qiskit/transpiler/passes/mapping/stochastic_swap.py
def _layer_permutation(self, layer_partition, layout, qubit_subset,
                           coupling, trials):
        """Find a swap circuit that implements a permutation for this layer.

        The goal is to swap qubits such that qubits in the same two-qubit gates
        are adjacent.

        Based on S. Bravyi's algorithm.

        layer_partition (list): The layer_partition is a list of (qu)bit
            lists and each qubit is a tuple (qreg, index).
        layout (Layout): The layout is a Layout object mapping virtual
            qubits in the input circuit to physical qubits in the coupling
            graph. It reflects the current positions of the data.
        qubit_subset (list): The qubit_subset is the set of qubits in
            the coupling graph that we have chosen to map into, as tuples
            (Register, index).
        coupling (CouplingMap): Directed graph representing a coupling map.
            This coupling map should be one that was provided to the
            stochastic mapper.
        trials (int): Number of attempts the randomized algorithm makes.

        Returns:
            Tuple: success_flag, best_circuit, best_depth, best_layout, trivial_flag

        If success_flag is True, then best_circuit contains a DAGCircuit with
        the swap circuit, best_depth contains the depth of the swap circuit,
        and best_layout contains the new positions of the data qubits after the
        swap circuit has been applied. The trivial_flag is set if the layer
        has no multi-qubit gates.

        Raises:
            TranspilerError: if anything went wrong.
     """
        return _layer_permutation(layer_partition, self.initial_layout,
                                  layout, qubit_subset,
                                  coupling, trials,
                                  self.qregs, self.rng)
def _layer_permutation(self, layer_partition, layout, qubit_subset,
                           coupling, trials):
        """Find a swap circuit that implements a permutation for this layer.

        The goal is to swap qubits such that qubits in the same two-qubit gates
        are adjacent.

        Based on S. Bravyi's algorithm.

        layer_partition (list): The layer_partition is a list of (qu)bit
            lists and each qubit is a tuple (qreg, index).
        layout (Layout): The layout is a Layout object mapping virtual
            qubits in the input circuit to physical qubits in the coupling
            graph. It reflects the current positions of the data.
        qubit_subset (list): The qubit_subset is the set of qubits in
            the coupling graph that we have chosen to map into, as tuples
            (Register, index).
        coupling (CouplingMap): Directed graph representing a coupling map.
            This coupling map should be one that was provided to the
            stochastic mapper.
        trials (int): Number of attempts the randomized algorithm makes.

        Returns:
            Tuple: success_flag, best_circuit, best_depth, best_layout, trivial_flag

        If success_flag is True, then best_circuit contains a DAGCircuit with
        the swap circuit, best_depth contains the depth of the swap circuit,
        and best_layout contains the new positions of the data qubits after the
        swap circuit has been applied. The trivial_flag is set if the layer
        has no multi-qubit gates.

        Raises:
            TranspilerError: if anything went wrong.
     """
        return _layer_permutation(layer_partition, self.initial_layout,
                                  layout, qubit_subset,
                                  coupling, trials,
                                  self.qregs, self.rng)
Find a swap circuit that implements a permutation for this layer.

        The goal is to swap qubits such that qubits in the same two-qubit gates
        are adjacent.

        Based on S. Bravyi's algorithm.

        layer_partition (list): The layer_partition is a list of (qu)bit
            lists and each qubit is a tuple (qreg, index).
        layout (Layout): The layout is a Layout object mapping virtual
            qubits in the input circuit to physical qubits in the coupling
            graph. It reflects the current positions of the data.
        qubit_subset (list): The qubit_subset is the set of qubits in
            the coupling graph that we have chosen to map into, as tuples
            (Register, index).
        coupling (CouplingMap): Directed graph representing a coupling map.
            This coupling map should be one that was provided to the
            stochastic mapper.
        trials (int): Number of attempts the randomized algorithm makes.

        Returns:
            Tuple: success_flag, best_circuit, best_depth, best_layout, trivial_flag

        If success_flag is True, then best_circuit contains a DAGCircuit with
        the swap circuit, best_depth contains the depth of the swap circuit,
        and best_layout contains the new positions of the data qubits after the
        swap circuit has been applied. The trivial_flag is set if the layer
        has no multi-qubit gates.

        Raises:
            TranspilerError: if anything went wrong.