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0.2.0 | Apr 25, 2021 |
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0.1.1 | Jun 17, 2019 |

0.1.0 | Jun 17, 2019 |

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# 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 ``=` `"`0.2.0`"`

### 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_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

object to execute multiple queries instead:`PathCalculator`

`//` ... 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_graph`)``;`
`let` shortest_path `=` path_calculator`.``calc_path``(``&`fast_graph`,` `8``,` `6``)``;`

### Serializing the prepared graph

implements standard Serde serialization.`FastGraph`

To be able to use the graph in a 32bit WebAssembly environment, it needs to be transformed to a 32bit representation when preparing it on a 64bit system. This can be achieved with the following two methods, but it will only work for graphs that do not exceed the 32bit limit, i.e. the number of nodes and edges and all weights must be below 2^32.

`use` `fast_paths``::``{`deserialize_32`,` serialize_32`,` FastGraph`}``;`
`#``[``derive``(``Serialize``,` Deserialize`)``]`
`struct` `YourData` `{`
`#``[``serde``(``serialize_with ``=` `"`serialize_32`"``,` deserialize_with `=` `"`deserialize_32`"``)``]`
`graph``:` FastGraph,
`//` the rest of your struct
`}`

### 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

must have the same number of nodes as `another_input_graph`

, otherwise `input_graph`

will return an error. Also performance will only be acceptable if `prepare_with_order`

and `input_graph`

are similar to each other, say you only changed a few edge weights.`another_input_graph`

### 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:

area | number of nodes | number of edges |
---|---|---|

New York | 264.347 | 730.100 |

California&Nevada | 1.890.816 | 4.630.444 |

USA | 23.947.348 | 57.708.624 |

graph | metric | preparation time | average query time (micros) |
---|---|---|---|

NY city | distance | 19 s | 108 |

CAL&NV | distance | 85 s | 243 |

USA | distance | 28 min | 1452 |

NY city | time | 12 s | 54 |

CAL&NV | time | 54 s | 149 |

USA | time | 12 min | 856 |

The shortest path calculation time was averaged over 100k random routing queries. The benchmarks were run using Rust 1.50.0

There are also some benchmarks using smaller maps included in the test suite. You can run them like this:

`export` `RUST_TEST_THREADS``=``1``;` `cargo`` test`` --`release` --`` --ignored --nocapture`

### 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

### Special Thanks

Thanks to Dustin Carlino from A/B Street!

#### License

This project is licensed under either of

- Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

#### Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in fast_paths by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.