Data update

Task/Dijkstras-algorithm/Crystal/dijkstras-algorithm.cr

@@ -0,0 +1,43 @@
+def shortest_paths (edges edgelist, start)
+  # put the edges in a hash for easier access
+  edges = Hash(Symbol, Array({Symbol, Int32})).new {|h, k| h[k] = [] of {Symbol, Int32} }
+  edgelist.each do |from, to, cost|
+    edges[from] << { to, cost }
+  end
+  vertices = { start => { cost: 0, prev: :"" } }
+  queue = Deque.new [start]
+  while queue.present?
+    node = queue.shift
+    current_cost = vertices[node][:cost]
+    if dests = edges[node]?
+      dests.each do |dest, cost|
+        if (v = vertices[dest]?).nil? || v[:cost] > current_cost + cost
+          vertices[dest] = { cost: current_cost + cost, prev: node }
+          queue.push dest
+        end
+      end
+    end
+  end
+  vertices.map {|vertex, t|
+    # reconstruct path
+    path = [vertex]
+    prev = t[:prev]
+    while prev != :""
+      path << prev
+      prev = vertices[prev][:prev]
+    end
+    { path.reverse, t[:cost] }
+  }
+end
+
+edges = [{: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}]
+
+# show the shortest path to each reachable vertex
+puts "Shortest paths from :a to each reachable vertex:"
+pp shortest_paths(edges, :a)
+
+# interpret the output from above and use it to output
+# the shortest path from a to e and f
+puts "\nShortest paths from :a to :e and :f:"
+pp shortest_paths(edges, :a).select {|path, _| path[0] == :a && path[-1].in? [:e, :f] }