-module(fabric_view_changes).

-export([go/5, start_update_notifier/1]).

-include("fabric.hrl").

go(DbName, Feed, Options, Callback, Acc0) when Feed == "continuous" orelse
        Feed == "longpoll" ->
    Args = make_changes_args(Options),
    {ok, Acc} = Callback(start, Acc0),
    Notifiers = start_update_notifiers(DbName),
    {Timeout, TimeoutFun} = couch_changes:get_changes_timeout(Args, Callback),
    try
        keep_sending_changes(
            DbName,
            Args,
            Callback,
            get_start_seq(DbName, Args),
            Acc,
            Timeout,
            TimeoutFun
        )
    after
        stop_update_notifiers(Notifiers),
        couch_changes:get_rest_db_updated()
    end;

go(DbName, "normal", Options, Callback, Acc0) ->
    Args = make_changes_args(Options),
    {ok, Acc} = Callback(start, Acc0),
    {ok, #collector{counters=Seqs, user_acc=AccOut}} = send_changes(
        DbName,
        Args,
        Callback,
        get_start_seq(DbName, Args),
        Acc
    ),
    Callback({stop, pack_seqs(Seqs)}, AccOut).

keep_sending_changes(DbName, Args, Callback, Seqs, AccIn, Timeout, TFun) ->
    #changes_args{limit=Limit, feed=Feed} = Args,
    {ok, Collector} = send_changes(DbName, Args, Callback, Seqs, AccIn),
    #collector{limit=Limit2, counters=NewSeqs, user_acc=AccOut} = Collector,
    LastSeq = pack_seqs(NewSeqs),
    if Limit > Limit2, Feed == "longpoll" ->
        Callback({stop, LastSeq}, AccOut);
    true ->
        case couch_changes:wait_db_updated(Timeout, TFun) of
        updated ->
            keep_sending_changes(
                DbName,
                Args#changes_args{limit=Limit2},
                Callback,
                LastSeq,
                AccIn,
                Timeout,
                TFun
            );
        stop ->
            Callback({stop, LastSeq}, AccOut)
        end
    end.

send_changes(DbName, ChangesArgs, Callback, PackedSeqs, AccIn) ->
    AllShards = partitions:all_parts(DbName),
    Seqs = lists:flatmap(fun({#shard{name=Name, node=N} = Shard, Seq}) ->
        case lists:member(Shard, AllShards) of
        true ->
            Ref = rexi:cast(N, {fabric_rpc, changes, [Name,ChangesArgs,Seq]}),
            [{Shard#shard{ref = Ref}, Seq}];
        false ->
            % Find some replacement shards to cover the missing range
            % TODO It's possible in rare cases of shard merging to end up
            % with overlapping shard ranges from this technique
            lists:map(fun(#shard{name=Name2, node=N2} = NewShard) ->
                Ref = rexi:cast(N2, {fabric_rpc, changes, [Name2,ChangesArgs,0]}),
                {NewShard#shard{ref = Ref}, 0}
            end, find_replacement_shards(Shard, AllShards))
        end
    end, unpack_seqs(PackedSeqs, DbName)),
    {Workers, _} = lists:unzip(Seqs),
    State = #collector{
        query_args = ChangesArgs,
        callback = Callback,
        counters = fabric_dict:init(Workers, 0),
        user_acc = AccIn,
        limit = ChangesArgs#changes_args.limit,
        rows = Seqs % store sequence positions instead
    },
    try fabric_util:receive_loop(Workers, #shard.ref, fun handle_message/3,
            State, infinity, 5000)
    after
        fabric_util:cleanup(Workers)
    end.

handle_message({rexi_DOWN, _, _, _}, nil, State) ->
    % TODO see if progress can be made here, possibly by removing all shards
    % from that node and checking is_progress_possible
    {ok, State};

handle_message({rexi_EXIT, Reason}, Worker, State) ->
    ?LOG_ERROR("~p rexi_EXIT ~p", [?MODULE, Reason]),
    #collector{
        callback=Callback,
        counters=Counters0,
        rows = Seqs0,
        user_acc=Acc
    } = State,
    Counters = fabric_dict:erase(Worker, Counters0),
    Seqs = fabric_dict:erase(Worker, Seqs0),
    case fabric_view:is_progress_possible(Counters) of
    true ->
        {ok, State#collector{counters = Counters, rows=Seqs}};
    false ->
        Callback({error, dead_shards}, Acc),
        {error, dead_shards}
    end;

handle_message(_, _, #collector{limit=0} = State) ->
    {stop, State};

handle_message(#view_row{key=Seq} = Row0, {Worker, From}, St) ->
    #collector{
        query_args = #changes_args{include_docs=IncludeDocs},
        callback = Callback,
        counters = S0,
        limit = Limit,
        user_acc = AccIn
    } = St,
    case fabric_dict:lookup_element(Worker, S0) of
    undefined ->
        % this worker lost the race with other partition copies, terminate it
        gen_server:reply(From, stop),
        {ok, St};
    _ ->
        S1 = fabric_dict:store(Worker, Seq, S0),
        S2 = fabric_view:remove_overlapping_shards(Worker, S1),
        Row = Row0#view_row{key = pack_seqs(S2)},
        {Go, Acc} = Callback(changes_row(Row, IncludeDocs), AccIn),
        gen_server:reply(From, Go),
        {Go, St#collector{counters=S2, limit=Limit-1, user_acc=Acc}}
    end;

handle_message({complete, EndSeq}, Worker, State) ->
    #collector{
        counters = S0,
        total_rows = Completed % override
    } = State,
    case fabric_dict:lookup_element(Worker, S0) of
    undefined ->
        {ok, State};
    _ ->
        S1 = fabric_dict:store(Worker, EndSeq, S0),
        % unlikely to have overlaps here, but possible w/ filters
        S2 = fabric_view:remove_overlapping_shards(Worker, S1),
        NewState = State#collector{counters=S2, total_rows=Completed+1},
        case fabric_dict:size(S2) =:= (Completed+1) of
        true ->
            {stop, NewState};
        false ->
            {ok, NewState}
        end
    end.

make_changes_args(Options) ->
    Options.

get_start_seq(_DbName, #changes_args{dir=fwd, since=Since}) ->
    Since;
get_start_seq(DbName, #changes_args{dir=rev}) ->
    Shards = partitions:all_parts(DbName),
    Workers = fabric_util:submit_jobs(Shards, get_update_seq, []),
    {ok, Since} = fabric_util:recv(Workers, #shard.ref,
        fun collect_update_seqs/3, fabric_dict:init(Workers, -1)),
    Since.

collect_update_seqs(Seq, Shard, Counters) when is_integer(Seq) ->
    case fabric_dict:lookup_element(Shard, Counters) of
    undefined ->
        % already heard from someone else in this range
        {ok, Counters};
    -1 ->
        C1 = fabric_dict:store(Shard, Seq, Counters),
        C2 = fabric_view:remove_overlapping_shards(Shard, C1),
        case fabric_dict:any(-1, C2) of
        true ->
            {ok, C2};
        false ->
            {stop, pack_seqs(C2)}
        end
    end.

pack_seqs(Workers) ->
    SeqList = [{N,R,S} || {#shard{node=N, range=R}, S} <- Workers],
    couch_util:encodeBase64Url(term_to_binary(SeqList, [compressed])).

unpack_seqs(0, DbName) ->
    fabric_dict:init(partitions:all_parts(DbName), 0);

unpack_seqs("0", DbName) ->
    fabric_dict:init(partitions:all_parts(DbName), 0);
    
unpack_seqs(Packed, DbName) ->
    % TODO relies on internal structure of fabric_dict as keylist
    lists:map(fun({Node, [A,B], Seq}) ->
        Name = partitions:shard_name(A, DbName),
        {#shard{node=Node, range=[A,B], dbname=DbName, name=Name}, Seq}
    end, binary_to_term(couch_util:decodeBase64Url(Packed))).

start_update_notifiers(DbName) ->
    lists:map(fun(#shard{node=Node, name=Name}) ->
        {Node, rexi:cast(Node, {?MODULE, start_update_notifier, [Name]})}
    end, partitions:all_parts(DbName)).

% rexi endpoint
start_update_notifier(DbName) ->
    {Caller, _} = get(rexi_from),
    Fun = fun({_, X}) when X == DbName -> Caller ! db_updated; (_) -> ok end,
    Id = {couch_db_update_notifier, make_ref()},
    ok = gen_event:add_sup_handler(couch_db_update, Id, Fun),
    receive {gen_event_EXIT, Id, Reason} ->
        rexi:reply({gen_event_EXIT, DbName, Reason})
    end.

stop_update_notifiers(Notifiers) ->
    [rexi:kill(Node, Ref) || {Node, Ref} <- Notifiers].

changes_row(#view_row{key=Seq, id=Id, value=Value, doc=deleted}, true) ->
    {change, {[{seq,Seq}, {id,Id}, {changes,Value}, {deleted, true}, {doc, null}]}};
changes_row(#view_row{key=Seq, id=Id, value=Value, doc=deleted}, false) ->
    {change, {[{seq,Seq}, {id,Id}, {changes,Value}, {deleted, true}]}};
changes_row(#view_row{key=Seq, id=Id, value=Value, doc={error,Reason}}, true) ->
    {change, {[{seq,Seq}, {id,Id}, {changes,Value}, {error,Reason}]}};
changes_row(#view_row{key=Seq, id=Id, value=Value, doc=Doc}, true) ->
    {change, {[{seq,Seq}, {id,Id}, {changes,Value}, {doc,Doc}]}};
changes_row(#view_row{key=Seq, id=Id, value=Value}, false) ->
    {change, {[{seq,Seq}, {id,Id}, {changes,Value}]}}.

find_replacement_shards(#shard{range=Range}, AllShards) ->
    % TODO make this moar betta -- we might have split or merged the partition
    [Shard || Shard <- AllShards, Shard#shard.range =:= Range].