problem2.erl

-module(problem2).
-include_lib("eunit/include/eunit.hrl").
-export([problem2/0]).
-import(euler_helper,[fib/1]).

even(X) ->
    X rem 2 == 0.

fibsum(Invariant, Termcond) ->
    fibsum(1, Invariant, Termcond).

fibsum(Y, Invariant, Termcond) ->
    X = fib(Y),
    case Termcond(X) of
        true ->
            case Invariant(X) of
                true ->
                    fib(Y) + fibsum(Y+1, Invariant, Termcond);
                false ->
                    fibsum(Y+1, Invariant, Termcond)
            end;
        false ->
            0
    end.

problem2() ->
    fibsum(fun even/1,fun(X) -> X < 4000000 end).

%% tests

even_test() ->
    ?assert(even(2)).

even_not_test() ->
    ?assertNot(even(1)).

fibsum_simple_test() ->
    ?assertEqual(6,fibsum(fun(_) -> true end, fun(X) -> X < 4 end)).

fibsum_longer_test() ->
    ?assertEqual(11,fibsum(1, fun(_) -> true end, fun(X) -> X < 6 end)).

fibsum_step_test() ->
    ?assertEqual(6,fibsum(1,fun(_) -> true end, fun(X) -> X < 4 end)).

fibsum_invariant_test() ->
    ?assertEqual(1,fibsum(1,fun(X) -> X == 1 end, fun(X) -> X < 4 end)).