collatz.erl

-module(collatz).
-export([find_len/2]).

find_len(Start, Len) when Start =:= 1 orelse Start =:= 0 ->
    Len;

find_len(Start, Len) when Start rem 2 =:= 1 ->
    find_len((Start * 3) + 1, Len + 1);

find_len(Start, Len) when Start rem 2 =:= 0 ->
    find_len((Start div 2), Len + 1).

find_longest(First, Last, Longest) when First < Last ->
    Longest;
find_longest(First, Last, Longest) ->
    Len = find_len(First, 1),
    case Len of
        {_, Len} when {_, Len} > Longest ->
            find_longest(First - 1, Last, {First, Len});
        _ ->
            find_longest(First - 1, Last, Longest)
    end.