Allow returning edge indices from random walk (#139)

This commit adds an optional argument in the `random_walk` function,
namely `return_edge_indices`. The default behaviour is not changed, but
if a user wants to directly use the edges visited by the random walker,
we can return the indices of those edges by setting
`return_edge_indices` to `True`. New cases are also added to the test
suite.

test/test_rw.py

@@ -6,7 +6,7 @@ from .utils import devices, tensor
 
 
 @pytest.mark.parametrize('device', devices)
-def test_rw(device):
+def test_rw_large(device):
     row = tensor([0, 1, 1, 1, 2, 2, 3, 3, 4, 4], torch.long, device)
     col = tensor([1, 0, 2, 3, 1, 4, 1, 4, 2, 3], torch.long, device)
     start = tensor([0, 1, 2, 3, 4], torch.long, device)
@@ -21,6 +21,9 @@ def test_rw(device):
             assert out[n, i].item() in col[row == cur].tolist()
             cur = out[n, i].item()
 
+
+@pytest.mark.parametrize('device', devices)
+def test_rw_small(device):
     row = tensor([0, 1], torch.long, device)
     col = tensor([1, 0], torch.long, device)
     start = tensor([0, 1, 2], torch.long, device)
@@ -28,3 +31,49 @@ def test_rw(device):
 
     out = random_walk(row, col, start, walk_length, num_nodes=3)
     assert out.tolist() == [[0, 1, 0, 1, 0], [1, 0, 1, 0, 1], [2, 2, 2, 2, 2]]
+
+
+@pytest.mark.parametrize('device', devices)
+def test_rw_large_with_edge_indices(device):
+    row = tensor([0, 1, 1, 1, 2, 2, 3, 3, 4, 4], torch.long, device)
+    col = tensor([1, 0, 2, 3, 1, 4, 1, 4, 2, 3], torch.long, device)
+    start = tensor([0, 1, 2, 3, 4], torch.long, device)
+    walk_length = 10
+
+    node_seq, edge_seq = random_walk(
+        row, col, start, walk_length,
+        return_edge_indices=True,
+    )
+    assert node_seq[:, 0].tolist() == start.tolist()
+
+    for n in range(start.size(0)):
+        cur = start[n].item()
+        for i in range(1, walk_length):
+            assert node_seq[n, i].item() in col[row == cur].tolist()
+            cur = node_seq[n, i].item()
+
+    assert (edge_seq != -1).all()
+
+
+@pytest.mark.parametrize('device', devices)
+def test_rw_small_with_edge_indices(device):
+    row = tensor([0, 1], torch.long, device)
+    col = tensor([1, 0], torch.long, device)
+    start = tensor([0, 1, 2], torch.long, device)
+    walk_length = 4
+
+    node_seq, edge_seq = random_walk(
+        row, col, start, walk_length,
+        num_nodes=3,
+        return_edge_indices=True,
+    )
+    assert node_seq.tolist() == [
+        [0, 1, 0, 1, 0],
+        [1, 0, 1, 0, 1],
+        [2, 2, 2, 2, 2],
+    ]
+    assert edge_seq.tolist() == [
+        [0, 1, 0, 1],
+        [1, 0, 1, 0],
+        [-1, -1, -1, -1],
+    ]

torch_cluster/rw.py

-from typing import Optional
+from typing import Optional, Tuple, Union
 
 import torch
 from torch import Tensor
 
 
 @torch.jit.script
-def random_walk(row: Tensor, col: Tensor, start: Tensor, walk_length: int,
-                p: float = 1, q: float = 1, coalesced: bool = True,
-                num_nodes: Optional[int] = None) -> Tensor:
+def random_walk(
+    row: Tensor,
+    col: Tensor,
+    start: Tensor,
+    walk_length: int,
+    p: float = 1,
+    q: float = 1,
+    coalesced: bool = True,
+    num_nodes: Optional[int] = None,
+    return_edge_indices: bool = False,
+) -> Union[Tensor, Tuple[Tensor, Tensor]]:
     """Samples random walks of length :obj:`walk_length` from all node indices
     in :obj:`start` in the graph given by :obj:`(row, col)` as described in the
     `"node2vec: Scalable Feature Learning for Networks"
@@ -28,6 +36,9 @@ def random_walk(row: Tensor, col: Tensor, start: Tensor, walk_length: int,
             the graph given by :obj:`(row, col)` according to :obj:`row`.
             (default: :obj:`True`)
         num_nodes (int, optional): The number of nodes. (default: :obj:`None`)
+        return_edge_indices (bool, optional): Whether to additionally return
+            the indices of edges traversed during the random walk.
+            (default: :obj:`False`)
 
     :rtype: :class:`LongTensor`
     """
@@ -43,5 +54,11 @@ def random_walk(row: Tensor, col: Tensor, start: Tensor, walk_length: int,
     rowptr = row.new_zeros(num_nodes + 1)
     torch.cumsum(deg, 0, out=rowptr[1:])
 
-    return torch.ops.torch_cluster.random_walk(rowptr, col, start, walk_length,
-                                               p, q)[0]
+    node_seq, edge_seq = torch.ops.torch_cluster.random_walk(
+        rowptr, col, start, walk_length, p, q,
+    )
+
+    if return_edge_indices:
+        return node_seq, edge_seq
+
+    return node_seq