-module(day23).

-export([solve/1]).

-compile(export_all).

solve(InputData) ->
    {part1(InputData), none}.

part1(Input) ->
    State = parse_input(Input),
    EndState = step(10, State),
    {XMin, XMax, YMin, YMax} = Bounds = get_bounds(EndState),
    %io:format("~p~n", [Bounds]),
    (XMax - XMin + 1) * (YMax - YMin + 1) - num_elves(State).

parse_input(Input) -> parse_input(Input, 0, 0, 1, {#{}, #{}}).

parse_input(<<>>, _X, _Y, _NumElves, State) ->
    State;
parse_input(<<$., Rest/binary>>, X, Y, ElfId, State) ->
    parse_input(Rest, X+1, Y, ElfId, State);
parse_input(<<$\n, Rest/binary>>, X, Y, ElfId, State) ->
    parse_input(Rest, 0, Y+1, ElfId, State);
parse_input(<<$#, Rest/binary>>, X, Y, ElfId, {ActiveElves, Positions}) ->
    parse_input(Rest, X+1, Y, ElfId+1, {ActiveElves#{ElfId => {X, Y}}, Positions#{{X, Y} => ElfId}}).

num_elves({_ActiveElves, Positions}) -> map_size(Positions).

board_put(X, Y, Char, {W, Data}) ->
    Index = Y * (W+1) + X,
    Before = binary_part(Data, {0, Index}),
    After = binary_part(Data, {Index + 1, byte_size(Data) - Index - 1}),
    {W, <<Before/bytes, Char, After/bytes>>}.

mkboard(W, H) ->
    mkboard(W, H, <<>>).

mkboard(W, 0, Acc) -> {W, Acc};
mkboard(W, H, Acc) ->
    MkRow = fun MkRow(0, R) -> R;
                MkRow(N, R) -> MkRow(N-1, <<R/binary, $.>>) end,
    WithRow = MkRow(W, Acc),
    mkboard(W, H-1, <<WithRow/binary, $\n>>).

render({_Active, Positions} = State) ->
    {MinX, MaxX, MinY, MaxY} = get_bounds(State),
    {_, Data} = maps:fold(fun ({X, Y}, Id, Board) -> board_put(X - MinX, Y - MinY, (Id rem 10) + $0, Board) end,
              mkboard(MaxX - MinX + 1, MaxY - MinY + 1),
              Positions),
    Data.

step(N, State) -> step(0, N, State).

step(N, N, State) -> State;
step(I, N, State) when N - I > 0 -> step(I+1, N, do_step(State, I)).

do_step({ActiveElves, PositionsIn}, Round) ->
    %io:format(" Stepping ~n", []),
    %io:format("~s~n", [render({ActiveElves, PositionsIn})]),
    {PlannedMoves, _Inactive} = plan_moves(ActiveElves, PositionsIn, Round),
    maps:fold(fun (NewPos, ElfId, {Active, Positions}) ->
                       #{ElfId := OldPos} = Active,
                       Updooted = maps:remove(OldPos, Positions),
                       {Active#{ElfId := NewPos}, Updooted#{NewPos => ElfId}}
               end,
               {ActiveElves, PositionsIn},
               PlannedMoves).

plan_moves(ActiveElves, Positions, Round) ->
    {_Collisions, Moves, Inactive} =
    maps:fold(
      fun (ElfId, Position, {Collisions, MoveSet, Inactive}) ->
              Move = get_move(Position, Positions, Round),
              %io:format(" ~p ~p ~n", [ElfId, Position]),
              %io:format(" ~p ~p ~n", [ElfId, Move]),
              % Todo: only mark this unit as inactive if it exited because it had no neighbors (update the signature of get_move/2)
              if 
                  Position == Move ->
                      {Collisions, MoveSet, Inactive#{ElfId => Position}};
                  is_map_key(Move, Collisions) ->
                      #{Move := Elves} = Collisions,
                      {Collisions#{Move:=[ElfId|Elves]}, MoveSet, Inactive};
                  is_map_key(Move, MoveSet) ->
                      #{Move := CollidingElf} = MoveSet,
                      {Collisions#{Move => [ElfId, CollidingElf]}, maps:remove(Move, MoveSet), Inactive};
                  true ->
                      {Collisions, MoveSet#{Move => ElfId}, Inactive}
              end
      end,
      {#{}, #{}, #{}},
      ActiveElves),
    {Moves, Inactive}.


open(Position, Positions) when is_map_key(Position, Positions) -> 0;
open(_Position, _Positions) -> 1.

% N S W NW SW E NE SE
get_move({ElfX, ElfY}, Positions, Round) ->
    [_|Adjacent] = [{X, Y} || X <- [ElfX, ElfX-1, ElfX+1], Y <- [ElfY, ElfY-1, ElfY+1]],
    OpenByte= lists:foldl(fun (Pos, Acc) -> (Acc bsl 1) + open(Pos, Positions) end, 0, Adjacent),
    %io:format("~.2B~n", [OpenByte]),
    case Round rem 4 of
        0 ->
            if
                2#11111111 == OpenByte -> {ElfX, ElfY};
                2#10010010 == (OpenByte band 2#10010010) -> {ElfX, ElfY-1};
                2#01001001 == (OpenByte band 2#01001001) -> {ElfX, ElfY+1};
                2#00111000 == (OpenByte band 2#00111000) -> {ElfX-1, ElfY};
                2#00000111 == (OpenByte band 2#00000111) -> {ElfX+1, ElfY};
                true -> {ElfX, ElfY}
            end;
        1 ->
            if
                2#11111111 == OpenByte -> {ElfX, ElfY};
                2#01001001 == (OpenByte band 2#01001001) -> {ElfX, ElfY+1};
                2#00111000 == (OpenByte band 2#00111000) -> {ElfX-1, ElfY};
                2#00000111 == (OpenByte band 2#00000111) -> {ElfX+1, ElfY};
                2#10010010 == (OpenByte band 2#10010010) -> {ElfX, ElfY-1};
                true -> {ElfX, ElfY}
            end;
        2 ->
            if
                2#11111111 == OpenByte -> {ElfX, ElfY};
                2#00111000 == (OpenByte band 2#00111000) -> {ElfX-1, ElfY};
                2#00000111 == (OpenByte band 2#00000111) -> {ElfX+1, ElfY};
                2#10010010 == (OpenByte band 2#10010010) -> {ElfX, ElfY-1};
                2#01001001 == (OpenByte band 2#01001001) -> {ElfX, ElfY+1};
                true -> {ElfX, ElfY}
            end;
        3 ->
            if
                2#11111111 == OpenByte -> {ElfX, ElfY};
                2#00000111 == (OpenByte band 2#00000111) -> {ElfX+1, ElfY};
                2#10010010 == (OpenByte band 2#10010010) -> {ElfX, ElfY-1};
                2#01001001 == (OpenByte band 2#01001001) -> {ElfX, ElfY+1};
                2#00111000 == (OpenByte band 2#00111000) -> {ElfX-1, ElfY};
                true -> {ElfX, ElfY}
            end
    end.

mark_inactive(ElfId, {ActiveElves, Positions}) -> maps:remove(ElfId, ActiveElves).
mark_active(ElfId, ElfPosition, {ActiveElves, Positions}) -> ActiveElves#{ElfId => ElfPosition}.

get_bounds({_ActiveElves, Positions}) ->
    maps:fold(fun ({X, Y}, _Value, {MinX, MaxX, MinY, MaxY}) ->
                      {min(X, MinX), max(X, MaxX), min(Y, MinY), max(Y, MaxY)}
              end,
              {50000000, 0, 50000000, 0},
              Positions).