Dijkstras Algorithm for shortest path.

dijkstra.py

"""
Dijkstra's algorithm for shortest paths.
http://code.activestate.com/recipes/577343-dijkstras-algorithm-for-shortest-paths/)

  - dijkstra(G, s) finds all shortest paths from s to each other vertex.
  - shortest_path(G, s, t) uses dijkstra to find the shortest path from s to t.

The input graph G is assumed to have the following representation:

  - A vertex can be any object that can be used as an index into a dictionary.
  - G is a dictionary, indexed by vertices.
  - For any vertex v, G[v] is itself a dictionary, indexed by  neighbors of v.
  - For any edge v->w, G[v][w] is the length of the edge.

"""
from priodict import priorityDictionary


def dijkstra(graph, start, end=None):
    final_distances = {}
    predecessors = {}
    estimated_distances = priorityDictionary()
    estimated_distances[start] = 0

    for vertex in estimated_distances:
        final_distances[vertex] = estimated_distances[vertex]
        if vertex == end:
            break

        for edge in graph[vertex]:
            path_distance = final_distances[vertex] + graph[vertex][edge]
            if edge in final_distances:
                if path_distance < final_distances[edge]:
                    raise ValueError("Better path to already-final vertex")
            elif edge not in estimated_distances or path_distance < estimated_distances[edge]:
                estimated_distances[edge] = path_distance
                predecessors[edge] = vertex

    return final_distances, predecessors


def shortest_path(graph, start, end):
    """
    Find a single shortest path from the given start vertex to the given end,
    output as list of vertices in order along the shortest path.
    """

    final_distances, predecessors = dijkstra(graph, start, end)
    path = []
    while 1:
        path.append(end)
        if end == start:
            break
        end = predecessors[end]
    path.reverse()
    return path

if __name__ == "____main__":
    pass
    # G = {'s':{'u':10, 'x':5}, 'u':{'v':1, 'x':2}, 'v':{'y':4},
    #      'x':{'u':3, 'v':9, 'y':2}, 'y':{'s':7, 'v':6}}
    # The shortest path from s to v is ['s', 'x', 'u', 'v'] and has length 9.