% Find the sum of all the even-valued terms in the Fibonacci sequence which do not exceed four million.

% Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
% 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
% Find the sum of all the even-valued terms in the sequence which do not exceed four million.

-module(e002).
-export([
        main/1,
        fibonacci_max/1
    ]).

fibonacci_max(Max) ->
    fibonacci_max_([], Max).

fibonacci_max_([], Max) ->
    if
        Max < 1 ->
            [];
        true ->
            fibonacci_max_([1], Max)
    end;
fibonacci_max_([N], Max) ->
    Next = N + N,
    if
        Max < Next ->
            [N];
        true ->
            fibonacci_max_([Next, N], Max)
    end;
fibonacci_max_([N1, N2 | Seq], Max) ->
    Next = N1 + N2,
    if
        Max < Next ->
            [N1, N2 | Seq];
        true ->
            fibonacci_max_([Next, N1, N2 | Seq], Max)
    end.

sum_if_even([]) ->
    0;
sum_if_even([N | Seq]) ->
    if
        N rem 2 == 0 ->
            N + sum_if_even(Seq);
        true ->
            sum_if_even(Seq)
    end.

main(_) ->
    Max = 4000000,
    io:format("Sum for fibonacci <= ~w: ~w~n", [Max, sum_if_even(fibonacci_max(Max))]).