erlang 99% exponential weighted moving average

update(Tag, NewVal, State) ->
    NewTree =
        case gb_trees:lookup(Tag, State#state.tree) of
            {value, Val} ->
                X = Val + (NewVal - Val) * 0.01,
                gb_trees:update(Tag, X, State#state.tree);
            none ->
                gb_trees:insert(Tag, NewVal, State#state.tree)
        end,
    State#state{tree = NewTree}.