connected_components.cc

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// Copyright 2010-2025 Google LLC
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
 
// The following uses disjoint-sets algorithms, see:
// https://en.wikipedia.org/wiki/Disjoint-set_data_structure#Disjoint-set_forests
 
#include "ortools/graph/connected_components.h"
 
#include <algorithm>
#include <numeric>
#include <vector>
 
#include "absl/log/check.h"
#include "ortools/base/stl_util.h"
 
void DenseConnectedComponentsFinder::SetNumberOfNodes(int num_nodes) {
  const int old_num_nodes = GetNumberOfNodes();
  if (num_nodes == old_num_nodes) {
    return;
  }
  CHECK_GE(num_nodes, 0) << "Number of nodes overflowed the `int` type.";
  CHECK_GT(num_nodes, old_num_nodes);
  // Each new node starts as an isolated component:
  // It has itself as root.
  parent_.resize(num_nodes);
  std::iota(parent_.begin() + old_num_nodes, parent_.end(), old_num_nodes);
  //  It's in an isolated component of size 1.
  component_size_.resize(num_nodes, 1);
  //  Its rank is 0.
  rank_.resize(num_nodes);
  // This introduces one extra component per added node.
  num_components_ += num_nodes - old_num_nodes;
}
 
int DenseConnectedComponentsFinder::FindRoot(int node) {
  DCHECK_GE(node, 0);
  DCHECK_LT(node, GetNumberOfNodes());
 
  // Search the root.
  int root = parent_[node];
  while (parent_[root] != root) {
    root = parent_[root];
  }
 
  // Apply path compression.
  while (node != root) {
    const int prev_parent = parent_[node];
    parent_[node] = root;
    node = prev_parent;
  }
  return root;
}
 
const std::vector<int>& DenseConnectedComponentsFinder::GetComponentRoots() {
  const int num_nodes = GetNumberOfNodes();
  if (num_nodes != num_nodes_at_last_get_roots_call_) {
    // Add potential roots for each new node that did not exist the last time
    // GetComponentRoots() was called.  The cost here is amortized against
    // adding the nodes in the first place.
    const int previous_num_roots = roots_.size();
    roots_.resize(previous_num_roots + num_nodes -
                  num_nodes_at_last_get_roots_call_);
    std::iota(roots_.begin() + previous_num_roots, roots_.end(),
              num_nodes_at_last_get_roots_call_);
  }
 
  // Remove the roots that have been merged with other components. Each node
  // only gets removed once from the roots vector, so the cost of FindRoot() is
  // amortized against adding the edge.
  gtl::STLEraseAllFromSequenceIf(
      &roots_, [&](const int node) { return node != FindRoot(node); });
 
  num_nodes_at_last_get_roots_call_ = num_nodes;
  return roots_;
}
 
bool DenseConnectedComponentsFinder::AddEdge(int node1, int node2) {
  // Grow if needed.
  const int min_num_nodes = std::max(node1, node2) + 1;
  if (min_num_nodes > GetNumberOfNodes()) {
    SetNumberOfNodes(min_num_nodes);
  }
 
  // Just union the sets for node1 and node2.
  int root1 = FindRoot(node1);
  int root2 = FindRoot(node2);
 
  // Already the same set.
  if (root1 == root2) {
    return false;
  }
 
  DCHECK_GE(num_components_, 2);
  --num_components_;
 
  const int component_size = component_size_[root1] + component_size_[root2];
 
  // Attach the shallowest tree to root of the deepest one. Note that this
  // operation grows the rank of the new common root by at most one (if the two
  // trees originally have the same rank).
  if (rank_[root1] > rank_[root2]) {
    parent_[root2] = root1;
    component_size_[root1] = component_size;
  } else {
    parent_[root1] = root2;
    component_size_[root2] = component_size;
    // If the ranks were the same then attaching just grew the rank by one.
    if (rank_[root1] == rank_[root2]) {
      ++rank_[root2];
    }
  }
  return true;
}
 
bool DenseConnectedComponentsFinder::Connected(int node1, int node2) {
  if (node1 < 0 || node1 >= GetNumberOfNodes() || node2 < 0 ||
      node2 >= GetNumberOfNodes()) {
    return false;
  }
  return FindRoot(node1) == FindRoot(node2);
}
 
int DenseConnectedComponentsFinder::GetSize(int node) {
  if (node < 0 || node >= GetNumberOfNodes()) {
    return 0;
  }
  return component_size_[FindRoot(node)];
}
 
std::vector<int> DenseConnectedComponentsFinder::GetComponentIds() {
  std::vector<int> component_ids(GetNumberOfNodes(), -1);
  int current_component = 0;
  for (int node = 0; node < GetNumberOfNodes(); ++node) {
    int& root_component = component_ids[FindRoot(node)];
    if (root_component < 0) {
      // This is the first node in a yet unseen component.
      root_component = current_component;
      ++current_component;
    }
    component_ids[node] = root_component;
  }
  return component_ids;
}