class RGL::DijkstraAlgorithm

Constants

DEFAULT_DISTANCE_COMBINATOR

Distance combinator is a lambda that accepts the distance (usually from the source) to vertex u and the weight of the edge connecting vertex u to another vertex v and returns the distance to vertex v if it's reached through the vertex u. By default, the distance to vertex u and the edge's weight are summed.

Public Class Methods

new(graph, edge_weights_map, visitor, distance_combinator = nil) click to toggle source

Initializes Dijkstra's algorithm for a graph with provided edges weights map.

# File lib/rgl/dijkstra.rb, line 18
def initialize(graph, edge_weights_map, visitor, distance_combinator = nil)
  @graph               = graph
  @edge_weights_map    = build_edge_weights_map(edge_weights_map)
  @visitor             = visitor
  @distance_combinator = distance_combinator || DEFAULT_DISTANCE_COMBINATOR
end

Public Instance Methods

find_shortest_paths(source) click to toggle source

Finds the shortest path from the source to every other vertex.

# File lib/rgl/dijkstra.rb, line 47
def find_shortest_paths(source)
  init(source)
  relax_edges
end
shortest_path(source, target) click to toggle source

Finds the shortest path from the source to the target in the graph.

Returns the shortest path, if it exists, as an Array of vertices. Otherwise, returns nil.

# File lib/rgl/dijkstra.rb, line 29
def shortest_path(source, target)
  init(source)
  relax_edges(target, true)
  PathBuilder.new(source, @visitor.parents_map).path(target)
end
shortest_paths(source) click to toggle source

Finds the shortest path form the source to every other vertex of the graph and builds shortest paths map.

Returns the shortest paths map that contains the shortest path (if it exists) from the source to any vertex of the graph.

# File lib/rgl/dijkstra.rb, line 40
def shortest_paths(source)
  find_shortest_paths(source)
  PathBuilder.new(source, @visitor.parents_map).paths(@graph.vertices)
end

Private Instance Methods

build_edge_weights_map(edge_weights_map) click to toggle source
# File lib/rgl/dijkstra.rb, line 100
def build_edge_weights_map(edge_weights_map)
  edge_weights_map.is_a?(EdgePropertiesMap) ? edge_weights_map : NonNegativeEdgePropertiesMap.new(edge_weights_map, @graph.directed?)
end
init(source) click to toggle source
# File lib/rgl/dijkstra.rb, line 54
def init(source)
  @visitor.set_source(source)

  @queue = MinPriorityQueue.new
  @queue.push(source, 0)
end
relax_edge(u, v) click to toggle source
# File lib/rgl/dijkstra.rb, line 78
def relax_edge(u, v)
  @visitor.handle_examine_edge(u, v)

  new_v_distance = @distance_combinator.call(@visitor.distance_map[u], @edge_weights_map.edge_property(u, v))

  if new_v_distance < @visitor.distance_map[v]
    @visitor.distance_map[v] = new_v_distance
    @visitor.parents_map[v]  = u

    if @visitor.color_map[v] == :WHITE
      @visitor.color_map[v] = :GRAY
      @queue.push(v, new_v_distance)
    elsif @visitor.color_map[v] == :GRAY
      @queue.decrease_key(v, new_v_distance)
    end

    @visitor.handle_edge_relaxed(u, v)
  else
    @visitor.handle_edge_not_relaxed(u, v)
  end
end
relax_edges(target = nil, break_on_target = false) click to toggle source
# File lib/rgl/dijkstra.rb, line 61
def relax_edges(target = nil, break_on_target = false)
  until @queue.empty?
    u = @queue.pop

    break if break_on_target && u == target

    @visitor.handle_examine_vertex(u)

    @graph.each_adjacent(u) do |v|
      relax_edge(u, v) unless @visitor.finished_vertex?(v)
    end

    @visitor.color_map[u] = :BLACK
    @visitor.handle_finish_vertex(u)
  end
end