% Licensed under the Apache License, Version 2.0 (the "License"); you may not
% use this file except in compliance with the License. You may obtain a copy of
% the License at
%
%   http://www.apache.org/licenses/LICENSE-2.0
%
% Unless required by applicable law or agreed to in writing, software
% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
% License for the specific language governing permissions and limitations under
% the License.

-module(fabric_view_changes).

-export([go/5, pack_seqs/1, unpack_seqs/2]).
-export([increment_changes_epoch/0]).

%% exported for upgrade purposes.
-export([keep_sending_changes/8]).

-include_lib("fabric/include/fabric.hrl").
-include_lib("mem3/include/mem3.hrl").
-include_lib("couch/include/couch_db.hrl").
-include_lib("eunit/include/eunit.hrl").

-import(fabric_db_update_listener, [wait_db_updated/1]).

go(DbName, Feed, Options, Callback, Acc0) when Feed == "continuous" orelse
        Feed == "longpoll" orelse Feed == "eventsource" ->
    Args = make_changes_args(Options),
    Since = get_start_seq(DbName, Args),
    case validate_start_seq(DbName, Since) of
    ok ->
        {ok, Acc} = Callback(start, Acc0),
        {Timeout, _} = couch_changes:get_changes_timeout(Args, Callback),
        Ref = make_ref(),
        Parent = self(),
        UpdateListener = {spawn_link(fabric_db_update_listener, go,
                                     [Parent, Ref, DbName, Timeout]),
                          Ref},
        put(changes_epoch, get_changes_epoch()),
        try
            keep_sending_changes(
                DbName,
                Args,
                Callback,
                Since,
                Acc,
                Timeout,
                UpdateListener,
                os:timestamp()
            )
        after
            fabric_db_update_listener:stop(UpdateListener)
        end;
    Error ->
        Callback(Error, Acc0)
    end;

go(DbName, "normal", Options, Callback, Acc0) ->
    Args = make_changes_args(Options),
    Since = get_start_seq(DbName, Args),
    case validate_start_seq(DbName, Since) of
    ok ->
        {ok, Acc} = Callback(start, Acc0),
        {ok, Collector} = send_changes(
            DbName,
            Args,
            Callback,
            Since,
            Acc,
            5000
        ),
        #collector{counters=Seqs, user_acc=AccOut, offset=Offset} = Collector,
        Callback({stop, pack_seqs(Seqs), pending_count(Offset)}, AccOut);
    Error ->
        Callback(Error, Acc0)
    end.

keep_sending_changes(DbName, Args, Callback, Seqs, AccIn, Timeout, UpListen, T0) ->
    #changes_args{limit=Limit, feed=Feed, heartbeat=Heartbeat} = Args,
    {ok, Collector} = send_changes(DbName, Args, Callback, Seqs, AccIn, Timeout),
    #collector{
        limit = Limit2,
        counters = NewSeqs,
        offset = Offset,
        user_acc = AccOut0
    } = Collector,
    LastSeq = pack_seqs(NewSeqs),
    MaintenanceMode = config:get("couchdb", "maintenance_mode"),
    NewEpoch = get_changes_epoch() > erlang:get(changes_epoch),
    if Limit > Limit2, Feed == "longpoll";
      MaintenanceMode == "true"; MaintenanceMode == "nolb"; NewEpoch ->
        Callback({stop, LastSeq, pending_count(Offset)}, AccOut0);
    true ->
        {ok, AccOut} = Callback(waiting_for_updates, AccOut0),
        WaitForUpdate = wait_db_updated(UpListen),
        AccumulatedTime = timer:now_diff(os:timestamp(), T0) div 1000,
        Max = case config:get("fabric", "changes_duration") of
        undefined ->
            infinity;
        MaxStr ->
            list_to_integer(MaxStr)
        end,
        case {Heartbeat, AccumulatedTime > Max, WaitForUpdate} of
        {_, _, changes_feed_died} ->
            Callback({stop, LastSeq, pending_count(Offset)}, AccOut);
        {undefined, _, timeout} ->
            Callback({stop, LastSeq, pending_count(Offset)}, AccOut);
        {_, true, timeout} ->
            Callback({stop, LastSeq, pending_count(Offset)}, AccOut);
        _ ->
            {ok, AccTimeout} = Callback(timeout, AccOut),
            ?MODULE:keep_sending_changes(
                DbName,
                Args#changes_args{limit=Limit2},
                Callback,
                LastSeq,
                AccTimeout,
                Timeout,
                UpListen,
                T0
            )
        end
    end.

send_changes(DbName, ChangesArgs, Callback, PackedSeqs, AccIn, Timeout) ->
    LiveNodes = [node() | nodes()],
    AllLiveShards = mem3:live_shards(DbName, LiveNodes),
    Seqs0 = unpack_seqs(PackedSeqs, DbName),
    {WSeqs0, Dead, Reps} = find_replacements(Seqs0, AllLiveShards),
    % Start workers which didn't need replacements
    WSeqs = lists:map(fun({#shard{name = Name, node = N} = S, Seq}) ->
         Ref = rexi:cast(N, {fabric_rpc, changes, [Name, ChangesArgs, Seq]}),
         {S#shard{ref = Ref}, Seq}
    end, WSeqs0),
    % For some dead workers see if they are a result of split shards. In that
    % case make a replacement argument so that local rexi workers can calculate
    % (hopefully) a > 0 update sequence.
    {WSplitSeqs0, Reps1} = find_split_shard_replacements(Dead, Reps),
    WSplitSeqs = lists:map(fun({#shard{name = Name, node = N} = S, Seq}) ->
         Arg = make_replacement_arg(N, Seq),
         Ref = rexi:cast(N, {fabric_rpc, changes, [Name, ChangesArgs, Arg]}),
         {S#shard{ref = Ref}, Seq}
    end, WSplitSeqs0),
    % For ranges that were not split start sequences from 0
    WReps = lists:map(fun(#shard{name = Name, node = N} = S) ->
         Ref = rexi:cast(N, {fabric_rpc, changes, [Name, ChangesArgs, 0]}),
         {S#shard{ref = Ref}, 0}
    end, Reps1),
    Seqs = WSeqs ++ WSplitSeqs ++ WReps,
    {Workers0, _} = lists:unzip(Seqs),
    Repls = fabric_ring:get_shard_replacements(DbName, Workers0),
    StartFun = fun(#shard{name=Name, node=N, range=R0}=Shard) ->
        %% Find the original shard copy in the Seqs array
        case lists:dropwhile(fun({S, _}) -> S#shard.range =/= R0 end, Seqs) of
            [{#shard{}, {replace, _, _, _}} | _] ->
                % Don't attempt to replace a replacement
                SeqArg = 0;
            [{#shard{node = OldNode}, OldSeq} | _] ->
                SeqArg = make_replacement_arg(OldNode, OldSeq);
            _ ->
                % TODO this clause is probably unreachable in the N>2
                % case because we compute replacements only if a shard has one
                % in the original set.
                couch_log:error("Streaming ~s from zero while replacing ~p",
                    [Name, PackedSeqs]),
                SeqArg = 0
        end,
        Ref = rexi:cast(N, {fabric_rpc, changes, [Name, ChangesArgs, SeqArg]}),
        Shard#shard{ref = Ref}
    end,
    RexiMon = fabric_util:create_monitors(Workers0),
    try
        case fabric_streams:start(Workers0, #shard.ref, StartFun, Repls) of
            {ok, Workers} ->
                try
                    LiveSeqs = lists:map(fun(W) ->
                        case lists:keyfind(W, 1, Seqs) of
                            {W, Seq} -> {W, Seq};
                            _ -> {W, 0}
                        end
                    end, Workers),
                    send_changes(DbName, Workers, LiveSeqs, ChangesArgs,
                            Callback, AccIn, Timeout)
                after
                    fabric_streams:cleanup(Workers)
                end;
            {timeout, NewState} ->
                DefunctWorkers = fabric_util:remove_done_workers(
                    NewState#stream_acc.workers,
                    waiting
                ),
                fabric_util:log_timeout(
                    DefunctWorkers,
                    "changes"
                ),
                throw({error, timeout});
            {error, Reason} ->
                throw({error, Reason});
            Else ->
                throw({error, Else})
        end
    after
        rexi_monitor:stop(RexiMon)
    end.

send_changes(DbName, Workers, Seqs, ChangesArgs, Callback, AccIn, Timeout) ->
    State = #collector{
        db_name = DbName,
        query_args = ChangesArgs,
        callback = Callback,
        counters = orddict:from_list(Seqs),
        user_acc = AccIn,
        limit = ChangesArgs#changes_args.limit,
        offset = fabric_dict:init(Workers, null),
        rows = Seqs % store sequence positions instead
    },
    %% TODO: errors need to be handled here
    receive_results(Workers, State, Timeout, Callback).

receive_results(Workers, State, Timeout, Callback) ->
    case rexi_utils:recv(Workers, #shard.ref, fun handle_message/3, State,
            Timeout, infinity) of
    {timeout, NewState0} ->
        {ok, AccOut} = Callback(timeout, NewState0#collector.user_acc),
        NewState = NewState0#collector{user_acc = AccOut},
        receive_results(Workers, NewState, Timeout, Callback);
    {_, NewState} ->
        {ok, NewState}
    end.

handle_message({rexi_DOWN, _, {_, NodeRef}, _}, _, State) ->
    fabric_view:check_down_shards(State, NodeRef);

handle_message({rexi_EXIT, Reason}, Worker, State) ->
    fabric_view:handle_worker_exit(State, Worker, Reason);

% Temporary upgrade clause - Case 24236
handle_message({complete, Key}, Worker, State) when is_tuple(Key) ->
    handle_message({complete, [{seq, Key}, {pending, 0}]}, Worker, State);

handle_message({change, Props}, {Worker, _}, #collector{limit=0} = State) ->
    O0 = State#collector.offset,
    O1 = case fabric_dict:lookup_element(Worker, O0) of
        null ->
            % Use Pending+1 because we're ignoring this row in the response
            Pending = couch_util:get_value(pending, Props, 0),
            fabric_dict:store(Worker, Pending+1, O0);
        _ ->
            O0
    end,
    maybe_stop(State#collector{offset = O1});

handle_message({complete, Props}, Worker, #collector{limit=0} = State) ->
    O0 = State#collector.offset,
    O1 = case fabric_dict:lookup_element(Worker, O0) of
        null ->
            fabric_dict:store(Worker, couch_util:get_value(pending,Props), O0);
        _ ->
            O0
    end,
    maybe_stop(State#collector{offset = O1});

handle_message({no_pass, Props}, {Worker, From}, #collector{limit=0} = State)
  when is_list(Props) ->
    #collector{counters = S0, offset = O0} = State,
    O1 = case fabric_dict:lookup_element(Worker, O0) of
        null ->
            fabric_dict:store(Worker, couch_util:get_value(pending, Props), O0);
        _ ->
            O0
    end,
    S1 = fabric_dict:store(Worker, couch_util:get_value(seq, Props), S0),
    rexi:stream_ack(From),
    maybe_stop(State#collector{counters = S1, offset = O1});

handle_message(#change{} = Row, {Worker, From}, St) ->
    Change = {change, [
        {seq, Row#change.key},
        {id, Row#change.id},
        {changes, Row#change.value},
        {deleted, Row#change.deleted},
        {doc, Row#change.doc}
    ]},
    handle_message(Change, {Worker, From}, St);

handle_message({change, Props}, {Worker, From}, St) ->
    #collector{
        callback = Callback,
        counters = S0,
        offset = O0,
        limit = Limit,
        user_acc = AccIn
    } = St,
    true = fabric_dict:is_key(Worker, S0),
    S1 = fabric_dict:store(Worker, couch_util:get_value(seq, Props), S0),
    O1 = fabric_dict:store(Worker, couch_util:get_value(pending, Props), O0),
    % Temporary hack for FB 23637
    Interval = erlang:get(changes_seq_interval),
    if (Interval == undefined) orelse (Limit rem Interval == 0) ->
        Props2 = lists:keyreplace(seq, 1, Props, {seq, pack_seqs(S1)});
    true ->
        Props2 = lists:keyreplace(seq, 1, Props, {seq, null})
    end,
    {Go, Acc} = Callback(changes_row(Props2), AccIn),
    rexi:stream_ack(From),
    {Go, St#collector{counters=S1, offset=O1, limit=Limit-1, user_acc=Acc}};

%% upgrade clause
handle_message({no_pass, Seq}, From, St) when is_integer(Seq) ->
    handle_message({no_pass, [{seq, Seq}]}, From, St);

handle_message({no_pass, Props}, {Worker, From}, St) ->
    Seq = couch_util:get_value(seq, Props),
    #collector{counters = S0} = St,
    true = fabric_dict:is_key(Worker, S0),
    S1 = fabric_dict:store(Worker, Seq, S0),
    rexi:stream_ack(From),
    {ok, St#collector{counters=S1}};

handle_message({complete, Props}, Worker, State) ->
    Key = couch_util:get_value(seq, Props),
    #collector{
        counters = S0,
        offset = O0,
        total_rows = Completed % override
    } = State,
    true = fabric_dict:is_key(Worker, S0),
    S1 = fabric_dict:store(Worker, Key, S0),
    O1 = fabric_dict:store(Worker, couch_util:get_value(pending, Props), O0),
    NewState = State#collector{counters=S1, offset=O1, total_rows=Completed+1},
    % We're relying on S1 having exactly the numnber of workers that
    % are participtaing in this response. With the new stream_start
    % that's a bit more obvious but historically it wasn't quite
    % so clear. The Completed variable is just a hacky override
    % of the total_rows field in the #collector{} record.
    NumWorkers = fabric_dict:size(S1),
    Go = case NumWorkers =:= (Completed+1) of
        true -> stop;
        false -> ok
    end,
    {Go, NewState}.


make_replacement_arg(Node, {Seq, Uuid}) ->
    {replace, Node, Uuid, Seq};
make_replacement_arg(_Node, {Seq, Uuid, EpochNode}) ->
    % The replacement should properly be computed aginst the node that owned
    % the sequence when it was written to disk (the EpochNode) rather than the
    % node we're trying to replace.
    {replace, EpochNode, Uuid, Seq};
make_replacement_arg(_, _) ->
    0.

maybe_stop(#collector{offset = Offset} = State) ->
    case fabric_dict:any(null, Offset) of
        false ->
            {stop, State};
        true ->
            % Wait till we've heard from everyone to compute pending count
            {ok, State}
    end.

make_changes_args(#changes_args{style=Style, filter_fun=undefined}=Args) ->
    Args#changes_args{filter_fun = {default, Style}};
make_changes_args(Args) ->
    Args.

get_start_seq(DbName, #changes_args{dir=Dir, since=Since})
  when Dir == rev; Since == "now" ->
    {ok, Info} = fabric:get_db_info(DbName),
    couch_util:get_value(update_seq, Info);
get_start_seq(_DbName, #changes_args{dir=fwd, since=Since}) ->
    Since.

pending_count(Dict) ->
    fabric_dict:fold(fun
        (_Worker, Count, Acc) when is_integer(Count), is_integer(Acc) ->
            Count + Acc;
        (_Worker, _Count, _Acc) ->
            null
    end, 0, Dict).

pack_seqs(Workers) ->
    SeqList = [{N,R,S} || {#shard{node=N, range=R}, S} <- Workers],
    SeqSum = lists:sum([seq(S) || {_,_,S} <- SeqList]),
    Opaque = couch_util:encodeBase64Url(term_to_binary(SeqList, [compressed])),
    ?l2b([integer_to_list(SeqSum), $-, Opaque]).

seq({Seq, _Uuid, _Node}) -> Seq;
seq({Seq, _Uuid}) -> Seq;
seq(Seq)          -> Seq.


unpack_seq_regex_match(Packed) ->
    NewPattern = "^\\[[0-9]+\s*,\s*\"(?<opaque>.*)\"\\]$",
    OldPattern = "^\"?([0-9]+-)?(?<opaque>.*?)\"?$",
    Options = [{capture, [opaque], binary}],
    case re:run(Packed, NewPattern, Options) of
        {match, Match} ->
            Match;
        nomatch ->
            {match, Match} = re:run(Packed, OldPattern, Options),
            Match
    end.


unpack_seq_decode_term(Opaque) ->
    binary_to_term(couch_util:decodeBase64Url(Opaque)).


unpack_seqs(0, DbName) ->
    fabric_dict:init(mem3:shards(DbName), 0);

unpack_seqs("0", DbName) ->
    fabric_dict:init(mem3:shards(DbName), 0);

unpack_seqs([_SeqNum, Opaque], DbName) -> % deprecated
    do_unpack_seqs(Opaque, DbName);

unpack_seqs(Packed, DbName) ->
    Opaque = unpack_seq_regex_match(Packed),
    do_unpack_seqs(Opaque, DbName).

do_unpack_seqs(Opaque, DbName) ->
    % A preventative fix for FB 13533 to remove duplicate shards.
    % This just picks each unique shard and keeps the largest seq
    % value recorded.
    Decoded = unpack_seq_decode_term(Opaque),
    DedupDict = lists:foldl(fun({Node, [A, B], Seq}, Acc) ->
        dict:append({Node, [A, B]}, Seq, Acc)
    end, dict:new(), Decoded),
    Deduped = lists:map(fun({{Node, [A, B]}, SeqList}) ->
        {Node, [A, B], lists:max(SeqList)}
    end, dict:to_list(DedupDict)),

    % Create a fabric_dict of {Shard, Seq} entries
    % TODO relies on internal structure of fabric_dict as keylist
    Unpacked = lists:flatmap(fun({Node, [A,B], Seq}) ->
        case mem3:get_shard(DbName, Node, [A,B]) of
        {ok, Shard} ->
            [{Shard, Seq}];
        {error, not_found} ->
            []
        end
    end, Deduped),

    % This just handles the case if the ring in the unpacked sequence
    % received is not complete and in that case tries to fill in the
    % missing ranges with shards from the shard map
    case fabric_ring:is_progress_possible(Unpacked) of
        true ->
            Unpacked;
        false ->
            PotentialWorkers = lists:map(fun({Node, [A, B], Seq}) ->
                case mem3:get_shard(DbName, Node, [A, B]) of
                    {ok, Shard} ->
                        {Shard, Seq};
                    {error, not_found} ->
                        {#shard{node = Node, range = [A, B]}, Seq}
                end
            end, Deduped),
            Shards = mem3:shards(DbName),
            {Unpacked1, Dead, Reps} = find_replacements(PotentialWorkers, Shards),
            {Splits, Reps1} = find_split_shard_replacements(Dead, Reps),
            RepSeqs = lists:map(fun(#shard{} = S) ->
                {S, get_old_seq(S, Deduped)}
            end, Reps1),
            Unpacked1 ++ Splits ++ RepSeqs
    end.


get_old_seq(#shard{range=R}=Shard, SinceSeqs) ->
    case lists:keyfind(R, 2, SinceSeqs) of
        {Node, R, Seq} when is_number(Seq) ->
            % Unfortunately we don't have access to the db
            % uuid so we can't set a replacememnt here.
            couch_log:warning("~p get_old_seq missing uuid "
                "node: ~p, range: ~p, seq: ~p", [?MODULE, Node, R, Seq]),
            0;
        {Node, R, {Seq, Uuid}} ->
            % This update seq is using the old format that
            % didn't include the node. This information is
            % important for replacement.
            {Seq, Uuid, Node};
        {_Node, R, {Seq, Uuid, EpochNode}} ->
            % This is the newest sequence format that we
            % can use for replacement.
            {Seq, Uuid, EpochNode};
        Error ->
            couch_log:warning("~p get_old_seq error: ~p, shard: ~p, seqs: ~p",
                [?MODULE, Error, Shard, SinceSeqs]),
            0
    end.


changes_row(Props0) ->
    Props1 = case couch_util:get_value(deleted, Props0) of
        true ->
            Props0;
        _ ->
            lists:keydelete(deleted, 1, Props0)
    end,
    Allowed = [seq, id, changes, deleted, doc, error],
    Props2 = lists:filter(fun({K,_V}) -> lists:member(K, Allowed) end, Props1),
    {change, {Props2}}.


find_replacements(Workers, AllShards) ->
    % Build map [B, E] => [Worker1, Worker2, ...] for all workers
    WrkMap = lists:foldl(fun({#shard{range = [B, E]}, _} = W, Acc) ->
         maps:update_with({B, E}, fun(Ws) -> [W | Ws] end, [W], Acc)
    end, #{}, fabric_dict:to_list(Workers)),

    % Build map [B, E] => [Shard1, Shard2, ...] for all shards
    AllMap = lists:foldl(fun(#shard{range = [B, E]} = S, Acc) ->
         maps:update_with({B, E}, fun(Ss) -> [S | Ss] end, [S], Acc)
    end, #{}, AllShards),

    % Custom sort function will prioritize workers over other shards.
    % The idea is to not unnecessarily kill workers if we don't have to
    SortFun = fun
        (R1 = {B, E1}, R2 = {B, E2}) ->
            case {maps:is_key(R1, WrkMap), maps:is_key(R2, WrkMap)} of
                {true, true} ->
                    % Both are workers, larger interval wins
                    E1 >= E2;
                {true, false} ->
                    % First element is a worker range, it wins
                    true;
                {false, true} ->
                    % Second element is a worker range, it wins
                    false;
                {false, false} ->
                    % Neither one is a worker interval, pick larger one
                    E1 >= E2
            end;
        ({B1, _}, {B2, _}) ->
            B1 =< B2
    end,
    Ring = mem3_util:get_ring(maps:keys(AllMap), SortFun),

    % Keep only workers in the ring  and from one of the available nodes
    Keep = fun(#shard{range = [B, E], node = N}) ->
        lists:member({B, E}, Ring) andalso lists:keyfind(N, #shard.node,
            maps:get({B, E}, AllMap)) =/= false
    end,
    Workers1 = fabric_dict:filter(fun(S, _) -> Keep(S) end, Workers),
    Removed = fabric_dict:filter(fun(S, _) -> not Keep(S) end, Workers),

    {Rep, _} = lists:foldl(fun(R, {RepAcc, AllMapAcc}) ->
        case maps:is_key(R, WrkMap)of
            true ->
                % It's a worker and in the map of available shards. Make sure
                % to keep it only if there is a range available on that node
                % only (reuse Keep/1 predicate from above)
                WorkersInRange = maps:get(R, WrkMap),
                case lists:any(fun({S, _}) -> Keep(S) end, WorkersInRange) of
                    true ->
                        {RepAcc, AllMapAcc};
                    false ->
                        [Shard | Rest] = maps:get(R, AllMapAcc),
                        {[Shard | RepAcc], AllMapAcc#{R := Rest}}
                end;
             false ->
                % No worker for this range. Replace from available shards
                [Shard | Rest] = maps:get(R, AllMapAcc),
                {[Shard | RepAcc], AllMapAcc#{R := Rest}}
         end
    end, {[], AllMap}, Ring),

    % Return the list of workers that are part of ring, list of removed workers
    % and a list of replacement shards that could be used to make sure the ring
    % completes.
    {Workers1, Removed, Rep}.


% From the list of dead workers determine if any are a result of a split shard.
% In that case perhaps there is a way to not rewind the changes feed back to 0.
% Returns {NewWorkers, Available} where NewWorkers is the list of
% viable workers Available is the list of still unused input Shards
find_split_shard_replacements(DeadWorkers, Shards) ->
    Acc0 = {[], Shards},
    AccF = fabric_dict:fold(fun(#shard{node = WN, range = R}, Seq, Acc) ->
        [B, E] = R,
        {SplitWorkers, Available} = Acc,
        ShardsOnSameNode = [S || #shard{node = N} = S <- Available, N =:= WN],
        SplitShards = mem3_util:non_overlapping_shards(ShardsOnSameNode, B, E),
        RepCount = length(SplitShards),
        NewWorkers = [{S, make_split_seq(Seq, RepCount)} || S <- SplitShards],
        NewAvailable = [S || S <- Available, not lists:member(S, SplitShards)],
        {NewWorkers ++ SplitWorkers, NewAvailable}
    end, Acc0, DeadWorkers),
    {Workers, Available} = AccF,
    {fabric_dict:from_list(Workers), Available}.


make_split_seq({Num, Uuid, Node}, RepCount) when RepCount > 1 ->
    {Num, {split, Uuid}, Node};
make_split_seq(Seq, _) ->
    Seq.


validate_start_seq(_DbName, "now") ->
    ok;
validate_start_seq(_DbName, 0) ->
    ok;
validate_start_seq(_DbName, "0") ->
    ok;
validate_start_seq(_DbName, Seq) ->
    try
        case Seq of
            [_SeqNum, Opaque] ->
                unpack_seq_decode_term(Opaque);
            Seq ->
                Opaque = unpack_seq_regex_match(Seq),
                unpack_seq_decode_term(Opaque)
        end,
        ok
    catch
        _:_ ->
            Reason = <<"Malformed sequence supplied in 'since' parameter.">>,
            {error, {bad_request, Reason}}
    end.

get_changes_epoch() ->
    case application:get_env(fabric, changes_epoch) of
        undefined ->
            increment_changes_epoch(),
            get_changes_epoch();
        {ok, Epoch} ->
            Epoch
    end.

increment_changes_epoch() ->
    application:set_env(fabric, changes_epoch, os:timestamp()).


unpack_seq_setup() ->
    meck:new(mem3),
    meck:new(fabric_view),
    meck:expect(mem3, get_shard, fun(_, _, _) -> {ok, #shard{}} end),
    meck:expect(fabric_ring, is_progress_possible, fun(_) -> true end),
    ok.


unpack_seqs_test_() ->
    {
        setup,
        fun unpack_seq_setup/0,
        fun (_) -> meck:unload() end,
        [
            t_unpack_seqs()
        ]
    }.


t_unpack_seqs() ->
    ?_test(begin
        % BigCouch 0.3 style.
        assert_shards("23423-g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA"),

        % BigCouch 0.4 style.
        assert_shards([23423,<<"g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA">>]),

        % BigCouch 0.4 style (as string).
        assert_shards("[23423,\"g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA\"]"),
        assert_shards("[23423 ,\"g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA\"]"),
        assert_shards("[23423, \"g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA\"]"),
        assert_shards("[23423 , \"g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA\"]"),

        % with internal hypen
        assert_shards("651-g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwNDLXMwBCwxygOFMiQ"
        "5L8____sxJTcalIUgCSSfZgReE4FTmAFMWDFYXgVJQAUlQPVuSKS1EeC5BkaABSQHXz8"
        "VgJUbgAonB_VqIPfoUHIArvE7T6AUQh0I1-WQAzp1XB"),
        assert_shards([651,"g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwNDLXMwBCwxygOFMiQ"
        "5L8____sxJTcalIUgCSSfZgReE4FTmAFMWDFYXgVJQAUlQPVuSKS1EeC5BkaABSQHXz8"
        "VgJUbgAonB_VqIPfoUHIArvE7T6AUQh0I1-WQAzp1XB"]),

        % CouchDB 1.2 style
        assert_shards("\"23423-g1AAAAE7eJzLYWBg4MhgTmHgS0ktM3QwND"
        "LXMwBCwxygOFMiQ5L8____sxIZcKlIUgCSSfZgRUw4FTmAFMWDFTHiVJQAUlSPX1Ee"
        "C5BkaABSQHXzsxKZ8StcAFG4H4_bIAoPQBTeJ2j1A4hCUJBkAQC7U1NA\"")
    end).


assert_shards(Packed) ->
    ?assertMatch([{#shard{},_}|_], unpack_seqs(Packed, <<"foo">>)).


find_replacements_test() ->
    % None of the workers are in the live list of shard but there is a
    % replacement on n3 for the full range. It should get picked instead of
    % the two smaller one on n2.
    Workers1 = mk_workers([{"n1", 0, 10}, {"n2", 11, ?RING_END}]),
    AllShards1 = [
        mk_shard("n1", 11, ?RING_END),
        mk_shard("n2", 0, 4),
        mk_shard("n2", 5, 10),
        mk_shard("n3", 0, ?RING_END)
    ],
    {WorkersRes1, Dead1, Reps1} = find_replacements(Workers1, AllShards1),
    ?assertEqual([], WorkersRes1),
    ?assertEqual(Workers1, Dead1),
    ?assertEqual([mk_shard("n3", 0, ?RING_END)], Reps1),

    % None of the workers are in the live list of shards and there is a
    % split replacement from n2 (range [0, 10] replaced with [0, 4], [5, 10])
    Workers2 = mk_workers([{"n1", 0, 10}, {"n2", 11, ?RING_END}]),
    AllShards2 = [
        mk_shard("n1", 11, ?RING_END),
        mk_shard("n2", 0, 4),
        mk_shard("n2", 5, 10)
    ],
    {WorkersRes2, Dead2, Reps2} = find_replacements(Workers2, AllShards2),
    ?assertEqual([], WorkersRes2),
    ?assertEqual(Workers2, Dead2),
    ?assertEqual([
        mk_shard("n1", 11, ?RING_END),
        mk_shard("n2", 0, 4),
        mk_shard("n2", 5, 10)
    ], lists:sort(Reps2)),

    % One worker is available and one needs to be replaced. Replacement will be
    % from two split shards
    Workers3 = mk_workers([{"n1", 0, 10}, {"n2", 11, ?RING_END}]),
    AllShards3 = [
        mk_shard("n1", 11, ?RING_END),
        mk_shard("n2", 0, 4),
        mk_shard("n2", 5, 10),
        mk_shard("n2", 11, ?RING_END)
    ],
    {WorkersRes3, Dead3, Reps3} = find_replacements(Workers3, AllShards3),
    ?assertEqual(mk_workers([{"n2", 11, ?RING_END}]), WorkersRes3),
    ?assertEqual(mk_workers([{"n1", 0, 10}]), Dead3),
    ?assertEqual([
        mk_shard("n2", 0, 4),
        mk_shard("n2", 5, 10)
    ], lists:sort(Reps3)),

    % All workers are available. Make sure they are not killed even if there is
    % a longer (single) shard to replace them.
    Workers4 = mk_workers([{"n1", 0, 10}, {"n1", 11, ?RING_END}]),
    AllShards4 = [
        mk_shard("n1", 0, 10),
        mk_shard("n1", 11, ?RING_END),
        mk_shard("n2", 0, 4),
        mk_shard("n2", 5, 10),
        mk_shard("n3", 0, ?RING_END)
    ],
    {WorkersRes4, Dead4, Reps4} = find_replacements(Workers4, AllShards4),
    ?assertEqual(Workers4, WorkersRes4),
    ?assertEqual([], Dead4),
    ?assertEqual([], Reps4).


mk_workers(NodesRanges) ->
    mk_workers(NodesRanges, nil).

mk_workers(NodesRanges, Val) ->
    orddict:from_list([{mk_shard(N, B, E), Val} || {N, B, E} <- NodesRanges]).


mk_shard(Name, B, E) ->
    Node = list_to_atom(Name),
    BName = list_to_binary(Name),
    #shard{name = BName, node = Node, range = [B, E]}.


find_split_shard_replacements_test() ->
    % One worker is can be replaced and one can't
    Dead1 = mk_workers([{"n1", 0, 10}, {"n2", 11, ?RING_END}], 42),
    Shards1 = [
        mk_shard("n1", 0, 4),
        mk_shard("n1", 5, 10),
        mk_shard("n3", 11, ?RING_END)
    ],
    {Workers1, ShardsLeft1} = find_split_shard_replacements(Dead1, Shards1),
    ?assertEqual(mk_workers([{"n1", 0, 4}, {"n1", 5, 10}], 42), Workers1),
    ?assertEqual([mk_shard("n3", 11, ?RING_END)], ShardsLeft1),

    % All workers can be replaced - one by 1 shard, another by 3 smaller shards
    Dead2 = mk_workers([{"n1", 0, 10}, {"n2", 11, ?RING_END}], 42),
    Shards2 = [
        mk_shard("n1", 0, 10),
        mk_shard("n2", 11, 12),
        mk_shard("n2", 13, 14),
        mk_shard("n2", 15, ?RING_END)
    ],
    {Workers2, ShardsLeft2} = find_split_shard_replacements(Dead2, Shards2),
    ?assertEqual(mk_workers([
       {"n1", 0, 10},
       {"n2", 11, 12},
       {"n2", 13, 14},
       {"n2", 15, ?RING_END}
    ], 42), Workers2),
    ?assertEqual([], ShardsLeft2),

    % No workers can be replaced. Ranges match but they are on different nodes
    Dead3 = mk_workers([{"n1", 0, 10}, {"n2", 11, ?RING_END}], 42),
    Shards3 = [
        mk_shard("n2", 0, 10),
        mk_shard("n3", 11, ?RING_END)
    ],
    {Workers3, ShardsLeft3} = find_split_shard_replacements(Dead3, Shards3),
    ?assertEqual([], Workers3),
    ?assertEqual(Shards3, ShardsLeft3).