noisy_circuit.h

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/*
Copyright 2025 SQUANDER Contributors

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

#pragma once

#include "density_matrix.h"
#include "density_operation.h"
#include <memory>
#include <vector>

// Forward declarations for gate types
class Gate;
class Gates_block;

namespace squander {
namespace density {

class NoisyCircuit {
public:
  // ================================================================
  // Constructors & Destructor
  // ================================================================

  explicit NoisyCircuit(int qbit_num);

  ~NoisyCircuit();

  // Disable copy (operations own their resources)
  NoisyCircuit(const NoisyCircuit &) = delete;
  NoisyCircuit &operator=(const NoisyCircuit &) = delete;

  // Move semantics
  NoisyCircuit(NoisyCircuit &&other) noexcept;
  NoisyCircuit &operator=(NoisyCircuit &&other) noexcept;

  // ================================================================
  // Single-Qubit Gates (constant - no parameters)
  // ================================================================

  void add_H(int target);
  void add_X(int target);
  void add_Y(int target);
  void add_Z(int target);
  void add_S(int target);
  void add_Sdg(int target);
  void add_T(int target);
  void add_Tdg(int target);
  void add_SX(int target);

  // ================================================================
  // Single-Qubit Parametric Gates
  // ================================================================

  void add_RX(int target); 
  void add_RY(int target); 
  void add_RZ(int target); 
  void add_U1(int target); 
  void add_U2(int target); 
  void add_U3(int target); 

  // ================================================================
  // Two-Qubit Gates (constant)
  // ================================================================

  void add_CNOT(int target, int control);
  void add_CZ(int target, int control);
  void add_CH(int target, int control);

  // ================================================================
  // Two-Qubit Parametric Gates
  // ================================================================

  void add_CRY(int target, int control); 
  void add_CRZ(int target, int control); 
  void add_CRX(int target, int control); 
  void add_CP(int target, int control);  

  // ================================================================
  // Noise Channel Addition
  // ================================================================

  void add_depolarizing(int qbit_num);

  void add_depolarizing(int qbit_num, double error_rate);

  void add_local_depolarizing(int target);

  void add_local_depolarizing(int target, double error_rate);

  void add_amplitude_damping(int target);

  void add_amplitude_damping(int target, double gamma);

  void add_phase_damping(int target);

  void add_phase_damping(int target, double lambda);

  // ================================================================
  // Circuit Execution
  // ================================================================

  void apply_to(const double *params, int param_count, DensityMatrix &rho);

  void apply_to(const matrix_base<double> &params, DensityMatrix &rho);

  // ================================================================
  // Properties
  // ================================================================

  int get_qbit_num() const { return qbit_num_; }

  int get_parameter_num() const { return total_params_; }

  size_t get_operation_count() const { return operations_.size(); }

  // ================================================================
  // Inspection
  // ================================================================

  struct OperationInfo {
    std::string name;
    bool is_unitary;
    int param_count;
    int param_start;
  };

  std::vector<OperationInfo> get_operation_info() const;

private:
  int qbit_num_;
  std::vector<std::unique_ptr<IDensityOperation>> operations_;
  std::vector<int> param_starts_;
  int total_params_ = 0;

  // Helper to add an operation and update parameter tracking
  void add_operation(std::unique_ptr<IDensityOperation> op);

  // Helper to add a gate by type
  void add_gate_internal(Gate *gate);
};

} // namespace density
} // namespace squander