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Description

This provides the ConnGraph class, which implements an undirected graph with dynamic connectivity. It supports adding and removing edges and determining whether two vertices are connected - whether there is a path between them. Adding and removing edges take O(log2N) amortized time with high probability, while checking whether two vertices are connected takes O(log N) time with high probability.

Features

  • Efficiently add and remove edges and determine whether vertices are connected.
  • A vertex can appear in multiple graphs, with a different set of adjacent vertices in each graph.
  • ConnGraph supports arbitrary vertex augmentation. Given a vertex V, ConnGraph can quickly report the result of combining the augmentations of all of the vertices in the connected component containing V, using a combining function provided to the constructor. For example, if a ConnGraph represents a game map, then given the location of the player, we can quickly determine the amount of gold the player can access, or the strongest monster that can reach him. Retrieving the combined augmentation for a connected component takes O(log N) time with high probability.
  • Compatible with Java 7.0 and above.

Limitations

  • ConnGraph does not directly support augmenting edges. However, this can be accomplished by imputing each edge's augmentation to an adjacent vertex. For example, if each edge contains a certain amount of gold, then we can augment each vertex with the amount of gold in the adjacent edges. We can then calculate the amount of gold in a connected component by retrieving the component's augmentation and dividing by two. A more general approach would be to store the edges adjacent to each vertex in an augmented self-balancing binary search tree (see RedBlackNode), and to use this to assign an augmentation to each vertex.
  • Careful attention has been paid to the asymptotic running time of each method. However, beyond this, no special effort has been made to optimize performance. (A big obstacle to optimizing ConnGraph is a lack of access to samples from real-world usage.)

Example usage

ConnGraph graph = new ConnGraph();
ConnVertex vertex1 = new ConnVertex();
ConnVertex vertex2 = new ConnVertex();
ConnVertex vertex3 = new ConnVertex();
graph.addEdge(vertex1, vertex2);
graph.addEdge(vertex2, vertex3);
graph.connected(vertex1, vertex3);  // Returns true
graph.removeEdge(vertex1, vertex2);
graph.connected(vertex1, vertex3);  // Returns false

Documentation

See https://btrekkie.github.io/dynamic-connectivity/index.html for API documentation.