Fix a CAGRA graph opt bug (#192)

There is a bug in the current CAGRA graph rank-based neighbor reordering process. A low recall or illegal memory access can occur if there are many detourable nodes from a node to its neighbors, e.g. there is a small subgraph in the initial kNN graph. This PR fixes this.

Authors:
  - tsuki (https://github.com/enp1s0)

Approvers:
  - Tamas Bela Feher (https://github.com/tfeher)

URL: #192

cpp/src/neighbors/detail/cagra/graph_core.cuh

@@ -435,24 +435,39 @@ void optimize(
     const auto num_full = host_stats.data_handle()[1];
 
     // Create pruned kNN graph
-    uint32_t max_detour = 0;
-#pragma omp parallel for reduction(max : max_detour)
+#pragma omp parallel for
     for (uint64_t i = 0; i < graph_size; i++) {
-      uint64_t pk = 0;
-      for (uint32_t num_detour = 0; num_detour < output_graph_degree; num_detour++) {
-        if (max_detour < num_detour) { max_detour = num_detour; /* stats */ }
+      // Find the `output_graph_degree` smallest detourable count nodes by checking the detourable
+      // count of the neighbors while increasing the target detourable count from zero.
+      uint64_t pk         = 0;
+      uint32_t num_detour = 0;
+      while (pk < output_graph_degree) {
+        uint32_t next_num_detour = std::numeric_limits<uint32_t>::max();
         for (uint64_t k = 0; k < input_graph_degree; k++) {
-          if (detour_count.data_handle()[k + (input_graph_degree * i)] != num_detour) { continue; }
+          const auto num_detour_k = detour_count.data_handle()[k + (input_graph_degree * i)];
+          // Find the detourable count to check in the next iteration
+          if (num_detour_k > num_detour) {
+            next_num_detour = std::min(static_cast<uint32_t>(num_detour_k), next_num_detour);
+          }
+
+          // Store the neighbor index if its detourable count is equal to `num_detour`.
+          if (num_detour_k != num_detour) { continue; }
           output_graph_ptr[pk + (output_graph_degree * i)] =
             input_graph_ptr[k + (input_graph_degree * i)];
           pk += 1;
           if (pk >= output_graph_degree) break;
         }
         if (pk >= output_graph_degree) break;
+
+        assert(next_num_detour != std::numeric_limits<uint32_t>::max());
+        num_detour = next_num_detour;
       }
-      assert(pk == output_graph_degree);
+      RAFT_EXPECTS(pk == output_graph_degree,
+                   "Couldn't find the output_graph_degree (%u) smallest detourable count nodes for "
+                   "node %lu in the rank-based node reranking process",
+                   output_graph_degree,
+                   static_cast<uint64_t>(i));
     }
-    // RAFT_LOG_DEBUG("# max_detour: %u\n", max_detour);
 
     const double time_prune_end = cur_time();
     RAFT_LOG_DEBUG(