sequential voxelmap accessor (#113)

* sequential voxelmap accessor

* capital

include/small_gicp/ann/incremental_voxelmap.hpp

@@ -34,6 +34,7 @@ public:
 ///        This class supports incremental point cloud insertion and LRU-based voxel deletion that removes voxels that are not recently referenced.
 /// @note  This class can be used as a point cloud as well as a neighbor search structure.
 /// @note  This class can handle arbitrary number of voxels and arbitrary range of voxel coordinates (in 32-bit int range).
+/// @note  To use this as a source point cloud for registration, use `SequentialVoxelMapAccessor`.
 template <typename VoxelContents>
 struct IncrementalVoxelMap {
 public:

include/small_gicp/ann/sequential_voxelmap_accessor.hpp

@@ -0,0 +1,58 @@
+// SPDX-FileCopyrightText: Copyright 2024 Kenji Koide
+// SPDX-License-Identifier: MIT
+#pragma once
+
+#include <small_gicp/points/traits.hpp>
+#include <small_gicp/ann/incremental_voxelmap.hpp>
+
+namespace small_gicp {
+
+/**
+ * @brief A wrapper class to sequentially access points in a voxelmap (e.g., using points in a voxelmap as source point cloud for registration).
+ * @note  If the contents of the voxelmap are changed, the accessor must be recreated.
+ * @example
+    small_gicp::IncrementalVoxelMap<small_gicp::FlatContainerCov>::Ptr last_voxelmap = ...;
+    small_gicp::IncrementalVoxelMap<small_gicp::FlatContainerCov>::Ptr voxelmap = ...;
+
+    // Create a sequential accessor
+    const auto accessor = small_gicp::create_sequential_accessor(*voxelmap);
+
+    // Use the voxelmap as a source point cloud
+    small_gicp::Registration<small_gicp::GICPFactor, small_gicp::ParallelReductionOMP> registration;
+    auto result = registration.align(*last_voxelmap, accessor, *last_voxelmap, Eigen::Isometry3d::Identity());
+ */
+template <typename VoxelMap>
+class SequentialVoxelMapAccessor {
+public:
+  /// @brief Constructor.
+  /// @param voxelmap Voxelmap
+  SequentialVoxelMapAccessor(const VoxelMap& voxelmap) : voxelmap(voxelmap), indices(traits::point_indices(voxelmap)) {}
+
+  /// @brief Number of points in the voxelmap.
+  size_t size() const { return indices.size(); }
+
+public:
+  const VoxelMap& voxelmap;           ///< Voxelmap
+  const std::vector<size_t> indices;  ///< Indices of points in the voxelmap
+};
+
+/// @brief Create a sequential voxelmap accessor.
+template <typename VoxelMap>
+SequentialVoxelMapAccessor<VoxelMap> create_sequential_accessor(const VoxelMap& voxelmap) {
+  return SequentialVoxelMapAccessor<VoxelMap>(voxelmap);
+}
+
+template <typename VoxelMap>
+struct traits::Traits<SequentialVoxelMapAccessor<VoxelMap>> {
+  static size_t size(const SequentialVoxelMapAccessor<VoxelMap>& accessor) { return accessor.size(); }
+
+  static bool has_points(const SequentialVoxelMapAccessor<VoxelMap>& accessor) { traits::has_points(accessor.voxelmap); }
+  static bool has_normals(const SequentialVoxelMapAccessor<VoxelMap>& accessor) { return traits::has_normals(accessor.voxelmap); }
+  static bool has_covs(const SequentialVoxelMapAccessor<VoxelMap>& accessor) { return traits::has_covs(accessor.voxelmap); }
+
+  static Eigen::Vector4d point(const SequentialVoxelMapAccessor<VoxelMap>& accessor, size_t i) { return traits::point(accessor.voxelmap, accessor.indices[i]); }
+  static Eigen::Vector4d normal(const SequentialVoxelMapAccessor<VoxelMap>& accessor, size_t i) { return traits::normal(accessor.voxelmap, accessor.indices[i]); }
+  static Eigen::Matrix4d cov(const SequentialVoxelMapAccessor<VoxelMap>& accessor, size_t i) { return traits::cov(accessor.voxelmap, accessor.indices[i]); }
+};
+
+}  // namespace small_gicp