# `bellman_ford_shortest_paths`

    template <class EdgeListGraph, class Size, class P, class T, class R>
    bool bellman_ford_shortest_paths(EdgeListGraph& g, Size N,
      const bgl_named_params<P, T, R>& params = all defaults)

The Bellman-Ford algorithm [4,11,20,8] solves the single-source shortest paths problem for a graph with both positive and negative edge weights. For the definition of the shortest paths problem see Section Shortest-Paths Algorithms. If you only need to solve the shortest paths problem for positive edge weights, Dijkstra's algorithm provides a more efficient alternative. If all the edge weights are all equal to one then breadth-first search provides an even more efficient alternative.

Before calling the `bellman_ford_shortest_paths()` function, the user must assign the source vertex a distance of zero and all other vertices a distance of infinity. The Bellman-Ford algorithm proceeds by looping through all of the edges in the graph, applying the relaxation operation to each edge. In the following pseudo-code, v is a vertex adjacent to u, w maps edges to their weight, and d is a distance map that records the length of the shortest path to each vertex seen so far. p is a predecessor map which records the parent of each vertex, which will ultimately be the parent in the shortest paths tree

    RELAX(u, v, w, d, p)
      if (w(u,v) + d[u] < d[v])
        d[v] := w(u,v) + d[u]
        p[v] := u
      else
        ...
    relax edge (u,v)

    edge (u,v) is not relaxed

The algorithm repeats this loop |V| times after which it is guaranteed that the distances to each vertex have been reduced to the minimum possible unless there is a negative cycle in the graph. If there is a negative cycle, then there will be edges in the graph that were not properly minimized. That is, there will be edges (u,v) such that w(u,v) + d[u] < d[v]. The algorithm loops over the edges in the graph one final time to check if all the edges were minimized, returning `true` if they were and returning `false` otherwise.

    BELLMAN-FORD(G)
      for each vertex u in V
        d[u] := infinity
        p[u] := u
      end for
      for i := 1 to |V|-1
        for each edge (u,v) in E
          RELAX(u, v, w, d, p)
        end for
      end for
      for each edge (u,v) in E
        if (w(u,v) + d[u] < d[v])
          return (false, , )
        else
          ...
      end for
      return (true, p, d)
    initialize vertex u

    examine edge (u,v)

    edge (u,v) was not minimized

    edge (u,v) was minimized

There are two main options for obtaining output from the `bellman_ford_shortest_paths()` function. If the user provides a distance property map through the `distance_map()` parameter then the shortest distance from the source vertex to every other vertex in the graph will be recorded in the distance map (provided the function returns `true`). The second option is recording the shortest paths tree in the `predecessor_map()`. For each vertex u in V, p[u] will be the predecessor of u in the shortest paths tree (unless p[u] = u, in which case u is either the source vertex or a vertex unreachable from the source). In addition to these two options, the user can provide there own custom-made visitor that can takes actions during any of the algorithm's event points.

### Parameters

IN: `EdgeListGraph& g`

> A directed or undirected graph whose type must be a model of Edge List Graph.

IN: `Size N`

> The number of vertices in the graph. The type `Size` must be an integer type.

### Named Parameters

IN: `weight_map(WeightMap w)`

> The weight (also know as ``length'' or ``cost'') of each edge in the graph. The `WeightMap` type must be a model of Readable Property Map. The key type for this property map must be the edge descriptor of the graph. The value type for the weight map must be Addable with the distance map's value type.
> Default: `get(edge_weight, g)`

OUT: `predecessor_map(PredecessorMap p_map)`

> The predecessor map records the edges in the minimum spanning tree. Upon completion of the algorithm, the edges (p[u],u) for all u in V are in the minimum spanning tree. If p[u] = u then u is either the source vertex or a vertex that is not reachable from the source. The `PredecessorMap` type must be a Read/Write Property Map which key and vertex types the same as the vertex descriptor type of the graph.
> Default: `dummy_property_map`

IN/OUT: `distance_map(DistanceMap d)`

> The shortest path weight from the source vertex to each vertex in the graph `g` is recorded in this property map. The type `DistanceMap` must be a model of Read/Write Property Map. The key type of the property map must be the vertex descriptor type of the graph, and the value type of the distance map must be Less Than Comparable.
> Default: `get(vertex_distance, g)`

IN: `visitor(BellmanFordVisitor v)`

> The visitor object, whose type must be a model of Bellman-Ford Visitor.
> Default: `bellman_visitor<null_visitor>`

IN: `distance_combine(BinaryFunction combine)`

> This function object replaces the role of addition in the relaxation step. The first argument type must match the distance map's value type and the second argument type must match the weight map's value type. The result type must be the same as the distance map's value type.
> Default:`std::plus<D>` with `D=typename property_traits<DistanceMap>::value_type`.

IN: `distance_compare(BinaryPredicate compare)`

> This function object replaces the role of the less-than operator that compares distances in the relaxation step. The argument types must match the distance map's value type.
> Default: `std::less<D>` with `D=typename property_traits<DistanceMap>::value_type`.

### Complexity

The time complexity is O(V E).

### Visitor Event Points

- `vis.examine_edge(e, g)` is invoked on every edge in the graph |V| times.
- `vis.edge_relaxed(e, g)` is invoked when the distance label for the target vertex is decreased. The edge (u,v) that participated in the last relaxation for vertex v is an edge in the shortest paths tree.
- `vis.edge_not_relaxed(e, g)` is invoked if the distance label for the target vertex is not decreased.
- `vis.edge_minimized(e, g)` is invoked during the second stage of the algorithm, during the test of whether each edge was minimized. If the edge is minimized then this function is invoked.
- `vis.edge_not_minimized(e, g)` is also invoked during the second stage of the algorithm, during the test of whether each edge was minimized. If the edge was not minimized, this function is invoked. This happens when there is a negative cycle in the graph.

### Example

An example of using the Bellman-Ford algorithm is in `examples/bellman_ford.cpp`.

Copyright © 2000 | Jeremy Siek, Univ.of Notre Dame (jsiek@lsc.nd.edu)