ceres-solver/internal/ceres/parameter_block_ordering.cc

// Ceres Solver - A fast non-linear least squares minimizer
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// http://ceres-solver.org/
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// Author: sameeragarwal@google.com (Sameer Agarwal)

#include "ceres/parameter_block_ordering.h"

#include <map>
#include <memory>
#include <set>
#include <unordered_set>
#include <vector>

#include "absl/log/check.h"
#include "ceres/graph.h"
#include "ceres/graph_algorithms.h"
#include "ceres/map_util.h"
#include "ceres/parameter_block.h"
#include "ceres/program.h"
#include "ceres/residual_block.h"
#include "ceres/wall_time.h"

namespace ceres::internal {

int ComputeStableSchurOrdering(const Program& program,
                               std::vector<ParameterBlock*>* ordering) {
  CHECK(ordering != nullptr);
  ordering->clear();
  EventLogger event_logger("ComputeStableSchurOrdering");
  auto graph = CreateHessianGraph(program);
  event_logger.AddEvent("CreateHessianGraph");

  const std::vector<ParameterBlock*>& parameter_blocks =
      program.parameter_blocks();
  const std::unordered_set<ParameterBlock*>& vertices = graph->vertices();
  for (auto* parameter_block : parameter_blocks) {
    if (vertices.count(parameter_block) > 0) {
      ordering->push_back(parameter_block);
    }
  }
  event_logger.AddEvent("Preordering");

  int independent_set_size = StableIndependentSetOrdering(*graph, ordering);
  event_logger.AddEvent("StableIndependentSet");

  // Add the excluded blocks to back of the ordering vector.
  for (auto* parameter_block : parameter_blocks) {
    if (parameter_block->IsConstant()) {
      ordering->push_back(parameter_block);
    }
  }
  event_logger.AddEvent("ConstantParameterBlocks");

  return independent_set_size;
}

int ComputeSchurOrdering(const Program& program,
                         std::vector<ParameterBlock*>* ordering) {
  CHECK(ordering != nullptr);
  ordering->clear();

  auto graph = CreateHessianGraph(program);
  int independent_set_size = IndependentSetOrdering(*graph, ordering);
  const std::vector<ParameterBlock*>& parameter_blocks =
      program.parameter_blocks();

  // Add the excluded blocks to back of the ordering vector.
  for (auto* parameter_block : parameter_blocks) {
    if (parameter_block->IsConstant()) {
      ordering->push_back(parameter_block);
    }
  }

  return independent_set_size;
}

void ComputeRecursiveIndependentSetOrdering(const Program& program,
                                            ParameterBlockOrdering* ordering) {
  CHECK(ordering != nullptr);
  ordering->Clear();
  const std::vector<ParameterBlock*> parameter_blocks =
      program.parameter_blocks();
  auto graph = CreateHessianGraph(program);

  int num_covered = 0;
  int round = 0;
  while (num_covered < parameter_blocks.size()) {
    std::vector<ParameterBlock*> independent_set_ordering;
    const int independent_set_size =
        IndependentSetOrdering(*graph, &independent_set_ordering);
    for (int i = 0; i < independent_set_size; ++i) {
      ParameterBlock* parameter_block = independent_set_ordering[i];
      ordering->AddElementToGroup(parameter_block->mutable_user_state(), round);
      graph->RemoveVertex(parameter_block);
    }
    num_covered += independent_set_size;
    ++round;
  }
}

std::unique_ptr<Graph<ParameterBlock*>> CreateHessianGraph(
    const Program& program) {
  auto graph = std::make_unique<Graph<ParameterBlock*>>();
  CHECK(graph != nullptr);
  const std::vector<ParameterBlock*>& parameter_blocks =
      program.parameter_blocks();
  for (auto* parameter_block : parameter_blocks) {
    if (!parameter_block->IsConstant()) {
      graph->AddVertex(parameter_block);
    }
  }

  const std::vector<ResidualBlock*>& residual_blocks =
      program.residual_blocks();
  for (auto* residual_block : residual_blocks) {
    const int num_parameter_blocks = residual_block->NumParameterBlocks();
    ParameterBlock* const* parameter_blocks =
        residual_block->parameter_blocks();
    for (int j = 0; j < num_parameter_blocks; ++j) {
      if (parameter_blocks[j]->IsConstant()) {
        continue;
      }

      for (int k = j + 1; k < num_parameter_blocks; ++k) {
        if (parameter_blocks[k]->IsConstant()) {
          continue;
        }

        graph->AddEdge(parameter_blocks[j], parameter_blocks[k]);
      }
    }
  }

  return graph;
}

void OrderingToGroupSizes(const ParameterBlockOrdering* ordering,
                          std::vector<int>* group_sizes) {
  CHECK(group_sizes != nullptr);
  group_sizes->clear();
  if (ordering == nullptr) {
    return;
  }

  // TODO(sameeragarwal): Investigate if this should be a set or an
  // unordered_set.
  const std::map<int, std::set<double*>>& group_to_elements =
      ordering->group_to_elements();
  for (const auto& g_t_e : group_to_elements) {
    group_sizes->push_back(g_t_e.second.size());
  }
}

}  // namespace ceres::internal