route.erl

-module(route).
-compile(export_all).


main(Filename) -> 
  % Filename = "road.txt",
  {ok, Binary} = file:read_file(Filename),
  NumberList = parse_input(Binary),
  RouteTuples = lists:reverse(split_by_three(NumberList, [])),
  Optimal  = calculate_route(RouteTuples),
  io:format("~p~n",[Optimal]),
  erlang:halt().

parse_input(Input)  when is_binary(Input) -> parse_input(binary_to_list(Input));
parse_input(Input)  when is_list(Input)   -> [ list_to_integer(X) || X <- string:tokens(Input, "\r\n\t") ].

split_by_three([], Acc) -> Acc;
split_by_three([A,B,C|Tail], Acc) -> split_by_three(Tail, [{A,B,C}|Acc]).


test() -> calculate_route([{50, 10, 30}, {5, 90, 20}, {40, 2, 25}, { 10, 8, 0}]).

calculate_route(List) -> {R1, _} = lists:foldl(fun step/2,{{0, []}, {0, []}}, List), R1.


step({A, B, X}, {{Acost, Astory}, {Bcost, Bstory}}) ->

    {
      pick_route(Bcost, Acost, B, A, X, Bstory, Astory, a, b),
      pick_route(Acost, Bcost, A, B, X, Astory, Bstory, b, a)
    }.

pick_route(FarthestPointPreviousCost, ClosestPointPreviousCost,
  LongestRoute1, ShortestRoute, LongestRoute2,
  StoryOfFarthestPoint, StoryOfClosestPoint,
  ShortestRouteName, LongestRouteName) ->

    NewShortestRoute = ClosestPointPreviousCost + ShortestRoute,
    NewLongestRoute = FarthestPointPreviousCost + LongestRoute1 + LongestRoute2,

    case NewShortestRoute > NewLongestRoute of
      true  -> {NewLongestRoute, [ {x, LongestRoute2},  {LongestRouteName, LongestRoute1} |StoryOfFarthestPoint]};
      _ -> {NewShortestRoute, [{ShortestRouteName, ShortestRoute}|StoryOfClosestPoint]}
    end.