Module: Algorithm

Defined in:
lib/ext/algorithm.rb,
lib/ext/shortest_path.rb

Class Method Summary collapse

Class Method Details

.follow(object, sym = nil, &block) ⇒ Object 



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# File 'lib/ext/algorithm.rb', line 3

def follow(object, sym = nil, &block)
  sym.nil? == block_given? or raise "Can't use both symbol and block"
  a = []
  while object
    a << object
    object = block_given? ? yield(object) : object.send(sym)
  end
  a
end

.shortest_path(edges, from, to) ⇒ Object

Return (one of) the shortest path from from to to as a list of edges. edges is a list of arrays with from-node and to-node as the first two elements. Returns nil if no route was found

The algorithm does slightly more work than it needs to because the same node can be inserted more than once into the queue of nodes to be processed



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# File 'lib/ext/shortest_path.rb', line 10

def self.shortest_path(edges, from, to)
  pool = {} # Map from 'from' to map from 'to' to edge. Eg. pool[from][to] == edge-between-them
  edges.each { |edge| (pool[edge[0]] ||= {})[edge[1]] = edge }

  visited = {}
  prev = {}

  todo = [from]
  while true
    return nil if todo.empty?

    current = todo.shift
    break if current == to

    next if visited[current]
    visited[current] = true

    next if pool[current].nil?
    pool[current].each { |node, edge|
      prev[node] ||= edge
      todo << node
    }
  end

  to_node = to
  route = []
  while edge = prev[to_node]
    to_node = edge.first
    route.unshift edge
    break if from == to_node
  end

  route
end