();
 
    if (simulationType == SimulationType::kMatrixProductState) {
      bool normal = true;
      if (useMPSMeasureNoCollapse) {
        // check to see if it can be used
        std::unordered_set qset(qubits.begin(), qubits.end());
        if (qset.size() == GetNumberOfQubits()) {
          // it can!
          normal = false;
          for (size_t shot = 0; shot < shots; ++shot) {
            const size_t measRaw = MeasureNoCollapse();
            size_t meas = 0;
            size_t mask = 1ULL;
 
            // translate the measurement
            for (auto q : qubits) {
              const size_t qubitMask = 1ULL << q;
              if (measRaw & qubitMask)
                meas |= mask;
              mask <<= 1ULL;
            }
 
            ++result[meas];
          }
        }
      }
 
      if (normal) {
        auto savedState = mpsSimulator->getState();
        for (size_t shot = 0; shot < shots; ++shot) {
          const size_t meas = Measure(qubits);
          ++result[meas];
          mpsSimulator->setState(savedState);
        }
      }
    } else if (simulationType == SimulationType::kStabilizer) {
      cliffordSimulator->SaveState();
      for (size_t shot = 0; shot < shots; ++shot) {
        const size_t meas = Measure(qubits);
        ++result[meas];
        cliffordSimulator->RestoreState();
      }
      cliffordSimulator->ClearSavedState();
    } else if (simulationType == SimulationType::kTensorNetwork) {
      tensorNetwork->SaveState();
      for (size_t shot = 0; shot < shots; ++shot) {
        const size_t meas = Measure(qubits);
        ++result[meas];
        tensorNetwork->RestoreState();
      }
      tensorNetwork->ClearSavedState();
    } else {
      if (shots > 1) {
        const auto &statev = state->getRegisterStorage();
 
        const Utils::Alias alias(statev);
 
        for (size_t shot = 0; shot < shots; ++shot) {
          const double prob = 1. - uniformZeroOne(rng);
          const size_t measRaw = alias.Sample(prob);
 
          size_t meas = 0;
          size_t mask = 1ULL;
          for (auto q : qubits) {
            const size_t qubitMask = 1ULL << q;
            if ((measRaw & qubitMask) != 0)
              meas |= mask;
            mask <<= 1ULL;
          }
 
          ++result[meas];
        }
      } else {
        for (size_t shot = 0; shot < shots; ++shot) {
          const size_t measRaw = MeasureNoCollapse();
          size_t meas = 0;
          size_t mask = 1ULL;
 
          for (auto q : qubits) {
            const size_t qubitMask = 1ULL << q;
            if ((measRaw & qubitMask) != 0)
              meas |= mask;
            mask <<= 1ULL;
          }
 
          ++result[meas];
        }
      }
    }
 
    Notify();
    NotifyObservers(qubits);
 
    return result;
  }

  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());
    }
 
    if (simulationType == SimulationType::kStabilizer)
      return cliffordSimulator->ExpectationValue(pauliString);
    else if (simulationType == SimulationType::kTensorNetwork)
      return tensorNetwork->ExpectationValue(pauliString);
 
    // statevector or mps
    static const QC::Gates::PauliXGate<> xgate;
    static const QC::Gates::PauliYGate<> ygate;
    static const QC::Gates::PauliZGate<> zgate;
 
    std::vector<QC::Gates::AppliedGate<Eigen::MatrixXcd>> pauliStringVec;
    pauliStringVec.reserve(pauliString.size());
 
    for (size_t q = 0; q < pauliString.size(); ++q) {
      switch (toupper(pauliString[q])) {
      case 'X': {
        QC::Gates::AppliedGate<Eigen::MatrixXcd> ag(
            xgate.getRawOperatorMatrix(), static_cast<Types::qubit_t>(q));
        pauliStringVec.emplace_back(std::move(ag));
      } break;
      case 'Y': {
        QC::Gates::AppliedGate<Eigen::MatrixXcd> ag(
            ygate.getRawOperatorMatrix(), static_cast<Types::qubit_t>(q));
        pauliStringVec.emplace_back(std::move(ag));
      } break;
      case 'Z': {
        QC::Gates::AppliedGate<Eigen::MatrixXcd> ag(
            zgate.getRawOperatorMatrix(), static_cast<Types::qubit_t>(q));
        pauliStringVec.emplace_back(std::move(ag));
      } break;
      case 'I':
        [[fallthrough]];
      default:
        break;
      }
    }
 
    if (pauliStringVec.empty())
      return 1.0;
 
    if (simulationType == SimulationType::kMatrixProductState)
      return mpsSimulator->ExpectationValue(pauliStringVec).real();
 
    return state->ExpectationValue(pauliStringVec).real();
  }

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

  SimulationType GetSimulationType() const override { return simulationType; }

  void Flush() override {}

  void SaveStateToInternalDestructive() override {}

  void RestoreInternalDestructiveSavedState() override {}

  void SaveState() override {
    if (simulationType == SimulationType::kMatrixProductState)
      mpsSimulator->SaveState();
    else if (simulationType == SimulationType::kStabilizer)
      cliffordSimulator->SaveState();
    else if (simulationType == SimulationType::kTensorNetwork)
      tensorNetwork->SaveState();
    else
      state->SaveState();
  }

  void RestoreState() override {
    if (simulationType == SimulationType::kMatrixProductState)
      mpsSimulator->RestoreState();
    else if (simulationType == SimulationType::kStabilizer)
      cliffordSimulator->RestoreState();
    else if (simulationType == SimulationType::kTensorNetwork)
      tensorNetwork->RestoreState();
    else
      state->RestoreState();
  }

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

  void SetMultithreading(bool multithreading = true) override {
    enableMultithreading = multithreading;
    if (state)
      state->SetMultithreading(multithreading);
    if (cliffordSimulator)
      cliffordSimulator->SetMultithreading(multithreading);
    if (tensorNetwork)
      tensorNetwork->SetMultithreading(multithreading);
  }

  bool GetMultithreading() const override { return enableMultithreading; }

  bool IsQcsim() const override { return true; }

  Types::qubit_t MeasureNoCollapse() override {
    if (simulationType == SimulationType::kStatevector)
      return state->MeasureNoCollapse();
    else if (simulationType == SimulationType::kMatrixProductState) {
      const auto measured = mpsSimulator->MeasureNoCollapse();
      Types::qubit_t result = 0;
      Types::qubit_t mask = 1;
      for (const auto &meas : measured) {
        if (meas)
          result |= mask;
        mask <<= 1;
      }
      return result;
    }
 
    throw std::runtime_error(
        "QCSimState::MeasureNoCollapse: Invalid simulation type for measuring "
        "all the qubits without collapsing the state.");
 
    return 0;
  }
 
protected:
  SimulationType simulationType =
      SimulationType::kStatevector; 
 
  std::unique_ptr<QC::QubitRegister<>> state; 
  std::unique_ptr<QC::TensorNetworks::MPSSimulator>
      mpsSimulator; 
  std::unique_ptr<QC::Clifford::StabilizerSimulator>
      cliffordSimulator; 
  std::unique_ptr<TensorNetworks::TensorNetwork>
      tensorNetwork; 
 
  size_t nrQubits = 0; 
  bool limitSize = false;
  bool limitEntanglement = false;
  Eigen::Index chi = 10;              // if limitSize is true
  double singularValueThreshold = 0.; // if limitEntanglement is true
  bool enableMultithreading = true;   
  bool useMPSMeasureNoCollapse =
      true; 
 
  std::mt19937_64 rng;
  std::uniform_real_distribution<double> uniformZeroOne;
};
 
} // namespace Private
} // namespace Simulators
 
#endif
 
#endif // !_QCSIMSTATE_H_