%% This Source Code Form is subject to the terms of the Mozilla Public
%% License, v. 2.0. If a copy of the MPL was not distributed with this
%% file, You can obtain one at https://mozilla.org/MPL/2.0/.
%%
%% Copyright (c) 2007-2023 Broadcom. All Rights Reserved. The term Broadcom refers to Broadcom Inc. and/or its subsidiaries.
%%

%% @hidden
-module(osiris_writer).

-behaviour(gen_batch_server).
-behaviour(osiris_member).

-include("osiris.hrl").

-export([start_link/1,
         start/1,
         overview/1,
         init_data_reader/3,
         register_data_listener/2,
         ack/2,
         write/2,
         write/5,
         write_tracking/3,
         read_tracking/3,
         read_tracking/1,
         query_writers/2,
         query_replication_state/1,
         init_fields_spec/0,
         init/1,
         handle_continue/2,
         handle_batch/2,
         terminate/2,
         format_status/1,
         stop/1,
         delete/1]).

%% osiris_member impl
-export([start/2,
         stop/2,
         delete/2]).

-define(C_COMMITTED_OFFSET, ?C_NUM_LOG_FIELDS + 1).
-define(C_READERS, ?C_NUM_LOG_FIELDS + 2).
-define(C_EPOCH, ?C_NUM_LOG_FIELDS + 3).
-define(C_COMMITTED_CHUNK_ID, ?C_NUM_LOG_FIELDS + 4).
-define(ADD_COUNTER_FIELDS,
        [
         {committed_offset, ?C_COMMITTED_OFFSET, counter, "Last committed offset"},
         {readers, ?C_READERS, counter, "Number of readers"},
         {epoch, ?C_EPOCH, counter, "Current epoch"},
         {committed_chunk_id, ?C_COMMITTED_CHUNK_ID, counter, "Last committed chunk ID"}
        ]
       ).
-define(FIELDSPEC_KEY, osiris_writer_seshat_fields_spec).

%% primary osiris process
%% batch writes incoming data
%% notifies replicator and reader processes of the new max index
%% manages incoming max index

-record(cfg,
        {name :: osiris:name(),
         reference :: term(),
         replicas = [] :: [node()],
         directory :: file:filename_all(),
         counter :: counters:counters_ref(),
         event_formatter :: undefined | mfa()}).
-record(?MODULE,
        {cfg :: #cfg{},
         log :: osiris_log:state(),
         tracking :: osiris_tracking:state(),
         replica_state = #{} :: #{node() => osiris:tail_info()},
         pending_corrs = queue:new() :: queue:queue(),
         duplicates = [] ::
             [{osiris:offset(), pid(), osiris:writer_id(), non_neg_integer()}],
         data_listeners = [] :: [{pid(), osiris:offset()}],
         offset_listeners = [] ::
             [{pid(), osiris:offset(), mfa() | undefined}],
         committed_chunk_id = -1 :: osiris:offset()}).

-opaque state() :: #?MODULE{}.

-export_type([state/0]).

-spec start(node(), Config :: osiris:config()) ->
    supervisor:startchild_ret().
start(Node, #{name := Name, leader_node := Node} = Config) ->
    supervisor:start_child({?SUP, Node},
                           #{id => osiris_util:normalise_name(Name),
                             start => {?MODULE, start_link, [Config]},
                             restart => temporary,
                             shutdown => 5000,
                             type => worker}).

-spec stop(node(), osiris:config()) ->
    ok | {error, not_found}.
stop(Node, #{leader_node := Node} = Config) ->
    ?SUP:stop_child(Node, Config).

-spec delete(node(), osiris:config()) ->
    ok | {error, term()}.
delete(Node, #{leader_node := Node} = Config) ->
    ?SUP:delete_child(Node, Config).

%% backwards compat
start(#{leader_node := LeaderNode} = Config) ->
    start(LeaderNode, Config).
%% backwards compat
stop(#{leader_node := Node} = Config) ->
    stop(Node, Config).
%% backwards compat
delete(#{leader_node := Node} = Config) ->
    ?SUP:delete_child(Node, Config).

-spec start_link(Config :: map()) ->
    {ok, pid()} | {error, {already_started, pid()}}.
start_link(Config) ->
    Mod = ?MODULE,
    Opts = [{reversed_batch, true},
            {flush_mailbox_on_terminate, {true, 10}}],
    gen_batch_server:start_link(undefined, Mod, Config, Opts).

overview(Pid) when node(Pid) == node() ->
    case erlang:is_process_alive(Pid) of
        true ->
            #{dir := Dir} = osiris_util:get_reader_context(Pid),
            {ok, osiris_log:overview(Dir)};
        false ->
            {error, no_process}
    end.

init_data_reader(Pid, TailInfo, Config)
  when node(Pid) == node() ->
    case erlang:is_process_alive(Pid) of
        true ->
            Ctx0 = osiris_util:get_reader_context(Pid),
            Ctx = maps:merge(Ctx0, Config),
            osiris_log:init_data_reader(TailInfo, Ctx);
        false ->
            {error, no_process}
    end.

register_data_listener(Pid, Offset) ->
    ok =
        gen_batch_server:cast(Pid, {register_data_listener, self(), Offset}).

-spec ack(identifier(), {osiris:offset(), osiris:timestamp()} |
          osiris:tail_info()) -> ok.
ack(LeaderPid, {Offset, _} = OffsetTs)
  when is_integer(Offset) andalso Offset >= 0 ->
    gen_batch_server:cast(LeaderPid, {ack, node(), OffsetTs});
ack(LeaderPid, ?TAIL_INFO(ChkId) = TailInfo)
  when is_integer(ChkId) andalso ChkId >= 0 ->
    gen_batch_server:cast(LeaderPid, {ack, node(), TailInfo}).

-spec write(Pid :: pid(), Data :: osiris:data()) -> ok.
write(Pid, Data)
    when is_pid(Pid) ->
    gen_batch_server:cast(Pid, {write, Data}).

-spec write(Pid :: pid(),
            Sender :: pid(),
            WriterId :: binary() | undefined,
            CorrOrSeq :: non_neg_integer() | term(),
            Data :: osiris:data()) -> ok.
write(Pid, Sender, WriterId, Corr, Data)
    when is_pid(Pid) andalso is_pid(Sender) ->
    gen_batch_server:cast(Pid, {write, Sender, WriterId, Corr, Data}).

-spec write_tracking(pid(), binary(), {offset | timestamp, osiris:offset() | osiris:timestamp()}) -> ok.
write_tracking(Pid, TrackingId, {TrackingType, TrackingData})
    when is_pid(Pid)
         andalso is_binary(TrackingId)
         andalso byte_size(TrackingId) =< 255
         andalso is_integer(TrackingData) ->
    gen_batch_server:cast(Pid, {write_tracking, TrackingId, TrackingType, TrackingData}).

read_tracking(Pid, TrackingType, TrackingId) ->
    gen_batch_server:call(Pid, {read_tracking, TrackingType, TrackingId}).

read_tracking(Pid) ->
    gen_batch_server:call(Pid, read_tracking).

query_writers(Pid, QueryFun) ->
    gen_batch_server:call(Pid, {query_writers, QueryFun}).

query_replication_state(Pid) when is_pid(Pid) ->
    gen_batch_server:call(Pid, query_replication_state).

init_fields_spec() ->
    persistent_term:put(?FIELDSPEC_KEY,
                        osiris_log:counter_fields() ++ ?ADD_COUNTER_FIELDS).

-spec init(osiris:config()) ->
    {ok, undefined, {continue, osiris:config()}}.
init(#{name := Name0,
       reference := ExtRef} = Config0) ->
    %% augment config
    Name = osiris_util:normalise_name(Name0),
    Shared = osiris_log_shared:new(),
    Dir = osiris_log:directory(Config0),
    CntName = {?MODULE, ExtRef},
    CntSpec = {CntName, {persistent_term, ?FIELDSPEC_KEY}},
    Config = Config0#{name => Name,
                      dir => Dir,
                      shared => Shared,
                      counter_spec => CntSpec},
    CntRef = osiris_log:make_counter(Config),
    {ok, undefined, {continue, Config#{counter => CntRef}}}.

handle_continue(#{name := Name,
                  dir := Dir,
                  epoch := Epoch,
                  reference := ExtRef,
                  shared := Shared,
                  counter := CntRef,
                  replica_nodes := Replicas} =
                Config, undefined)
  when ?IS_STRING(Name) ->
    process_flag(trap_exit, true),
    process_flag(message_queue_data, off_heap),
    process_flag(fullsweep_after, 0),
    Log = osiris_log:init(Config),
    %% reader context can only be cached _after_ log init as we need to ensure
    %% there is at least 1 segment/index pair and also that the log has been
    %% truncated such that only valid index / segment data remains.
    osiris_util:cache_reader_context(self(), Dir, Name, Shared, ExtRef,
                                     fun(Inc) ->
                                             counters:add(CntRef, ?C_READERS, Inc)
                                     end),
    Trk = osiris_log:recover_tracking(Log),
    {LastChunkId, NextOffset} =
        case osiris_log:tail_info(Log) of
            ?TAIL_INFO_NEXT(TailNextOffs, TailChId) when Replicas == [] ->
                %% only when there are no replicas can we
                %% recover the committed offset from the last
                %% batch offset in the log
                {TailChId, TailNextOffs};
            _ ->
                {-1, 0}
        end,
    LastOffset = NextOffset - 1,
    ok = osiris_log:set_committed_chunk_id(Log, LastChunkId),
    ok = osiris_log:set_committed_offset(Log, LastOffset),
    counters:put(CntRef, ?C_COMMITTED_CHUNK_ID, LastChunkId),
    counters:put(CntRef, ?C_COMMITTED_OFFSET, LastOffset),
    counters:put(CntRef, ?C_EPOCH, Epoch),
    EvtFmt = maps:get(event_formatter, Config, undefined),
    ?INFO("osiris_writer:init/1: name: ~ts committed offset: ~b "
          "committed chunk id: ~b epoch: ~b",
          [Name, LastOffset, LastChunkId, Epoch]),
    {ok,
     #?MODULE{cfg =
                  #cfg{name = Name,
                       %% reference used for notification
                       %% if not provided use the name
                       reference = ExtRef,
                       event_formatter = EvtFmt,
                       replicas = Replicas,
                       directory = Dir,
                       counter = CntRef},
              committed_chunk_id = LastChunkId,
              replica_state = maps:from_list([{R, {0, {0, -1, 0}}} || R <- Replicas]),
              log = Log,
              tracking = Trk}}.

handle_batch(Commands,
             #?MODULE{cfg = #cfg{counter = Cnt} = Cfg,
                      duplicates = Dupes0,
                      committed_chunk_id = ChkId0,
                      tracking = Trk0} =
                 State0) ->

    %% process commands in reverse order
    case catch lists:foldr(fun handle_command/2,
                           {State0, [], [], #{}, Trk0, []}, Commands) of
        {#?MODULE{log = Log0} = State1, Entries, Replies, Corrs, Trk1, Dupes} ->
            {Log1, Trk2} = osiris_log:evaluate_tracking_snapshot(Log0, Trk1),
            Now = erlang:system_time(millisecond),
            ThisBatchOffs = osiris_log:next_offset(Log1),
            {TrkData, Trk} = osiris_tracking:flush(Trk2),
            Log = case Entries of
                      [] when TrkData =/= [] ->
                          %% TODO: we could set a timer for explicit tracking delta
                          %% chunks in order to writer fewer of them
                          osiris_log:write([TrkData],
                                           ?CHNK_TRK_DELTA,
                                           Now,
                                           <<>>,
                                           Log1);
                      _ ->
                          osiris_log:write(Entries,
                                           ?CHNK_USER,
                                           Now,
                                           TrkData,
                                           Log1)
                  end,
            State2 =
                update_pending(ThisBatchOffs, Corrs,
                               State1#?MODULE{tracking = Trk,
                                              log = Log}),

            TailInfo =
                case osiris_log:tail_info(State2#?MODULE.log) of
                    ?TAIL_INFO(_) = TI ->
                        TI;
                    _ ->
                        {-1, {-1, -1, 0}}
                end,

            ReplicaTI = maps:values(State2#?MODULE.replica_state),
            {NOffs, {_, ChkId, _}} = agreed_commit([TailInfo | ReplicaTI]),

            RemDupes = handle_duplicates(ChkId, Dupes ++ Dupes0, Cfg),
            %% if committed offset has increased - update
            State = case ChkId > ChkId0 of
                        true ->
                            P = State2#?MODULE.pending_corrs,
                            LastOffset = NOffs - 1,
                            ok = osiris_log:set_committed_chunk_id(Log, ChkId),
                            ok = osiris_log:set_committed_offset(Log, LastOffset),
                            counters:put(Cnt, ?C_COMMITTED_CHUNK_ID, ChkId),
                            counters:put(Cnt, ?C_COMMITTED_OFFSET, LastOffset),
                            Pending = notify_writers(P, ChkId, Cfg),
                            State2#?MODULE{committed_chunk_id = ChkId,
                                           duplicates = RemDupes,
                                           pending_corrs = Pending};
                        false ->
                            State2#?MODULE{duplicates = RemDupes}
                    end,
            {ok, [garbage_collect | Replies],
             notify_offset_listeners(notify_data_listeners(State))};
        {stop, normal} ->
            {stop, normal}
    end.

terminate(Reason,
          #?MODULE{log = Log,
                   data_listeners = Listeners,
                   cfg = #cfg{name = Name}}) ->
    ?INFO("osiris_writer:terminate/2: name ~ts reason: ~w",
          [Name, Reason]),
    _ = ets:delete(osiris_reader_context_cache, self()),
    ok = osiris_log:close(Log),
    [osiris_replica_reader:stop(Pid) || {Pid, _} <- Listeners],
    ok.

format_status(#?MODULE{cfg = #cfg{name = Name,
                                  replicas = Replicas,
                                  reference = ExtRef},
                       log = Log,
                       tracking = Trk,
                       pending_corrs = PendingCorrs,
                       replica_state = ReplicaState,
                       data_listeners = DataListeners,
                       offset_listeners = OffsetListeners,
                       committed_chunk_id = CommittedChunkId}) ->
    #{name => Name,
      external_reference => ExtRef,
      replica_nodes => Replicas,
      log => osiris_log:format_status(Log),
      tracking => osiris_tracking:overview(Trk),
      replica_state => ReplicaState,
      %%TODO make lqueue for performance
      num_pending_correlations => queue:len(PendingCorrs),
      num_data_listeners => length(DataListeners),
      num_offset_listeners => length(OffsetListeners),
      committed_chunk_id => CommittedChunkId
     }.

%% Internal

update_pending(BatchOffs, Corrs,
               #?MODULE{cfg = #cfg{}, pending_corrs = Pending0} = State) ->
    case Corrs of
        _ when map_size(Corrs) == 0 ->
            State;
        _ ->
            State#?MODULE{pending_corrs =
                              queue:in({BatchOffs, Corrs}, Pending0)}
    end.

put_writer(undefined, _ChId, _Corr, Trk) ->
    Trk;
put_writer(WriterId, ChunkId, Corr, Trk) when is_binary(WriterId) ->
    osiris_tracking:add(WriterId, sequence, Corr, ChunkId, Trk).

handle_duplicates(_CommittedOffset, [], _Cfg) ->
    [];
handle_duplicates(CommittedOffset, Dupes, #cfg{} = Cfg)
    when is_list(Dupes) ->
    %% turn list of dupes into corr map
    {Rem, Corrs} =
        lists:foldr(fun ({ChId, Pid, WriterId, Seq}, {Rem, Corrs0})
                            when ChId =< CommittedOffset ->
                            Corrs =
                                maps:update_with({Pid, WriterId},
                                                 fun(C) -> [Seq | C] end,
                                                 [Seq],
                                                 Corrs0),
                            {Rem, Corrs};
                        (Dupe, {Rem, Corrs}) ->
                            {[Dupe | Rem], Corrs}
                    end,
                    {[], #{}}, Dupes),
    send_written_events(Cfg, Corrs),
    Rem.

handle_command({cast, {write, Pid, WriterId, Corr, R}},
               {#?MODULE{log = Log} = State, Records, Replies, Corrs0, Trk, Dupes}) ->
    case is_duplicate(WriterId, Corr, State, Trk) of
        {false, _} ->
            Corrs =
                maps:update_with({Pid, WriterId},
                                 fun(C) -> [Corr | C] end,
                                 [Corr],
                                 Corrs0),
            ChId = osiris_log:next_offset(Log),
            {State,
             [R | Records],
             Replies,
             Corrs,
             put_writer(WriterId, ChId, Corr, Trk),
             Dupes};
        {true, ChId} ->
            %% add write to duplications list
            {State,
             Records,
             Replies,
             Corrs0,
             Trk,
             [{ChId, Pid, WriterId, Corr} | Dupes]}
    end;
handle_command({cast, {write, R}},
               {#?MODULE{} = State, Records, Replies, Corrs0, Trk, Dupes}) ->
    {State, [R | Records], Replies, Corrs0, Trk, Dupes};
handle_command({cast, {write_tracking, TrackingId, TrackingType, TrackingData}},
               {#?MODULE{log = Log} = State, Records, Replies, Corrs, Trk0, Dupes}) ->
    ChunkId = osiris_log:next_offset(Log),
    Trk = osiris_tracking:add(TrackingId, TrackingType, TrackingData, ChunkId, Trk0),
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({call, From, {read_tracking, TrackingType, TrackingId}},
               {State, Records, Replies0, Corrs, Trk, Dupes}) ->
    %% need to merge pending tracking entries before read
    %%
    Tracking = case osiris_tracking:query(TrackingId, TrackingType, Trk) of
                   {ok, T} ->
                       {TrackingType, T};
                   {error, not_found} ->
                       undefined
               end,
    Replies = [{reply, From, Tracking} | Replies0],
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({call, From, read_tracking},
               {State, Records, Replies0, Corrs, Trk, Dupes}) ->
    Tracking = osiris_tracking:overview(Trk),
    Replies = [{reply, From, Tracking} | Replies0],
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({cast, {register_data_listener, Pid, Offset}},
               {#?MODULE{data_listeners = Listeners} = State0,
                Records,
                Replies,
                Corrs,
                Trk,
                Dupes}) ->
    State = State0#?MODULE{data_listeners = [{Pid, Offset} | Listeners]},
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({cast,
                {register_offset_listener, Pid, EvtFormatter, Offset}},
               {#?MODULE{offset_listeners = Listeners} = State0,
                Records,
                Replies,
                Corrs,
                Trk,
                Dupes}) ->
    State =
        State0#?MODULE{offset_listeners =
                           [{Pid, Offset, EvtFormatter} | Listeners]},
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({cast, {ack, ReplicaNode, TailInfoOrOffsTs}},
               {#?MODULE{replica_state = ReplicaState0} = State0,
                Records,
                Replies,
                Corrs,
                Trk,
                Dupes}) ->
    % ?DEBUG("osiris_writer ack from ~w at ~b", [ReplicaNode, Offset]),
    %% ack message can be a tail_info or a 2-term tuple
    {Offset, TI} = case TailInfoOrOffsTs of
                       ?TAIL_INFO(ChkId) ->
                           {ChkId, TailInfoOrOffsTs};
                       {Offs, Ts} ->
                           {Offs, {0, {0, Offs, Ts}}}
                   end,
    ReplicaState =
        case ReplicaState0 of
            #{ReplicaNode := ?TAIL_INFO(O)} when Offset > O ->
                ReplicaState0#{ReplicaNode => TI};
            _ ->
                %% ignore anything else including acks from unknown replicas
                ReplicaState0
        end,
    State = State0#?MODULE{replica_state = ReplicaState},
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({call, From, get_reader_context},
               {#?MODULE{cfg =
                             #cfg{reference = Ref,
                                  name = Name,
                                  directory = Dir,
                                  counter = CntRef},
                         committed_chunk_id = CChkId,
                         log = Log} =
                    State,
                Records,
                Replies,
                Corrs,
                Trk,
                Dupes}) ->
    Shared = osiris_log:get_shared(Log),
    Reply =
        {reply, From,
         #{dir => Dir,
           name => Name,
           committed_chunk_id => max(0, CChkId),
           committed_offset => max(0, osiris_log:committed_offset(Log)),
           shared => Shared,
           reference => Ref,
           readers_counter_fun => fun(Inc) -> counters:add(CntRef, ?C_READERS, Inc) end
          }},
    {State, Records, [Reply | Replies], Corrs, Trk, Dupes};
handle_command({call, From, {query_writers, QueryFun}},
               {State, Records, Replies0, Corrs, Trk, Dupes}) ->
    %% need to merge pending tracking entries before read
    #{sequences := Writers} = osiris_tracking:overview(Trk),
    Result =
        try QueryFun(Writers) of
            R ->
                R
        catch
            Err ->
                Err
        end,
    Replies = [{reply, From, Result} | Replies0],
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({call, From, query_replication_state},
               {#?MODULE{log = Log,
                         replica_state = R} = State, Records, Replies0,
                Corrs, Trk, Dupes}) ->
    Result0 = maps:map(fun(_, ?TAIL_INFO(O, T)) ->
                               {O, T}
                       end, R),
    %% need to merge pending tracking entries before read
    Result1 = case osiris_log:tail_info(Log) of
                  {0, empty} ->
                      Result0#{node() => {-1, 0}};
                  ?TAIL_INFO(O, T) ->
                      Result0#{node() => {O, T}}
              end,
    Replies = [{reply, From, Result1} | Replies0],
    {State, Records, Replies, Corrs, Trk, Dupes};
handle_command({call, From, {update_retention, Retention}},
               {#?MODULE{log = Log0} = State,
                Records,
                Replies0,
                Corrs,
                Trk,
                Dupes}) ->
    Log = osiris_log:update_retention(Retention, Log0),
    Replies = [{reply, From, ok} | Replies0],
    {State#?MODULE{log = Log}, Records, Replies, Corrs, Trk, Dupes};
handle_command(osiris_stop, _Acc) ->
    throw({stop, normal});
handle_command(_Unk, Acc) ->
    ?DEBUG("osiris_writer: unknown command ~w", [_Unk]),
    Acc.

notify_data_listeners(#?MODULE{log = Seg, data_listeners = L0} =
                          State) ->
    NextOffset = osiris_log:next_offset(Seg),
    {Notify, L} =
        lists:partition(fun({_Pid, O}) -> O < NextOffset end, L0),
    [gen_server:cast(P, {more_data, NextOffset}) || {P, _} <- Notify],
    State#?MODULE{data_listeners = L}.

notify_offset_listeners(#?MODULE{cfg =
                                     #cfg{reference = Ref,
                                          event_formatter = EvtFmt},
                                 committed_chunk_id = COffs,
                                 offset_listeners = L0} =
                            State) ->
    {Notify, L} =
        lists:partition(fun({_Pid, O, _}) -> O =< COffs end, L0),
    _ = [begin
             Evt =
                 %% the per offset listener event formatter takes precedence of
                 %% the process scoped one
                 wrap_osiris_event(
                   select_formatter(Fmt, EvtFmt),
                   {osiris_offset, Ref, COffs}),
             P ! Evt
         end
         || {P, _, Fmt} <- Notify],
    State#?MODULE{offset_listeners = L}.

select_formatter(undefined, Fmt) ->
    Fmt;
select_formatter(Fmt, _) ->
    Fmt.

notify_writers(Q0, COffs, Cfg) ->
    case queue:peek(Q0) of
        {value, {O, Corrs}} when O =< COffs ->
            send_written_events(Cfg, Corrs),
            {_, Q} = queue:out(Q0),
            notify_writers(Q, COffs, Cfg);
        _ ->
            Q0
    end.

send_written_events(#cfg{reference = ExtRef,
                         event_formatter = Fmt},
                    Corrs) ->
    maps:fold(fun({P, WId}, V, Acc) ->
                     %% TODO: if the writer is on a remote node this could block
                     %% which is bad but we'd have to consider the downsides of using
                     %% send with noconnect and nosuspend here
                     % ?DEBUG("send_written_events ~s ~w", [ExtRef, V]),
                     P ! wrap_osiris_event(Fmt,
                                           {osiris_written,
                                            ExtRef,
                                            WId,
                                            lists:reverse(V)}),
                     Acc
             end, ok,
             Corrs),
    ok.

wrap_osiris_event(undefined, Evt) ->
    Evt;
wrap_osiris_event({M, F, A}, Evt) ->
    apply(M, F, [Evt | A]).

-spec agreed_commit([osiris:tail_info()]) -> osiris:tail_info().
agreed_commit(Indexes) ->
    SortedIdxs = lists:sort(fun(?TAIL_INFO(ChkId0), ?TAIL_INFO(ChkId1)) ->
                                    ChkId0 > ChkId1
                            end, Indexes),
    Nth = length(SortedIdxs) div 2 + 1,
    lists:nth(Nth, SortedIdxs).

is_duplicate(undefined, _, _, _) ->
    {false, 0};
is_duplicate(WriterId, Corr, #?MODULE{log = _Log}, Trk) ->
    case osiris_tracking:query(WriterId, sequence, Trk) of
        {ok, {ChunkId, Seq}} ->
            {Corr =< Seq, ChunkId};
        {error, not_found} ->
            {false, 0}
    end.
