-module(np).
-export([main/0]).

-define(LIST_LENGTH, 100).

main() ->
    Source = create_test_list(?LIST_LENGTH),
    product_all_lists(Source, 0).

product_all_lists([], Count) ->
    receive_product_lists_result([], 0, Count);
product_all_lists([L1], Count) ->
    receive_product_lists_result([L1], 0, Count);
product_all_lists([L1, L2 | T], Count) ->
    Pid = self(),
    spawn_link(fun() ->
                       product_lists(Pid, [L1, L2])
               end),
    product_all_lists(T, Count + 1).

receive_product_lists_result([L1], Count, Limit) when Count == Limit ->
    io:format("Result: ~p~n", [L1]);
receive_product_lists_result(Result, Count, Limit) when Count == Limit ->
    product_all_lists(Result, 0);
receive_product_lists_result(Result, Count, Limit) ->
    receive
        List ->
            receive_product_lists_result([List | Result], Count + 1, Limit)
    end.


product_lists(ParentPid, [L1, L2]) ->
    Result = product_lists(L1, L2, []),
    ParentPid ! Result.

product_lists([], _, Result) ->
    Result;
product_lists([H | T], L2, Result) ->
    case lists:member(H, L2) of
        true ->
            product_lists(T, L2, [H | Result]);
        false ->
            product_lists(T, L2, Result)
    end.

%% Creating test data.

create_test_list(Length) ->
    Seq = lists:seq(1, Length),
    create_test_list(Seq, [], Length, 0).

create_test_list(_, Result, Length, Count) when Length == Count ->
    Result;
create_test_list(Seq, Result, Length, Count) ->
    Sorted = create_rand_list(Seq),
    create_test_list(Seq, [Sorted | Result], Length, Count + 1).

create_rand_list(Source) ->
    create_rand_list(Source, []).

create_rand_list([], Result) ->
    Length = crypto:rand_uniform(?LIST_LENGTH - 5, ?LIST_LENGTH),
    lists:sublist(Result, Length);
create_rand_list(Source, Result) ->
    Length = length(Source),
    Position = crypto:rand_uniform(0, Length),
    Value = get_value_at(Source, Position),
    create_rand_list(lists:delete(Value, Source), [Value | Result]).

get_value_at(Source, Position) ->
    get_value_at(Source, Position, 0).

get_value_at([], _, _) ->
    throw("Position too big.");
get_value_at([H | _], Position, Count) when Position == Count ->
    H;
get_value_at([_ | T], Position, Count) ->
    get_value_at(T, Position, Count + 1).