qiskit/transpiler/passes/mapping/stochastic_swap.py
def _mapper(self, circuit_graph, coupling_graph,
                trials=20):
        """Map a DAGCircuit onto a CouplingMap using swap gates.

        Use self.initial_layout for the initial layout.

        Args:
            circuit_graph (DAGCircuit): input DAG circuit
            coupling_graph (CouplingMap): coupling graph to map onto
            trials (int): number of trials.

        Returns:
            DAGCircuit: object containing a circuit equivalent to
                circuit_graph that respects couplings in coupling_graph
            Layout: a layout object mapping qubits of circuit_graph into
                qubits of coupling_graph. The layout may differ from the
                initial_layout if the first layer of gates cannot be
                executed on the initial_layout, since in this case
                it is more efficient to modify the layout instead of swapping
            Dict: a final-layer qubit permutation

        Raises:
            TranspilerError: if there was any error during the mapping
                or with the parameters.
        """
        # Schedule the input circuit by calling layers()
        layerlist = list(circuit_graph.layers())
        logger.debug("schedule:")
        for i, v in enumerate(layerlist):
            logger.debug("    %d: %s", i, v["partition"])

        if self.initial_layout is not None:
            qubit_subset = self.initial_layout.get_virtual_bits().keys()
        else:
            # Supply a default layout for this dag
            self.initial_layout = Layout()
            physical_qubit = 0
            for qreg in circuit_graph.qregs.values():
                for index in range(qreg.size):
                    self.initial_layout[(qreg, index)] = physical_qubit
                    physical_qubit += 1
            qubit_subset = self.initial_layout.get_virtual_bits().keys()
            # Restrict the coupling map to the image of the layout
            coupling_graph = coupling_graph.subgraph(
                self.initial_layout.get_physical_bits().keys())
            if coupling_graph.size() < len(self.initial_layout):
                raise TranspilerError("Coupling map too small for default layout")
            self.input_layout = self.initial_layout.copy()

        # Find swap circuit to preceed to each layer of input circuit
        layout = self.initial_layout.copy()

        # Construct an empty DAGCircuit with the same set of
        # qregs and cregs as the input circuit
        dagcircuit_output = DAGCircuit()
        dagcircuit_output.name = circuit_graph.name
        for qreg in circuit_graph.qregs.values():
            dagcircuit_output.add_qreg(qreg)
        for creg in circuit_graph.cregs.values():
            dagcircuit_output.add_creg(creg)

        # Make a trivial wire mapping between the subcircuits
        # returned by _layer_update and the circuit we build
        identity_wire_map = {}
        for qubit in circuit_graph.qubits():
            identity_wire_map[qubit] = qubit
        for bit in circuit_graph.clbits():
            identity_wire_map[bit] = bit

        first_layer = True  # True until first layer is output
        logger.debug("initial_layout = %s", layout)

        # Iterate over layers
        for i, layer in enumerate(layerlist):

            # Attempt to find a permutation for this layer
            success_flag, best_circuit, best_depth, best_layout, trivial_flag \
                = self._layer_permutation(layer["partition"], layout,
                                          qubit_subset, coupling_graph,
                                          trials)
            logger.debug("mapper: layer %d", i)
            logger.debug("mapper: success_flag=%s,best_depth=%s,trivial_flag=%s",
                         success_flag, str(best_depth), trivial_flag)

            # If this fails, try one gate at a time in this layer
            if not success_flag:
                logger.debug("mapper: failed, layer %d, "
                             "retrying sequentially", i)
                serial_layerlist = list(layer["graph"].serial_layers())

                # Go through each gate in the layer
                for j, serial_layer in enumerate(serial_layerlist):

                    success_flag, best_circuit, best_depth, best_layout, trivial_flag = \
                        self._layer_permutation(
                            serial_layer["partition"],
                            layout, qubit_subset,
                            coupling_graph,
                            trials)
                    logger.debug("mapper: layer %d, sublayer %d", i, j)
                    logger.debug("mapper: success_flag=%s,best_depth=%s,"
                                 "trivial_flag=%s",
                                 success_flag, str(best_depth), trivial_flag)

                    # Give up if we fail again
                    if not success_flag:
                        raise TranspilerError("swap mapper failed: " +
                                              "layer %d, sublayer %d" % (i, j))

                    # If this layer is only single-qubit gates,
                    # and we have yet to see multi-qubit gates,
                    # continue to the next inner iteration
                    if trivial_flag and first_layer:
                        logger.debug("mapper: skip to next sublayer")
                        continue

                    if first_layer:
                        self.initial_layout = layout

                    # Update the record of qubit positions
                    # for each inner iteration
                    layout = best_layout
                    # Update the DAG
                    dagcircuit_output.extend_back(
                        self._layer_update(j,
                                           first_layer,
                                           best_layout,
                                           best_depth,
                                           best_circuit,
                                           serial_layerlist),
                        identity_wire_map)
                    if first_layer:
                        first_layer = False

            else:
                # Update the record of qubit positions for each iteration
                layout = best_layout

                if first_layer:
                    self.initial_layout = layout

                # Update the DAG
                dagcircuit_output.extend_back(
                    self._layer_update(i,
                                       first_layer,
                                       best_layout,
                                       best_depth,
                                       best_circuit,
                                       layerlist),
                    identity_wire_map)

                if first_layer:
                    first_layer = False

        # This is the final edgemap. We might use it to correctly replace
        # any measurements that needed to be removed earlier.
        logger.debug("mapper: self.initial_layout = %s", pformat(self.initial_layout))
        logger.debug("mapper: layout = %s", pformat(layout))
        last_edgemap = layout.combine_into_edge_map(self.initial_layout)
        logger.debug("mapper: last_edgemap = %s", pformat(last_edgemap))

        # If first_layer is still set, the circuit only has single-qubit gates
        # so we can use the initial layout to output the entire circuit
        # This code is dead due to changes to first_layer above.
        if first_layer:
            logger.debug("mapper: first_layer flag still set")
            layout = self.initial_layout
            for i, layer in enumerate(layerlist):
                edge_map = layout.combine_into_edge_map(self.initial_layout)
                dagcircuit_output.compose_back(layer["graph"], edge_map)

        return dagcircuit_output
def _mapper(self, circuit_graph, coupling_graph,
                trials=20):
        """Map a DAGCircuit onto a CouplingMap using swap gates.

        Use self.initial_layout for the initial layout.

        Args:
            circuit_graph (DAGCircuit): input DAG circuit
            coupling_graph (CouplingMap): coupling graph to map onto
            trials (int): number of trials.

        Returns:
            DAGCircuit: object containing a circuit equivalent to
                circuit_graph that respects couplings in coupling_graph
            Layout: a layout object mapping qubits of circuit_graph into
                qubits of coupling_graph. The layout may differ from the
                initial_layout if the first layer of gates cannot be
                executed on the initial_layout, since in this case
                it is more efficient to modify the layout instead of swapping
            Dict: a final-layer qubit permutation

        Raises:
            TranspilerError: if there was any error during the mapping
                or with the parameters.
        """
        # Schedule the input circuit by calling layers()
        layerlist = list(circuit_graph.layers())
        logger.debug("schedule:")
        for i, v in enumerate(layerlist):
            logger.debug("    %d: %s", i, v["partition"])

        if self.initial_layout is not None:
            qubit_subset = self.initial_layout.get_virtual_bits().keys()
        else:
            # Supply a default layout for this dag
            self.initial_layout = Layout()
            physical_qubit = 0
            for qreg in circuit_graph.qregs.values():
                for index in range(qreg.size):
                    self.initial_layout[(qreg, index)] = physical_qubit
                    physical_qubit += 1
            qubit_subset = self.initial_layout.get_virtual_bits().keys()
            # Restrict the coupling map to the image of the layout
            coupling_graph = coupling_graph.subgraph(
                self.initial_layout.get_physical_bits().keys())
            if coupling_graph.size() < len(self.initial_layout):
                raise TranspilerError("Coupling map too small for default layout")
            self.input_layout = self.initial_layout.copy()

        # Find swap circuit to preceed to each layer of input circuit
        layout = self.initial_layout.copy()

        # Construct an empty DAGCircuit with the same set of
        # qregs and cregs as the input circuit
        dagcircuit_output = DAGCircuit()
        dagcircuit_output.name = circuit_graph.name
        for qreg in circuit_graph.qregs.values():
            dagcircuit_output.add_qreg(qreg)
        for creg in circuit_graph.cregs.values():
            dagcircuit_output.add_creg(creg)

        # Make a trivial wire mapping between the subcircuits
        # returned by _layer_update and the circuit we build
        identity_wire_map = {}
        for qubit in circuit_graph.qubits():
            identity_wire_map[qubit] = qubit
        for bit in circuit_graph.clbits():
            identity_wire_map[bit] = bit

        first_layer = True  # True until first layer is output
        logger.debug("initial_layout = %s", layout)

        # Iterate over layers
        for i, layer in enumerate(layerlist):

            # Attempt to find a permutation for this layer
            success_flag, best_circuit, best_depth, best_layout, trivial_flag \
                = self._layer_permutation(layer["partition"], layout,
                                          qubit_subset, coupling_graph,
                                          trials)
            logger.debug("mapper: layer %d", i)
            logger.debug("mapper: success_flag=%s,best_depth=%s,trivial_flag=%s",
                         success_flag, str(best_depth), trivial_flag)

            # If this fails, try one gate at a time in this layer
            if not success_flag:
                logger.debug("mapper: failed, layer %d, "
                             "retrying sequentially", i)
                serial_layerlist = list(layer["graph"].serial_layers())

                # Go through each gate in the layer
                for j, serial_layer in enumerate(serial_layerlist):

                    success_flag, best_circuit, best_depth, best_layout, trivial_flag = \
                        self._layer_permutation(
                            serial_layer["partition"],
                            layout, qubit_subset,
                            coupling_graph,
                            trials)
                    logger.debug("mapper: layer %d, sublayer %d", i, j)
                    logger.debug("mapper: success_flag=%s,best_depth=%s,"
                                 "trivial_flag=%s",
                                 success_flag, str(best_depth), trivial_flag)

                    # Give up if we fail again
                    if not success_flag:
                        raise TranspilerError("swap mapper failed: " +
                                              "layer %d, sublayer %d" % (i, j))

                    # If this layer is only single-qubit gates,
                    # and we have yet to see multi-qubit gates,
                    # continue to the next inner iteration
                    if trivial_flag and first_layer:
                        logger.debug("mapper: skip to next sublayer")
                        continue

                    if first_layer:
                        self.initial_layout = layout

                    # Update the record of qubit positions
                    # for each inner iteration
                    layout = best_layout
                    # Update the DAG
                    dagcircuit_output.extend_back(
                        self._layer_update(j,
                                           first_layer,
                                           best_layout,
                                           best_depth,
                                           best_circuit,
                                           serial_layerlist),
                        identity_wire_map)
                    if first_layer:
                        first_layer = False

            else:
                # Update the record of qubit positions for each iteration
                layout = best_layout

                if first_layer:
                    self.initial_layout = layout

                # Update the DAG
                dagcircuit_output.extend_back(
                    self._layer_update(i,
                                       first_layer,
                                       best_layout,
                                       best_depth,
                                       best_circuit,
                                       layerlist),
                    identity_wire_map)

                if first_layer:
                    first_layer = False

        # This is the final edgemap. We might use it to correctly replace
        # any measurements that needed to be removed earlier.
        logger.debug("mapper: self.initial_layout = %s", pformat(self.initial_layout))
        logger.debug("mapper: layout = %s", pformat(layout))
        last_edgemap = layout.combine_into_edge_map(self.initial_layout)
        logger.debug("mapper: last_edgemap = %s", pformat(last_edgemap))

        # If first_layer is still set, the circuit only has single-qubit gates
        # so we can use the initial layout to output the entire circuit
        # This code is dead due to changes to first_layer above.
        if first_layer:
            logger.debug("mapper: first_layer flag still set")
            layout = self.initial_layout
            for i, layer in enumerate(layerlist):
                edge_map = layout.combine_into_edge_map(self.initial_layout)
                dagcircuit_output.compose_back(layer["graph"], edge_map)

        return dagcircuit_output
Map a DAGCircuit onto a CouplingMap using swap gates.

        Use self.initial_layout for the initial layout.

        Args:
            circuit_graph (DAGCircuit): input DAG circuit
            coupling_graph (CouplingMap): coupling graph to map onto
            trials (int): number of trials.

        Returns:
            DAGCircuit: object containing a circuit equivalent to
                circuit_graph that respects couplings in coupling_graph
            Layout: a layout object mapping qubits of circuit_graph into
                qubits of coupling_graph. The layout may differ from the
                initial_layout if the first layer of gates cannot be
                executed on the initial_layout, since in this case
                it is more efficient to modify the layout instead of swapping
            Dict: a final-layer qubit permutation

        Raises:
            TranspilerError: if there was any error during the mapping
                or with the parameters.