Fast Paths

The most famous algorithms used to calculate shortest paths are probably Dijkstra's algorithm and A*. However, shortest path calculation can be done much faster by preprocessing the graph.

Fast Paths uses Contraction Hierarchies, one of the best known speed-up techniques for shortest path calculation. It is especially suited to calculate shortest paths in road networks, but can be used for any directed graph with positive, non-zero edge weights.

Installation

In Cargo.toml

[dependencies]
fast_paths = "0.1.0"

Basic usage

// begin with an empty graph
let mut input_graph = InputGraph::new();

// add an edge between nodes with ID 0 and 6, the weight of the edge is 12.
// Note that the node IDs should be consecutive, if your graph has N nodes use 0...N-1 as node IDs,
// otherwise performance will degrade.
input_graph.add_edge(0, 6, 12);
// ... add many more edges here

// freeze the graph before using it (you cannot add more edges afterwards, unless you call thaw() first)
input_graph.freeze();

// prepare the graph for fast shortest path calculations. note that you have to do this again if you want to change the
// graph topology or any of the edge weights
let fast_graph = fast_paths::prepare(&input_graph);

// calculate the shortest path between nodes with ID 8 and 6 
let shortest_path = fast_paths::calc_path(&fast_path_graph, 8, 6);

match shortest_path {
    Some(p) => {
        // the weight of the shortest path
        let weight = p.get_weight();
        
        // all nodes of the shortest path (including source and target)
        let nodes = p.get_nodes();
    },
    None => {
        // no path has been found (nodes are not connected in this graph)
    }
}

Batch-wise shortest path calculation

For batch-wise calculation of shortest paths the method described above is inefficient. You should keep the PathCalculator object to execute multiple queries instead:

// ... see above
// create a path calculator (note: not thread-safe, use a separate object per thread)
let mut path_calculator = fast_paths::create_calculator(&fast_path_graph);
let shortest_path = path_calculator.calc_path(&fast_path_graph, 8, 6);

Saving the prepared graph to disk

fast_paths::save_to_disk(&fast_path_graph, "fast_path_graph.fp");
let fast_path_graph = fast_paths::load_from_disk("fast_path_graph.fp");

Preparing the graph after changes

The graph preparation can be done much faster using a fixed node ordering, which is just a permutation of node ids. This can be done like this:

let fast_graph = fast_paths::prepare(&input_graph);
let node_ordering = fast_graph.get_node_ordering();

let another_fast_graph = fast_paths::prepare_with_order(&another_input_graph, &node_ordering);

For this to work another_input_graph must have the same number of nodes as input_graph, otherwise prepare_with_order will return an error. Also performance will only be acceptable if input_graph and another_input_graph are similar to each other, say you only changed a few edge weights.

Benchmarks

FastPaths was run on a single core on a consumer-grade laptop using the road networks provided for the DIMACS implementation challenge graphs. The following graphs were used for the benchmark:

The shortest path calculation time was averaged over 100k random routing queries.

Graph limitations

• loop-edges (from node A to node A) will be ignored, because since we are only considering positive non-zero edge-weights they cannot be part of a shortest path
• in case the graph has duplicate edges (multiple edges from node A to node B) only the edge with the lowest weight will be considered

License

Apache 2.0