% 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(couch_mrview).

-export([validate/2]).
-export([query_all_docs/2, query_all_docs/4]).
-export([query_view/3, query_view/4, query_view/6, get_view_index_pid/4]).
-export([view_changes_since/5]).
-export([view_changes_since/6, view_changes_since/7]).
-export([count_view_changes_since/4, count_view_changes_since/5]).
-export([get_info/2]).
-export([trigger_update/2, trigger_update/3]).
-export([get_view_info/3]).
-export([refresh/2]).
-export([compact/2, compact/3, cancel_compaction/2]).
-export([cleanup/1]).

-include_lib("couch/include/couch_db.hrl").
-include_lib("couch_mrview/include/couch_mrview.hrl").

-record(mracc, {
    db,
    meta_sent=false,
    total_rows,
    offset,
    limit,
    skip,
    group_level,
    doc_info,
    callback,
    user_acc,
    last_go=ok,
    reduce_fun,
    finalizer,
    update_seq,
    args
}).


validate_ddoc_fields(DDoc) ->
    MapFuncType = map_function_type(DDoc),
    lists:foreach(fun(Path) ->
        validate_ddoc_fields(DDoc, Path)
    end, [
        [{<<"filters">>, object}, {any, [object, string]}],
        [{<<"language">>, string}],
        [{<<"lists">>, object}, {any, [object, string]}],
        [{<<"options">>, object}],
        [{<<"rewrites">>, [string, array]}],
        [{<<"shows">>, object}, {any, [object, string]}],
        [{<<"updates">>, object}, {any, [object, string]}],
        [{<<"validate_doc_update">>, string}],
        [{<<"views">>, object}, {<<"lib">>, object}],
        [{<<"views">>, object}, {any, object}, {<<"map">>, MapFuncType}],
        [{<<"views">>, object}, {any, object}, {<<"reduce">>, string}]
    ]),
    require_map_function_for_views(DDoc),
    ok.

require_map_function_for_views({Props}) ->
    case couch_util:get_value(<<"views">>, Props) of
        undefined -> ok;
        {Views} ->
            lists:foreach(fun
                ({<<"lib">>, _}) -> ok;
                ({Key, {Value}}) ->
                    case couch_util:get_value(<<"map">>, Value) of
                        undefined -> throw({invalid_design_doc,
                            <<"View `", Key/binary, "` must contain map function">>});
                        _ -> ok
                    end
            end, Views),
            ok
    end.

validate_ddoc_fields(DDoc, Path) ->
    case validate_ddoc_fields(DDoc, Path, []) of
        ok -> ok;
        {error, {FailedPath0, Type0}} ->
            FailedPath = iolist_to_binary(join(FailedPath0, <<".">>)),
            Type = format_type(Type0),
            throw({invalid_design_doc,
                  <<"`", FailedPath/binary, "` field must have ",
                     Type/binary, " type">>})
    end.

validate_ddoc_fields(undefined, _, _) ->
    ok;
validate_ddoc_fields(_, [], _) ->
    ok;
validate_ddoc_fields({KVS}=Props, [{any, Type} | Rest], Acc) ->
    lists:foldl(fun
        ({Key, _}, ok) ->
            validate_ddoc_fields(Props, [{Key, Type} | Rest], Acc);
        ({_, _}, {error, _}=Error) ->
            Error
    end, ok, KVS);
validate_ddoc_fields({KVS}=Props, [{Key, Type} | Rest], Acc) ->
    case validate_ddoc_field(Props, {Key, Type}) of
        ok ->
            validate_ddoc_fields(couch_util:get_value(Key, KVS),
                                 Rest,
                                 [Key | Acc]);
        error ->
            {error, {[Key | Acc], Type}};
        {error, Key1} ->
            {error, {[Key1 | Acc], Type}}
    end.

validate_ddoc_field(undefined, Type) when is_atom(Type) ->
    ok;
validate_ddoc_field(_, any) ->
    ok;
validate_ddoc_field(Value, Types) when is_list(Types) ->
    lists:foldl(fun
        (_, ok) -> ok;
        (Type, _) -> validate_ddoc_field(Value, Type)
    end, error, Types);
validate_ddoc_field(Value, string) when is_binary(Value) ->
    ok;
validate_ddoc_field(Value, array) when is_list(Value) ->
    ok;
validate_ddoc_field({Value}, object) when is_list(Value) ->
    ok;
validate_ddoc_field({Props}, {any, Type}) ->
    validate_ddoc_field1(Props, Type);
validate_ddoc_field({Props}, {Key, Type}) ->
    validate_ddoc_field(couch_util:get_value(Key, Props), Type);
validate_ddoc_field(_, _) ->
    error.

validate_ddoc_field1([], _) ->
    ok;
validate_ddoc_field1([{Key, Value} | Rest], Type) ->
    case validate_ddoc_field(Value, Type) of
        ok ->
            validate_ddoc_field1(Rest, Type);
        error ->
            {error, Key}
    end.

map_function_type({Props}) ->
    case couch_util:get_value(<<"language">>, Props) of
        <<"query">> -> object;
        _ -> string
    end.

format_type(Type) when is_atom(Type) ->
    ?l2b(atom_to_list(Type));
format_type(Types) when is_list(Types) ->
    iolist_to_binary(join(lists:map(fun atom_to_list/1, Types), <<" or ">>)).

join(L, Sep) ->
    join(L, Sep, []).
join([H|[]], _, Acc) ->
    [H | Acc];
join([H|T], Sep, Acc) ->
    join(T, Sep, [Sep, H | Acc]).


validate(DbName,  DDoc) ->
    ok = validate_ddoc_fields(DDoc#doc.body),
    GetName = fun
        (#mrview{map_names = [Name | _]}) -> Name;
        (#mrview{reduce_funs = [{Name, _} | _]}) -> Name;
        (_) -> null
    end,
    ValidateView = fun(Proc, #mrview{def=MapSrc, reduce_funs=Reds}=View) ->
        couch_query_servers:try_compile(Proc, map, GetName(View), MapSrc),
        lists:foreach(fun
            ({_RedName, <<"_sum", _/binary>>}) ->
                ok;
            ({_RedName, <<"_count", _/binary>>}) ->
                ok;
            ({_RedName, <<"_stats", _/binary>>}) ->
                ok;
            ({_RedName, <<"_approx_count_distinct", _/binary>>}) ->
                ok;
            ({_RedName, <<"_", _/binary>> = Bad}) ->
                Msg = ["`", Bad, "` is not a supported reduce function."],
                throw({invalid_design_doc, Msg});
            ({RedName, RedSrc}) ->
                couch_query_servers:try_compile(Proc, reduce, RedName, RedSrc)
        end, Reds)
    end,
    {ok, #mrst{language=Lang, views=Views}}
            = couch_mrview_util:ddoc_to_mrst(DbName, DDoc),
    try Views =/= [] andalso couch_query_servers:get_os_process(Lang) of
        false ->
            ok;
        Proc ->
            try
                lists:foreach(fun(V) -> ValidateView(Proc, V) end, Views)
            after
                couch_query_servers:ret_os_process(Proc)
            end
    catch {unknown_query_language, _Lang} ->
        %% Allow users to save ddocs written in uknown languages
        ok
    end.


query_all_docs(Db, Args) ->
    query_all_docs(Db, Args, fun default_cb/2, []).


query_all_docs(Db, Args, Callback, Acc) when is_list(Args) ->
    query_all_docs(Db, to_mrargs(Args), Callback, Acc);
query_all_docs(Db, Args0, Callback, Acc) ->
    Sig = couch_util:with_db(Db, fun(WDb) ->
        {ok, Info} = couch_db:get_db_info(WDb),
        couch_index_util:hexsig(couch_hash:md5_hash(term_to_binary(Info)))
    end),
    Args1 = Args0#mrargs{view_type=map},
    Args2 = couch_mrview_util:validate_args(Args1),
    {ok, Acc1} = case Args2#mrargs.preflight_fun of
        PFFun when is_function(PFFun, 2) -> PFFun(Sig, Acc);
        _ -> {ok, Acc}
    end,
    all_docs_fold(Db, Args2, Callback, Acc1).


query_view(Db, DDoc, VName) ->
    query_view(Db, DDoc, VName, #mrargs{}).


query_view(Db, DDoc, VName, Args) when is_list(Args) ->
    query_view(Db, DDoc, VName, to_mrargs(Args), fun default_cb/2, []);
query_view(Db, DDoc, VName, Args) ->
    query_view(Db, DDoc, VName, Args, fun default_cb/2, []).


query_view(Db, DDoc, VName, Args, Callback, Acc) when is_list(Args) ->
    query_view(Db, DDoc, VName, to_mrargs(Args), Callback, Acc);
query_view(Db, DDoc, VName, Args0, Callback, Acc0) ->
    case couch_mrview_util:get_view(Db, DDoc, VName, Args0) of
        {ok, VInfo, Sig, Args} ->
            {ok, Acc1} = case Args#mrargs.preflight_fun of
                PFFun when is_function(PFFun, 2) -> PFFun(Sig, Acc0);
                 _ -> {ok, Acc0}
            end,
            query_view(Db, VInfo, Args, Callback, Acc1);
        ddoc_updated ->
            Callback(ok, ddoc_updated)
    end.


get_view_index_pid(Db, DDoc, ViewName, Args0) ->
    couch_mrview_util:get_view_index_pid(Db, DDoc, ViewName, Args0).


query_view(Db, {Type, View, Ref}, Args, Callback, Acc) ->
    try
        case Type of
            map -> map_fold(Db, View, Args, Callback, Acc);
            red -> red_fold(Db, View, Args, Callback, Acc)
        end
    after
        erlang:demonitor(Ref, [flush])
    end.

view_changes_since(View, StartSeq, Fun, Opts0, Acc) ->
    Wrapper = fun(KV, _, Acc1) ->
        Fun(KV, Acc1)
    end,
    Opts = [{start_key, {StartSeq + 1, <<>>}}] ++ Opts0,
    {ok, _LastRed, AccOut} = couch_btree:fold(View#mrview.seq_btree, Wrapper, Acc, Opts),
    {ok, AccOut}.

view_changes_since(Db, DDoc, VName, StartSeq, Fun, Acc) ->
    view_changes_since(Db, DDoc, VName, StartSeq, Fun, [], Acc).

view_changes_since(Db, DDoc, VName, StartSeq, Fun, Options, Acc) ->
    Args0 = make_view_changes_args(Options),
    {ok, {_, View, _}, _, Args} = couch_mrview_util:get_view(Db, DDoc, VName,
                                                             Args0),
    #mrview{seq_indexed=SIndexed, keyseq_indexed=KSIndexed} = View,
    IsKSQuery = is_key_byseq(Options),
    if (SIndexed andalso not IsKSQuery) orelse (KSIndexed andalso IsKSQuery) ->
        OptList = make_view_changes_opts(StartSeq, Options, Args),
        Btree = case IsKSQuery of
            true -> View#mrview.key_byseq_btree;
            _ -> View#mrview.seq_btree
        end,
        AccOut = lists:foldl(fun(Opts, Acc0) ->
            {ok, _R, A} = couch_mrview_util:fold_changes(
                Btree, Fun, Acc0, Opts),
            A
        end, Acc, OptList),
        {ok, AccOut};
    true ->
        {error, seqs_not_indexed}
    end.

count_view_changes_since(Db, DDoc, VName, SinceSeq) ->
    count_view_changes_since(Db, DDoc, VName, SinceSeq, []).

count_view_changes_since(Db, DDoc, VName, SinceSeq, Options) ->
    Args0 = make_view_changes_args(Options),
    {ok, {_Type, View, _Ref}, _, Args} = couch_mrview_util:get_view(
        Db, DDoc, VName, Args0),
    case View#mrview.seq_indexed of
        true ->
            OptList = make_view_changes_opts(SinceSeq, Options, Args),
            Btree = case is_key_byseq(Options) of
                true -> View#mrview.key_byseq_btree;
                _ -> View#mrview.seq_btree
            end,
            RedFun = fun(_SeqStart, PartialReds, 0) ->
                {ok, couch_btree:final_reduce(Btree, PartialReds)}
            end,
            lists:foldl(fun(Opts, Acc0) ->
                case couch_btree:fold_reduce(Btree, RedFun, 0, Opts) of
                    {ok, N} when is_integer(N) ->
                        Acc0 + N;
                    {ok, N} when is_tuple(N) ->
                        Acc0 + element(1, N)
                end
            end, 0, OptList);
        _ ->
            {error, seqs_not_indexed}
    end.


get_info(Db, DDoc) ->
    {ok, Pid} = couch_index_server:get_index(couch_mrview_index, Db, DDoc),
    couch_index:get_info(Pid).


trigger_update(Db, DDoc) ->
    trigger_update(Db, DDoc, couch_db:get_update_seq(Db)).

trigger_update(Db, DDoc, UpdateSeq) ->
    {ok, Pid} = couch_index_server:get_index(couch_mrview_index, Db, DDoc),
    couch_index:trigger_update(Pid, UpdateSeq).

%% get informations on a view
get_view_info(Db, DDoc, VName) ->
    {ok, {_, View, _}, _, _Args} = couch_mrview_util:get_view(Db, DDoc, VName,
                                                              #mrargs{}),

    %% get the total number of rows
    {ok, TotalRows} =  couch_mrview_util:get_row_count(View),

    %% get the total number of sequence logged in this view
    SeqBtree = View#mrview.seq_btree,
    {ok, TotalSeqs} = case SeqBtree of
        nil -> {ok, 0};
        _ ->
            couch_btree:full_reduce(SeqBtree)
    end,

    {ok, [{seq_indexed, View#mrview.seq_indexed},
          {update_seq, View#mrview.update_seq},
          {purge_seq, View#mrview.purge_seq},
          {total_rows, TotalRows},
          {total_seqs, TotalSeqs}]}.


%% @doc refresh a view index
refresh(DbName, DDoc) when is_binary(DbName)->
    UpdateSeq = couch_util:with_db(DbName, fun(WDb) ->
                    couch_db:get_update_seq(WDb)
            end),

    case couch_index_server:get_index(couch_mrview_index, DbName, DDoc) of
        {ok, Pid} ->
            case catch couch_index:get_state(Pid, UpdateSeq) of
                {ok, _} -> ok;
                Error -> {error, Error}
            end;
        Error ->
            {error, Error}
    end;

refresh(Db, DDoc) ->
    refresh(couch_db:name(Db), DDoc).

compact(Db, DDoc) ->
    compact(Db, DDoc, []).


compact(Db, DDoc, Opts) ->
    {ok, Pid} = couch_index_server:get_index(couch_mrview_index, Db, DDoc),
    couch_index:compact(Pid, Opts).


cancel_compaction(Db, DDoc) ->
    {ok, IPid} = couch_index_server:get_index(couch_mrview_index, Db, DDoc),
    {ok, CPid} = couch_index:get_compactor_pid(IPid),
    ok = couch_index_compactor:cancel(CPid),

    % Cleanup the compaction file if it exists
    {ok, #mrst{sig=Sig, db_name=DbName}} = couch_index:get_state(IPid, 0),
    couch_mrview_util:delete_compaction_file(DbName, Sig),
    ok.


cleanup(Db) ->
    couch_mrview_cleanup:run(Db).


all_docs_fold(Db, #mrargs{keys=undefined}=Args, Callback, UAcc) ->
    ReduceFun = get_reduce_fun(Args),
    Total = get_total_rows(Db, Args),
    UpdateSeq = get_update_seq(Db, Args),
    Acc = #mracc{
        db=Db,
        total_rows=Total,
        limit=Args#mrargs.limit,
        skip=Args#mrargs.skip,
        callback=Callback,
        user_acc=UAcc,
        reduce_fun=ReduceFun,
        update_seq=UpdateSeq,
        args=Args
    },
    [Opts1] = couch_mrview_util:all_docs_key_opts(Args),
    % TODO: This is a terrible hack for now. We'll probably have
    % to rewrite _all_docs to not be part of mrview and not expect
    % a btree. For now non-btree's will just have to pass 0 or
    % some fake reductions to get an offset.
    Opts2 = [include_reductions | Opts1],
    FunName = case couch_util:get_value(namespace, Args#mrargs.extra) of
        <<"_design">> -> fold_design_docs;
        <<"_local">> -> fold_local_docs;
        _ -> fold_docs
    end,
    {ok, Offset, FinalAcc} = couch_db:FunName(Db, fun map_fold/3, Acc, Opts2),
    finish_fold(FinalAcc, [{total, Total}, {offset, Offset}]);
all_docs_fold(Db, #mrargs{direction=Dir, keys=Keys0}=Args, Callback, UAcc) ->
    ReduceFun = get_reduce_fun(Args),
    Total = get_total_rows(Db, Args),
    UpdateSeq = get_update_seq(Db, Args),
    Acc = #mracc{
        db=Db,
        total_rows=Total,
        limit=Args#mrargs.limit,
        skip=Args#mrargs.skip,
        callback=Callback,
        user_acc=UAcc,
        reduce_fun=ReduceFun,
        update_seq=UpdateSeq,
        args=Args
    },
    % Backwards compatibility hack. The old _all_docs iterates keys
    % in reverse if descending=true was passed. Here we'll just
    % reverse the list instead.
    Keys = if Dir =:= fwd -> Keys0; true -> lists:reverse(Keys0) end,

    FoldFun = fun(Key, Acc0) ->
        DocInfo = (catch couch_db:get_doc_info(Db, Key)),
        {Doc, Acc1} = case DocInfo of
            {ok, #doc_info{id=Id, revs=[RevInfo | _RestRevs]}=DI} ->
                Rev = couch_doc:rev_to_str(RevInfo#rev_info.rev),
                Props = [{rev, Rev}] ++ case RevInfo#rev_info.deleted of
                    true -> [{deleted, true}];
                    false -> []
                end,
                {{{Id, Id}, {Props}}, Acc0#mracc{doc_info=DI}};
            not_found ->
                {{{Key, error}, not_found}, Acc0}
        end,
        {_, Acc2} = map_fold(Doc, {[], [{0, 0, 0}]}, Acc1),
        Acc2
    end,
    FinalAcc = lists:foldl(FoldFun, Acc, Keys),
    finish_fold(FinalAcc, [{total, Total}]).


map_fold(Db, View, Args, Callback, UAcc) ->
    {ok, Total} = couch_mrview_util:get_row_count(View),
    Acc = #mracc{
        db=Db,
        total_rows=Total,
        limit=Args#mrargs.limit,
        skip=Args#mrargs.skip,
        callback=Callback,
        user_acc=UAcc,
        reduce_fun=fun couch_mrview_util:reduce_to_count/1,
        update_seq=View#mrview.update_seq,
        args=Args
    },
    OptList = couch_mrview_util:key_opts(Args),
    {Reds, Acc2} = lists:foldl(fun(Opts, {_, Acc0}) ->
        {ok, R, A} = couch_mrview_util:fold(View, fun map_fold/3, Acc0, Opts),
        {R, A}
    end, {nil, Acc}, OptList),
    Offset = couch_mrview_util:reduce_to_count(Reds),
    finish_fold(Acc2, [{total, Total}, {offset, Offset}]).


map_fold(#full_doc_info{} = FullDocInfo, OffsetReds, Acc) ->
    % matches for _all_docs and translates #full_doc_info{} -> KV pair
    case couch_doc:to_doc_info(FullDocInfo) of
        #doc_info{id=Id, revs=[#rev_info{deleted=false, rev=Rev}|_]} = DI ->
            Value = {[{rev, couch_doc:rev_to_str(Rev)}]},
            map_fold({{Id, Id}, Value}, OffsetReds, Acc#mracc{doc_info=DI});
        #doc_info{revs=[#rev_info{deleted=true}|_]} ->
            {ok, Acc}
    end;
map_fold(_KV, _Offset, #mracc{skip=N}=Acc) when N > 0 ->
    {ok, Acc#mracc{skip=N-1, last_go=ok}};
map_fold(KV, OffsetReds, #mracc{offset=undefined}=Acc) ->
    #mracc{
        total_rows=Total,
        callback=Callback,
        user_acc=UAcc0,
        reduce_fun=Reduce,
        update_seq=UpdateSeq,
        args=Args
    } = Acc,
    Offset = Reduce(OffsetReds),
    Meta = make_meta(Args, UpdateSeq, [{total, Total}, {offset, Offset}]),
    {Go, UAcc1} = Callback(Meta, UAcc0),
    Acc1 = Acc#mracc{meta_sent=true, offset=Offset, user_acc=UAcc1, last_go=Go},
    case Go of
        ok -> map_fold(KV, OffsetReds, Acc1);
        stop -> {stop, Acc1}
    end;
map_fold(_KV, _Offset, #mracc{limit=0}=Acc) ->
    {stop, Acc};
map_fold({{Key, Id}, Val}, _Offset, Acc) ->
    #mracc{
        db=Db,
        limit=Limit,
        doc_info=DI,
        callback=Callback,
        user_acc=UAcc0,
        args=Args
    } = Acc,
    Doc = case DI of
        #doc_info{} -> couch_mrview_util:maybe_load_doc(Db, DI, Args);
        _ -> couch_mrview_util:maybe_load_doc(Db, Id, Val, Args)
    end,
    Row = [{id, Id}, {key, Key}, {value, Val}] ++ Doc,
    {Go, UAcc1} = Callback({row, Row}, UAcc0),
    {Go, Acc#mracc{
        limit=Limit-1,
        doc_info=undefined,
        user_acc=UAcc1,
        last_go=Go
    }};
map_fold(#doc{id = <<"_local/", _/binary>>} = Doc, _Offset, #mracc{} = Acc) ->
    #mracc{
        limit=Limit,
        callback=Callback,
        user_acc=UAcc0,
        args=Args
    } = Acc,
    #doc{
        id = DocId,
        revs = {Pos, [RevId | _]}
    } = Doc,
    Rev = {Pos, RevId},
    Row = [
        {id, DocId},
        {key, DocId},
        {value, {[{rev, couch_doc:rev_to_str(Rev)}]}}
    ] ++ if not Args#mrargs.include_docs -> []; true ->
        [{doc, couch_doc:to_json_obj(Doc, Args#mrargs.doc_options)}]
    end,
    {Go, UAcc1} = Callback({row, Row}, UAcc0),
    {Go, Acc#mracc{
        limit=Limit-1,
        reduce_fun=undefined,
        doc_info=undefined,
        user_acc=UAcc1,
        last_go=Go
    }}.

red_fold(Db, {NthRed, _Lang, View}=RedView, Args, Callback, UAcc) ->
    Finalizer = case couch_util:get_value(finalizer, Args#mrargs.extra) of
        undefined ->
            {_, FunSrc} = lists:nth(NthRed, View#mrview.reduce_funs),
            FunSrc;
        CustomFun->
            CustomFun
    end,
    Acc = #mracc{
        db=Db,
        total_rows=null,
        limit=Args#mrargs.limit,
        skip=Args#mrargs.skip,
        group_level=Args#mrargs.group_level,
        callback=Callback,
        user_acc=UAcc,
        update_seq=View#mrview.update_seq,
        finalizer=Finalizer,
        args=Args
    },
    Grouping = {key_group_level, Args#mrargs.group_level},
    OptList = couch_mrview_util:key_opts(Args, [Grouping]),
    Acc2 = lists:foldl(fun(Opts, Acc0) ->
        {ok, Acc1} =
            couch_mrview_util:fold_reduce(RedView, fun red_fold/3,  Acc0, Opts),
        Acc1
    end, Acc, OptList),
    finish_fold(Acc2, []).

red_fold(_Key, _Red, #mracc{skip=N}=Acc) when N > 0 ->
    {ok, Acc#mracc{skip=N-1, last_go=ok}};
red_fold(Key, Red, #mracc{meta_sent=false}=Acc) ->
    #mracc{
        args=Args,
        callback=Callback,
        user_acc=UAcc0,
        update_seq=UpdateSeq
    } = Acc,
    Meta = make_meta(Args, UpdateSeq, []),
    {Go, UAcc1} = Callback(Meta, UAcc0),
    Acc1 = Acc#mracc{user_acc=UAcc1, meta_sent=true, last_go=Go},
    case Go of
        ok -> red_fold(Key, Red, Acc1);
        _ -> {Go, Acc1}
    end;
red_fold(_Key, _Red, #mracc{limit=0} = Acc) ->
    {stop, Acc};
red_fold(_Key, Red, #mracc{group_level=0} = Acc) ->
    #mracc{
        finalizer=Finalizer,
        limit=Limit,
        callback=Callback,
        user_acc=UAcc0
    } = Acc,
    Row = [{key, null}, {value, maybe_finalize(Red, Finalizer)}],
    {Go, UAcc1} = Callback({row, Row}, UAcc0),
    {Go, Acc#mracc{user_acc=UAcc1, limit=Limit-1, last_go=Go}};
red_fold(Key, Red, #mracc{group_level=exact} = Acc) ->
    #mracc{
        finalizer=Finalizer,
        limit=Limit,
        callback=Callback,
        user_acc=UAcc0
    } = Acc,
    Row = [{key, Key}, {value, maybe_finalize(Red, Finalizer)}],
    {Go, UAcc1} = Callback({row, Row}, UAcc0),
    {Go, Acc#mracc{user_acc=UAcc1, limit=Limit-1, last_go=Go}};
red_fold(K, Red, #mracc{group_level=I} = Acc) when I > 0, is_list(K) ->
    #mracc{
        finalizer=Finalizer,
        limit=Limit,
        callback=Callback,
        user_acc=UAcc0
    } = Acc,
    Row = [{key, lists:sublist(K, I)}, {value, maybe_finalize(Red, Finalizer)}],
    {Go, UAcc1} = Callback({row, Row}, UAcc0),
    {Go, Acc#mracc{user_acc=UAcc1, limit=Limit-1, last_go=Go}};
red_fold(K, Red, #mracc{group_level=I} = Acc) when I > 0 ->
    #mracc{
        finalizer=Finalizer,
        limit=Limit,
        callback=Callback,
        user_acc=UAcc0
    } = Acc,
    Row = [{key, K}, {value, maybe_finalize(Red, Finalizer)}],
    {Go, UAcc1} = Callback({row, Row}, UAcc0),
    {Go, Acc#mracc{user_acc=UAcc1, limit=Limit-1, last_go=Go}}.

maybe_finalize(Red, null) ->
    Red;
maybe_finalize(Red, RedSrc) ->
    {ok, Finalized} = couch_query_servers:finalize(RedSrc, Red),
    Finalized.

finish_fold(#mracc{last_go=ok, update_seq=UpdateSeq}=Acc,  ExtraMeta) ->
    #mracc{callback=Callback, user_acc=UAcc, args=Args}=Acc,
    % Possible send meta info
    Meta = make_meta(Args, UpdateSeq, ExtraMeta),
    {Go, UAcc1} = case Acc#mracc.meta_sent of
        false -> Callback(Meta, UAcc);
        _ -> {ok, Acc#mracc.user_acc}
    end,
    % Notify callback that the fold is complete.
    {_, UAcc2} = case Go of
        ok -> Callback(complete, UAcc1);
        _ -> {ok, UAcc1}
    end,
    {ok, UAcc2};
finish_fold(#mracc{user_acc=UAcc}, _ExtraMeta) ->
    {ok, UAcc}.


make_meta(Args, UpdateSeq, Base) ->
    case Args#mrargs.update_seq of
        true -> {meta, Base ++ [{update_seq, UpdateSeq}]};
        _ -> {meta, Base}
    end.


get_reduce_fun(#mrargs{extra = Extra}) ->
    case couch_util:get_value(namespace, Extra) of
        <<"_local">> ->
            fun(_) -> null end;
        _ ->
            fun couch_mrview_util:all_docs_reduce_to_count/1
    end.


get_total_rows(Db, #mrargs{extra = Extra}) ->
    case couch_util:get_value(namespace, Extra) of
        <<"_local">> ->
            null;
        _ ->
            {ok, Info} = couch_db:get_db_info(Db),
            couch_util:get_value(doc_count, Info)
    end.


get_update_seq(Db, #mrargs{extra = Extra}) ->
    case couch_util:get_value(namespace, Extra) of
        <<"_local">> ->
            null;
        _ ->
            couch_db:get_update_seq(Db)
    end.


default_cb(complete, Acc) ->
    {ok, lists:reverse(Acc)};
default_cb({final, Info}, []) ->
    {ok, [Info]};
default_cb({final, _}, Acc) ->
    {ok, Acc};
default_cb(ok, ddoc_updated) ->
    {ok, ddoc_updated};
default_cb(Row, Acc) ->
    {ok, [Row | Acc]}.


to_mrargs(KeyList) ->
    lists:foldl(fun({Key, Value}, Acc) ->
        Index = lookup_index(couch_util:to_existing_atom(Key)),
        setelement(Index, Acc, Value)
    end, #mrargs{}, KeyList).


lookup_index(Key) ->
    Index = lists:zip(
        record_info(fields, mrargs), lists:seq(2, record_info(size, mrargs))
    ),
    couch_util:get_value(Key, Index).


is_key_byseq(Options) ->
    lists:any(fun({K, _}) ->
                lists:member(K, [start_key, end_key, start_key_docid,
                                 end_key_docid, keys])
        end, Options).

make_view_changes_args(Options) ->
    case is_key_byseq(Options) of
        true ->
            to_mrargs(Options);
        false ->
            #mrargs{}
    end.

make_view_changes_opts(StartSeq, Options, Args) ->
    case is_key_byseq(Options) of
        true ->
            couch_mrview_util:changes_key_opts(StartSeq, Args);
        false ->
            [[{start_key, {StartSeq+1, <<>>}}] ++ Options]
    end.