"""Choose a Layout via iterative bidirectional routing of the input circuit.

Starting with a random initial `Layout`, the algorithm does a full routing

of the circuit (via the `routing_pass` method) to end up with a

`final_layout`. This final_layout is then used as the initial_layout for

routing the reverse circuit. The algorithm iterates a number of times until

it finds an initial_layout that reduces full routing cost.

This method exploits the reversibility of quantum circuits, and tries to

include global circuit information in the choice of initial_layout.

By default, this pass will run both layout and routing and will transform the

circuit so that the layout is applied to the input dag (meaning that the output

circuit will have ancilla qubits allocated for unused qubits on the coupling map

and the qubits will be reordered to match the mapped physical qubits) and then

routing will be applied (inserting :class:`~.SwapGate`s to account for limited

connectivity). This is unlike most other layout passes which are :class:`~.AnalysisPass`

objects and just find an initial layout and set that on the property set. This is

done because by default the pass will run parallel seed trials with different random

seeds for selecting the random initial layout and then selecting the routed output

which results in the least number of swap gates needed.

You can use the ``routing_pass`` argument to have this pass operate as a typical

layout pass. When specified this will use the specified routing pass to select an

initial layout only and will not run multiple seed trials.

In addition to starting with a random initial `Layout` the pass can also take in

an additional list of starting layouts which will be used for additional

trials. If the ``sabre_starting_layouts`` is present in the property set

when this pass is run, that will be used for additional trials. There will still

be ``layout_trials`` of full random starting layouts run and the contents of

``sabre_starting_layouts`` will be run in addition to those. The output which results

in the lowest amount of swap gates (whether from the random trials or the property

set starting point) will be used. The value for this property set field should be a

list of :class:`.Layout` objects representing the starting layouts to use. If a

virtual qubit is missing from an :class:`.Layout` object in the list a random qubit

will be selected.

Property Set Fields Read

------------------------

``sabre_starting_layouts`` (``list[Layout]``)

An optional list of :class:`~.Layout` objects to use for additional layout trials. This is

in addition to the full random trials specified with the ``layout_trials`` argument.

Property Set Values Written

---------------------------

``layout`` (:class:`.Layout`)

The chosen initial mapping of virtual to physical qubits, including the ancilla allocation.

``final_layout`` (:class:`.Layout`)

A permutation of how swaps have been applied to the input qubits at the end of the circuit.

**References:**

[1] Li, Gushu, Yufei Ding, and Yuan Xie. "Tackling the qubit mapping problem

for NISQ-era quantum devices." ASPLOS 2019.

`arXiv:1809.02573 <https://arxiv.org/pdf/1809.02573.pdf>`_

"""

def __init__(

self,

coupling_map,

routing_pass=None,

seed=None,

max_iterations=3,

swap_trials=None,

layout_trials=None,

skip_routing=False,

):

"""SabreLayout initializer.

Args:

coupling_map (Union[CouplingMap, Target]): directed graph representing a coupling map.

routing_pass (BasePass): the routing pass to use while iterating.

If specified this pass operates as an :class:`~.AnalysisPass` and

will only populate the ``layout`` field in the property set and

the input dag is returned unmodified. This argument is mutually

exclusive with the ``swap_trials`` and the ``layout_trials``

arguments and if this is specified at the same time as either

argument an error will be raised.

seed (int): seed for setting a random first trial layout.

max_iterations (int): number of forward-backward iterations.

swap_trials (int): The number of trials to run of

:class:`~.SabreSwap` for each iteration. This is equivalent to

the ``trials`` argument on :class:`~.SabreSwap`. If this is not

specified (and ``routing_pass`` isn't set) by default the number

of physical CPUs on your local system will be used. For

reproducibility between environments it is best to set this

to an explicit number because the output will potentially depend

on the number of trials run. This option is mutually exclusive

with the ``routing_pass`` argument and an error will be raised

if both are used.

layout_trials (int): The number of random seed trials to run

layout with. When > 1 the trial that resuls in the output with

the fewest swap gates will be selected. If this is not specified

(and ``routing_pass`` is not set) then the number of local

physical CPUs will be used as the default value. This option is

mutually exclusive with the ``routing_pass`` argument and an error

will be raised if both are used.

skip_routing (bool): If this is set ``True`` and ``routing_pass`` is not used

then routing will not be applied to the output circuit. Only the layout

will be set in the property set. This is a tradeoff to run custom

routing with multiple layout trials, as using this option will cause

SabreLayout to run the routing stage internally but not use that result.

Raises:

TranspilerError: If both ``routing_pass`` and ``swap_trials`` or

both ``routing_pass`` and ``layout_trials`` are specified

"""

super().__init__()

if isinstance(coupling_map, Target):

self.target = coupling_map

self.coupling_map = self.target.build_coupling_map()

else:

self.target = None

self.coupling_map = coupling_map

self._neighbor_table = None

if routing_pass is not None and (swap_trials is not None or layout_trials is not None):

raise TranspilerError("Both routing_pass and swap_trials can't be set at the same time")

self.routing_pass = routing_pass

self.seed = seed

self.max_iterations = max_iterations

self.trials = swap_trials

if swap_trials is None:

self.swap_trials = CPU_COUNT

else:

self.swap_trials = swap_trials

if layout_trials is None:

self.layout_trials = CPU_COUNT

else:

self.layout_trials = layout_trials

self.skip_routing = skip_routing

if self.coupling_map is not None:

if not self.coupling_map.is_symmetric:

# deepcopy is needed here if we don't own the coupling map (i.e. we were passed it

# directly) to avoid modifications updating shared references in passes which

# require directional constraints

if isinstance(coupling_map, CouplingMap):

self.coupling_map = copy.deepcopy(self.coupling_map)

self.coupling_map.make_symmetric()

self._neighbor_table = NeighborTable(rx.adjacency_matrix(self.coupling_map.graph))

def run(self, dag):

"""Run the SabreLayout pass on `dag`.

Args:

dag (DAGCircuit): DAG to find layout for.

Returns:

DAGCircuit: The output dag if swap mapping was run

(otherwise the input dag is returned unmodified).

Raises:

TranspilerError: if dag wider than self.coupling_map

"""

if len(dag.qubits) > self.coupling_map.size():

raise TranspilerError("More virtual qubits exist than physical.")

# Choose a random initial_layout.

if self.routing_pass is not None:

if not self.coupling_map.is_connected():

raise TranspilerError(

"The routing_pass argument cannot be used with disjoint coupling maps."

)

if self.seed is None:

seed = np.random.randint(0, np.iinfo(np.int32).max)

else:

seed = self.seed

rng = np.random.default_rng(seed)

physical_qubits = rng.choice(self.coupling_map.size(), len(dag.qubits), replace=False)

physical_qubits = rng.permutation(physical_qubits)

initial_layout = Layout({q: dag.qubits[i] for i, q in enumerate(physical_qubits)})

self.routing_pass.fake_run = True

# Do forward-backward iterations.

circ = dag_to_circuit(dag)

rev_circ = circ.reverse_ops()

for _ in range(self.max_iterations):

for _ in ("forward", "backward"):

pm = self._layout_and_route_passmanager(initial_layout)

new_circ = pm.run(circ)

# Update initial layout and reverse the unmapped circuit.

pass_final_layout = pm.property_set["final_layout"]

final_layout = self._compose_layouts(

initial_layout, pass_final_layout, new_circ.qregs

)

initial_layout = final_layout

circ, rev_circ = rev_circ, circ

# Diagnostics

logger.info("new initial layout")

logger.info(initial_layout)

for qreg in dag.qregs.values():

initial_layout.add_register(qreg)

self.property_set["layout"] = initial_layout

self.routing_pass.fake_run = False

return dag

# Combined

if self.target is not None:

# This is a special case SABRE only works with a bidirectional coupling graph

# which we explicitly can't create from the target. So do this manually here

# to avoid altering the shared state with the unfiltered indices.

target = self.target.build_coupling_map(filter_idle_qubits=True)

target.make_symmetric()

else:

target = self.coupling_map

inner_run = self._inner_run

if "sabre_starting_layouts" in self.property_set:

inner_run = functools.partial(

self._inner_run, starting_layouts=self.property_set["sabre_starting_layouts"]

)

components = disjoint_utils.run_pass_over_connected_components(dag, target, inner_run)

self.property_set["layout"] = Layout(

{

component.dag.qubits[logic]: component.coupling_map.graph[phys]

for component in components

for logic, phys in component.initial_layout.layout_mapping()

# A physical component of the coupling map might be wider than the DAG that we're

# laying out onto it. We shouldn't include these implicit ancillas right now as the

# ancilla-allocation pass will run on the whole map in one go.

if logic < len(component.dag.qubits)

}

)

# Add the existing registers to the layout

for qreg in dag.qregs.values():

self.property_set["layout"].add_register(qreg)

# If skip_routing is set then return the layout in the property set

# and throwaway the extra work we did to compute the swap map.

# We also skip routing here if there is more than one connected

# component we ran layout on. We can only reliably route the full dag

# in this case if there is any dependency between the components

# (typically shared classical data or barriers).

if self.skip_routing or len(components) > 1:

return dag

# At this point, we become a transformation pass, and apply the layout and the routing to

# the DAG directly. This includes filling in the `property_set` data of the embed passes.

dag = self._ancilla_allocation_no_pass_manager(dag)

# The ancilla-allocation pass has expanded this since we set it above.

full_initial_layout = self.property_set["layout"]

# Set up a physical DAG to apply the Sabre result onto. We do not need to run the

# `ApplyLayout` transpiler pass (which usually does this step), because we're about to apply

# the layout and routing together as part of resolving the Sabre result.

physical_qubits = QuantumRegister(self.coupling_map.size(), "q")

mapped_dag = DAGCircuit()

mapped_dag.add_qreg(physical_qubits)

mapped_dag.add_clbits(dag.clbits)

for creg in dag.cregs.values():

mapped_dag.add_creg(creg)

mapped_dag._global_phase = dag._global_phase

self.property_set["original_qubit_indices"] = {

bit: index for index, bit in enumerate(dag.qubits)

}

final_layout = Layout(

{

mapped_dag.qubits[

component.coupling_map.graph[initial]

]: component.coupling_map.graph[final]

for component in components

for initial, final in enumerate(component.final_permutation)

}

)

if self.property_set["final_layout"] is None:

self.property_set["final_layout"] = final_layout

else:

self.property_set["final_layout"] = final_layout.compose(

self.property_set["final_layout"], dag.qubits

)

for component in components:

# Sabre routing still returns all its swaps as on virtual qubits, so we need to expand

# each component DAG with the virtual ancillas that were allocated to it, so the layout

# application can succeed. This is the last thing we do with the component DAG, so it's

# ok for us to modify it.

component_size = component.coupling_map.size()

dag_size = component.dag.num_qubits()

if component_size > dag_size:

used_physical = {full_initial_layout[logic] for logic in component.dag.qubits}

component.dag.add_qubits(

[

full_initial_layout[component.coupling_map.graph[phys]]

for phys in range(component.dag.num_qubits(), component_size)

if component.coupling_map.graph[phys] not in used_physical

]

)

mapped_dag = _apply_sabre_result(

mapped_dag,

component.dag,

component.sabre_result,

component.initial_layout,

[

mapped_dag.qubits[component.coupling_map.graph[phys]]

for phys in range(component_size)

],

component.circuit_to_dag_dict,

)

disjoint_utils.combine_barriers(mapped_dag, retain_uuid=False)

return mapped_dag

def _inner_run(self, dag, coupling_map, starting_layouts=None):

if not coupling_map.is_symmetric:

# deepcopy is needed here to avoid modifications updating

# shared references in passes which require directional

# constraints

coupling_map = copy.deepcopy(coupling_map)

coupling_map.make_symmetric()

neighbor_table = NeighborTable(rx.adjacency_matrix(coupling_map.graph))

dist_matrix = coupling_map.distance_matrix

original_qubit_indices = {bit: index for index, bit in enumerate(dag.qubits)}

partial_layouts = []

if starting_layouts is not None:

coupling_map_reverse_mapping = {

coupling_map.graph[x]: x for x in coupling_map.graph.node_indices()

}

for layout in starting_layouts:

virtual_bits = layout.get_virtual_bits()

out_layout = [None] * len(dag.qubits)

for bit, phys in virtual_bits.items():

pos = original_qubit_indices.get(bit, None)

if pos is None:

continue

out_layout[pos] = coupling_map_reverse_mapping[phys]

partial_layouts.append(out_layout)

sabre_dag, circuit_to_dag_dict = _build_sabre_dag(

dag,

coupling_map.size(),

original_qubit_indices,

)

sabre_start = time.perf_counter()

(initial_layout, final_permutation, sabre_result) = sabre_layout_and_routing(

sabre_dag,

neighbor_table,

dist_matrix,

Heuristic.Decay,

self.max_iterations,

self.swap_trials,

self.layout_trials,

self.seed,

partial_layouts,

)

sabre_stop = time.perf_counter()

logger.debug(

"Sabre layout algorithm execution for a connected component complete in: %s sec.",

sabre_stop - sabre_start,

)

return _DisjointComponent(

dag,

coupling_map,

initial_layout,

final_permutation,

sabre_result,

circuit_to_dag_dict,

)

def _ancilla_allocation_no_pass_manager(self, dag):

"""Run the ancilla-allocation and -enlargment passes on the DAG chained onto our

``property_set``, skipping the DAG-to-circuit conversion cost of using a ``PassManager``."""

ancilla_pass = FullAncillaAllocation(self.coupling_map)

ancilla_pass.property_set = self.property_set

dag = ancilla_pass.run(dag)

enlarge_pass = EnlargeWithAncilla()

enlarge_pass.property_set = ancilla_pass.property_set

dag = enlarge_pass.run(dag)

self.property_set = enlarge_pass.property_set

return dag

def _layout_and_route_passmanager(self, initial_layout):

"""Return a passmanager for a full layout and routing.

We use a factory to remove potential statefulness of passes.

"""

layout_and_route = [

SetLayout(initial_layout),

FullAncillaAllocation(self.coupling_map),

EnlargeWithAncilla(),

ApplyLayout(),

self.routing_pass,

]

pm = PassManager(layout_and_route)

return pm

def _compose_layouts(self, initial_layout, pass_final_layout, qregs):

"""Return the real final_layout resulting from the composition

of an initial_layout with the final_layout reported by a pass.

The routing passes internally start with a trivial layout, as the

layout gets applied to the circuit prior to running them. So the

``"final_layout"`` they report must be amended to account for the actual

initial_layout that was selected.

"""

trivial_layout = Layout.generate_trivial_layout(*qregs)

qubit_map = Layout.combine_into_edge_map(initial_layout, trivial_layout)

final_layout = {v: pass_final_layout._v2p[qubit_map[v]] for v in initial_layout._v2p}

__slots__ = (

"dag",

"coupling_map",

"initial_layout",

"final_permutation",

"sabre_result",

"circuit_to_dag_dict",

)

dag: DAGCircuit

coupling_map: CouplingMap

initial_layout: NLayout

final_permutation: "list[int]"

sabre_result: "tuple[SwapMap, Sequence[int], NodeBlockResults]"