-module(pb_lite).
-compile(no_auto_import).

-export([dvarint/1, dtag_type/1, decode/1, decode_1/2]).
-export_type([value_type/0]).

-type value_type() :: {var_int, integer()} |
    {int64, integer()} |
    {binary, bitstring()} |
    {group, list({integer(), value_type()})} |
    {int32, integer()}.

-spec dvarint(bitstring()) -> {ok, {integer(), bitstring()}} | {error, nil}.
dvarint(Bin) ->
    dvarint_1(Bin, 0, 0).

-spec dvarint_1(bitstring(), integer(), integer()) -> {ok,
                                                       {integer(), bitstring()}} |
    {error, nil}.
dvarint_1(Bin, Count, Acc) ->
    case Bin of
        <<1:1, I:7, R/binary>> ->
            New_acc = Acc + helper:f_bsl(I, Count * 7),
            dvarint_1(R, Count + 1, New_acc);

        <<0:1, I@1:7, R@1/binary>> ->
            Num = Acc + helper:f_bsl(I@1, Count * 7),
            <<Signed:64/signed>> = <<Num:64>>,
            {ok, {Signed, R@1}};

        <<>> ->
            {error, nil}
    end.

-spec dbit64(bitstring()) -> {ok, {integer(), bitstring()}} | {error, nil}.
dbit64(Bin) ->
    case Bin of
        <<Int64:64/little-signed, R/binary>> ->
            {ok, {Int64, R}};

        _@1 ->
            {error, nil}
    end.

-spec dlen_delimited(bitstring()) -> {ok, {bitstring(), bitstring()}} |
    {error, nil}.
dlen_delimited(Bin) ->
    case dvarint(Bin) of
        {error, _try} -> {error, _try};
        {ok, {Value, R}} ->
            case R of
                <<V:Value/binary, R@1/binary>> ->
                    {ok, {V, R@1}};

                _@1 ->
                    {error, nil}
            end
    end.

-spec dbit32(bitstring()) -> {ok, {integer(), bitstring()}} | {error, nil}.
dbit32(Bin) ->
    case Bin of
        <<Int64:32/little-signed, R/binary>> ->
            {ok, {Int64, R}};

        _@1 ->
            {error, nil}
    end.

-spec dtag_type(bitstring()) -> {ok, {integer(), integer(), bitstring()}} |
    {error, nil}.
dtag_type(Bin) ->
    case Bin of
        <<Tag:5, Ftype:3, R/binary>> ->
            case helper:f_band(Tag, 16) of
                16 ->
                    case dvarint(R) of
                        {error, _try} -> {error, _try};
                        {ok, {Value, R@1}} ->
                            V = helper:f_bsl(Value, 4) + helper:f_band(
                                Tag,
                                16#F
                            ),
                            {ok, {V, Ftype, R@1}}
                    end;

                _@1 ->
                    {ok, {Tag, Ftype, R}}
            end
    end.

-spec decode(bitstring()) -> {ok,
                              {list({integer(), value_type()}), bitstring()}} |
    {error, nil}.
decode(Bin) ->
    decode_1(Bin, []).

-spec decode_1(bitstring(), list({integer(), value_type()})) -> {ok,
                                                                 {list({integer(),
                                                                        value_type()}),
                                                                  bitstring()}} |
    {error, nil}.
decode_1(Bin, Acc) ->
    case Bin of
        <<>> ->
            {ok, {gleam@list:reverse(Acc), <<>>}};

        _@1 ->
            case dtag_type(Bin) of
                {error, _try} -> {error, _try};
                {ok, {Tag, F_type, R}} ->
                    case F_type of
                        0 ->
                            case dvarint(R) of
                                {error, _try@1} -> {error, _try@1};
                                {ok, {Value, R@1}} ->
                                    V = {Tag, {var_int, Value}},
                                    decode_1(R@1, [V | Acc])
                            end;

                        1 ->
                            case dbit64(R) of
                                {error, _try@2} -> {error, _try@2};
                                {ok, {Value@1, R@2}} ->
                                    V@1 = {Tag, {int64, Value@1}},
                                    decode_1(R@2, [V@1 | Acc])
                            end;

                        2 ->
                            case dlen_delimited(R) of
                                {error, _try@3} -> {error, _try@3};
                                {ok, {Value@2, R@3}} ->
                                    V@2 = {Tag, {binary, Value@2}},
                                    decode_1(R@3, [V@2 | Acc])
                            end;

                        3 ->
                            case decode_1(R, []) of
                                {error, _try@4} -> {error, _try@4};
                                {ok, {Value@3, R@4}} ->
                                    V@3 = {Tag, {group, Value@3}},
                                    decode_1(R@4, [V@3 | Acc])
                            end;

                        4 ->
                            {ok, {gleam@list:reverse(Acc), R}};

                        5 ->
                            case dbit32(R) of
                                {error, _try@5} -> {error, _try@5};
                                {ok, {Value@4, R@5}} ->
                                    V@4 = {Tag, {int32, Value@4}},
                                    decode_1(R@5, [V@4 | Acc])
                            end
                    end
            end
    end.