Data commit

Task/Dijkstras-algorithm/Ruby/dijkstras-algorithm.rb

@@ -0,0 +1,71 @@
+class Graph
+  Vertex = Struct.new(:name, :neighbours, :dist, :prev)
+
+  def initialize(graph)
+    @vertices = Hash.new{|h,k| h[k]=Vertex.new(k,[],Float::INFINITY)}
+    @edges = {}
+    graph.each do |(v1, v2, dist)|
+      @vertices[v1].neighbours << v2
+      @vertices[v2].neighbours << v1
+      @edges[[v1, v2]] = @edges[[v2, v1]] = dist
+    end
+    @dijkstra_source = nil
+  end
+
+  def dijkstra(source)
+    return  if @dijkstra_source == source
+    q = @vertices.values
+    q.each do |v|
+      v.dist = Float::INFINITY
+      v.prev = nil
+    end
+    @vertices[source].dist = 0
+    until q.empty?
+      u = q.min_by {|vertex| vertex.dist}
+      break if u.dist == Float::INFINITY
+      q.delete(u)
+      u.neighbours.each do |v|
+        vv = @vertices[v]
+        if q.include?(vv)
+          alt = u.dist + @edges[[u.name, v]]
+          if alt < vv.dist
+            vv.dist = alt
+            vv.prev = u.name
+          end
+        end
+      end
+    end
+    @dijkstra_source = source
+  end
+
+  def shortest_path(source, target)
+    dijkstra(source)
+    path = []
+    u = target
+    while u
+      path.unshift(u)
+      u = @vertices[u].prev
+    end
+    return path, @vertices[target].dist
+  end
+
+  def to_s
+    "#<%s vertices=%p edges=%p>" % [self.class.name, @vertices.values, @edges]
+  end
+end
+
+g = Graph.new([ [:a, :b, 7],
+                [:a, :c, 9],
+                [:a, :f, 14],
+                [:b, :c, 10],
+                [:b, :d, 15],
+                [:c, :d, 11],
+                [:c, :f, 2],
+                [:d, :e, 6],
+                [:e, :f, 9],
+              ])
+
+start, stop = :a, :e
+path, dist = g.shortest_path(start, stop)
+puts "shortest path from #{start} to #{stop} has cost #{dist}:"
+puts path.join(" -> ")