-module(day13).

-export([solve/1]).

-compile(export_all).

solve(Input) ->
    PacketPairs = binary:split(Input, <<"\n\n">>, [global, trim_all]),
    Part1 = part1(PacketPairs),
    {Part1, none}.

part1(PacketPairs) -> part1(PacketPairs, {1, 0}).

part1([], {_Index, Score}) -> Score;
part1([PacketPair|Rest], {Index, Score}) ->
    [LeftStr, RightStr] = binary:split(PacketPair, <<$\n>>, [trim_all]),
    Left = parse_packet(LeftStr),
    Right = parse_packet(RightStr),
    NewScore = case compare(Left, Right) of
                   true -> Score + Index;
                   false -> Score
               end,
    part1(Rest, {Index+1, NewScore}).

compare([], []) -> undecided;
compare([], [_|_Rest]) -> true;
compare([_|_Rest], []) -> false;
compare([Left | LeftRest], [Right | RightRest]) when is_list(Left) and is_list(Right) -> 
    case compare(Left, Right) of
        undecided -> compare(LeftRest, RightRest);
        Result -> Result
    end;
compare([Left | _] = Lefts, [Right | RightRest]) when is_list(Left) and is_integer(Right) ->
    compare(Lefts, [[Right] | RightRest]);
compare([Left | LeftRest], [Right | _] = Rights) when is_integer(Left) and is_list(Right) ->
    compare([[Left] | LeftRest], Rights);
compare([Value | Left], [Value | Right]) when is_integer(Value) -> compare(Left, Right);
compare([Left|_], [Right|_]) when is_integer(Left) and is_integer(Right) -> Left < Right;
compare(Left, Right) when is_integer(Left) and is_integer(Right) -> Left < Right.

parse_packet(Packet) ->
    {ok, Tokens, _EndLine} = erl_scan:string(unicode:characters_to_list([Packet,$.])),
    {ok, Parsed} = erl_parse:parse_term(Tokens),
    Parsed.