Fun equation solving today

```
main(File) ->
{ok, RawData} = file:read_file(File),
Data = maps:from_list([ parse_monkey(X) || X <- binary:split(RawData, <<"\n">>, [global, trim]) ]),
io:format("part 1: ~p~n", [solve1(Data)]),
io:format("part 2: ~p~n", [solve2(Data)]).
parse_monkey(<<Name:4/binary, ": ", Rest/binary>>) ->
{Name, parse_monkey_value(Rest)}.
parse_monkey_value(<<N1:4/binary, " ", Op:1/binary, " ", N2:4/binary>>) -> {N1, Op, N2};
parse_monkey_value(V) -> binary_to_integer(V).
solve1(Data) ->
stack(<<"root">>, Data).
solve2(Data0) ->
Data = maps:remove(<<"humn">>, Data0),
{R1, _, R2} = maps:get(<<"root">>, Data),
S1 = stack(R1, Data),
S2 = stack(R2, Data),
inverse(S1, S2).
stack(M, Data) ->
case maps:get(M, Data, null) of
null ->
null;
Integer when is_integer(Integer) ->
Integer;
{M1, Op, M2} ->
S1 = stack(M1, Data),
S2 = stack(M2, Data),
case is_integer(S1) andalso is_integer(S2) of
true -> op(Op, S1, S2);
false -> {Op, S1, S2}
end
end.
op(<<"+">>, X, Y) -> X + Y;
op(<<"-">>, X, Y) -> X - Y;
op(<<"*">>, X, Y) -> X * Y;
op(<<"/">>, X, Y) -> X div Y.
inverse(S1, V) when is_integer(V) ->
inverse(V, S1);
inverse(V, {Op, V1, V2}) ->
case Op of
<<"+">> when is_integer(V1) -> inverse(V - V1, V2);
<<"+">> -> inverse(V - V2, V1);
<<"*">> when is_integer(V1) -> inverse(V div V1, V2);
<<"*">> -> inverse(V div V2, V1);
<<"-">> when is_integer(V1) -> inverse(V1 - V, V2);
<<"-">> -> inverse(V + V2, V1);
<<"/">> when is_integer(V1) -> inverse(V1 div V, V2);
<<"/">> -> inverse(V * V2, V1)
end;
inverse(V, null) ->
V.
```