Project Euler Problem #2

problem_2.erl

-module(problem_2).
-export([solve/1]).
-define(LIMIT, 4000000).

%% Normalizing the start value, need to start with an odd value and enter tail recursion
solve(Start) when Start rem 2 == 0, Start > 0 -> solve(Start - 1, Start, 0);
solve(Start) -> solve(Start, Start + 1, 0).
%% guard against the limit and drop out of recursion if we hit it
solve(Prev, Next, Sum) when Prev >= ?LIMIT; Next >= ?LIMIT -> Sum;
%% Sum even values
solve(Prev, Next, Sum) when Next rem 2 == 0 -> solve(Next, Prev + Next, Sum + Next);
%% Move onto the next value in the fib series
solve(Prev, Next, Sum) -> solve(Next, Prev + Next, Sum).

% To run it, you'll need Erlang ofcourse.. Duh. R15+ would do just fine
% From your terminal, compile
% $ erlc problem_2.erl

% Pop open erlang's shell
% $ erl

% Have fun
% > problem_2:solve(1).

%% Change Log:
%% - revision 1: solution per specs.
%% - revision 2: an unused variable slipped through the cracks, removed it.
%% - revision 3: covered a case of which the start variable isn't odd, only other way to cover such case is with another guard
%% on Prev which will eventually give us an off by -Next-value- error.
%% - revision 4: fixed a typo. Note to self: this was a rather shit idea to hack on a code snippet at 5am after a sprint