% SPDX-FileCopyrightText: 2025 Łukasz Niemier <~@hauleth.dev>
%
% SPDX-License-Identifier: Apache-2.0

%% @doc Protocol messages parsing and encoding.
%% @end
-module(e9p_msg).

-export([parse/1, encode/2, encode_stat/1, parse_stat/1]).

-export_type([tag/0,
              message/0,
              request_message/0,
              response_message/0
             ]).

-include("e9p_internal.hrl").

-type tag() :: 16#0000..16#FFFF.

-type request_message() ::
    #tversion{} |
    #tauth{} |
    #tattach{} |
    #tflush{} |
    #twalk{} |
    #topen{} |
    #tcreate{} |
    #tread{} |
    #twrite{} |
    #tclunk{} |
    #tremove{} |
    #tstat{} |
    #twstat{}.

-type response_message() ::
    #rversion{} |
    #rauth{} |
    #rattach{} |
    #rerror{} |
    #rflush{} |
    #rwalk{} |
    #ropen{} |
    #rcreate{} |
    #rread{} |
    #rwrite{} |
    #rclunk{} |
    #rstat{} |
    #rwstat{}.

-type message() :: request_message() | response_message().

-spec parse(binary()) -> {ok, tag(), message()} | {error, term()}.
parse(<<Type:1/?int, Tag:2/?int, Data/binary>>) ->
    case do_parse(Type, Data) of
        {ok, Parsed} ->
            {ok, Tag, Parsed};
        {error, Reason} ->
            {error, Reason}
    end.

%% version - negotiate protocol version
do_parse(?Tversion, <<MSize:4/?int, VSize:?len, Version:VSize/binary>>) ->
    {ok, #tversion{max_packet_size = MSize, version = Version}};
do_parse(?Rversion, <<MSize:4/?int, VSize:?len, Version:VSize/binary>>) ->
    {ok, #rversion{max_packet_size = MSize, version = Version}};

%% attach, auth - messages to establish a connection
do_parse(?Tauth, <<AFID:4/?int,
                   UnameLen:?len, Uname:UnameLen/binary,
                   AnameLen:?len, Aname:AnameLen/binary>>) ->
    {ok, #tauth{afid = AFID,
                  uname = Uname,
                  aname = Aname}};
do_parse(?Rauth, <<AQID:13/binary>>) ->
    {ok, #rauth{aqid = binary_to_qid(AQID)}};

do_parse(?Tattach, <<FID:4/?int,
                     AFID:4/?int,
                     ULen:?len, Uname:ULen/binary,
                     ALen:?len, Aname:ALen/binary>>) ->
    {ok, #tattach{fid = FID,
                    afid = AFID,
                    uname = Uname,
                    aname = Aname}};
do_parse(?Rattach, <<QID:13/binary>>) ->
    {ok, #rattach{qid = binary_to_qid(QID)}};

%% clunk - forget about a fid
do_parse(?Tclunk, <<FID:4/?int>>) ->
    {ok, #tclunk{fid = FID}};
do_parse(?Rclunk, <<>>) ->
    {ok, #rclunk{}};

%% error - return an error
do_parse(?Rerror, <<ELen:?len, Error:ELen/binary>>) ->
    {ok, #rerror{msg = Error}};

%% flush - abort a message
do_parse(?Tflush, <<Tag:2/?int>>) ->
    {ok, #tflush{tag = Tag}};
do_parse(?Rflush, <<>>) ->
    {ok, #rflush{}};

%% open, create - prepare a fid for I/O on an existing or new file
do_parse(?Topen, <<FID:4/?int, Mode:1/?int>>) ->
    {ok, #topen{fid = FID, mode = Mode}};
do_parse(?Ropen, <<QID:13/binary, IOUnit:4/?int>>) ->
    {ok, #ropen{qid = binary_to_qid(QID), io_unit = IOUnit}};

do_parse(?Tcreate, <<FID:4/?int,
                     NLen:?len, Name:NLen/binary,
                     Perm:4/?int,
                     Mode:1/?int>>) ->
    {ok, #tcreate{fid = FID, name = Name, perm = Perm, mode = Mode}};
do_parse(?Rcreate, <<QID:13/binary, IOUnit:4/?int>>) ->
    {ok, #rcreate{qid = binary_to_qid(QID), io_unit = IOUnit}};

%% remove - remove a file from a server
do_parse(?Tremove, <<FID:4/?int>>) ->
    {ok, #tremove{fid = FID}};
do_parse(?Rremove, <<>>) ->
    {ok, #rremove{}};

%% stat, wstat - inquire or change file attributes
do_parse(?Tstat, <<FID:4/?int>>) ->
    {ok, #tstat{fid = FID}};
do_parse(?Rstat, <<DLen:?len, Data:DLen/binary>>) ->
    case parse_stat(Data) of
        {ok, Stat} ->
            {ok, #rstat{stat = Stat}};

        {error, _} = Error ->
            Error
    end;

do_parse(?Twstat, <<FID:4/?int, DLen:?len, Data:DLen/binary>>) ->
    case parse_stat(Data) of
        {ok, Stat} ->
            {ok, #twstat{fid = FID, stat = Stat}};

        {error, _} = Error ->
            Error
    end;
do_parse(?Rwstat, <<>>) ->
    {ok, #rwstat{}};

%% walk - descend a directory hierarchy
do_parse(?Twalk, <<FID:4/?int, NewFID:4/?int, NWNLen:?len, Rest/binary>>) ->
    NWNames = [Name || <<NLen:?len, Name:NLen/binary>> <= Rest],
    Len = length(NWNames),
    if
        Len == NWNLen ->
            {ok, #twalk{fid = FID, new_fid = NewFID, names = NWNames}};
        true ->
            {error, {invalid_walk_length, NWNLen, Len}}
    end;
do_parse(?Rwalk, <<NWQLen:?len, QIDs:(NWQLen * 13)/binary>>) ->
    {ok, #rwalk{qids = [binary_to_qid(QID) || <<QID:13/binary>> <= QIDs]}};

do_parse(?Tread, <<FID:4/?int, Offset:8/?int, Len:4/?int>>) ->
    {ok, #tread{fid = FID, offset = Offset, len = Len}};
do_parse(?Rread, <<Count:4/?int, Data:Count/binary>>) ->
    {ok, #rread{data = Data}};
do_parse(?Twrite, <<FID:4/?int, Offset:8/?int, Len:4/?int, Data:Len/binary>>) ->
    {ok, #twrite{fid = FID, offset = Offset, data = Data}};
do_parse(?Rwrite, <<Len:4/?int>>) ->
    {ok, #rwrite{len = Len}};

do_parse(Type, Data) ->
    {error, {invalid_message, Type, Data}}.

parse_stat(<<_Size:2/?int,
             Type:2/?int,
             Dev:4/?int,
             RawQID:13/binary,
             Mode:4/?int,
             Atime:4/?int,
             Mtime:4/?int,
             Len:8/?int,
             NLen:?len, Name:NLen/binary,
             ULen:?len, Uid:ULen/binary,
             GLen:?len, Gid:GLen/binary,
             MULen:?len, MUid:MULen/binary>>)
->
    QID = binary_to_qid(RawQID),
    Flags = qid_to_mode_flags(QID),
    {ok, #{
           type => Type,
           dev => Dev,
           qid => QID,
           mode => Mode band (bnot Flags),
           atime => Atime,
           mtime => Mtime,
           length => Len,
           name => Name,
           uid => Uid,
           gid => Gid,
           muid => MUid
          }};
parse_stat(_) -> {error, invalid_stat_data}.

-spec encode(Tag :: tag() | notag, Data :: message()) -> iodata().
encode(Tag, Data) ->
    {MT, Encoded} = do_encode(Data),
    Tag0 = case Tag of
               notag -> ?notag;
               V -> V
           end,
    [<<MT:1/?int, Tag0:2/?int>> | Encoded].

do_encode(#tversion{max_packet_size = MSize, version = Version}) ->
    {?Tversion, [<<MSize:4/?int>> | encode_str(Version)]};
do_encode(#rversion{max_packet_size = MSize, version = Version}) ->
    {?Rversion, [<<MSize:4/?int>> | encode_str(Version)]};

do_encode(#tauth{afid = AFID, uname = Uname, aname = Aname}) ->
    {?Tauth, [<<AFID:4/?int>>, encode_str(Uname), encode_str(Aname)]};
do_encode(#rauth{aqid = AQID}) ->
    {?Rauth, qid_to_binary(AQID)};

do_encode(#tattach{fid = FID, afid = AFID, uname = Uname, aname = Aname}) ->
    {?Tattach, [<<FID:4/?int, AFID:4/?int>>, encode_str(Uname), encode_str(Aname)]};
do_encode(#rattach{qid = QID}) ->
    {?Rattach, qid_to_binary(QID)};

do_encode(#tclunk{fid = FID}) ->
    {?Tclunk, <<FID:4/?int>>};
do_encode(#rclunk{}) ->
    {?Rclunk, []};

do_encode(#rerror{msg = Error}) ->
    {?Rerror, encode_str(Error)};

do_encode(#tflush{tag = Tag}) ->
    {?Tflush, <<Tag:2/?int>>};
do_encode(#rflush{}) ->
    {?Rflush, []};

do_encode(#topen{fid = FID, mode = Mode}) ->
    {?Topen, <<FID:4/?int, Mode:1/?int>>};
do_encode(#ropen{qid = QID, io_unit = IOUnit}) ->
    {?Ropen, [qid_to_binary(QID), <<IOUnit:4/?int>>]};

do_encode(#tcreate{fid = FID, name = Name, perm = Perm, mode = Mode}) ->
    {?Tcreate, [<<FID:4/?int>>, encode_str(Name), <<Perm:4/?int, Mode:1/?int>>]};
do_encode(#rcreate{qid = QID, io_unit = IOUnit}) ->
    {?Rcreate, [qid_to_binary(QID), <<IOUnit:4/?int>>]};

do_encode(#tremove{fid = FID}) ->
    {?Tremove, <<FID:4/?int>>};
do_encode(#rremove{}) ->
    {?Rremove, []};

do_encode(#tstat{fid = FID}) ->
    {?Tstat, <<FID:4/?int>>};
do_encode(#rstat{stat = Stat}) ->
    Encoded = encode_stat(Stat),
    Len = iolist_size(Encoded),
    {?Rstat, [<<Len:?len>> | encode_stat(Stat)]};

do_encode(#twstat{fid = FID, stat = Stat}) ->
    Encoded = encode_stat(Stat),
    Len = iolist_size(Encoded),
    {?Twstat, [<<FID:4/?int, Len:?len>> | encode_stat(Stat)]};
do_encode(#rwstat{}) ->
    {?Rwstat, []};

do_encode(#twalk{fid = FID, new_fid = NewFID, names = Names}) ->
    ENames = [encode_str(Name) || Name <- Names],
    Len = length(ENames),
    {?Twalk, [<<FID:4/?int, NewFID:4/?int, Len:?len>> | ENames]};
do_encode(#rwalk{qids = QIDs}) ->
    EQIDs = [qid_to_binary(QID) || QID <- QIDs],
    Len = length(EQIDs),
    {?Rwalk, [<<Len:?len>> | EQIDs]};

do_encode(#tread{fid = FID, offset = Offset, len = Len}) ->
    {?Tread, <<FID:4/?int, Offset:8/?int, Len:4/?int>>};
do_encode(#rread{data = Data}) ->
    Len = iolist_size(Data),
    {?Rread, [<<Len:4/?int>> | Data]};
do_encode(#twrite{fid = FID, offset = Offset, data = Data}) ->
    Len = iolist_size(Data),
    {?Twrite, [<<FID:4/?int, Offset:8/?int, Len:4/?int>>, Data]};
do_encode(#rwrite{len = Len}) ->
    {?Rwrite, [<<Len:4/?int>>]}.

encode_stat(Stat) ->
    #{
      type := Type,
      dev := Dev,
      qid := QID,
      mode := Mode,
      atime := Atime,
      mtime := Mtime,
      length := Len,
      name := Name,
      uid := Uid,
      gid := Gid,
      muid := MUid
     } = maps:merge(
           #{
             type => 0,
             dev => 0,
             mode => 0,
             atime => 0,
             mtime => 0,
             uid => ~"",
             gid => ~"",
             muid => ~""
            }, Stat),
    FullMode = qid_to_mode_flags(QID) bor Mode,
    Encoded = [<<
                 Type:2/?int,
                 Dev:4/?int
               >>,
               qid_to_binary(QID),
               <<FullMode:4/?int>>,
               time_to_encoded_sec(Atime),
               time_to_encoded_sec(Mtime),
               <<Len:8/?int>>,
               encode_str(Name),
               encode_str(Uid),
               encode_str(Gid),
               encode_str(MUid)
              ],
    ELen = iolist_size(Encoded),
    [<<ELen:?len>> | Encoded].

qid_to_mode_flags(#qid{type = Type}) ->
    (Type band 2#11100100) bsl 24.

%% ========== Utilities ==========

encode_str(Data0) ->
    Data = unicode:characters_to_binary(Data0),
    true = is_binary(Data),
    Len = iolist_size(Data),
    [<<Len:?len>>, Data].

binary_to_qid(<<Type:1/?int, Version:4/?int, Path:8/?int>>) ->
    #qid{type = Type, version = Version, path = Path}.

qid_to_binary(#qid{type = Type, version = Version, path = Path}) ->
    <<Type:1/?int, Version:4/?int, Path:8/?int>>.

time_to_encoded_sec(Sec) when is_integer(Sec) -> <<Sec:4/?int>>;
time_to_encoded_sec(Time) ->
    Sec = calendar:universal_time_to_system_time(Time, [{unit, second}]),
    <<Sec:4/?int>>.