From 9aa1dcbac5ceaac769bed25d29256997967f8472 Mon Sep 17 00:00:00 2001
From: Da Zheng <zhengda1936@gmail.com>
Date: Sat, 3 Nov 2018 01:06:54 +0000
Subject: [PATCH] accelerate subgraph construction.

---
 src/operator/dgl_graph.cc | 118 ++++++++++++++++++++++++++++++++------
 1 file changed, 101 insertions(+), 17 deletions(-)

diff --git a/src/operator/dgl_graph.cc b/src/operator/dgl_graph.cc
index 11a782a49229..ce7c884ac391 100644
--- a/src/operator/dgl_graph.cc
+++ b/src/operator/dgl_graph.cc
@@ -103,16 +103,103 @@ static bool DGLSubgraphType(const nnvm::NodeAttrs& attrs,
 
 typedef int64_t dgl_id_t;
 
+class Bitmap {
+  const size_t size = 1024 * 1024 * 4;
+  const size_t mask = size - 1;
+  std::vector<bool> map;
+
+  size_t hash(dgl_id_t id) const {
+    return id & mask;
+  }
+ public:
+  Bitmap(const dgl_id_t *vid_data, int64_t len): map(size) {
+    for (int64_t i = 0; i < len; ++i) {
+      map[hash(vid_data[i])] = 1;
+    }
+  }
+
+  bool test(dgl_id_t id) const {
+    return map[hash(id)];
+  }
+};
+
+/*
+ * This uses a hashtable to check if a node is in the given node list.
+ */
+class HashTableChecker {
+  std::unordered_map<dgl_id_t, dgl_id_t> oldv2newv;
+  Bitmap map;
+ public:
+  HashTableChecker(const dgl_id_t *vid_data, int64_t len): map(vid_data, len) {
+    oldv2newv.reserve(len);
+    for (int64_t i = 0; i < len; ++i) {
+      oldv2newv[vid_data[i]] = i;
+    }
+  }
+
+  void CollectOnRow(const dgl_id_t col_idx[], const dgl_id_t eids[], size_t row_len,
+                    std::vector<dgl_id_t> *new_col_idx,
+                    std::vector<dgl_id_t> *orig_eids) {
+    // TODO(zhengda) I need to make sure the column index in each row is sorted.
+    for (size_t j = 0; j < row_len; ++j) {
+      const dgl_id_t oldsucc = col_idx[j];
+      const dgl_id_t eid = eids[j];
+      Collect(oldsucc, eid, new_col_idx, orig_eids);
+    }
+  }
+
+  void Collect(const dgl_id_t old_id, const dgl_id_t old_eid,
+               std::vector<dgl_id_t> *col_idx,
+               std::vector<dgl_id_t> *orig_eids) {
+    if (!map.test(old_id))
+      return;
+
+    auto it = oldv2newv.find(old_id);
+    if (it != oldv2newv.end()) {
+      const dgl_id_t new_id = it->second;
+      col_idx->push_back(new_id);
+      if (orig_eids)
+        orig_eids->push_back(old_eid);
+    }
+  }
+};
+
+class ScanChecker {
+  const dgl_id_t *vid_data;
+  size_t len;
+ public:
+  ScanChecker(const dgl_id_t *vid_data, size_t len) {
+    this->vid_data = vid_data;
+    this->len = len;
+  }
+
+  void CollectOnRow(const dgl_id_t col_idx[], const dgl_id_t eids[], size_t row_len,
+                    std::vector<dgl_id_t> *new_col_idx,
+                    std::vector<dgl_id_t> *orig_eids) {
+    for (size_t v_idx = 0, r_idx = 0; v_idx < len && r_idx < row_len; ) {
+      if (col_idx[r_idx] == vid_data[v_idx]) {
+        new_col_idx->push_back(vid_data[v_idx]);
+        if (orig_eids)
+          orig_eids->push_back(eids[r_idx]);
+        r_idx++;
+        v_idx++;
+      } else if (col_idx[r_idx] < vid_data[v_idx]) {
+        r_idx++;
+      } else {
+        v_idx++;
+      }
+    }
+  }
+};
+
 static void GetSubgraph(const NDArray &csr_arr, const NDArray &varr,
                         const NDArray &sub_csr, const NDArray *old_eids) {
   TBlob data = varr.data();
   int64_t num_vertices = csr_arr.shape()[0];
-  const auto len = varr.shape()[0];
+  const size_t len = varr.shape()[0];
   const dgl_id_t *vid_data = data.dptr<dgl_id_t>();
-  std::unordered_map<dgl_id_t, dgl_id_t> oldv2newv;
-  for (int64_t i = 0; i < len; ++i) {
-    oldv2newv[vid_data[i]] = i;
-  }
+  HashTableChecker def_check(vid_data, len);
+  ScanChecker scan_check(vid_data, len);
 
   // Collect the non-zero entries in from the original graph.
   std::vector<dgl_id_t> row_idx(len + 1);
@@ -123,24 +210,21 @@ static void GetSubgraph(const NDArray &csr_arr, const NDArray &varr,
   const dgl_id_t *eids = csr_arr.data().dptr<dgl_id_t>();
   const dgl_id_t *indptr = csr_arr.aux_data(csr::kIndPtr).dptr<dgl_id_t>();
   const dgl_id_t *indices = csr_arr.aux_data(csr::kIdx).dptr<dgl_id_t>();
-  for (int64_t i = 0; i < len; ++i) {
+  for (size_t i = 0; i < len; ++i) {
     const dgl_id_t oldvid = vid_data[i];
     CHECK_LT(oldvid, num_vertices) << "Vertex Id " << oldvid << " isn't in a graph of "
         << num_vertices << " vertices";
     size_t row_start = indptr[oldvid];
     size_t row_len = indptr[oldvid + 1] - indptr[oldvid];
-    // TODO(zhengda) I need to make sure the column index in each row is sorted.
-    for (size_t j = 0; j < row_len; ++j) {
-      const dgl_id_t oldsucc = indices[row_start + j];
-      const dgl_id_t eid = eids[row_start + j];
-      auto it = oldv2newv.find(oldsucc);
-      if (it != oldv2newv.end()) {
-        const dgl_id_t newsucc = it->second;
-        col_idx.push_back(newsucc);
-        if (old_eids)
-          orig_eids.push_back(eid);
-      }
+    // If there aren't so many elements in a row
+    if (row_len < len / 2) {
+      def_check.CollectOnRow(indices + row_start, eids + row_start, row_len,
+                             &col_idx, old_eids == nullptr ? nullptr : &orig_eids);
+    } else {
+      scan_check.CollectOnRow(indices + row_start, eids + row_start, row_len,
+                              &col_idx, old_eids == nullptr ? nullptr : &orig_eids);
     }
+
     row_idx[i + 1] = col_idx.size();
   }