# Source code for dgl.transform

"""Module for graph transformation utilities."""

from collections.abc import Iterable, Mapping
from collections import defaultdict
import numpy as np
from scipy import sparse
from ._ffi.function import _init_api
from .graph import DGLGraph
from .heterograph import DGLHeteroGraph
from . import ndarray as nd
from . import backend as F
from .graph_index import from_coo
from .graph_index import _get_halo_subgraph_inner_node
from .graph_index import _get_halo_subgraph_inner_edge
from .graph import unbatch
from .convert import graph, bipartite
from . import utils
from .base import EID, NID
from . import ndarray as nd

__all__ = [
'line_graph',
'khop_graph',
'reverse',
'to_simple_graph',
'to_bidirected',
'laplacian_lambda_max',
'knn_graph',
'segmented_knn_graph',
'remove_self_loop',
'metapath_reachable_graph',
'compact_graphs',
'to_block',
'to_simple',
'in_subgraph',
'out_subgraph',
'remove_edges',
'as_immutable_graph',
'as_heterograph']

def pairwise_squared_distance(x):
"""
x : (n_samples, n_points, dims)
return : (n_samples, n_points, n_points)
"""
x2s = F.sum(x * x, -1, True)
# assuming that __matmul__ is always implemented (true for PyTorch, MXNet and Chainer)
return x2s + F.swapaxes(x2s, -1, -2) - 2 * x @ F.swapaxes(x, -1, -2)

#pylint: disable=invalid-name
[docs]def knn_graph(x, k):
"""Transforms the given point set to a directed graph, whose coordinates
are given as a matrix. The predecessors of each point are its k-nearest
neighbors.

If a 3D tensor is given instead, then each row would be transformed into
a separate graph.  The graphs will be unioned.

Parameters
----------
x : Tensor
The input tensor.

If 2D, each row of x corresponds to a node.

If 3D, a k-NN graph would be constructed for each row.  Then
the graphs are unioned.
k : int
The number of neighbors

Returns
-------
DGLGraph
The graph.  The node IDs are in the same order as x.
"""
if F.ndim(x) == 2:
x = F.unsqueeze(x, 0)
n_samples, n_points, _ = F.shape(x)

dist = pairwise_squared_distance(x)
k_indices = F.argtopk(dist, k, 2, descending=False)
dst = F.copy_to(k_indices, F.cpu())

src = F.zeros_like(dst) + F.reshape(F.arange(0, n_points), (1, -1, 1))

per_sample_offset = F.reshape(F.arange(0, n_samples) * n_points, (-1, 1, 1))
dst += per_sample_offset
src += per_sample_offset
dst = F.reshape(dst, (-1,))
src = F.reshape(src, (-1,))
adj = sparse.csr_matrix((F.asnumpy(F.zeros_like(dst) + 1), (F.asnumpy(dst), F.asnumpy(src))))

return g

#pylint: disable=invalid-name
[docs]def segmented_knn_graph(x, k, segs):
"""Transforms the given point set to a directed graph, whose coordinates
are given as a matrix.  The predecessors of each point are its k-nearest
neighbors.

The matrices are concatenated along the first axis, and are segmented by
segs.  Each block would be transformed into a separate graph.  The
graphs will be unioned.

Parameters
----------
x : Tensor
The input tensor.
k : int
The number of neighbors
segs : iterable of int
Number of points of each point set.
Must sum up to the number of rows in x.

Returns
-------
DGLGraph
The graph.  The node IDs are in the same order as x.
"""
n_total_points, _ = F.shape(x)
offset = np.insert(np.cumsum(segs), 0, 0)

h_list = F.split(x, segs, 0)
dst = [
F.argtopk(pairwise_squared_distance(h_g), k, 1, descending=False) +
offset[i]
for i, h_g in enumerate(h_list)]
dst = F.cat(dst, 0)
src = F.arange(0, n_total_points).unsqueeze(1).expand(n_total_points, k)

dst = F.reshape(dst, (-1,))
src = F.reshape(src, (-1,))
adj = sparse.csr_matrix((F.asnumpy(F.zeros_like(dst) + 1), (F.asnumpy(dst), F.asnumpy(src))))

return g

[docs]def line_graph(g, backtracking=True, shared=False):
"""Return the line graph of this graph.

Parameters
----------
g : dgl.DGLGraph
The input graph.
backtracking : bool, optional
Whether the returned line graph is backtracking.
shared : bool, optional
Whether the returned line graph shares representations with self.

Returns
-------
DGLGraph
The line graph of this graph.
"""
graph_data = g._graph.line_graph(backtracking)
node_frame = g._edge_frame if shared else None
return DGLGraph(graph_data, node_frame)

"""Return the matrix of :math:A^k where :math:A is the adjacency matrix of :math:g,
where a row represents the destination and a column represents the source.

Parameters
----------
g : dgl.DGLGraph
The input graph.
k : int
The :math:k in :math:A^k.

Returns
-------
tensor
The returned tensor, dtype is np.float32.

Examples
--------

>>> import dgl
>>> g = dgl.DGLGraph()
tensor([[1., 0., 0., 0., 1.],
[1., 1., 0., 0., 0.],
[0., 1., 1., 0., 0.],
[0., 0., 1., 1., 0.],
[0., 0., 0., 1., 1.]])
tensor([[1., 0., 1., 3., 3.],
[3., 1., 0., 1., 3.],
[3., 3., 1., 0., 1.],
[1., 3., 3., 1., 0.],
[0., 1., 3., 3., 1.]])
"""

[docs]def khop_graph(g, k):
"""Return the graph that includes all :math:k-hop neighbors of the given graph as edges.
The adjacency matrix of the returned graph is :math:A^k
(where :math:A is the adjacency matrix of :math:g).

Parameters
----------
g : dgl.DGLGraph
The input graph.
k : int
The :math:k in k-hop graph.

Returns
-------
dgl.DGLGraph
The returned DGLGraph.

Examples
--------

>>> import dgl
>>> g = dgl.DGLGraph()
>>> dgl.khop_graph(g, 1)
DGLGraph(num_nodes=5, num_edges=10,
ndata_schemes={}
edata_schemes={})
>>> dgl.khop_graph(g, 3)
DGLGraph(num_nodes=5, num_edges=40,
ndata_schemes={}
edata_schemes={})
"""
n = g.number_of_nodes()
# TODO(zihao): we should support creating multi-graph from scipy sparse matrix
# in the future.
return DGLGraph(from_coo(n, row, col, True))

[docs]def reverse(g, share_ndata=False, share_edata=False):
"""Return the reverse of a graph

The reverse (also called converse, transpose) of a directed graph is another directed
graph on the same nodes with edges reversed in terms of direction.

Given a :class:DGLGraph object, we return another :class:DGLGraph object
representing its reverse.

Notes
-----
* We do not dynamically update the topology of a graph once that of its reverse changes.
This can be particularly problematic when the node/edge attrs are shared. For example,
if the topology of both the original graph and its reverse get changed independently,
you can get a mismatched node/edge feature.

Parameters
----------
g : dgl.DGLGraph
The input graph.
share_ndata: bool, optional
If True, the original graph and the reversed graph share memory for node attributes.
Otherwise the reversed graph will not be initialized with node attributes.
share_edata: bool, optional
If True, the original graph and the reversed graph share memory for edge attributes.
Otherwise the reversed graph will not have edge attributes.

Examples
--------
Create a graph to reverse.

>>> import dgl
>>> import torch as th
>>> g = dgl.DGLGraph()
>>> g.add_edges([0, 1, 2], [1, 2, 0])
>>> g.ndata['h'] = th.tensor([[0.], [1.], [2.]])
>>> g.edata['h'] = th.tensor([[3.], [4.], [5.]])

Reverse the graph and examine its structure.

>>> rg = g.reverse(share_ndata=True, share_edata=True)
>>> print(rg)
DGLGraph with 3 nodes and 3 edges.
Node data: {'h': Scheme(shape=(1,), dtype=torch.float32)}
Edge data: {'h': Scheme(shape=(1,), dtype=torch.float32)}

The edges are reversed now.

>>> rg.has_edges_between([1, 2, 0], [0, 1, 2])
tensor([1, 1, 1])

Reversed edges have the same feature as the original ones.

>>> g.edges[[0, 2], [1, 0]].data['h'] == rg.edges[[1, 0], [0, 2]].data['h']
tensor([[1],
[1]], dtype=torch.uint8)

The node/edge features of the reversed graph share memory with the original
graph, which is helpful for both forward computation and back propagation.

>>> g.ndata['h'] = g.ndata['h'] + 1
>>> rg.ndata['h']
tensor([[1.],
[2.],
[3.]])
"""
g_reversed = DGLGraph()
g_edges = g.all_edges(order='eid')
g_reversed._batch_num_nodes = g._batch_num_nodes
g_reversed._batch_num_edges = g._batch_num_edges
if share_ndata:
g_reversed._node_frame = g._node_frame
if share_edata:
g_reversed._edge_frame = g._edge_frame
return g_reversed

[docs]def to_simple_graph(g):
"""Convert the graph to a simple graph with no multi-edge.

The function generates a new *readonly* graph with no node/edge feature.

Parameters
----------
g : DGLGraph
The input graph.

Returns
-------
DGLGraph
A simple graph.
"""
gidx = _CAPI_DGLToSimpleGraph(g._graph)

"""Convert the graph to a bidirected graph.

The function generates a new graph with no node/edge feature.
If g has an edge for i->j but no edge for j->i, then the
returned graph will have both i->j and j->i.

If the input graph is a multigraph (there are multiple edges from node i to node j),
the returned graph isn't well defined.

Parameters
----------
g : DGLGraph
The input graph.
readonly : bool, default to be True
Whether the returned bidirected graph is readonly or not.

Notes
-----
Please make sure g is a single graph, otherwise the return value is undefined.

Returns
-------
DGLGraph

Examples
--------
The following two examples use PyTorch backend, one for non-multi graph
and one for multi-graph.

>>> g = dgl.DGLGraph()
>>> bg1 = dgl.to_bidirected(g)
>>> bg1.edges()
(tensor([0, 1, 0]), tensor([0, 0, 1]))
"""
newgidx = _CAPI_DGLToBidirectedImmutableGraph(g._graph)
else:
newgidx = _CAPI_DGLToBidirectedMutableGraph(g._graph)
return DGLGraph(newgidx)

[docs]def laplacian_lambda_max(g):
"""Return the largest eigenvalue of the normalized symmetric laplacian of g.

The eigenvalue of the normalized symmetric of any graph is less than or equal to 2,
ref: https://en.wikipedia.org/wiki/Laplacian_matrix#Properties

Parameters
----------
g : DGLGraph
The input graph, it should be an undirected graph.

Returns
-------
list :
Return a list, where the i-th item indicates the largest eigenvalue
of i-th graph in g.

Examples
--------

>>> import dgl
>>> g = dgl.DGLGraph()
>>> g.add_edges([0, 1, 2, 3, 4, 0, 1, 2, 3, 4], [1, 2, 3, 4, 0, 4, 0, 1, 2, 3])
>>> dgl.laplacian_lambda_max(g)
[1.809016994374948]
"""
g_arr = unbatch(g)
rst = []
for g_i in g_arr:
n = g_i.number_of_nodes()
norm = sparse.diags(F.asnumpy(g_i.in_degrees()).clip(1) ** -0.5, dtype=float)
laplacian = sparse.eye(n) - norm * adj * norm
rst.append(sparse.linalg.eigs(laplacian, 1, which='LM',
return_eigenvectors=False)[0].real)
return rst

[docs]def metapath_reachable_graph(g, metapath):
"""Return a graph where the successors of any node u are nodes reachable from u by
the given metapath.

If the beginning node type s and ending node type t are the same, it will return
a homogeneous graph with node type s = t.  Otherwise, a unidirectional bipartite graph
with source node type s and destination node type t is returned.

In both cases, two nodes u and v will be connected with an edge (u, v) if
there exists one path matching the metapath from u to v.

The result graph keeps the node set of type s and t in the original graph even if
they might have no neighbor.

The features of the source/destination node type in the original graph would be copied to
the new graph.

Parameters
----------
g : DGLHeteroGraph
The input graph
metapath : list[str or tuple of str]
Metapath in the form of a list of edge types

Returns
-------
DGLHeteroGraph
A homogeneous or bipartite graph.
"""
for etype in metapath:

srctype = g.to_canonical_etype(metapath[0])[0]
dsttype = g.to_canonical_etype(metapath[-1])[2]
if srctype == dsttype:
else:

for key, value in g.nodes[srctype].data.items():
new_g.nodes[srctype].data[key] = value
if srctype != dsttype:
for key, value in g.nodes[dsttype].data.items():
new_g.nodes[dsttype].data[key] = value

return new_g

"""Return a new graph containing all the edges in the input graph plus self loops
of every nodes.
No duplicate self loop will be added for nodes already having self loops.
Self-loop edges id are not preserved. All self-loop edges would be added at the end.

Examples
---------

>>> g = DGLGraph()
>>> g.add_edges([0, 1, 2], [1, 1, 2])
>>> new_g = dgl.transform.add_self_loop(g) # Nodes 0, 3, 4 don't have self-loop
>>> new_g.edges()
(tensor([0, 0, 1, 2, 3, 4]), tensor([1, 0, 1, 2, 3, 4]))

Parameters
------------
g: DGLGraph

Returns
--------
DGLGraph
"""
new_g = DGLGraph()
src, dst = g.all_edges(order="eid")
src = F.zerocopy_to_numpy(src)
dst = F.zerocopy_to_numpy(dst)
non_self_edges_idx = src != dst
nodes = np.arange(g.number_of_nodes())
return new_g

[docs]def remove_self_loop(g):
"""Return a new graph with all self-loop edges removed

Examples
---------

>>> g = DGLGraph()
>>> g.add_edges([0, 1, 2], [1, 1, 2])
>>> new_g = dgl.transform.remove_self_loop(g) # Nodes 1, 2 have self-loop
>>> new_g.edges()
(tensor([0]), tensor([1]))

Parameters
------------
g: DGLGraph

Returns
--------
DGLGraph
"""
new_g = DGLGraph()
src, dst = g.all_edges(order="eid")
src = F.zerocopy_to_numpy(src)
dst = F.zerocopy_to_numpy(dst)
non_self_edges_idx = src != dst
return new_g

def partition_graph_with_halo(g, node_part, num_hops):
'''
This is to partition a graph. Each partition contains HALO nodes
so that we can generate NodeFlow in each partition correctly.

Parameters
------------
g: DGLGraph
The graph to be partitioned

node_part: 1D tensor
Specify which partition a node is assigned to. The length of this tensor
needs to be the same as the number of nodes of the graph. Each element
indicates the partition Id of a node.

num_hops: int
The number of hops a HALO node can be accessed.

Returns
--------
a dict of DGLGraphs
The key is the partition Id and the value is the DGLGraph of the partition.
'''
assert len(node_part) == g.number_of_nodes()
node_part = utils.toindex(node_part)
subgs = _CAPI_DGLPartitionWithHalo(g._graph, node_part.todgltensor(), num_hops)
subg_dict = {}
for i, subg in enumerate(subgs):
inner_node = _get_halo_subgraph_inner_node(subg)
inner_edge = _get_halo_subgraph_inner_edge(subg)
subg = g._create_subgraph(subg, subg.induced_nodes, subg.induced_edges)
inner_node = F.zerocopy_from_dlpack(inner_node.to_dlpack())
subg.ndata['inner_node'] = inner_node
inner_edge = F.zerocopy_from_dlpack(inner_edge.to_dlpack())
subg.edata['inner_edge'] = inner_edge
subg_dict[i] = subg
return subg_dict

def metis_partition(g, k, extra_cached_hops=0):
'''
This is to partition a graph with Metis partitioning.

Metis assigns vertices to partitions. This API constructs graphs with the vertices assigned
to the partitions and their incoming edges.

The partitioned graph is stored in DGLGraph. The DGLGraph has the part_id
node data that indicates the partition a node belongs to.

Parameters
------------
g: DGLGraph
The graph to be partitioned

k: int
The number of partitions.

extra_cached_hops: int
The number of hops a HALO node can be accessed.

Returns
--------
a dict of DGLGraphs
The key is the partition Id and the value is the DGLGraph of the partition.
'''
# METIS works only on symmetric graphs.
# The METIS runs on the symmetric graph to generate the node assignment to partitions.
node_part = _CAPI_DGLMetisPartition(sym_g._graph, k)
if len(node_part) == 0:
return None

node_part = utils.toindex(node_part)
# Then we split the original graph into parts based on the METIS partitioning results.
parts = partition_graph_with_halo(g, node_part, extra_cached_hops)
node_part = node_part.tousertensor()
for part_id in parts:
part = parts[part_id]
part.ndata['part_id'] = F.gather_row(node_part, part.parent_nid)
return parts

def compact_graphs(graphs, always_preserve=None):
"""Given a list of graphs with the same set of nodes, find and eliminate the common
isolated nodes across all graphs.

This function requires the graphs to have the same set of nodes (i.e. the node types
must be the same, and the number of nodes of each node type must be the same).  The
metagraph does not have to be the same.

It finds all the nodes that have zero in-degree and zero out-degree in all the given
graphs, and eliminates them from all the graphs.

Useful for graph sampling where we have a giant graph but we only wish to perform
message passing on a smaller graph with a (tiny) subset of nodes.

The node and edge features are not preserved.

Parameters
----------
graphs : DGLHeteroGraph or list[DGLHeteroGraph]
The graph, or list of graphs
always_preserve : Tensor or dict[str, Tensor], optional
If a dict of node types and node ID tensors is given, the nodes of given
node types would not be removed, regardless of whether they are isolated.
If a Tensor is given, assume that all the graphs have one (same) node type.

Returns
-------
DGLHeteroGraph or list[DGLHeteroGraph]
The compacted graph or list of compacted graphs.

Each returned graph would have a feature dgl.NID containing the mapping
of node IDs for each type from the compacted graph(s) to the original graph(s).
Note that the mapping is the same for all the compacted graphs.

Bugs
----
This function currently requires that the same node type of all graphs should have
the same node type ID, i.e. the node types are *ordered* the same.

Examples
--------
The following code constructs a bipartite graph with 20 users and 10 games, but
only user #1 and #3, as well as game #3 and #5, have connections:

>>> g = dgl.bipartite([(1, 3), (3, 5)], 'user', 'plays', 'game', num_nodes=(20, 10))

The following would compact the graph above to another bipartite graph with only
two users and two games.

>>> new_g, induced_nodes = dgl.compact_graphs(g)
>>> induced_nodes
{'user': tensor([1, 3]), 'game': tensor([3, 5])}

The mapping tells us that only user #1 and #3 as well as game #3 and #5 are kept.
Furthermore, the first user and second user in the compacted graph maps to
user #1 and #3 in the original graph.  Games are similar.

One can verify that the edge connections are kept the same in the compacted graph.

>>> new_g.edges(form='all', order='eid', etype='plays')
(tensor([0, 1]), tensor([0, 1]), tensor([0, 1]))

When compacting multiple graphs, nodes that do not have any connections in any
of the given graphs are removed.  So if we compact g and the following g2
graphs together:

>>> g2 = dgl.bipartite([(1, 6), (6, 8)], 'user', 'plays', 'game', num_nodes=(20, 10))
>>> (new_g, new_g2), induced_nodes = dgl.compact_graphs([g, g2])
>>> induced_nodes
{'user': tensor([1, 3, 6]), 'game': tensor([3, 5, 6, 8])}

Then one can see that user #1 from both graphs, users #3 from the first graph, as
well as user #6 from the second graph, are kept.  Games are similar.

Similarly, one can also verify the connections:

>>> new_g.edges(form='all', order='eid', etype='plays')
(tensor([0, 1]), tensor([0, 1]), tensor([0, 1]))
>>> new_g2.edges(form='all', order='eid', etype='plays')
(tensor([0, 2]), tensor([2, 3]), tensor([0, 1]))
"""
return_single = False
if not isinstance(graphs, Iterable):
graphs = [graphs]
return_single = True
if len(graphs) == 0:
return []

# Ensure the node types are ordered the same.
# TODO(BarclayII): we ideally need to remove this constraint.
ntypes = graphs[0].ntypes
graph_dtype = graphs[0]._graph.dtype()
graph_ctx = graphs[0]._graph.ctx()
for g in graphs:
assert ntypes == g.ntypes, \
("All graphs should have the same node types in the same order, got %s and %s" %
ntypes, g.ntypes)
assert graph_dtype == g._graph.dtype(), "Graph data type mismatch"
assert graph_ctx == g._graph.ctx(), "Graph device mismatch"

# Process the dictionary or tensor of "always preserve" nodes
if always_preserve is None:
always_preserve = {}
elif not isinstance(always_preserve, Mapping):
if len(ntypes) > 1:
raise ValueError("Node type must be given if multiple node types exist.")
always_preserve = {ntypes[0]: always_preserve}

always_preserve_nd = []
for ntype in ntypes:
nodes = always_preserve.get(ntype, None)
if nodes is None:
nodes = nd.empty([0], graph_dtype, graph_ctx)
else:
nodes = F.zerocopy_to_dgl_ndarray(nodes)
always_preserve_nd.append(nodes)

# Compact and construct heterographs
new_graph_indexes, induced_nodes = _CAPI_DGLCompactGraphs(
[g._graph for g in graphs], always_preserve_nd)
induced_nodes = [F.zerocopy_from_dgl_ndarray(nodes.data) for nodes in induced_nodes]

new_graphs = [
DGLHeteroGraph(new_graph_index, graph.ntypes, graph.etypes)
for new_graph_index, graph in zip(new_graph_indexes, graphs)]
for g in new_graphs:
for i, ntype in enumerate(graphs[0].ntypes):
g.nodes[ntype].data[NID] = induced_nodes[i]
if return_single:
new_graphs = new_graphs[0]

return new_graphs

def to_block(g, dst_nodes=None, include_dst_in_src=True):
"""Convert a graph into a bipartite-structured "block" for message passing.

A block graph is uni-directional bipartite graph consisting of two sets of nodes
SRC and DST. Each set can have many node types while all the edges are from SRC
nodes to DST nodes.

Specifically, for each relation graph of canonical edge type (utype, etype, vtype),
node type utype belongs to SRC while vtype belongs to DST.
Edges from node type utype to node type vtype are preserved. If
utype == vtype, the result graph will have two node types of the same name utype,
but one belongs to SRC while the other belongs to DST. This is because although
they have the same name, their node ids are relabeled differently (see below). In
both cases, the canonical edge type in the new graph is still
(utype, etype, vtype), so there is no difference when referring to it.

Moreover, the function also relabels node ids in each type to make the graph more compact.
Specifically, the nodes of type vtype would contain the nodes that have at least one
inbound edge of any type, while utype would contain all the DST nodes of type vtype,
as well as the nodes that have at least one outbound edge to any DST node.

Since DST nodes are included in SRC nodes, a common requirement is to fetch
the DST node features from the SRC nodes features. To avoid expensive sparse lookup,
the function assures that the DST nodes in both SRC and DST sets have the same ids.
As a result, given the node feature tensor X of type utype,
the following code finds the corresponding DST node features of type vtype:

.. code::

X[:block.number_of_nodes('DST/vtype')]

If the dst_nodes argument is given, the DST nodes would contain the given nodes.
Otherwise, the DST nodes would be determined by DGL via the rules above.

Parameters
----------
graph : DGLHeteroGraph
The graph.
dst_nodes : Tensor or dict[str, Tensor], optional
Optional DST nodes. If a tensor is given, the graph must have only one node type.
include_dst_in_src : bool, default True
If False, do not include DST nodes in SRC nodes.

Returns
-------
DGLHeteroGraph
The new graph describing the block.

The node IDs induced for each type in both sides would be stored in feature
dgl.NID.

The edge IDs induced for each type would be stored in feature dgl.EID.

Notes
-----
This function is primarily for creating the structures for efficient
computation of message passing.  See [TODO] for a detailed example.

Examples
--------
Converting a homogeneous graph to a block as described above:
>>> g = dgl.graph([(0, 1), (1, 2), (2, 3)])
>>> block = dgl.to_block(g, torch.LongTensor([3, 2]))

The right hand side nodes would be exactly the same as the ones given: [3, 2].
>>> induced_dst = block.dstdata[dgl.NID]
>>> induced_dst
tensor([3, 2])

The first few nodes of the left hand side nodes would also be exactly the same as
the ones given.  The rest of the nodes are the ones necessary for message passing
into nodes 3, 2.  This means that the node 1 would be included.
>>> induced_src = block.srcdata[dgl.NID]
>>> induced_src
tensor([3, 2, 1])

We can notice that the first two nodes are identical to the given nodes as well as
the right hand side nodes.

The induced edges can also be obtained by the following:
>>> block.edata[dgl.EID]
tensor([2, 1])

This indicates that edge (2, 3) and (1, 2) are included in the result graph.  We can
verify that the first edge in the block indeed maps to the edge (2, 3), and the
second edge in the block indeed maps to the edge (1, 2):
>>> src, dst = block.edges(order='eid')
>>> induced_src[src], induced_dst[dst]
(tensor([2, 1]), tensor([3, 2]))

Converting a heterogeneous graph to a block is similar, except that when specifying
the right hand side nodes, you have to give a dict:
>>> g = dgl.bipartite([(0, 1), (1, 2), (2, 3)], utype='A', vtype='B')

If you don't specify any node of type A on the right hand side, the node type A
in the block would have zero nodes.
>>> block = dgl.to_block(g, {'B': torch.LongTensor([3, 2])})
>>> block.number_of_nodes('A')
0
>>> block.number_of_nodes('B')
2
>>> block.nodes['B'].data[dgl.NID]
tensor([3, 2])

The left hand side would contain all the nodes on the right hand side:
>>> block.nodes['B'].data[dgl.NID]
tensor([3, 2])

As well as all the nodes that have connections to the nodes on the right hand side:
>>> block.nodes['A'].data[dgl.NID]
tensor([2, 1])
"""
if dst_nodes is None:
# Find all nodes that appeared as destinations
dst_nodes = defaultdict(list)
for etype in g.canonical_etypes:
_, dst = g.edges(etype=etype)
dst_nodes[etype[2]].append(dst)
dst_nodes = {ntype: F.unique(F.cat(values, 0)) for ntype, values in dst_nodes.items()}
elif not isinstance(dst_nodes, Mapping):
# dst_nodes is a Tensor, check if the g has only one type.
if len(g.ntypes) > 1:
raise ValueError(
'Graph has more than one node type; please specify a dict for dst_nodes.')
dst_nodes = {g.ntypes[0]: dst_nodes}

# dst_nodes is now a dict
dst_nodes_nd = []
for ntype in g.ntypes:
nodes = dst_nodes.get(ntype, None)
if nodes is not None:
dst_nodes_nd.append(F.zerocopy_to_dgl_ndarray(nodes))
else:
dst_nodes_nd.append(nd.null())

new_graph_index, src_nodes_nd, induced_edges_nd = _CAPI_DGLToBlock(
g._graph, dst_nodes_nd, include_dst_in_src)
src_nodes = [F.zerocopy_from_dgl_ndarray(nodes_nd.data) for nodes_nd in src_nodes_nd]
dst_nodes = [F.zerocopy_from_dgl_ndarray(nodes_nd) for nodes_nd in dst_nodes_nd]

# The new graph duplicates the original node types to SRC and DST sets.
new_ntypes = ([ntype for ntype in g.ntypes], [ntype for ntype in g.ntypes])
new_graph = DGLHeteroGraph(new_graph_index, new_ntypes, g.etypes)
assert new_graph.is_unibipartite  # sanity check

for i, ntype in enumerate(g.ntypes):
new_graph.srcnodes[ntype].data[NID] = src_nodes[i]
new_graph.dstnodes[ntype].data[NID] = dst_nodes[i]

for i, canonical_etype in enumerate(g.canonical_etypes):
induced_edges = F.zerocopy_from_dgl_ndarray(induced_edges_nd[i].data)
utype, etype, vtype = canonical_etype
new_canonical_etype = (utype, etype, vtype)
new_graph.edges[new_canonical_etype].data[EID] = induced_edges

return new_graph

def remove_edges(g, edge_ids):
"""Return a new graph with given edge IDs removed.

The nodes are preserved.

Parameters
----------
graph : DGLHeteroGraph
The graph
edge_ids : Tensor or dict[etypes, Tensor]
The edge IDs for each edge type.

Returns
-------
DGLHeteroGraph
The new graph.
The edge ID mapping from the new graph to the original graph is stored as
dgl.EID on edge features.
"""
if not isinstance(edge_ids, Mapping):
if len(g.etypes) != 1:
raise ValueError(
"Graph has more than one edge type; specify a dict for edge_id instead.")
edge_ids = {g.canonical_etypes[0]: edge_ids}

edge_ids_nd = [nd.null()] * len(g.etypes)
for key, value in edge_ids.items():
edge_ids_nd[g.get_etype_id(key)] = F.zerocopy_to_dgl_ndarray(value)
new_graph_index, induced_eids_nd = _CAPI_DGLRemoveEdges(g._graph, edge_ids_nd)

new_graph = DGLHeteroGraph(new_graph_index, g.ntypes, g.etypes)
for i, canonical_etype in enumerate(g.canonical_etypes):
data = induced_eids_nd[i].data
if len(data) == 0:
# Empty means that either
# (1) no edges are removed and edges are not shuffled.
# (2) all edges are removed.
# The following statement deals with both cases.
new_graph.edges[canonical_etype].data[EID] = F.arange(
0, new_graph.number_of_edges(canonical_etype))
else:
new_graph.edges[canonical_etype].data[EID] = F.zerocopy_from_dgl_ndarray(data)

return new_graph

def in_subgraph(g, nodes):
"""Extract the subgraph containing only the in edges of the given nodes.

The subgraph keeps the same type schema and the cardinality of the original one.
Node/edge features are not preserved. The original IDs
the extracted edges are stored as the dgl.EID feature in the returned graph.

Parameters
----------
g : DGLHeteroGraph
Full graph structure.
nodes : tensor or dict
Node ids to sample neighbors from. The allowed types
are dictionary of node types to node id tensors, or simply node id tensor if
the given graph g has only one type of nodes.

Returns
-------
DGLHeteroGraph
The subgraph.
"""
if not isinstance(nodes, dict):
if len(g.ntypes) > 1:
raise DGLError("Must specify node type when the graph is not homogeneous.")
nodes = {g.ntypes[0] : nodes}
nodes_all_types = []
for ntype in g.ntypes:
if ntype in nodes:
nodes_all_types.append(utils.toindex(nodes[ntype]).todgltensor())
else:
nodes_all_types.append(nd.array([], ctx=nd.cpu()))

subgidx = _CAPI_DGLInSubgraph(g._graph, nodes_all_types)
induced_edges = subgidx.induced_edges
ret = DGLHeteroGraph(subgidx.graph, g.ntypes, g.etypes)
for i, etype in enumerate(ret.canonical_etypes):
ret.edges[etype].data[EID] = induced_edges[i].tousertensor()
return ret

def out_subgraph(g, nodes):
"""Extract the subgraph containing only the out edges of the given nodes.

The subgraph keeps the same type schema and the cardinality of the original one.
Node/edge features are not preserved. The original IDs
the extracted edges are stored as the dgl.EID feature in the returned graph.

Parameters
----------
g : DGLHeteroGraph
Full graph structure.
nodes : tensor or dict
Node ids to sample neighbors from. The allowed types
are dictionary of node types to node id tensors, or simply node id tensor if
the given graph g has only one type of nodes.

Returns
-------
DGLHeteroGraph
The subgraph.
"""
if not isinstance(nodes, dict):
if len(g.ntypes) > 1:
raise DGLError("Must specify node type when the graph is not homogeneous.")
nodes = {g.ntypes[0] : nodes}
nodes_all_types = []
for ntype in g.ntypes:
if ntype in nodes:
nodes_all_types.append(utils.toindex(nodes[ntype]).todgltensor())
else:
nodes_all_types.append(nd.array([], ctx=nd.cpu()))

subgidx = _CAPI_DGLOutSubgraph(g._graph, nodes_all_types)
induced_edges = subgidx.induced_edges
ret = DGLHeteroGraph(subgidx.graph, g.ntypes, g.etypes)
for i, etype in enumerate(ret.canonical_etypes):
ret.edges[etype].data[EID] = induced_edges[i].tousertensor()
return ret

def to_simple(g, return_counts='count', writeback_mapping=None):
"""Convert a heterogeneous multigraph to a heterogeneous simple graph, coalescing
duplicate edges into one.

This function does not preserve node and edge features.

Parameters
----------
g : DGLHeteroGraph
The heterogeneous graph
return_counts : str, optional
If given, the returned graph would have a column with the same name that stores
the number of duplicated edges from the original graph.
writeback_mapping : str, optional
If given, the mapping from the edge IDs of original graph to those of the returned
graph would be written into edge feature with this name in the original graph for
each edge type.

Returns
-------
DGLHeteroGraph
The new heterogeneous simple graph.

Examples
--------
Consider the following graph
>>> g = dgl.graph([(0, 1), (1, 3), (2, 2), (1, 3), (1, 4), (1, 4)])
>>> sg = dgl.to_simple(g, return_counts='weights', writeback_mapping='new_eid')

The returned graph would have duplicate edges connecting (1, 3) and (1, 4) removed:
>>> sg.all_edges(form='uv', order='eid')
(tensor([0, 1, 1, 2]), tensor([1, 3, 4, 2]))

If return_counts is set, the returned graph will also return how many edges
in the original graph are connecting the endpoints of the edges in the new graph:
>>> sg.edata['weights']
tensor([1, 2, 2, 1])

This essentially reads that one edge is connecting (0, 1) in g, whereas 2 edges
are connecting (1, 3) in g, etc.

One can also retrieve the mapping from the edges in the original graph to edges in
the new graph by setting writeback_mapping and running
>>> g.edata['new_eid']
tensor([0, 1, 3, 1, 2, 2])

This tells us that the first edge in g is mapped to the first edge in sg, and
the second and the fourth edge are mapped to the second edge in sg, etc.
"""
simple_graph_index, counts, edge_maps = _CAPI_DGLToSimpleHetero(g._graph)
simple_graph = DGLHeteroGraph(simple_graph_index, g.ntypes, g.etypes)
counts = [F.zerocopy_from_dgl_ndarray(count.data) for count in counts]
edge_maps = [F.zerocopy_from_dgl_ndarray(edge_map.data) for edge_map in edge_maps]

if return_counts is not None:
for count, canonical_etype in zip(counts, g.canonical_etypes):
simple_graph.edges[canonical_etype].data[return_counts] = count

if writeback_mapping is not None:
for edge_map, canonical_etype in zip(edge_maps, g.canonical_etypes):
g.edges[canonical_etype].data[writeback_mapping] = edge_map

return simple_graph

def as_heterograph(g, ntype='_U', etype='_E'):
"""Convert a DGLGraph to a DGLHeteroGraph with one node and edge type.

Node and edge features are preserved.

Parameters
----------
g : DGLGraph
The graph
ntype : str, optional
The node type name
etype : str, optional
The edge type name

Returns
-------
DGLHeteroGraph
The heterograph.
"""
hgi = _CAPI_DGLAsHeteroGraph(g._graph)
hg = DGLHeteroGraph(hgi, [ntype], [etype])
hg.ndata.update(g.ndata)
hg.edata.update(g.edata)
return hg

def as_immutable_graph(hg):
"""Convert a DGLHeteroGraph with one node and edge type into a DGLGraph.

Node and edge features are preserved.

Parameters
----------
g : DGLHeteroGraph
The heterograph

Returns
-------
DGLGraph
The graph.
"""
gidx = _CAPI_DGLAsImmutableGraph(hg._graph)
g = DGLGraph(gidx)
g.ndata.update(hg.ndata)
g.edata.update(hg.edata)
return g

_init_api("dgl.transform")