[BUGFIX] fix the order of the seed nodes in NodeFlow (#626)

* fix.

* add comments.

src/graph/sampler.cc

@@ -276,13 +276,17 @@ NodeFlow ConstructNodeFlow(std::vector<dgl_id_t> neighbor_list,
   size_t out_node_idx = 0;
   for (int layer_id = num_hops - 1; layer_id >= 0; layer_id--) {
     // We sort the vertices in a layer so that we don't need to sort the neighbor Ids
-    // after remap to a subgraph.
+    // after remap to a subgraph. However, we don't need to sort the first layer
+    // because we want the order of the nodes in the first layer is the same as
+    // the input seed nodes.
+    if (layer_id > 0) {
       std::sort(sub_vers->begin() + layer_offsets[layer_id],
                 sub_vers->begin() + layer_offsets[layer_id + 1],
                 [](const std::pair<dgl_id_t, dgl_id_t> &a1,
                    const std::pair<dgl_id_t, dgl_id_t> &a2) {
         return a1.first < a2.first;
       });
+    }
 
     // Save the sampled vertices and its layer Id.
     for (size_t i = layer_offsets[layer_id]; i < layer_offsets[layer_id + 1]; i++) {
@@ -305,11 +309,15 @@ NodeFlow ConstructNodeFlow(std::vector<dgl_id_t> neighbor_list,
   layer_off_data[1] = layer_offsets[num_hops] - layer_offsets[num_hops - 1];
   int out_layer_idx = 1;
   for (int layer_id = num_hops - 2; layer_id >= 0; layer_id--) {
+    // Because we don't sort the vertices in the first layer above, we can't sort
+    // the neighbor positions of the vertices in the first layer either.
+    if (layer_id > 0) {
       std::sort(neigh_pos->begin() + layer_offsets[layer_id],
                 neigh_pos->begin() + layer_offsets[layer_id + 1],
                 [](const neighbor_info &a1, const neighbor_info &a2) {
                   return a1.id < a2.id;
                 });
+    }
 
     for (size_t i = layer_offsets[layer_id]; i < layer_offsets[layer_id + 1]; i++) {
       dgl_id_t dst_id = sub_vers->at(i).first;

tests/compute/test_nodeflow.py

@@ -39,11 +39,12 @@ def test_self_loop():
         assert F.array_equal(in_deg, deg)
 
 def create_mini_batch(g, num_hops, add_self_loop=False):
-    seed_ids = np.array([0, 1, 2, 3])
+    seed_ids = np.array([1, 2, 0, 3])
     sampler = NeighborSampler(g, batch_size=4, expand_factor=g.number_of_nodes(),
             num_hops=num_hops, seed_nodes=seed_ids, add_self_loop=add_self_loop)
     nfs = list(sampler)
     assert len(nfs) == 1
+    assert np.array_equal(F.asnumpy(nfs[0].layer_parent_nid(-1)), seed_ids)
     return nfs[0]
 
 def check_basic(g, nf):