Since this one was pretty much all parsing (again), I made a slightly more robust parser this time
e.g.
defp parse(%{todo: [h | t]} = state) when h in @digits do
state
|> add_to_buffer(h)
|> increment(:x)
|> Map.put(:todo, t)
|> parse()
end
Worked well for both parts. Full solution code here on Github
