Add brute-force kNN graph construction

c/src/neighbors/cagra.cpp

@@ -757,11 +757,7 @@ static void update_dataset(raft::resources* res_ptr,
 }
 
 static void _set_graph_build_params(
-  std::variant<std::monostate,
-               cuvs::neighbors::cagra::graph_build_params::ivf_pq_params,
-               cuvs::neighbors::cagra::graph_build_params::nn_descent_params,
-               cuvs::neighbors::cagra::graph_build_params::ace_params,
-               cuvs::neighbors::cagra::graph_build_params::iterative_search_params>& out_params,
+  cuvs::neighbors::cagra::graph_build_params_t& out_params,
   cuvsCagraIndexParams& params,
   cuvsCagraGraphBuildAlgo algo,
   int64_t n_rows,

cpp/include/cuvs/neighbors/cagra.hpp

@@ -8,6 +8,7 @@
 #include <cuvs/core/bitset.hpp>
 #include <cuvs/core/export.hpp>
 #include <cuvs/distance/distance.hpp>
+#include <cuvs/neighbors/brute_force.hpp>
 #include <cuvs/neighbors/common.hpp>
 #include <cuvs/neighbors/ivf_pq.hpp>
 #include <cuvs/neighbors/nn_descent.hpp>
@@ -254,7 +255,8 @@ using graph_build_params_t = std::variant<std::monostate,
                                           graph_build_params::ivf_pq_params,
                                           graph_build_params::nn_descent_params,
                                           graph_build_params::ace_params,
-                                          graph_build_params::iterative_search_params>;
+                                          graph_build_params::iterative_search_params,
+                                          graph_build_params::brute_force_params>;
 
 /**
  * @brief A strategy for selecting the graph build parameters based on similar HNSW index
@@ -317,6 +319,9 @@ struct index_params : cuvs::neighbors::index_params {
    * // 4. Choose iterative graph building using CAGRA's search() and optimize()  [Experimental]
    * params.graph_build_params =
    * cagra::graph_build_params::iterative_search_params();
+   *
+   * // 5. Choose exact brute-force kNN construction (float and half datasets)
+   * params.graph_build_params = cagra::graph_build_params::brute_force_params{};
    * @endcode
    */
   graph_build_params_t graph_build_params;
@@ -4415,6 +4420,90 @@ void distribute(const raft::resources& clique,
                 const std::string& filename,
                 cuvs::neighbors::mg_index<cagra::device_padded_index<T, IdxT>, T, IdxT>* index);
 
+/**
+ * @brief Build an exact kNN graph using brute-force search.
+ * @ingroup cagra_cpp_index_build
+ *
+ * Each row of the output contains the nearest neighbor indices for the corresponding dataset
+ * point, ordered by distance and excluding the point itself. The graph is stored in host memory.
+ * The dataset is processed in chunks, allowing graphs larger than available device memory to be
+ * built.
+ *
+ * @param[in] res RAFT resources.
+ * @param[in] dataset Row-major host dataset of float values, with shape [n_rows, dim].
+ * @param[out] knn_graph Host output with shape [n_rows, degree], where degree is in [1, n_rows -
+ * 1].
+ * @param[in] build_params Brute-force index and search parameters. Supported metrics are
+ * L2Expanded, L2SqrtExpanded, InnerProduct and CosineExpanded.
+ */
+void build_knn_graph(raft::resources const& res,
+                     raft::host_matrix_view<const float, int64_t, raft::row_major> dataset,
+                     raft::host_matrix_view<uint32_t, int64_t, raft::row_major> knn_graph,
+                     graph_build_params::brute_force_params build_params);
+
+/**
+ * @brief Build an exact kNN graph using brute-force search.
+ * @ingroup cagra_cpp_index_build
+ *
+ * Each row of the output contains the nearest neighbor indices for the corresponding dataset
+ * point, ordered by distance and excluding the point itself. The graph is stored in host memory.
+ * The dataset is processed in chunks, allowing graphs larger than available device memory to be
+ * built.
+ *
+ * @param[in] res RAFT resources.
+ * @param[in] dataset Row-major device dataset of float values, with shape [n_rows, dim].
+ * @param[out] knn_graph Host output with shape [n_rows, degree], where degree is in [1, n_rows -
+ * 1].
+ * @param[in] build_params Brute-force index and search parameters. Supported metrics are
+ * L2Expanded, L2SqrtExpanded, InnerProduct and CosineExpanded.
+ */
+void build_knn_graph(raft::resources const& res,
+                     raft::device_matrix_view<const float, int64_t, raft::row_major> dataset,
+                     raft::host_matrix_view<uint32_t, int64_t, raft::row_major> knn_graph,
+                     graph_build_params::brute_force_params build_params);
+
+/**
+ * @brief Build an exact kNN graph using brute-force search.
+ * @ingroup cagra_cpp_index_build
+ *
+ * Each row of the output contains the nearest neighbor indices for the corresponding dataset
+ * point, ordered by distance and excluding the point itself. The graph is stored in host memory.
+ * The dataset is processed in chunks, allowing graphs larger than available device memory to be
+ * built.
+ *
+ * @param[in] res RAFT resources.
+ * @param[in] dataset Row-major host dataset of half values, with shape [n_rows, dim].
+ * @param[out] knn_graph Host output with shape [n_rows, degree], where degree is in [1, n_rows -
+ * 1].
+ * @param[in] build_params Brute-force index and search parameters. Supported metrics are
+ * L2Expanded, L2SqrtExpanded, InnerProduct and CosineExpanded.
+ */
+void build_knn_graph(raft::resources const& res,
+                     raft::host_matrix_view<const half, int64_t, raft::row_major> dataset,
+                     raft::host_matrix_view<uint32_t, int64_t, raft::row_major> knn_graph,
+                     graph_build_params::brute_force_params build_params);
+
+/**
+ * @brief Build an exact kNN graph using brute-force search.
+ * @ingroup cagra_cpp_index_build
+ *
+ * Each row of the output contains the nearest neighbor indices for the corresponding dataset
+ * point, ordered by distance and excluding the point itself. The graph is stored in host memory.
+ * The dataset is processed in chunks, allowing graphs larger than available device memory to be
+ * built.
+ *
+ * @param[in] res RAFT resources.
+ * @param[in] dataset Row-major device dataset of half values, with shape [n_rows, dim].
+ * @param[out] knn_graph Host output with shape [n_rows, degree], where degree is in [1, n_rows -
+ * 1].
+ * @param[in] build_params Brute-force index and search parameters. Supported metrics are
+ * L2Expanded, L2SqrtExpanded, InnerProduct and CosineExpanded.
+ */
+void build_knn_graph(raft::resources const& res,
+                     raft::device_matrix_view<const half, int64_t, raft::row_major> dataset,
+                     raft::host_matrix_view<uint32_t, int64_t, raft::row_major> knn_graph,
+                     graph_build_params::brute_force_params build_params);
+
 /**
  * @brief Build a kNN graph using IVF-PQ.
  *

cpp/src/neighbors/cagra.cuh

@@ -191,6 +191,16 @@ void build_knn_graph(
   detail::build_knn_graph<DataT, IdxT>(res, dataset, knn_graph, build_params);
 }
 
+template <typename DataT, typename IdxT, typename Accessor>
+void build_knn_graph(
+  raft::resources const& res,
+  raft::mdspan<const DataT, raft::matrix_extent<int64_t>, raft::row_major, Accessor> dataset,
+  raft::host_matrix_view<IdxT, int64_t, raft::row_major> knn_graph,
+  graph_build_params::brute_force_params params)
+{
+  detail::build_knn_graph(res, dataset, knn_graph, params);
+}
+
 /**
  * @brief Sort a KNN graph index.
  * Preprocessing step for `cagra::optimize`: If a KNN graph is not built using

cpp/src/neighbors/cagra_build_inst.cu.in

@@ -43,6 +43,22 @@ void build_knn_graph(raft::resources const& handle,
   cuvs::neighbors::cagra::build_knn_graph<data_t, index_t>(handle, dataset, knn_graph, params);
 }
 
+void build_knn_graph(raft::resources const& res,
+                     raft::host_matrix_view<const data_t, int64_t, raft::row_major> dataset,
+                     raft::host_matrix_view<index_t, int64_t, raft::row_major> knn_graph,
+                     graph_build_params::brute_force_params params)
+{
+  detail::build_knn_graph(res, dataset, knn_graph, params);
+}
+
+void build_knn_graph(raft::resources const& res,
+                     raft::device_matrix_view<const data_t, int64_t, raft::row_major> dataset,
+                     raft::host_matrix_view<index_t, int64_t, raft::row_major> knn_graph,
+                     graph_build_params::brute_force_params params)
+{
+  detail::build_knn_graph(res, dataset, knn_graph, params);
+}
+
 #define CUVS_INST_CAGRA_BUILD(DatasetViewT)                                   \
   auto build(raft::resources const& res,                                      \
              const cuvs::neighbors::cagra::index_params& params,              \

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

@@ -8,6 +8,7 @@
 #include "../../../util/kvikio_io.hpp"
 #include "../../ivf_pq/ivf_pq_fp16_overflow.cuh"
 #include "cagra_ace.cuh"
+#include "cagra_build_brute_force.cuh"
 #include "cagra_search.cuh"
 #include "graph_core.cuh"
 #include <cuvs/preprocessing/quantize/pq.hpp>
@@ -982,11 +983,12 @@ inline void validate_cagra_knn_graph_build_constraints(index_params const& param
       std::holds_alternative<cagra::graph_build_params::iterative_search_params>(
         knn_build_params) ||
       std::holds_alternative<cagra::graph_build_params::nn_descent_params>(knn_build_params),
-    "IVF_PQ for CAGRA graph build does not support BitwiseHamming as a metric. Please "
+    "The selected CAGRA graph builder does not support BitwiseHamming as a metric. Please "
     "use nn-descent or the iterative CAGRA search build.");
   RAFT_EXPECTS(
     params.metric != cuvs::distance::DistanceType::CosineExpanded ||
       std::holds_alternative<cagra::graph_build_params::ivf_pq_params>(knn_build_params) ||
+      std::holds_alternative<cagra::graph_build_params::brute_force_params>(knn_build_params) ||
       std::holds_alternative<cagra::graph_build_params::nn_descent_params>(knn_build_params),
     "CosineExpanded distance is not supported for iterative CAGRA graph build.");
 
@@ -997,7 +999,8 @@ inline void validate_cagra_knn_graph_build_constraints(index_params const& param
 }
 
 /**
- * Iterative / IVF-PQ / NN-descent KNN graph construction and `optimize` → final host CAGRA graph.
+ * Iterative / IVF-PQ / NN-descent / brute-force KNN graph construction and `optimize` → final host
+ * CAGRA graph.
  *
  * @param knn_graph_dataset  mdspan passed to IVF-PQ / NN-descent `build_knn_graph` (any stride).
  */
@@ -1026,6 +1029,10 @@ auto build_cagra_host_graph_from_knn_params(raft::resources const& res,
       ivf_pq_params.build_params.metric = params.metric;
     }
     build_knn_graph(res, knn_graph_dataset, knn_graph->view(), ivf_pq_params);
+  } else if (std::holds_alternative<graph_build_params::brute_force_params>(knn_build_params)) {
+    auto brute_force_params = std::get<graph_build_params::brute_force_params>(knn_build_params);
+    brute_force_params.build_params.metric = params.metric;
+    build_knn_graph(res, knn_graph_dataset, knn_graph->view(), brute_force_params);
   } else {
     auto nn_descent_params =
       std::get<cagra::graph_build_params::nn_descent_params>(knn_build_params);