internal/ceres/single_linkage_clustering_test.cc

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

#include "ceres/single_linkage_clustering.h"

#include "absl/container/flat_hash_map.h"
#include "ceres/graph.h"
#include "gtest/gtest.h"

namespace ceres::internal {

TEST(SingleLinkageClustering, GraphHasTwoComponents) {
  WeightedGraph<int> graph;
  const int kNumVertices = 6;
  for (int i = 0; i < kNumVertices; ++i) {
    graph.AddVertex(i);
  }
  // Graph structure:
  //
  //  0-1-2-3 4-5
  graph.AddEdge(0, 1, 1.0);
  graph.AddEdge(1, 2, 1.0);
  graph.AddEdge(2, 3, 1.0);
  graph.AddEdge(4, 5, 1.0);

  SingleLinkageClusteringOptions options;
  absl::flat_hash_map<int, int> membership;
  ComputeSingleLinkageClustering(options, graph, &membership);
  EXPECT_EQ(membership.size(), kNumVertices);

  EXPECT_EQ(membership[1], membership[0]);
  EXPECT_EQ(membership[2], membership[0]);
  EXPECT_EQ(membership[3], membership[0]);
  EXPECT_NE(membership[4], membership[0]);
  EXPECT_NE(membership[5], membership[0]);
  EXPECT_EQ(membership[4], membership[5]);
}

TEST(SingleLinkageClustering, ComponentWithWeakLink) {
  WeightedGraph<int> graph;
  const int kNumVertices = 6;
  for (int i = 0; i < kNumVertices; ++i) {
    graph.AddVertex(i);
  }
  // Graph structure:
  //
  //  0-1-2-3 4-5
  graph.AddEdge(0, 1, 1.0);
  graph.AddEdge(1, 2, 1.0);
  graph.AddEdge(2, 3, 1.0);

  // This component should break up into two.
  graph.AddEdge(4, 5, 0.5);

  SingleLinkageClusteringOptions options;
  absl::flat_hash_map<int, int> membership;
  ComputeSingleLinkageClustering(options, graph, &membership);
  EXPECT_EQ(membership.size(), kNumVertices);

  EXPECT_EQ(membership[1], membership[0]);
  EXPECT_EQ(membership[2], membership[0]);
  EXPECT_EQ(membership[3], membership[0]);
  EXPECT_NE(membership[4], membership[0]);
  EXPECT_NE(membership[5], membership[0]);
  EXPECT_NE(membership[4], membership[5]);
}

TEST(SingleLinkageClustering, ComponentWithWeakLinkAndStrongLink) {
  WeightedGraph<int> graph;
  const int kNumVertices = 6;
  for (int i = 0; i < kNumVertices; ++i) {
    graph.AddVertex(i);
  }
  // Graph structure:
  //
  //  0-1-2-3 4-5
  graph.AddEdge(0, 1, 1.0);
  graph.AddEdge(1, 2, 1.0);
  graph.AddEdge(2, 3, 0.5);  // Weak link
  graph.AddEdge(0, 3, 1.0);

  // This component should break up into two.
  graph.AddEdge(4, 5, 1.0);

  SingleLinkageClusteringOptions options;
  absl::flat_hash_map<int, int> membership;
  ComputeSingleLinkageClustering(options, graph, &membership);
  EXPECT_EQ(membership.size(), kNumVertices);

  EXPECT_EQ(membership[1], membership[0]);
  EXPECT_EQ(membership[2], membership[0]);
  EXPECT_EQ(membership[3], membership[0]);
  EXPECT_EQ(membership[4], membership[5]);
}

}  // namespace ceres::internal