Data update

Task/Dijkstras-algorithm/OCaml/dijkstras-algorithm-1.ml

@@ -0,0 +1,87 @@
+let list_vertices graph =
+  List.fold_left (fun acc ((a, b), _) ->
+    let acc = if List.mem b acc then acc else b::acc in
+    let acc = if List.mem a acc then acc else a::acc in
+    acc
+  ) [] graph
+
+let neighbors v =
+  List.fold_left (fun acc ((a, b), d) ->
+    if a = v then (b, d)::acc else acc
+  ) []
+
+let remove_from v lst =
+  let rec aux acc = function [] -> failwith "remove_from"
+  | x::xs -> if x = v then List.rev_append acc xs else aux (x::acc) xs
+  in aux [] lst
+
+let with_smallest_distance q dist =
+  match q with
+  | [] -> assert false
+  | x::xs ->
+      let rec aux distance v = function
+      | x::xs ->
+          let d = Hashtbl.find dist x in
+          if d < distance
+          then aux d x xs
+          else aux distance v xs
+      | [] -> (v, distance)
+      in
+      aux (Hashtbl.find dist x) x xs
+
+let dijkstra max_val zero add graph source target =
+  let vertices = list_vertices graph in
+  let dist_between u v =
+    try List.assoc (u, v) graph
+    with _ -> zero
+  in
+  let dist = Hashtbl.create 1 in
+  let previous = Hashtbl.create 1 in
+  List.iter (fun v ->                  (* initializations *)
+    Hashtbl.add dist v max_val         (* unknown distance function from source to v *)
+  ) vertices;
+  Hashtbl.replace dist source zero;    (* distance from source to source *)
+  let rec loop = function [] -> ()
+  | q ->
+      let u, dist_u =
+        with_smallest_distance q dist in   (* vertex in q with smallest distance in dist *)
+      if dist_u = max_val then
+        failwith "vertices inaccessible";  (* all remaining vertices are inaccessible from source *)
+      if u = target then () else begin
+        let q = remove_from u q in
+        List.iter (fun (v, d) ->
+          if List.mem v q then begin
+            let alt = add dist_u (dist_between u v) in
+            let dist_v = Hashtbl.find dist v in
+            if alt < dist_v then begin       (* relax (u,v,a) *)
+              Hashtbl.replace dist v alt;
+              Hashtbl.replace previous v u;  (* previous node in optimal path from source *)
+            end
+          end
+        ) (neighbors u graph);
+        loop q
+      end
+  in
+  loop vertices;
+  let s = ref [] in
+  let u = ref target in
+  while Hashtbl.mem previous !u do
+    s := !u :: !s;
+    u := Hashtbl.find previous !u
+  done;
+  (source :: !s)
+
+let () =
+  let graph =
+    [ ("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; ]
+  in
+  let p = dijkstra max_int 0 (+) graph "a" "e" in
+  print_endline (String.concat " -> " p)