rht
diff --git a/‎cirq-core/cirq/__init__.py
+6 b/‎cirq-core/cirq/__init__.py
+6
diff --git a/‎cirq-core/cirq/devices/__init__.py
+9 b/‎cirq-core/cirq/devices/__init__.py
+9
diff --git a/‎cirq-core/cirq/devices/named_topologies.py
+316 b/‎cirq-core/cirq/devices/named_topologies.py
+316
@@ -91,6 +91,12 @@
     NoiseModel,
     SymmetricalQidPair,
     UNCONSTRAINED_DEVICE,
+    NamedTopology,
+    draw_gridlike,
+    LineTopology,
+    TiltedSquareLattice,
+    get_placements,
+    draw_placements,
 )
 
 from cirq.experiments import (
 
@@ -38,3 +38,12 @@
     NoiseModel,
     ConstantQubitNoiseModel,
 )
+
+from cirq.devices.named_topologies import (
+    NamedTopology,
+    draw_gridlike,
+    LineTopology,
+    TiltedSquareLattice,
+    get_placements,
+    draw_placements,
+)
@@ -0,0 +1,316 @@
+# Copyright 2021 The Cirq Developers
+#
+# 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
+#
+#     https://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.
+
+import abc
+import dataclasses
+import warnings
+from dataclasses import dataclass
+from typing import Dict, List, Tuple, Any, Sequence, Union, Iterable, TYPE_CHECKING
+
+import networkx as nx
+from cirq.devices import GridQubit
+from cirq.protocols.json_serialization import obj_to_dict_helper
+from matplotlib import pyplot as plt
+
+if TYPE_CHECKING:
+    import cirq
+
+
+def dataclass_json_dict(obj: Any, namespace: str = None) -> Dict[str, Any]:
+    return obj_to_dict_helper(obj, [f.name for f in dataclasses.fields(obj)], namespace=namespace)
+
+
+class NamedTopology(metaclass=abc.ABCMeta):
+    """A topology (graph) with a name.
+
+    "Named topologies" provide a mapping from a simple dataclass to a unique graph for categories
+    of relevant topologies. Relevant topologies may be hardware dependant, but common topologies
+    are linear (1D) and rectangular grid topologies.
+    """
+
+    name: str = NotImplemented
+    """A name that uniquely identifies this topology."""
+
+    n_nodes: int = NotImplemented
+    """The number of nodes in the topology."""
+
+    graph: nx.Graph = NotImplemented
+    """A networkx graph representation of the topology."""
+
+
+_GRIDLIKE_NODE = Union['cirq.GridQubit', Tuple[int, int]]
+
+
+def _node_and_coordinates(
+    nodes: Iterable[_GRIDLIKE_NODE],
+) -> Iterable[Tuple[_GRIDLIKE_NODE, Tuple[int, int]]]:
+    """Yield tuples whose first element is the input node and the second is guaranteed to be a tuple
+    of two integers. The input node can be a tuple of ints or a GridQubit."""
+    for node in nodes:
+        if isinstance(node, GridQubit):
+            yield node, (node.row, node.col)
+        else:
+            x, y = node
+            yield node, (x, y)
+
+
+def draw_gridlike(
+    graph: nx.Graph, ax: plt.Axes = None, tilted: bool = True, **kwargs
+) -> Dict[Any, Tuple[int, int]]:
+    """Draw a grid-like graph using Matplotlib.
+
+    This wraps nx.draw_networkx to produce a matplotlib drawing of the graph. Nodes
+    should be two-dimensional gridlike objects.
+
+    Args:
+        graph: A NetworkX graph whose nodes are (row, column) coordinates or cirq.GridQubits.
+        ax: Optional matplotlib axis to use for drawing.
+        tilted: If True, directly position as (row, column); otherwise,
+            rotate 45 degrees to accommodate google-style diagonal grids.
+        kwargs: Additional arguments to pass to `nx.draw_networkx`.
+
+    Returns:
+        A positions dictionary mapping nodes to (x, y) coordinates suitable for future calls
+        to NetworkX plotting functionality.
+    """
+    if ax is None:
+        ax = plt.gca()  # coverage: ignore
+
+    if tilted:
+        pos = {node: (y, -x) for node, (x, y) in _node_and_coordinates(graph.nodes)}
+    else:
+        pos = {node: (x + y, y - x) for node, (x, y) in _node_and_coordinates(graph.nodes)}
+
+    nx.draw_networkx(graph, pos=pos, ax=ax, **kwargs)
+    ax.axis('equal')
+    return pos
+
+
+@dataclass(frozen=True)
+class LineTopology(NamedTopology):
+    """A 1D linear topology.
+
+    Node indices are contiguous integers starting from 0 with edges between
+    adjacent integers.
+
+    Args:
+        n_nodes: The number of nodes in a line.
+    """
+
+    n_nodes: int
+
+    def __post_init__(self):
+        if self.n_nodes <= 1:
+            raise ValueError("`n_nodes` must be greater than 1.")
+        object.__setattr__(self, 'name', f'line-{self.n_nodes}')
+        graph = nx.from_edgelist(
+            [(i1, i2) for i1, i2 in zip(range(self.n_nodes), range(1, self.n_nodes))]
+        )
+        object.__setattr__(self, 'graph', graph)
+
+    def draw(self, ax=None, tilted: bool = True, **kwargs) -> Dict[Any, Tuple[int, int]]:
+        """Draw this graph using Matplotlib.
+
+        Args:
+            ax: Optional matplotlib axis to use for drawing.
+            tilted: If True, draw as a horizontal line. Otherwise, draw on a diagonal.
+            kwargs: Additional arguments to pass to `nx.draw_networkx`.
+        """
+        g2 = nx.relabel_nodes(self.graph, {n: (n, 1) for n in self.graph.nodes})
+        return draw_gridlike(g2, ax=ax, tilted=tilted, **kwargs)
+
+    def _json_dict_(self) -> Dict[str, Any]:
+        return dataclass_json_dict(self)
+
+
+@dataclass(frozen=True)
+class TiltedSquareLattice(NamedTopology):
+    """A grid lattice rotated 45-degrees.
+
+    This topology is based on Google devices where plaquettes consist of four qubits in a square
+    connected to a central qubit:
+
+        x   x
+          x
+        x   x
+
+    The corner nodes are not connected to each other. `width` and `height` refer to the rectangle
+    formed by rotating the lattice 45 degrees. `width` and `height` are measured in half-unit
+    cells, or equivalently half the number of central nodes.
+    An example diagram of this topology is shown below. It is a
+    "tilted-square-lattice-6-4" with width 6 and height 4.
+
+              x
+              │
+         x────X────x
+         │    │    │
+    x────X────x────X────x
+         │    │    │    │
+         x────X────x────X───x
+              │    │    │
+              x────X────x
+                   │
+                   x
+
+    Nodes are 2-tuples of integers which may be negative. Please see `get_placements` for
+    mapping this topology to a GridQubit Device.
+    """
+
+    width: int
+    height: int
+
+    def __post_init__(self):
+        if self.width <= 0:
+            raise ValueError("Width must be a positive integer")
+        if self.height <= 0:
+            raise ValueError("Height must be a positive integer")
+
+        object.__setattr__(self, 'name', f'tilted-square-lattice-{self.width}-{self.height}')
+
+        rect1 = set(
+            (i + j, i - j) for i in range(self.width // 2 + 1) for j in range(self.height // 2 + 1)
+        )
+        rect2 = set(
+            ((i + j) // 2, (i - j) // 2)
+            for i in range(1, self.width + 1, 2)
+            for j in range(1, self.height + 1, 2)
+        )
+        nodes = rect1 | rect2
+        g = nx.Graph()
+        for node in nodes:
+            for dx, dy in [(1, 0), (-1, 0), (0, 1), (0, -1)]:
+                neighbor = (node[0] + dx, node[1] + dy)
+                if neighbor in nodes:
+                    g.add_edge(node, neighbor)
+
+        object.__setattr__(self, 'graph', g)
+
+        # The number of edges = width * height (see unit tests). This can be seen if you remove
+        # all vertices and replace edges with dots.
+        # The formula for the number of vertices is not that nice, but you can derive it by
+        # summing big and small Xes in the asciiart in the docstring.
+        # There are (width//2 + 1) * (height//2 + 1) small xes and
+        # ((width + 1)//2) * ((height + 1)//2) big ones.
+        n_nodes = (self.width // 2 + 1) * (self.height // 2 + 1)
+        n_nodes += ((self.width + 1) // 2) * ((self.height + 1) // 2)
+        object.__setattr__(self, 'n_nodes', n_nodes)
+
+    def draw(self, ax=None, tilted=True, **kwargs):
+        """Draw this graph using Matplotlib.
+
+        Args:
+            ax: Optional matplotlib axis to use for drawing.
+            tilted: If True, directly position as (row, column); otherwise,
+                rotate 45 degrees to accommodate the diagonal nature of this topology.
+            kwargs: Additional arguments to pass to `nx.draw_networkx`.
+        """
+        return draw_gridlike(self.graph, ax=ax, tilted=tilted, **kwargs)
+
+    def nodes_as_gridqubits(self) -> List['cirq.GridQubit']:
+        """Get the graph nodes as cirq.GridQubit"""
+        return [GridQubit(r, c) for r, c in sorted(self.graph.nodes)]
+
+    def _json_dict_(self) -> Dict[str, Any]:
+        return dataclass_json_dict(self)
+
+
+def get_placements(
+    big_graph: nx.Graph, small_graph: nx.Graph, max_placements=100_000
+) -> List[Dict]:
+    """Get 'placements' mapping small_graph nodes onto those of `big_graph`.
+
+    This function considers monomorphisms with a restriction: we restrict only to unique set
+    of `big_graph` qubits. Some monomorphisms may be basically
+    the same mapping just rotated/flipped which we purposefully exclude. This could
+    exclude meaningful differences like using the same qubits but having the edges assigned
+    differently, but it prevents the number of placements from blowing up.
+
+    Args:
+        big_graph: The parent, super-graph. We often consider the case where this is a
+            nx.Graph representation of a Device whose nodes are `cirq.Qid`s like `GridQubit`s.
+        small_graph: The subgraph. We often consider the case where this is a NamedTopology
+            graph.
+        max_placements: Raise a value error if there are more than this many placement
+            possibilities. It is possible to use `big_graph`, `small_graph` combinations
+            that result in an intractable number of placements.
+
+    Raises:
+        ValueError: if the number of placements exceeds `max_placements`.
+
+    Returns:
+        A list of placement dictionaries. Each dictionary maps the nodes in `small_graph` to
+        nodes in `big_graph` with a monomorphic relationship. That's to say: if an edge exists
+        in `small_graph` between two nodes, it will exist in `big_graph` between the mapped nodes.
+    """
+    matcher = nx.algorithms.isomorphism.GraphMatcher(big_graph, small_graph)
+
+    # de-duplicate rotations, see docstring.
+    dedupe = {}
+    for big_to_small_map in matcher.subgraph_monomorphisms_iter():
+        dedupe[frozenset(big_to_small_map.keys())] = big_to_small_map
+        if len(dedupe) > max_placements:
+            # coverage: ignore
+            raise ValueError(
+                f"We found more than {max_placements} placements. Please use a "
+                f"more constraining `big_graph` or a more constrained `small_graph`."
+            )
+
+    small_to_bigs = []
+    for big in sorted(dedupe.keys()):
+        big_to_small_map = dedupe[big]
+        small_to_big_map = {v: k for k, v in big_to_small_map.items()}
+        small_to_bigs.append(small_to_big_map)
+    return small_to_bigs
+
+
+def draw_placements(
+    big_graph: nx.Graph,
+    small_graph: nx.Graph,
+    small_to_big_mappings,
+    max_plots=20,
+    axes: Sequence[plt.Axes] = None,
+):
+    """Draw a visualization of placements from small_graph onto big_graph using Matplotlib.
+
+    The entire `big_graph` will be drawn with default blue colored nodes. `small_graph` nodes
+    and edges will be highlighted with a red color.
+    """
+    if len(small_to_big_mappings) > max_plots:
+        # coverage: ignore
+        warnings.warn(f"You've provided a lot of mappings. Only plotting the first {max_plots}")
+        small_to_big_mappings = small_to_big_mappings[:max_plots]
+
+    call_show = False
+    if axes is None:
+        # coverage: ignore
+        call_show = True
+
+    for i, small_to_big_map in enumerate(small_to_big_mappings):
+        if axes is not None:
+            ax = axes[i]
+        else:
+            # coverage: ignore
+            ax = plt.gca()
+
+        small_mapped = nx.relabel_nodes(small_graph, small_to_big_map)
+        draw_gridlike(big_graph, ax=ax)
+        draw_gridlike(
+            small_mapped, node_color='red', edge_color='red', width=2, with_labels=False, ax=ax
+        )
+        ax.axis('equal')
+        if call_show:
+            # coverage: ignore
+            # poor man's multi-axis figure: call plt.show() after each plot
+            # and jupyter will put the plots one after another.
+            plt.show()