[Feature] FindEdge/FindEdges for Immutable Graph (#404)

* fix rgcn tutorial

* small fix

* upd

* findedge/s

* upd

* upd

* upd

* upd

* add test

* remove redundancy

* upd

* upd

* upd

* upd

* add edge_subgraph

* explicit cast

* add test immutable subg

* reformat

* reformat

* fix bug

* upd

include/dgl/immutable_graph.h

@@ -32,6 +32,30 @@ class ImmutableGraph: public GraphInterface {
     dgl_id_t edge_id;
   };
 
+  // Edge list indexed by edge id;
+  struct EdgeList {
+    typedef std::shared_ptr<EdgeList> Ptr;
+    std::vector<dgl_id_t> src_points;
+    std::vector<dgl_id_t> dst_points;
+
+    EdgeList(int64_t len, dgl_id_t val) {
+      src_points.resize(len, val);
+      dst_points.resize(len, val);
+    }
+
+    void register_edge(dgl_id_t eid, dgl_id_t src, dgl_id_t dst) {
+      CHECK_LT(eid, src_points.size()) << "Invalid edge id " << eid;
+      src_points[eid] = src;
+      dst_points[eid] = dst;
+    }
+
+    static EdgeList::Ptr FromCSR(
+        const std::vector<int64_t>& indptr,
+        const std::vector<dgl_id_t>& indices,
+        const std::vector<dgl_id_t>& edge_ids,
+        bool in_csr);
+  };
+
   struct CSR {
     typedef std::shared_ptr<CSR> Ptr;
     std::vector<int64_t> indptr;
@@ -79,6 +103,7 @@ class ImmutableGraph: public GraphInterface {
     void ReadAllEdges(std::vector<Edge> *edges) const;
     CSR::Ptr Transpose() const;
     std::pair<CSR::Ptr, IdArray> VertexSubgraph(IdArray vids) const;
+    std::pair<CSR::Ptr, IdArray> EdgeSubgraph(IdArray eids, EdgeList::Ptr edge_list) const;
     /*
      * Construct a CSR from a list of edges.
      *
@@ -261,20 +286,14 @@ class ImmutableGraph: public GraphInterface {
    * \param eid The edge ID
    * \return a pair whose first element is the source and the second the destination.
    */
-  std::pair<dgl_id_t, dgl_id_t> FindEdge(dgl_id_t eid) const {
-    LOG(FATAL) << "FindEdge isn't supported in ImmutableGraph";
-    return std::pair<dgl_id_t, dgl_id_t>();
-  }
+  std::pair<dgl_id_t, dgl_id_t> FindEdge(dgl_id_t eid) const;
 
   /*!
    * \brief Find the edge IDs and return their source and target node IDs.
    * \param eids The edge ID array.
    * \return EdgeArray containing all edges with id in eid.  The order is preserved.
    */
-  EdgeArray FindEdges(IdArray eids) const {
-    LOG(FATAL) << "FindEdges isn't supported in ImmutableGraph";
-    return EdgeArray();
-  }
+  EdgeArray FindEdges(IdArray eids) const;
 
   /*!
    * \brief Get the in edges of the vertex.
@@ -496,6 +515,25 @@ class ImmutableGraph: public GraphInterface {
     }
   }
 
+  /*
+   * The edge list is required for FindEdge/FindEdges/EdgeSubgraph, if no such function is called, we would not create edge list.
+   * if such function is called the first time, we create a edge list from one of the graph's csr representations,
+   * if we have called such function before, we get the one cached in the structure.
+   */
+  EdgeList::Ptr GetEdgeList() const {
+    if (edge_list_)
+      return edge_list_;
+    if (in_csr_) {
+      const_cast<ImmutableGraph *>(this)->edge_list_ =\
+        EdgeList::FromCSR(in_csr_->indptr, in_csr_->indices, in_csr_->edge_ids, true);
+    } else {
+      CHECK(out_csr_ != nullptr) << "one of the CSRs must exist";
+      const_cast<ImmutableGraph *>(this)->edge_list_ =\
+        EdgeList::FromCSR(out_csr_->indptr, out_csr_->indices, out_csr_->edge_ids, false);
+    }
+    return edge_list_;
+  }
+
  protected:
   DGLIdIters GetInEdgeIdRef(dgl_id_t src, dgl_id_t dst) const;
   DGLIdIters GetOutEdgeIdRef(dgl_id_t src, dgl_id_t dst) const;
@@ -525,6 +563,8 @@ class ImmutableGraph: public GraphInterface {
   CSR::Ptr in_csr_;
   // Store the out-edges.
   CSR::Ptr out_csr_;
+  // Store the edge list indexed by edge id
+  EdgeList::Ptr edge_list_;
   /*!
    * \brief Whether if this is a multigraph.
    *

python/dgl/graph.py

@@ -3070,10 +3070,8 @@ class DGLGraph(DGLBaseGraph):
         >>> G.number_of_nodes()
         8
         """
-        if readonly_state == self._graph.is_readonly():
-            return self
+        if readonly_state != self.is_readonly:
             self._graph.readonly(readonly_state)
-        return self
 
     def __repr__(self):
         ret = ('DGLGraph(num_nodes={node}, num_edges={edge},\n'

src/graph/graph.cc

@@ -252,6 +252,7 @@ Graph::EdgeArray Graph::EdgeIds(IdArray src_ids, IdArray dst_ids) const {
 }
 
 Graph::EdgeArray Graph::FindEdges(IdArray eids) const {
+  CHECK(IsValidIdArray(eids)) << "Invalid edge id array";
   int64_t len = eids->shape[0];
 
   IdArray rst_src = IdArray::Empty({len}, eids->dtype, eids->ctx);
@@ -472,7 +473,7 @@ Subgraph Graph::VertexSubgraph(IdArray vids) const {
 }
 
 Subgraph Graph::EdgeSubgraph(IdArray eids) const {
-  CHECK(IsValidIdArray(eids)) << "Invalid vertex id array.";
+  CHECK(IsValidIdArray(eids)) << "Invalid edge id array.";
 
   const auto len = eids->shape[0];
   std::unordered_map<dgl_id_t, dgl_id_t> oldv2newv;

src/graph/immutable_graph.cc

@@ -158,6 +158,25 @@ class HashTableChecker {
   }
 };
 
+ImmutableGraph::EdgeList::Ptr ImmutableGraph::EdgeList::FromCSR(
+    const std::vector<int64_t>& indptr,
+    const std::vector<dgl_id_t>& indices,
+    const std::vector<dgl_id_t>& edge_ids,
+    bool in_csr) {
+  const auto n = indptr.size() - 1;
+  const auto len = edge_ids.size();
+  auto t = std::make_shared<EdgeList>(len, n);
+  for (size_t i = 0; i < indptr.size() - 1; i++) {
+    for (int64_t j = indptr[i]; j < indptr[i + 1]; j++) {
+      dgl_id_t row = i, col = indices[j];
+      if (in_csr)
+        std::swap(row, col);
+      t->register_edge(edge_ids[j], row, col);
+    }
+  }
+  return t;
+}
+
 std::pair<ImmutableGraph::CSR::Ptr, IdArray> ImmutableGraph::CSR::VertexSubgraph(
     IdArray vids) const {
   CHECK(IsValidIdArray(vids)) << "Invalid vertex id array.";
@@ -165,7 +184,7 @@ std::pair<ImmutableGraph::CSR::Ptr, IdArray> ImmutableGraph::CSR::VertexSubgraph
   const int64_t len = vids->shape[0];
 
   HashTableChecker def_check(vid_data, len);
-  // check if varr is sorted.
+  // check if vid_data is sorted.
   CHECK(std::is_sorted(vid_data, vid_data + len)) << "The input vertex list has to be sorted";
 
   // Collect the non-zero entries in from the original graph.
@@ -197,6 +216,42 @@ std::pair<ImmutableGraph::CSR::Ptr, IdArray> ImmutableGraph::CSR::VertexSubgraph
   return std::pair<ImmutableGraph::CSR::Ptr, IdArray>(sub_csr, rst_eids);
 }
 
+std::pair<ImmutableGraph::CSR::Ptr, IdArray> ImmutableGraph::CSR::EdgeSubgraph(
+    IdArray eids, EdgeList::Ptr edge_list) const {
+  // Return sub_csr and vids array.
+  CHECK(IsValidIdArray(eids)) << "Invalid edge id array.";
+  const dgl_id_t* eid_data = static_cast<dgl_id_t*>(eids->data);
+  const int64_t len = eids->shape[0];
+  std::vector<dgl_id_t> nodes;
+  std::unordered_map<dgl_id_t, dgl_id_t> oldv2newv;
+  std::vector<Edge> edges;
+
+  for (int64_t i = 0; i < len; i++) {
+    dgl_id_t src_id = edge_list->src_points[eid_data[i]];
+    dgl_id_t dst_id = edge_list->dst_points[eid_data[i]];
+
+    // pair<iterator, bool>, the second indicates whether the insertion is successful or not.
+    auto src_pair = oldv2newv.insert(std::make_pair(src_id, oldv2newv.size()));
+    auto dst_pair = oldv2newv.insert(std::make_pair(dst_id, oldv2newv.size()));
+    if (src_pair.second)
+      nodes.push_back(src_id);
+    if (dst_pair.second)
+      nodes.push_back(dst_id);
+    edges.push_back(Edge{src_pair.first->second, dst_pair.first->second, static_cast<dgl_id_t>(i)});
+  }
+
+  const size_t n = oldv2newv.size();
+  auto sub_csr = CSR::FromEdges(&edges, 0, n);
+
+  IdArray rst_vids = IdArray::Empty({static_cast<int64_t>(nodes.size())},
+      DLDataType{kDLInt, 64, 1}, DLContext{kDLCPU, 0});
+  dgl_id_t* vid_data = static_cast<dgl_id_t*>(rst_vids->data);
+  std::copy(nodes.begin(), nodes.end(), vid_data);
+
+  return std::make_pair(sub_csr, rst_vids);
+}
+
+
 ImmutableGraph::CSR::Ptr ImmutableGraph::CSR::FromEdges(std::vector<Edge> *edges,
                                                         int sort_on, uint64_t num_nodes) {
   CHECK(sort_on == 0 || sort_on == 1) << "we must sort on the first or the second vector";
@@ -449,6 +504,34 @@ ImmutableGraph::EdgeArray ImmutableGraph::EdgeIds(IdArray src_ids, IdArray dst_i
   return ImmutableGraph::EdgeArray{rst_src, rst_dst, rst_eid};
 }
 
+std::pair<dgl_id_t, dgl_id_t> ImmutableGraph::FindEdge(dgl_id_t eid) const {
+  dgl_id_t row = 0, col = 0;
+  auto edge_list = GetEdgeList();
+  CHECK(eid < NumEdges()) << "Invalid edge id " << eid;
+  row = edge_list->src_points[eid];
+  col = edge_list->dst_points[eid];
+  CHECK(row < NumVertices() && col < NumVertices()) << "Invalid edge id " << eid;
+  return std::pair<dgl_id_t, dgl_id_t>(row, col);
+}
+
+ImmutableGraph::EdgeArray ImmutableGraph::FindEdges(IdArray eids) const {
+  CHECK(IsValidIdArray(eids)) << "Invalid edge id array";
+  dgl_id_t* eid_data = static_cast<dgl_id_t*>(eids->data);
+  int64_t len = eids->shape[0];
+  IdArray rst_src = IdArray::Empty({len}, eids->dtype, eids->ctx);
+  IdArray rst_dst = IdArray::Empty({len}, eids->dtype, eids->ctx);
+  dgl_id_t* rst_src_data = static_cast<dgl_id_t*>(rst_src->data);
+  dgl_id_t* rst_dst_data = static_cast<dgl_id_t*>(rst_dst->data);
+
+  for (int64_t i = 0; i < len; i++) {
+    auto edge = ImmutableGraph::FindEdge(eid_data[i]);
+    rst_src_data[i] = edge.first;
+    rst_dst_data[i] = edge.second;
+  }
+
+  return ImmutableGraph::EdgeArray{rst_src, rst_dst, eids};
+}
+
 ImmutableGraph::EdgeArray ImmutableGraph::Edges(const std::string &order) const {
   int64_t rstlen = NumEdges();
   IdArray rst_src = IdArray::Empty({rstlen}, DLDataType{kDLInt, 64, 1}, DLContext{kDLCPU, 0});
@@ -506,8 +589,19 @@ Subgraph ImmutableGraph::VertexSubgraph(IdArray vids) const {
 }
 
 Subgraph ImmutableGraph::EdgeSubgraph(IdArray eids) const {
-  LOG(FATAL) << "EdgeSubgraph isn't implemented in immutable graph";
-  return Subgraph();
+  Subgraph subg;
+  std::pair<CSR::Ptr, IdArray> ret;
+  auto edge_list = GetEdgeList();
+  if (out_csr_) {
+    ret = out_csr_->EdgeSubgraph(eids, edge_list);
+    subg.graph = GraphPtr(new ImmutableGraph(nullptr, ret.first, IsMultigraph()));
+  } else {
+    ret = in_csr_->EdgeSubgraph(eids, edge_list);
+    subg.graph = GraphPtr(new ImmutableGraph(ret.first, nullptr, IsMultigraph()));
+  }
+  subg.induced_edges = eids;
+  subg.induced_vertices = ret.second;
+  return subg;
 }
 
 ImmutableGraph::CSRArray ImmutableGraph::GetInCSRArray() const {

tests/compute/test_graph.py

@@ -184,6 +184,35 @@ def test_readonly():
         assert g.number_of_edges() == 14
         assert F.shape(g.edata['x']) == (14, 4)
 
+def test_find_edges():
+    g = dgl.DGLGraph()
+    g.add_nodes(10)
+    g.add_edges(range(9), range(1, 10))
+    e = g.find_edges([1, 3, 2, 4])
+    assert e[0][0] == 1 and e[0][1] == 3 and e[0][2] == 2 and e[0][3] == 4
+    assert e[1][0] == 2 and e[1][1] == 4 and e[1][2] == 3 and e[1][3] == 5
+
+    try:
+        g.find_edges([10])
+        fail = False
+    except DGLError:
+        fail = True
+    finally:
+        assert fail
+
+    g.readonly()
+    e = g.find_edges([1, 3, 2, 4])
+    assert e[0][0] == 1 and e[0][1] == 3 and e[0][2] == 2 and e[0][3] == 4
+    assert e[1][0] == 2 and e[1][1] == 4 and e[1][2] == 3 and e[1][3] == 5
+
+    try:
+        g.find_edges([10])
+        fail = False
+    except DGLError:
+        fail = True
+    finally:
+        assert fail
+
 if __name__ == '__main__':
     test_graph_creation()
     test_create_from_elist()
@@ -191,3 +220,4 @@ if __name__ == '__main__':
     test_incmat()
     test_incmat_cache()
     test_readonly()
+    test_find_edges()

tests/graph_index/test_subgraph.py

@@ -32,7 +32,24 @@ def test_edge_subgraph():
         assert sgi.induced_edges[e] in gi.edge_id(
                 sgi.induced_nodes[s], sgi.induced_nodes[d])
 
+def test_immutable_edge_subgraph():
+    gi = create_graph_index()
+    gi.add_nodes(4)
+    gi.add_edge(0, 1)
+    gi.add_edge(0, 1)
+    gi.add_edge(0, 2)
+    gi.add_edge(2, 3)
+    gi.readonly() # Make the graph readonly
+
+    sub2par_edgemap = [3, 2]
+    sgi = gi.edge_subgraph(toindex(sub2par_edgemap))
+
+    for s, d, e in zip(*sgi.edges()):
+        assert sgi.induced_edges[e] in gi.edge_id(
+                sgi.induced_nodes[s], sgi.induced_nodes[d])
+
 
 if __name__ == '__main__':
     test_node_subgraph()
     test_edge_subgraph()
+    test_immutable_edge_subgraph()