第9回オフラインリアルタイムどう書く Erlang による実装 ref: http://qiita.com/pazworld/items/e641fd71af82a99f92e2

bus.erl

-module(bus).
-compile(export_all).

solve(Data) -> adult_fee(Data) + child_fee(Data) + infant_fee(Data).

adult_fee(Data) -> lists:sum(fees($A, Data)).

child_fee(Data) -> lists:sum(fees($C, Data)).

infant_fee(Data) -> lists:sum(dropn(num_adult(Data) * 2, rev_sort(fees($I, Data)))).

num_adult(Data) -> length(fees($A, Data)).

fees(Mark, Data) -> [mark_to_fee(X, base_fee(Data)) || X <- only(Mark, Data)].

mark_to_fee(Mark, BaseFee) -> roundup10(marks_to_ratio(Mark) * BaseFee).

roundup10(X) -> trunc(X / 10 + 0.99) * 10.

marks_to_ratio(Marks) -> lists:foldl(fun(X, S) -> mark_to_ratio(X) * S end, 1, Marks).

mark_to_ratio(Mark) -> element(2, find_mark(Mark)).

find_mark(Mark) -> hd(lists:dropwhile(fun({M, _}) -> M =/= Mark end, ratio_data())).

ratio_data() -> [{$A, 1}, {$C, 0.5}, {$I, 0.5}, {$n, 1}, {$p, 0}, {$w, 0.5}].

base_fee(Data) -> list_to_integer(hd(string:tokens(Data, ":"))).

only(Mark, Data) -> lists:filter(fun(X) -> hd(X) =:= Mark end, marks(Data)).

marks(Data) -> string:tokens(lists:nth(2, string:tokens(Data, ":")), ",").

dropn(N, L) -> case length(L) >= N of true -> lists:nthtail(N, L); false -> [] end.

rev_sort(L) -> lists:reverse(lists:sort(L)).

tests() ->
  test("210:Cn,In,Iw,Ap,Iw", 170), %0
  test("220:Cp,In", 110), %1
  test("440:In,Ip,Cp,Aw,Iw,In,An", 660), %39
  test("1270:Ap,In,An,Ip,In,Ip,Ip", 1270). %40

test(Data, Expected) ->
  Result = solve(Data),
  OkNg = case Result =:= Expected of true -> ok; false -> ng end,
  io:fwrite("~s: ~s -> ~w~n", [OkNg, Data, Result]).