centralized intermediate graph degree recommendation

Author: landrumb

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

@@ -69,7 +69,7 @@ void check_graph_degree(size_t& intermediate_degree, size_t& graph_degree, size_
     graph_degree = intermediate_degree;
   }
 
-  const size_t recommended_intermediate = graph_degree + ((graph_degree + 1) / 2);
+  const size_t recommended_intermediate = recommended_intermediate_graph_degree(graph_degree);
   if (intermediate_degree < recommended_intermediate) {
     RAFT_LOG_WARN(
       "CAGRA: intermediate_graph_degree (%lu) is below the recommended minimum (%lu, i.e. "

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

@@ -1679,7 +1679,8 @@ void prune_graph_gpu(
   raft::copy(res, host_stats.view(), raft::make_const_mdspan(dev_stats.view()));
   raft::resource::sync_stream(res);
 
-  const size_t recommended_intermediate = output_graph_degree + ((output_graph_degree + 1) / 2);
+  const size_t recommended_intermediate =
+    recommended_intermediate_graph_degree(output_graph_degree);
   RAFT_EXPECTS(
     invalid_neighbor_list == 0,
     "CAGRA graph pruning failed: could not select graph_degree (%lu) distinct neighbors for "

cpp/src/neighbors/detail/cagra/utils.hpp

@@ -301,4 +301,9 @@ void copy_with_padding(
   }
 }
 
+constexpr size_t recommended_intermediate_graph_degree(size_t graph_degree) noexcept
+{
+  return graph_degree + ((graph_degree + 1) / 2);
+}
+
 }  // namespace cuvs::neighbors::cagra::detail