gproc.erl

%% ``The contents of this file are subject to the Erlang Public License,
%% Version 1.1, (the "License"); you may not use this file except in
%% compliance with the License. You should have received a copy of the
%% Erlang Public License along with this software. If not, it can be
%% retrieved via the world wide web at http://www.erlang.org/.
%%
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and limitations
%% under the License.
%%
%% The Initial Developer of the Original Code is Ericsson Utvecklings AB.
%% Portions created by Ericsson are Copyright 1999, Ericsson Utvecklings
%% AB. All Rights Reserved.''
%%
%% @author Ulf Wiger <ulf@wiger.net>
%%
%% @doc Extended process registry
%% This module implements an extended process registry
%%
%% For a detailed description, see
%% <a href="erlang07-wiger.pdf">erlang07-wiger.pdf</a>.
%%
%% <h2>Tuning Gproc performance</h2>
%%
%% Gproc relies on a central server and an ordered-set ets table.
%% Effort is made to perform as much work as possible in the client without
%% sacrificing consistency. A few things can be tuned by setting the following
%% application environment variables in the top application of `gproc'
%% (usually `gproc'):
%%
%% * `{ets_options, list()}' - Currently, the options `{write_concurrency, F}'
%%   and `{read_concurrency, F}' are allowed. The default is
%%   `[{write_concurrency, true}, {read_concurrency, true}]'
%% * `{server_options, list()}' - These will be passed as spawn options when
%%   starting the `gproc' and `gproc_dist' servers. Default is `[]'. It is
%%   likely that `{priority, high | max}' and/or increasing `min_heap_size'
%%   will improve performance.
%%
%% @end

%% @type type()  = n | p | c | a. n = name; p = property; c = counter;
%%                                a = aggregate_counter
%% @type scope() = l | g. l = local registration; g = global registration
%%
%% @type reg_id() = {type(), scope(), any()}.
%% @type unique_id() = {n | a, scope(), any()}.
%%
%% @type sel_scope() = scope | all | global | local.
%% @type sel_type() = type() | names | props | counters | aggr_counters.
%% @type context() = {scope(), type()} | type(). {'all','all'} is the default
%%
%% @type headpat() = {keypat(),pidpat(),ValPat}.
%% @type keypat() = {sel_type() | sel_var(),
%%                   l | g | sel_var(),
%%                   any()}.
%% @type pidpat() = pid() | sel_var().
%% @type sel_var() = DollarVar | '_'.
%% @type sel_pattern() = [{headpat(), Guards, Prod}].
%% @type key()   = {type(), scope(), any()}.
%%
%% update_counter increment
%% @type ctr_incr()   = integer().
%% @type ctr_thr()    = integer().
%% @type ctr_setval() = integer().
%% @type ctr_update()  = ctr_incr()
%% 		     | {ctr_incr(), ctr_thr(), ctr_setval()}.
%% @type increment() = ctr_incr() | ctr_update() | [ctr_update()].

-module(gproc).
-behaviour(gen_server).

-export([start_link/0,
         reg/1, reg/2, unreg/1,
	 reg_or_locate/1, reg_or_locate/2, reg_or_locate/3,
	 reg_shared/1, reg_shared/2, unreg_shared/1,
         mreg/3,
         munreg/3,
         set_value/2,
         get_value/1, get_value/2,
         where/1,
         await/1, await/2, await/3,
	 wide_await/3,
         nb_wait/1, nb_wait/2,
         cancel_wait/2, cancel_wait/3,
	 cancel_wait_or_monitor/1,
	 monitor/1,
	 demonitor/2,
         lookup_pid/1,
         lookup_pids/1,
         lookup_value/1,
         lookup_values/1,
         update_counter/2,
	 update_counters/2,
	 reset_counter/1,
	 update_shared_counter/2,
         give_away/2,
         goodbye/0,
         send/2,
	 bcast/2, bcast/3,
         info/1, info/2,
	 i/0,
         select/1, select/2, select/3,
         select_count/1, select_count/2,
         first/1,
         next/2,
         prev/2,
         last/1,
         table/0, table/1, table/2]).

%% Environment handling
-export([get_env/3, get_env/4,
         get_set_env/3, get_set_env/4,
         set_env/5]).

%% Convenience functions
-export([add_local_name/1,
         add_global_name/1,
         add_local_property/2,
         add_global_property/2,
         add_local_counter/2,
         add_global_counter/2,
         add_local_aggr_counter/1,
         add_global_aggr_counter/1,
	 add_shared_local_counter/2,
         lookup_local_name/1,
         lookup_global_name/1,
         lookup_local_properties/1,
         lookup_global_properties/1,
         lookup_local_counters/1,
         lookup_global_counters/1,
         lookup_local_aggr_counter/1,
         lookup_global_aggr_counter/1]).

%% Callbacks for behaviour support
-export([whereis_name/1,
	 register_name/2,
         unregister_name/1]).

-export([default/1]).

%%% internal exports
-export([init/1,
         handle_cast/2,
         handle_call/3,
         handle_info/2,
         code_change/3,
         terminate/2]).

%% this shouldn't be necessary
-export([audit_process/1]).

-include("gproc_int.hrl").
-include("gproc.hrl").
-include_lib("eunit/include/eunit.hrl").

-define(SERVER, ?MODULE).
%%-define(l, l(?LINE)). % when activated, calls a traceable empty function
-define(l, ignore).

-define(CHK_DIST,
        case whereis(gproc_dist) of
            undefined ->
		?THROW_GPROC_ERROR(local_only);
            _ ->
                ok
        end).

-define(PID_IS_DEAD(Pid),
	(node(Pid) == node() andalso is_process_alive(Pid) == false)).

-record(state, {}).

%% @spec () -> {ok, pid()}
%%
%% @doc Starts the gproc server.
%%
%% This function is intended to be called from gproc_sup, as part of
%% starting the gproc application.
%% @end
start_link() ->
    _ = create_tabs(),
    SpawnOpts = gproc_lib:valid_opts(server_options, []),
    gen_server:start_link({local, ?SERVER}, ?MODULE, [],
			  [{spawn_opt, SpawnOpts}]).

%% spec(Name::any()) -> true
%%
%% @doc Registers a local (unique) name. @equiv reg({n,l,Name})
%% @end
%%
add_local_name(Name)  -> ?CATCH_GPROC_ERROR(reg1({n,l,Name}, undefined), [Name]).


%% spec(Name::any()) -> true
%%
%% @doc Registers a global (unique) name. @equiv reg({n,g,Name})
%% @end
%%
add_global_name(Name) -> ?CATCH_GPROC_ERROR(reg1({n,g,Name}, undefined), [Name]).


%% spec(Name::any(), Value::any()) -> true
%%
%% @doc Registers a local (non-unique) property. @equiv reg({p,l,Name},Value)
%% @end
%%
add_local_property(Name , Value) ->
    ?CATCH_GPROC_ERROR(reg1({p,l,Name}, Value), [Name, Value]).

%% spec(Name::any(), Value::any()) -> true
%%
%% @doc Registers a global (non-unique) property. @equiv reg({p,g,Name},Value)
%% @end
%%
add_global_property(Name, Value) ->
    ?CATCH_GPROC_ERROR(reg1({p,g,Name}, Value), [Name, Value]).

%% spec(Name::any(), Initial::integer()) -> true
%%
%% @doc Registers a local (non-unique) counter. @equiv reg({c,l,Name},Value)
%% @end
%%
add_local_counter(Name, Initial) when is_integer(Initial) ->
    ?CATCH_GPROC_ERROR(reg1({c,l,Name}, Initial), [Name, Initial]).


%% spec(Name::any(), Initial::integer()) -> true
%%
%% @doc Registers a local shared (unique) counter.
%% @equiv reg_shared({c,l,Name},Value)
%% @end
%%
add_shared_local_counter(Name, Initial) when is_integer(Initial) ->
    reg_shared({c,l,Name}, Initial).


%% spec(Name::any(), Initial::integer()) -> true
%%
%% @doc Registers a global (non-unique) counter. @equiv reg({c,g,Name},Value)
%% @end
%%
add_global_counter(Name, Initial) when is_integer(Initial) ->
    ?CATCH_GPROC_ERROR(reg1({c,g,Name}, Initial), [Name, Initial]).

%% spec(Name::any()) -> true
%%
%% @doc Registers a local (unique) aggregated counter.
%% @equiv reg({a,l,Name})
%% @end
%%
add_local_aggr_counter(Name)  -> ?CATCH_GPROC_ERROR(reg1({a,l,Name}), [Name]).

%% spec(Name::any()) -> true
%%
%% @doc Registers a global (unique) aggregated counter.
%% @equiv reg({a,g,Name})
%% @end
%%
add_global_aggr_counter(Name) -> ?CATCH_GPROC_ERROR(reg1({a,g,Name}), [Name]).


%% @spec (Name::any()) -> pid()
%%
%% @doc Lookup a local unique name. Fails if there is no such name.
%% @equiv where({n,l,Name})
%% @end
%%
lookup_local_name(Name)   -> where({n,l,Name}).

%% @spec (Name::any()) -> pid()
%%
%% @doc Lookup a global unique name. Fails if there is no such name.
%% @equiv where({n,g,Name})
%% @end
%%
lookup_global_name(Name)  -> where({n,g,Name}).

%% @spec (Name::any()) -> integer()
%%
%% @doc Lookup a local (unique) aggregated counter and returns its value.
%% Fails if there is no such object.
%% @equiv where({a,l,Name})
%% @end
%%
lookup_local_aggr_counter(Name)  -> lookup_value({a,l,Name}).

%% @spec (Name::any()) -> integer()
%%
%% @doc Lookup a global (unique) aggregated counter and returns its value.
%% Fails if there is no such object.
%% @equiv where({a,g,Name})
%% @end
%%
lookup_global_aggr_counter(Name) -> lookup_value({a,g,Name}).


%% @spec (Property::any()) -> [{pid(), Value}]
%%
%% @doc Look up all local (non-unique) instances of a given Property.
%% Returns a list of {Pid, Value} tuples for all matching objects.
%% @equiv lookup_values({p, l, Property})
%% @end
%%
lookup_local_properties(P)  -> lookup_values({p,l,P}).

%% @spec (Property::any()) -> [{pid(), Value}]
%%
%% @doc Look up all global (non-unique) instances of a given Property.
%% Returns a list of {Pid, Value} tuples for all matching objects.
%% @equiv lookup_values({p, g, Property})
%% @end
%%
lookup_global_properties(P) -> lookup_values({p,g,P}).


%% @spec (Counter::any()) -> [{pid(), Value::integer()}]
%%
%% @doc Look up all local (non-unique) instances of a given Counter.
%% Returns a list of {Pid, Value} tuples for all matching objects.
%% @equiv lookup_values({c, l, Counter})
%% @end
%%
lookup_local_counters(P)    -> lookup_values({c,l,P}).


%% @spec (Counter::any()) -> [{pid(), Value::integer()}]
%%
%% @doc Look up all global (non-unique) instances of a given Counter.
%% Returns a list of {Pid, Value} tuples for all matching objects.
%% @equiv lookup_values({c, g, Counter})
%% @end
%%
lookup_global_counters(P)   -> lookup_values({c,g,P}).

%% @spec get_env(Scope::scope(), App::atom(), Key::atom()) -> term()
%% @equiv get_env(Scope, App, Key, [app_env])
get_env(Scope, App, Key) ->
    get_env(Scope, App, Key, [app_env]).

%% @spec (Scope::scope(), App::atom(), Key::atom(), Strategy) -> term()
%%   Strategy = [Alternative]
%%   Alternative = app_env
%%               | os_env
%%               | inherit | {inherit, pid()} | {inherit, unique_id()}
%%               | init_arg
%%               | {mnesia, ActivityType, Oid, Pos}
%%               | {default, term()}
%%               | error
%% @doc Read an environment value, potentially cached as a `gproc_env' property.
%%
%% This function first tries to read the value of a cached property,
%% `{p, Scope, {gproc_env, App, Key}}'. If this fails, it will try the provided
%% alternative strategy. `Strategy' is a list of alternatives, tried in order.
%% Each alternative can be one of:
%%
%% * `app_env' - try `application:get_env(App, Key)'
%% * `os_env' - try `os:getenv(ENV)', where `ENV' is `Key' converted into an
%%   uppercase string
%% * `{os_env, ENV}' - try `os:getenv(ENV)'
%% * `inherit' - inherit the cached value, if any, held by the parent process.
%% * `{inherit, Pid}' - inherit the cached value, if any, held by `Pid'.
%% * `{inherit, Id}' - inherit the cached value, if any, held by the process
%%    registered in `gproc' as `Id'.
%% * `init_arg' - try `init:get_argument(Key)'; expects a single value, if any.
%% * `{mnesia, ActivityType, Oid, Pos}' - try
%%   `mnesia:activity(ActivityType, fun() -> mnesia:read(Oid) end)'; retrieve
%%    the value in position `Pos' if object found.
%% * `{default, Value}' - set a default value to return once alternatives have
%%    been exhausted; if not set, `undefined' will be returned.
%% * `error' - raise an exception, `erlang:error(gproc_env, [App, Key, Scope])'.
%%
%% While any alternative can occur more than once, the only one that might make
%% sense to use multiple times is `{default, Value}'.
%%
%% The return value will be one of:
%%
%% * The value of the first matching alternative, or `error' eception,
%%   whichever comes first
%% * The last instance of `{default, Value}', or `undefined', if there is no
%%   matching alternative, default or `error' entry in the list.
%%
%% The `error' option can be used to assert that a value has been previously
%% cached. Alternatively, it can be used to assert that a value is either cached
%% or at least defined somewhere,
%% e.g. `get_env(l, mnesia, dir, [app_env, error])'.
%% @end
get_env(Scope, App, Key, Strategy)
  when Scope==l, is_atom(App), is_atom(Key);
       Scope==g, is_atom(App), is_atom(Key) ->
    do_get_env(Scope, App, Key, Strategy, false).

%% @spec get_set_env(Scope::scope(), App::atom(), Key::atom()) -> term()
%% @equiv get_set_env(Scope, App, Key, [app_env])
get_set_env(Scope, App, Key) ->
    get_set_env(Scope, App, Key, [app_env]).

%% @spec get_set_env(Scope::scope(), App::atom(), Key::atom(), Strategy) ->
%%           Value
%% @doc Fetch and cache an environment value, if not already cached.
%%
%% This function does the same thing as {@link get_env/4}, but also updates the
%% cache. Note that the cache will be updated even if the result of the lookup
%% is `undefined'.
%%
%% @see get_env/4.
%% @end
%%
get_set_env(Scope, App, Key, Strategy)
  when Scope==l, is_atom(App), is_atom(Key);
       Scope==g, is_atom(App), is_atom(Key) ->
    do_get_env(Scope, App, Key, Strategy, true).

do_get_env(Context, App, Key, Alternatives, Set) ->
    case lookup_env(Context, App, Key, self()) of
        undefined ->
            check_alternatives(Alternatives, Context, App, Key, undefined, Set);
        {ok, Value} ->
            Value
    end.

%% @spec set_env(Scope::scope(), App::atom(),
%%               Key::atom(), Value::term(), Strategy) -> Value
%%   Strategy = [Alternative]
%%   Alternative = app_env | os_env | {os_env, VAR}
%%                | {mnesia, ActivityType, Oid, Pos}
%%
%% @doc Updates the cached value as well as underlying environment.
%%
%% This function should be exercised with caution, as it affects the larger
%% environment outside gproc. This function modifies the cached value, and then
%% proceeds to update the underlying environment (OS environment variable or
%% application environment variable).
%%
%% When the `mnesia' alternative is used, gproc will try to update any existing
%% object, changing only the `Pos' position. If no such object exists, it will
%% create a new object, setting any other attributes (except `Pos' and the key)
%% to `undefined'.
%% @end
%%
set_env(Scope, App, Key, Value, Strategy)
  when Scope==l, is_atom(App), is_atom(Key);
       Scope==g, is_atom(App), is_atom(Key) ->
    case is_valid_set_strategy(Strategy, Value) of
        true ->
            update_cached_env(Scope, App, Key, Value),
            set_strategy(Strategy, App, Key, Value);
        false ->
            erlang:error(badarg)
    end.

check_alternatives([{default, Val}|Alts], Scope, App, Key, _, Set) ->
    check_alternatives(Alts, Scope, App, Key, Val, Set);
check_alternatives([H|T], Scope, App, Key, Def, Set) ->
    case try_alternative(H, App, Key, Scope) of
        undefined ->
            check_alternatives(T, Scope, App, Key, Def, Set);
        {ok, Value} ->
            if Set ->
                    cache_env(Scope, App, Key, Value),
                    Value;
               true ->
                    Value
            end
    end;
check_alternatives([], Scope, App, Key, Def, Set) ->
    if Set ->
            cache_env(Scope, App, Key, Def);
       true ->
            ok
    end,
    Def.

try_alternative(error, App, Key, Scope) ->
    erlang:error(gproc_env, [App, Key, Scope]);
try_alternative(inherit, App, Key, Scope) ->
    case get('$ancestors') of
        [P|_] ->
            lookup_env(Scope, App, Key, P);
        _ ->
            undefined
    end;
try_alternative({inherit, P}, App, Key, Scope) when is_pid(P) ->
    lookup_env(Scope, App, Key, P);
try_alternative({inherit, P}, App, Key, Scope) ->
    case where(P) of
        undefined -> undefined;
        Pid when is_pid(Pid) ->
            lookup_env(Scope, App, Key, Pid)
    end;
try_alternative(app_env, App, Key, _Scope) ->
    case application:get_env(App, Key) of
        undefined       -> undefined;
        {ok, undefined} -> undefined;
        {ok, Value}     -> {ok, Value}
    end;
try_alternative(os_env, _App, Key, _) ->
    case os:getenv(os_env_key(Key)) of
        false  -> undefined;
        Val -> {ok, Val}
    end;
try_alternative({os_env, Key}, _, _, _) ->
    case os:getenv(Key) of
        false  -> undefined;
        Val -> {ok, Val}
    end;
try_alternative(init_arg, _, Key, _) ->
    case init:get_argument(Key) of
        {ok, [[Value]]} ->
            {ok, Value};
        error ->
            undefined
    end;
try_alternative({mnesia,Type,Key,Pos}, _, _, _) ->
    case mnesia:activity(Type, fun() -> mnesia:read(Key) end) of
        [] -> undefined;
        [Found] ->
            {ok, element(Pos, Found)}
    end.

os_env_key(Key) ->
    string:to_upper(atom_to_list(Key)).

lookup_env(Scope, App, Key, P) ->
    case ets:lookup(?TAB, {{p, Scope, {gproc_env, App, Key}}, P}) of
        [] ->
            undefined;
        [{_, _, Value}] ->
            {ok, Value}
    end.

cache_env(Scope, App, Key, Value) ->
    ?CATCH_GPROC_ERROR(
       reg1({p, Scope, {gproc_env, App, Key}}, Value),
       [Scope,App,Key,Value]).

update_cached_env(Scope, App, Key, Value) ->
    case lookup_env(Scope, App, Key, self()) of
        undefined ->
            cache_env(Scope, App, Key, Value);
        {ok, _} ->
            set_value({p, Scope, {gproc_env, App, Key}}, Value)
    end.

is_valid_set_strategy([os_env|T], Value) ->
    is_string(Value) andalso is_valid_set_strategy(T, Value);
is_valid_set_strategy([{os_env, _}|T], Value) ->
    is_string(Value) andalso is_valid_set_strategy(T, Value);
is_valid_set_strategy([app_env|T], Value) ->
    is_valid_set_strategy(T, Value);
is_valid_set_strategy([{mnesia,_Type,_Oid,_Pos}|T], Value) ->
    is_valid_set_strategy(T, Value);
is_valid_set_strategy([], _) ->
    true;
is_valid_set_strategy(_, _) ->
    false.

set_strategy([H|T], App, Key, Value) ->
    case H of
        app_env ->
            application:set_env(App, Key, Value);
        os_env ->
            os:putenv(os_env_key(Key), Value);
        {os_env, ENV} ->
            os:putenv(ENV, Value);
        {mnesia,Type,Oid,Pos} ->
            mnesia:activity(
              Type,
              fun() ->
                      Rec = case mnesia:read(Oid) of
                                [] ->
                                    {Tab,K} = Oid,
                                    Tag = mnesia:table_info(Tab, record_name),
                                    Attrs = mnesia:table_info(Tab, attributes),
                                    list_to_tuple(
                                      [Tag,K |
                                       [undefined || _ <- tl(Attrs)]]);
                                [Old] ->
                                    Old
                            end,
                      mnesia:write(setelement(Pos, Rec, Value))
              end)
    end,
    set_strategy(T, App, Key, Value);
set_strategy([], _, _, Value) ->
    Value.

is_string(S) ->
    try begin _ = iolist_to_binary(S),
              true
        end
    catch
        error:_ ->
            false
    end.

%% @spec reg(Key::key()) -> true
%%
%% @doc
%% @equiv reg(Key, default(Key))
%% @end
reg(Key) ->
    ?CATCH_GPROC_ERROR(reg1(Key), [Key]).

reg1(Key) ->
    reg1(Key, default(Key)).

%% @spec reg_or_locate(Key::key()) -> true
%%
%% @doc
%% @equiv reg_or_locate(Key, default(Key))
%% @end
reg_or_locate(Key) ->
    ?CATCH_GPROC_ERROR(reg_or_locate1(Key), [Key]).

reg_or_locate1(Key) ->
    reg_or_locate1(Key, default(Key), self()).

default({T,_,_}) when T==c -> 0;
default(_) -> undefined.

%% @spec await(Key::key()) -> {pid(),Value}
%% @equiv await(Key,infinity)
%%
await(Key) ->
    ?CATCH_GPROC_ERROR(await1(Key, infinity), [Key]).

%% @spec await(Key::key(), Timeout) -> {pid(),Value}
%%   Timeout = integer() | infinity
%%
%% @doc Wait for a local name to be registered.
%% The function raises an exception if the timeout expires. Timeout must be
%% either an interger &gt; 0 or 'infinity'.
%% A small optimization: we first perform a lookup, to see if the name
%% is already registered. This way, the cost of the operation will be
%% roughly the same as of where/1 in the case where the name is already
%% registered (the difference: await/2 also returns the value).
%% @end
%%
await(Key, Timeout) ->
    ?CATCH_GPROC_ERROR(await1(Key, Timeout), [Key, Timeout]).

%% @spec await(Node::node(), Key::key(), Timeout) -> {pid(),Value}
%%   Timeout = integer() | infinity
%%
%% @doc Wait for a local name to be registered on `Node'.
%% This function works exactly like {@link await/2}, but queries a remote
%% node instead. An exception is thrown if `Node' cannot be reached. If gproc
%% is not running on a given node, this is treated the same as the node being
%% down.
%% @end
%%
await(Node, Key, Timeout) when Node == node() ->
    await(Key, Timeout);
await(Node, Key, Timeout) when is_atom(Node) ->
    ?CATCH_GPROC_ERROR(await1(Node, Key, Timeout), [Node, Key, Timeout]).

await1({n,g,_} = Key, Timeout) ->
    ?CHK_DIST,
    request_wait(Key, Timeout);
await1({n,l,_} = Key, Timeout) ->
    case ets:lookup(?TAB, {Key, n}) of
        [{_, Pid, Value}] ->
	    case is_process_alive(Pid) of
		true ->
		    {Pid, Value};
		false ->
		    %% we can send an asynchronous audit request, since the purpose is
		    %% only to ensure that the server handles the audit before it serves
		    %% our 'await' request. Strictly speaking, we could allow the bad Pid
		    %% to be returned, as there are no guarantees that whatever Pid we return
		    %% will still be alive when addressed. Still, we don't want to knowingly
		    %% serve bad data.
		    nb_audit_process(Pid),
		    request_wait(Key, Timeout)
	    end;
        _ ->
            request_wait(Key, Timeout)
    end;
await1(_, _) ->
    throw(badarg).

await1(N, {n,l,_} = Key, Timeout) when is_atom(N) ->
    request_wait(N, Key, Timeout);
await1(_, _, _) ->
    throw(badarg).


request_wait({_,g,_} = Key, Timeout) ->
    request_wait(undefined, Key, Timeout);
request_wait(Key, Timeout) ->
    request_wait(node(), Key, Timeout).

request_wait(N, {n,C,_} = Key, Timeout) when C==l; C==g ->
    TRef = case Timeout of
               infinity -> no_timer;
               T when is_integer(T), T > 0 ->
                   erlang:start_timer(T, self(), gproc_timeout);
               _ ->
                   ?THROW_GPROC_ERROR(badarg)
           end,
    WRef = case {call(N, {await,Key,self()}, C), C} of
               {{R, {Kg,Pg,Vg}}, g} ->
                   self() ! {gproc, R, registered, {Kg,Pg,Vg}},
                   R;
               {R,_} ->
                   R
           end,
    receive
        {gproc, WRef, registered, {_K, Pid, V}} ->
            _ = case TRef of
                    no_timer -> ignore;
                    _ -> erlang:cancel_timer(TRef)
                end,
            {Pid, V};
        {timeout, TRef, gproc_timeout} ->
            cancel_wait(N, Key, WRef),
            ?THROW_GPROC_ERROR(timeout)
    end.

%% @spec wide_await(Nodes::[node()], Key::key(), Timeout) -> {pid(),Value}
%%   Timeout = integer() | infinity
%%
%% @doc Wait for a local name to be registered on any of `Nodes'.
%% This function works rather like {@link await/2}, but queries all nodes in
%% the `Nodes' list at the same time. The first node to respond with a
%% process registered as `Key' will provide the result. Other results are
%% ignored. `Key' must be a unique name with local scope, i.e. `{n,l,Name}'.
%%
%% An exception is thrown upon timeout, or if no node can be reached (if gproc is
%% not running on a given node, this is treated the same as the node being down).
%% @end
%%
wide_await(Nodes, Key, Timeout) ->
    ?CATCH_GPROC_ERROR(wide_await1(Nodes, Key, Timeout), [Nodes, Key, Timeout]).

wide_await1(Nodes, {n,l,_} = Key, Timeout) ->
    {_, Ref} = spawn_monitor(fun() ->
				     wide_request_wait(Nodes, Key, Timeout)
			     end),
    receive
	{'DOWN', Ref, _, _, Reason} ->
	    case Reason of
		{ok, {gproc,_,registered,{_,Pid,V}}} ->
		    {Pid, V};
		Other ->
		    ?THROW_GPROC_ERROR(Other)
	    end
    end;
wide_await1(_, _, _) ->
    ?THROW_GPROC_ERROR(badarg).


wide_request_wait(Nodes, {n,l,_} = Key, Timeout) ->
        TRef = case Timeout of
               infinity -> no_timer;
               T when is_integer(T), T > 0 ->
                   erlang:start_timer(T, self(), gproc_timeout);
               _ ->
                   exit(badarg)
           end,
    Req = {await, Key, self()},
    Refs = lists:map(
	     fun(Node) ->
		     S = {?MODULE, Node},
		     Ref = erlang:monitor(process, S),
		     catch erlang:send(S, {'$gen_call', {self(), Ref}, Req},
				       [noconnect]),
		     {Node, Ref}
	     end, Nodes),
    collect_replies(Refs, Key, TRef).

collect_replies(Refs, Key, TRef) ->
    receive
	{gproc, _Ref, registered, {_, _, _}} = Result ->
	    exit({ok, Result});
	{'DOWN', Ref, _, _, _} ->
	    case lists:keydelete(Ref, 2, Refs) of
		[] ->
		    exit(nodedown);
		Refs1 ->
		    collect_replies(Refs1, Key, TRef)
	    end;
	{timeout, TRef, gproc_timeout} ->
	    exit(timeout);
	{Ref, Ref} ->
	    %% ignore
	    collect_replies(Refs, Key, TRef)
    end.


%% @spec nb_wait(Key::key()) -> Ref
%%
%% @doc Wait for a local name to be registered.
%% The caller can expect to receive a message,
%% {gproc, Ref, registered, {Key, Pid, Value}}, once the name is registered.
%% @end
%%
nb_wait(Key) ->
    ?CATCH_GPROC_ERROR(nb_wait1(Key), [Key]).

%% @spec nb_wait(Node::node(), Key::key()) -> Ref
%%
%% @doc Wait for a local name to be registered on `Node'.
%% The caller can expect to receive a message,
%% {gproc, Ref, registered, {Key, Pid, Value}}, once the name is registered.
%% @end
%%
nb_wait(Node, {n,l,_} = Key) when is_atom(Node) ->
    ?CATCH_GPROC_ERROR(nb_wait1(Node, Key), [Node, Key]).

nb_wait1({n,g,_} = Key) ->
    ?CHK_DIST,
    call({await, Key, self()}, g);
nb_wait1({n,l,_} = Key) ->
    call({await, Key, self()}, l);
nb_wait1(_) ->
    ?THROW_GPROC_ERROR(badarg).

nb_wait1(Node, {n,l,_} = Key) when is_atom(Node) ->
    call(Node, {await, Key, self()}, l).


%% @spec cancel_wait(Key::key(), Ref) -> ok
%%    Ref = all | reference()
%%
%% @doc Cancels a previous call to nb_wait/1
%%
%% If `Ref = all', all wait requests on `Key' from the calling process
%% are canceled.
%% @end
%%
cancel_wait(Key, Ref) ->
    ?CATCH_GPROC_ERROR(cancel_wait1(Key, Ref), [Key, Ref]).

%% @spec cancel_wait(Node::node(), Key::key(), Ref) -> ok
%%    Ref = all | reference()
%%
%% @doc Cancels a previous call to nb_wait/2
%%
%% This function works just like {@link cancel_wait/2}, but talks to a remote
%% node.
%% @end
%%
cancel_wait(N, Key, Ref) when N == node() ->
    cancel_wait(Key, Ref);
cancel_wait(N, Key, Ref) ->
    ?CATCH_GPROC_ERROR(cancel_wait1(N, Key, Ref), [N, Key, Ref]).


cancel_wait1({_,g,_} = Key, Ref) ->
    ?CHK_DIST,
    cast({cancel_wait, self(), Key, Ref}, g),
    ok;
cancel_wait1({_,l,_} = Key, Ref) ->
    cast({cancel_wait, self(), Key, Ref}, l),
    ok.

cancel_wait1(undefined, {_,g,_} = Key, Ref) ->
    cast({cancel_wait, self(), Key, Ref}, g);
cancel_wait1(N, {_,l,_} = Key, Ref) ->
    cast(N, {cancel_wait, self(), Key, Ref}, l).

cancel_wait_or_monitor(Key) ->
    ?CATCH_GPROC_ERROR(cancel_wait_or_monitor1(Key), [Key]).

cancel_wait_or_monitor1({_,g,_} = Key) ->
    ?CHK_DIST,
    cast({cancel_wait_or_monitor, self(), Key}, g),
    ok;
cancel_wait_or_monitor1({_,l,_} = Key) ->
    cast({cancel_wait_or_monitor, self(), Key}, l),
    ok.


%% @spec monitor(key()) -> reference()
%%
%% @doc monitor a registered name
%% This function works much like erlang:monitor(process, Pid), but monitors
%% a unique name registered via gproc. A message, `{gproc, unreg, Ref, Key}'
%% will be sent to the requesting process, if the name is unregistered or
%% the registered process dies.
%%
%% If the name is not yet registered, the same message is sent immediately.
%% @end
monitor(Key) ->
    ?CATCH_GPROC_ERROR(monitor1(Key), [Key]).

monitor1({T,g,_} = Key) when T==n; T==a ->
    ?CHK_DIST,
    call({monitor, Key, self()}, g);
monitor1({T,l,_} = Key) when T==n; T==a ->
    call({monitor, Key, self()}, l);
monitor1(_) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec demonitor(key(), reference()) -> ok
%%
%% @doc Remove a monitor on a registered name
%% This function is the reverse of monitor/1. It removes a monitor previously
%% set on a unique name. This function always succeeds given legal input.
%% @end
demonitor(Key, Ref) ->
    ?CATCH_GPROC_ERROR(demonitor1(Key, Ref), [Key, Ref]).

demonitor1({T,g,_} = Key, Ref) when T==n; T==a ->
    ?CHK_DIST,
    call({demonitor, Key, Ref, self()}, g);
demonitor1({T,l,_} = Key, Ref) when T==n; T==a ->
    call({demonitor, Key, Ref, self()}, l);
demonitor1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec reg(Key::key(), Value) -> true
%%
%% @doc Register a name or property for the current process
%%
%%
reg(Key, Value) ->
    ?CATCH_GPROC_ERROR(reg1(Key, Value), [Key, Value]).

reg1({_,g,_} = Key, Value) ->
    %% anything global
    ?CHK_DIST,
    gproc_dist:reg(Key, Value);
reg1({p,l,_} = Key, Value) ->
    local_reg(Key, Value);
reg1({a,l,_} = Key, undefined) ->
    call({reg, Key, undefined});
reg1({c,l,_} = Key, Value) when is_integer(Value) ->
    call({reg, Key, Value});
reg1({n,l,_} = Key, Value) ->
    call({reg, Key, Value});
reg1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec reg_or_locate(Key::key(), Value) -> {pid(), NewValue}
%%
%% @doc Try registering a unique name, or return existing registration.
%%
%% This function tries to register the name `Key', if available.
%% If such a registration already exists, the pid and value of
%% the current registration is returned instead.
%% @end
reg_or_locate(Key, Value) ->
    ?CATCH_GPROC_ERROR(reg_or_locate1(Key, Value, self()), [Key, Value]).

%% @spec reg_or_locate(Key::key(), Value, Fun::fun()) -> {pid(), NewValue}
%%
%% @doc Spawn a process with a registered name, or return existing registration.
%%
%% This function checks whether a local name is registered; if not, it spawns
%% a new process (with `spawn(Fun)') and gives it the name.
%% The pid and value of the resulting registration is returned.
%%
%% This function is only available for local registration. While it could
%% theoretically be done in the global case, the spawning of a new process
%% on a remote node by the leader instance is more problematic.
%% @end
reg_or_locate({_,l,_} = Key, Value, F) when is_function(F, 0) ->
    ?CATCH_GPROC_ERROR(reg_or_locate1(Key, Value, F), [Key, Value, F]).

reg_or_locate1({_,g,_} = Key, Value, P) ->
    ?CHK_DIST,
    gproc_dist:reg_or_locate(Key, Value, P);
reg_or_locate1({n,l,_} = Key, Value, P) ->
    call({reg_or_locate, Key, Value, P});
reg_or_locate1(_, _, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec reg_shared(Key::key()) -> true
%%
%% @doc Register a resource, but don't tie it to a particular process.
%%
%% `reg_shared({c,l,C}) -> reg_shared({c,l,C}, 0).'
%% `reg_shared({a,l,A}) -> reg_shared({a,l,A}, undefined).'
%% @end
reg_shared(Key) ->
    ?CATCH_GPROC_ERROR(reg_shared1(Key), [Key]).

reg_shared1({c,_,_} = Key) ->
    reg_shared(Key, 0);
reg_shared1({a,_,_} = Key) ->
    reg_shared(Key, undefined).


%% @spec reg_shared(Key::key(), Value) -> true
%%
%% @doc Register a resource, but don't tie it to a particular process.
%%
%% Shared resources are all unique. They remain until explicitly unregistered
%% (using {@link unreg_shared/1}). The types of shared resources currently
%% supported are `counter' and `aggregated counter'. In listings and query
%% results, shared resources appear as other similar resources, except that
%% `Pid == shared'. To wit, update_counter({c,l,myCounter}, 1, shared) would
%% increment the shared counter `myCounter' with 1, provided it exists.
%%
%% A shared aggregated counter will track updates in exactly the same way as
%% an aggregated counter which is owned by a process.
%% @end
%%
reg_shared(Key, Value) ->
    ?CATCH_GPROC_ERROR(reg_shared1(Key, Value), [Key, Value]).

reg_shared1({_,g,_} = Key, Value) ->
    %% anything global
    ?CHK_DIST,
    gproc_dist:reg_shared(Key, Value);
reg_shared1({a,l,_} = Key, undefined) ->
    call({reg_shared, Key, undefined});
reg_shared1({c,l,_} = Key, Value) when is_integer(Value) ->
    call({reg_shared, Key, Value});
reg_shared1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec mreg(type(), scope(), [{Key::any(), Value::any()}]) -> true
%%
%% @doc Register multiple {Key,Value} pairs of a given type and scope.
%%
%% This function is more efficient than calling {@link reg/2} repeatedly.
%% It is also atomic in regard to unique names; either all names are registered
%% or none are.
%% @end
mreg(T, C, KVL) ->
    ?CATCH_GPROC_ERROR(mreg1(T, C, KVL), [T, C, KVL]).

mreg1(T, g, KVL) ->
    ?CHK_DIST,
    gproc_dist:mreg(T, KVL);
mreg1(T, l, KVL) when T==a; T==n ->
    if is_list(KVL) ->
            call({mreg, T, l, KVL});
       true ->
            erlang:error(badarg)
    end;
mreg1(p, l, KVL) ->
    local_mreg(p, KVL);
mreg1(_, _, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec munreg(type(), scope(), [Key::any()]) -> true
%%
%% @doc Unregister multiple Key items of a given type and scope.
%%
%% This function is usually more efficient than calling {@link unreg/1}
%% repeatedly.
%% @end
munreg(T, C, L) ->
    ?CATCH_GPROC_ERROR(munreg1(T, C, L), [T, C, L]).

munreg1(T, g, L) ->
    ?CHK_DIST,
    gproc_dist:munreg(T, existing(T,g,L));
munreg1(T, l, L) when T==a; T==n ->
    if is_list(L) ->
            call({munreg, T, l, existing(T,l,L)});
       true ->
            erlang:error(badarg)
    end;
munreg1(p, l, L) ->
    local_munreg(p, existing(p,l,L));
munreg1(_, _, _) ->
    ?THROW_GPROC_ERROR(badarg).

existing(T,Scope,L) ->
    Keys = if T==p; T==c ->
                   [{{T,Scope,K}, self()} || K <- L];
              T==a; T==n ->
                   [{{T,Scope,K}, T} || K <- L]
           end,
    _ = [case ets:member(?TAB, K) of
             false -> erlang:error(badarg);
             true  -> true
         end || K <- Keys],
    L.


%% @spec (Key:: key()) -> true
%%
%% @doc Unregister a name or property.
%% @end
unreg(Key) ->
    ?CATCH_GPROC_ERROR(unreg1(Key), [Key]).

unreg1(Key) ->
    case Key of
        {_, g, _} ->
            ?CHK_DIST,
            gproc_dist:unreg(Key);
        {T, l, _} when T == n;
                       T == a -> call({unreg, Key});
        {_, l, _} ->
            case ets:member(?TAB, {Key,self()}) of
                true ->
                    _ = gproc_lib:remove_reg(Key, self(), unreg),
                    true;
                false ->
                    ?THROW_GPROC_ERROR(badarg)
            end
    end.

%% @spec (Key:: key()) -> true
%%
%% @doc Unregister a shared resource.
%% @end
unreg_shared(Key) ->
    ?CATCH_GPROC_ERROR(unreg_shared1(Key), [Key]).

unreg_shared1(Key) ->
    case Key of
        {_, g, _} ->
            ?CHK_DIST,
            gproc_dist:unreg_shared(Key);
        {T, l, _} when T == c;
                       T == a -> call({unreg_shared, Key});
        _ ->
	    ?THROW_GPROC_ERROR(badarg)
    end.

%% @spec (key(), pid()) -> yes | no
%%
%% @doc Behaviour support callback
%% @end
register_name({n,_,_} = Name, Pid) when Pid == self() ->
    try reg(Name), yes
    catch
	error:_ ->
	    no
    end.

%% @equiv unreg/1
unregister_name(Key) ->
    unreg(Key).

%% @spec select(Arg) -> [Match] | {[Match], Continuation} | '$end_of_table'
%%   where  Arg = Continuation
%%                | sel_pattern()
%%          Match = {Key, Pid, Value}
%% @doc Perform a select operation on the process registry
%%
%% When Arg = Contination, resume a gproc:select/1 operation
%% (see {@link //stdlib/ets:select/1. ets:select/1}
%%
%% When Arg = {@type sel_pattern()}, this function executes a select operation,
%% emulating ets:select/1
%%
%% {@link select/2} offers the opportunity to narrow the search
%% (by limiting to only global or local scope, or a single type of object).
%% When only a pattern as single argument is given, both global and local scope,
%% as well as all types of object can be searched. Note that the pattern may
%% still limit the select operation so that scanning the entire table is avoided.
%%
%% The physical representation in the registry may differ from the above,
%% but the select patterns are transformed appropriately. The logical
%% representation for the gproc select operations is given by
%% {@type headpat()}.
%% @end
select({?TAB, _, _, _, _, _, _, _} = Continuation) ->
    ets:select(Continuation);
select(Pat) ->
    select(all, Pat).

%% @spec (Context::context(), Pat::sel_pattern()) -> [{Key, Pid, Value}]
%%
%% @doc Perform a select operation with limited context on the process registry
%%
%% The physical representation in the registry may differ from the above,
%% but the select patterns are transformed appropriately.
%%
%% Note that limiting the context is just a convenience function, allowing you
%% to write a simpler select pattern and still avoid searching the entire
%% registry. Whenever variables are used in the head pattern, this will result
%% in a wider scan, even if the values are restricted through a guard (e.g.
%% <code>select([{'$1','$2','$3'}, [{'==', '$1', p}], ...])</code> will count as a wild
%% pattern on the key and result in a full scan). In this case, specifying a
%% Context will allow gproc to perform some variable substitution and ensure
%% that the scan is limited.
%% @end
select(Context, Pat) ->
    ets:select(?TAB, pattern(Pat, Context)).

%% @spec (Context::context(), Pat::sel_patten(), Limit::integer()) ->
%%          {[Match],Continuation} | '$end_of_table'
%% @doc Like {@link select/2} but returns Limit objects at a time.
%%
%% See [http://www.erlang.org/doc/man/ets.html#select-3].
%% @end
select(Context, Pat, Limit) ->
    ets:select(?TAB, pattern(Pat, Context), Limit).


%% @spec (sel_pattern()) -> list(sel_object())
%% @doc
%% @equiv select_count(all, Pat)
%% @end
select_count(Pat) ->
    select_count(all, Pat).

%% @spec (Context::context(), Pat::sel_pattern()) -> [{Key, Pid, Value}]
%%
%% @doc Perform a select_count operation on the process registry.
%%
%% The physical representation in the registry may differ from the above,
%% but the select patterns are transformed appropriately.
%% @end
select_count(Context, Pat) ->
    ets:select_count(?TAB, pattern(Pat, Context)).


%%% Local properties can be registered in the local process, since
%%% no other process can interfere.
%%%
local_reg(Key, Value) ->
    case gproc_lib:insert_reg(Key, Value, self(), l) of
        false -> ?THROW_GPROC_ERROR(badarg);
        true  -> monitor_me()
    end.

local_mreg(_, []) -> true;
local_mreg(T, [_|_] = KVL) ->
    case gproc_lib:insert_many(T, l, KVL, self()) of
        false     -> ?THROW_GPROC_ERROR(badarg);
        {true,_}  -> monitor_me()
    end.

local_munreg(T, L) when T==p; T==c ->
    _ = [gproc_lib:remove_reg({T,l,K}, self(), unreg) || K <- L],
    true.

%% @spec (Key :: key(), Value) -> true
%% @doc Sets the value of the registeration entry given by Key
%%
%% Key is assumed to exist and belong to the calling process.
%% If it doesn't, this function will exit.
%%
%% Value can be any term, unless the object is a counter, in which case
%% it must be an integer.
%% @end
%%
set_value(Key, Value) ->
    ?CATCH_GPROC_ERROR(set_value1(Key, Value), [Key, Value]).

set_value1({_,g,_} = Key, Value) ->
    ?CHK_DIST,
    gproc_dist:set_value(Key, Value);
set_value1({a,l,_} = Key, Value) when is_integer(Value) ->
    call({set, Key, Value});
set_value1({n,l,_} = Key, Value) ->
    %% we cannot do this locally, since we have to check that the object
    %% exists first - not an atomic update.
    call({set, Key, Value});
set_value1({p,l,_} = Key, Value) ->
    %% we _can_ to this locally, since there is no race condition - no
    %% other process can update our properties.
    case gproc_lib:do_set_value(Key, Value, self()) of
        true -> true;
        false ->
            erlang:error(badarg)
    end;
set_value1({c,l,_} = Key, Value) when is_integer(Value) ->
    gproc_lib:do_set_counter_value(Key, Value, self());
set_value1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec (Key) -> Value
%% @doc Reads the value stored with a key registered to the current process.
%%
%% If no such key is registered to the current process, this function exits.
%% @end
get_value(Key) ->
    ?CATCH_GPROC_ERROR(get_value1(Key, self()), [Key]).

%% @spec (Key, Pid) -> Value
%% @doc Reads the value stored with a key registered to the process Pid.
%%
%% If `Pid == shared', the value of a shared key (see {@link reg_shared/1})
%% will be read.
%% @end
%%
get_value(Key, Pid) ->
    ?CATCH_GPROC_ERROR(get_value1(Key, Pid), [Key, Pid]).

get_value1({T,_,_} = Key, Pid) when is_pid(Pid) ->
    if T==n orelse T==a ->
            case ets:lookup(?TAB, {Key, T}) of
                [{_, P, Value}] when P == Pid -> Value;
                _ -> ?THROW_GPROC_ERROR(badarg)
            end;
       true ->
            ets:lookup_element(?TAB, {Key, Pid}, 3)
    end;
get_value1({T,_,_} = K, shared) when T==c; T==a ->
    Key = case T of
	      c -> {K, shared};
	      a -> {K, a}
	  end,
    case ets:lookup(?TAB, Key) of
	[{_, shared, Value}] -> Value;
	_ -> ?THROW_GPROC_ERROR(badarg)
    end;
get_value1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).


%% @spec (Key) -> Pid
%% @doc Lookup the Pid stored with a key.
%%
lookup_pid({_T,_,_} = Key) ->
    case where(Key) of
        undefined -> erlang:error(badarg);
        P -> P
    end.

%% @spec (Key) -> Value
%% @doc Lookup the value stored with a key.
%%
lookup_value({T,_,_} = Key) ->
    if T==n orelse T==a ->
            ets:lookup_element(?TAB, {Key,T}, 3);
       true ->
            erlang:error(badarg)
    end.

%% @spec (Key::key()) -> pid()
%%
%% @doc Returns the pid registered as Key
%%
%% The type of registration entry must be either name or aggregated counter.
%% Otherwise this function will exit. Use {@link lookup_pids/1} in these
%% cases.
%% @end
%%
where(Key) ->
    ?CATCH_GPROC_ERROR(where1(Key), [Key]).

where1({T,_,_}=Key) ->
    if T==n orelse T==a ->
            case ets:lookup(?TAB, {Key,T}) of
                [{_, P, _Value}] ->
                    case my_is_process_alive(P) of
                        true -> P;
                        false ->
                            undefined
                    end;
                _ ->  % may be [] or [{Key,Waiters}]
                    undefined
            end;
       true ->
            ?THROW_GPROC_ERROR(badarg)
    end.

%% @equiv where/1
whereis_name(Key) ->
    ?CATCH_GPROC_ERROR(where1(Key), [Key]).

%% @spec (Key::key()) -> [pid()]
%%
%% @doc Returns a list of pids with the published key Key
%%
%% If the type of registration entry is either name or aggregated counter,
%% this function will return either an empty list, or a list of one pid.
%% For non-unique types, the return value can be a list of any length.
%% @end
%%
lookup_pids({T,_,_} = Key) ->
    L = if T==n orelse T==a ->
                ets:select(?TAB, [{{{Key,T}, '$1', '_'},[],['$1']}]);
           true ->
                ets:select(?TAB, [{{{Key,'_'}, '$1', '_'},[],['$1']}])
        end,
    [P || P <- L, my_is_process_alive(P)].


%% @spec (pid()) -> boolean()
%%
my_is_process_alive(P) when node(P) =:= node() ->
    is_process_alive(P);
my_is_process_alive(_) ->
    %% remote pid - assume true (too costly to find out)
    true.

%% @spec (Key::key()) -> [{pid(), Value}]
%%
%% @doc Retrieve the `{Pid,Value}' pairs corresponding to Key.
%%
%% Key refer to any type of registry object. If it refers to a unique
%% object, the list will be of length 0 or 1. If it refers to a non-unique
%% object, the return value can be a list of any length.
%% @end
%%
lookup_values({T,_,_} = Key) ->
    L = if T==n orelse T==a ->
                ets:select(?TAB, [{{{Key,T}, '$1', '$2'},[],[{{'$1','$2'}}]}]);
           true ->
                ets:select(?TAB, [{{{Key,'_'}, '$1', '$2'},[],[{{'$1','$2'}}]}])
        end,
    [Pair || {P,_} = Pair <- L, my_is_process_alive(P)].

%% @ spec (Key::key(), Incr) -> integer() | [integer()]
%%    Incr = IncrVal | UpdateOp | [UpdateOp]
%%    UpdateOp = IncrVal | {IncrVal, Threshold, SetValue}
%%    IncrVal = integer()
%%
%% @doc Updates the counter registered as Key for the current process.
%%
%% This function works almost exactly like ets:update_counter/3
%% (see [http://www.erlang.org/doc/man/ets.html#update_counter-3]), but
%% will fail if the type of object referred to by Key is not a counter.
%%
%% Aggregated counters with the same name will be updated automatically.
%% The `UpdateOp' patterns are the same as for `ets:update_counter/3', except
%% that the position is omitted; in gproc, the value position is always `3'.
%% @end
%%
-spec update_counter(key(), increment()) -> integer().
update_counter(Key, Incr) ->
    ?CATCH_GPROC_ERROR(update_counter1(Key, Incr), [Key, Incr]).

update_counter1({c,l,_} = Key, Incr) ->
    gproc_lib:update_counter(Key, Incr, self());
update_counter1({c,g,_} = Key, Incr) ->
    ?CHK_DIST,
    gproc_dist:update_counter(Key, Incr);
update_counter1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @doc Update a list of counters
%%
%% This function is not atomic, except (in a sense) for global counters. For local counters,
%% it is more of a convenience function. For global counters, it is much more efficient
%% than calling `gproc:update_counter/2' for each individual counter.
%%
%% The return value is the corresponding list of `[{Counter, Pid, NewValue}]'.
%% @end
-spec update_counters(scope(), [{key(), pid(), increment()}]) ->
			     [{key(), pid(), integer()}].
update_counters(_, []) ->
    [];
update_counters(l, [_|_] = Cs) ->
    ?CATCH_GPROC_ERROR(update_counters1(Cs), [Cs]);
update_counters(g, [_|_] = Cs) ->
    ?CHK_DIST,
    gproc_dist:update_counters(Cs).


update_counters1([{{c,l,_} = Key, Pid, Incr}|T]) ->
    [{Key, Pid, gproc_lib:update_counter(Key, Incr, Pid)}|update_counters1(T)];
update_counters1([]) ->
    [];
update_counters1(_) ->
    ?THROW_GPROC_ERROR(badarg).




%% @spec (Key) -> {ValueBefore, ValueAfter}
%%   Key   = {c, Scope, Name}
%%   Scope = l | g
%%   ValueBefore = integer()
%%   ValueAfter  = integer()
%%
%% @doc Reads and resets a counter in a "thread-safe" way
%%
%% This function reads the current value of a counter and then resets it to its
%% initial value. The reset operation is done using {@link update_counter/2},
%% which allows for concurrent calls to {@link update_counter/2} without losing
%% updates. Aggregated counters are updated accordingly.
%% @end
%%
reset_counter(Key) ->
    ?CATCH_GPROC_ERROR(reset_counter1(Key), [Key]).

reset_counter1({c,g,_} = Key) ->
    ?CHK_DIST,
    gproc_dist:reset_counter(Key);
reset_counter1({c,l,_} = Key) ->
    Current = ets:lookup_element(?TAB, {Key, self()}, 3),
    Initial = case ets:lookup(?TAB, {self(), Key}) of
		  [{_, r}] -> 0;
		  [{_, Opts}] ->
		      proplists:get_value(initial, Opts, 0)
	      end,
    {Current, update_counter(Key, Initial - Current)}.

%% @spec (Key::key(), Incr) -> integer() | [integer()]
%%    Incr = IncrVal | UpdateOp | [UpdateOp]
%%    UpdateOp = IncrVal | {IncrVal, Threshold, SetValue}
%%    IncrVal = integer()
%%
%% @doc Updates the shared counter registered as Key.
%%
%% This function works almost exactly like ets:update_counter/3
%% (see [http://www.erlang.org/doc/man/ets.html#update_counter-3]), but
%% will fail if the type of object referred to by Key is not a counter.
%%
%% Aggregated counters with the same name will be updated automatically.
%% The `UpdateOp' patterns are the same as for `ets:update_counter/3', except
%% that the position is omitted; in gproc, the value position is always `3'.
%% @end
%%
update_shared_counter(Key, Incr) ->
    ?CATCH_GPROC_ERROR(update_shared_counter1(Key, Incr), [Key, Incr]).

update_shared_counter1({c,g,_} = Key, Incr) ->
    ?CHK_DIST,
    gproc_dist:update_shared_counter(Key, Incr);
update_shared_counter1({c,l,_} = Key, Incr) ->
    gproc_lib:update_counter(Key, Incr, shared).

%% @spec (From::key(), To::pid() | key()) -> undefined | pid()
%%
%% @doc Atomically transfers the key `From' to the process identified by `To'.
%%
%% This function transfers any gproc key (name, property, counter, aggr counter)
%% from one process to another, and returns the pid of the new owner.
%%
%% `To' must be either a pid or a unique name (name or aggregated counter), but
%% does not necessarily have to resolve to an existing process. If there is
%% no process registered with the `To' key, `give_away/2' returns `undefined',
%% and the `From' key is effectively unregistered.
%%
%% It is allowed to give away a key to oneself, but of course, this operation
%% will have no effect.
%%
%% Fails with `badarg' if the calling process does not have a `From' key
%% registered.
%% @end
give_away(Key, ToPid) ->
    ?CATCH_GPROC_ERROR(give_away1(Key, ToPid), [Key, ToPid]).

give_away1({_,l,_} = Key, ToPid) when is_pid(ToPid), node(ToPid) == node() ->
    call({give_away, Key, ToPid});
give_away1({_,l,_} = Key, {n,l,_} = ToKey) ->
    call({give_away, Key, ToKey});
give_away1({_,g,_} = Key, To) ->
    ?CHK_DIST,
    gproc_dist:give_away(Key, To).

%% @spec () -> ok
%%
%% @doc Unregister all items of the calling process and inform gproc
%% to forget about the calling process.
%%
%% This function is more efficient than letting gproc perform these
%% cleanup operations.
%% @end
goodbye() ->
    process_is_down(self()).

%% @spec (Key::key(), Msg::any()) -> Msg
%%
%% @doc Sends a message to the process, or processes, corresponding to Key.
%%
%% If Key belongs to a unique object (name or aggregated counter), this
%% function will send a message to the corresponding process, or fail if there
%% is no such process. If Key is for a non-unique object type (counter or
%% property), Msg will be send to all processes that have such an object.
%% @end
%%
send(Key, Msg) ->
    ?CATCH_GPROC_ERROR(send1(Key, Msg), [Key, Msg]).

send1({T,C,_} = Key, Msg) when C==l; C==g ->
    if T == n orelse T == a ->
            case ets:lookup(?TAB, {Key, T}) of
                [{_, Pid, _}] ->
                    Pid ! Msg;
                _ ->
                    ?THROW_GPROC_ERROR(badarg)
            end;
       T==p orelse T==c ->
            %% BUG - if the key part contains select wildcards, we may end up
            %% sending multiple messages to the same pid
            lists:foreach(fun(Pid) ->
                                  Pid ! Msg
                          end, lookup_pids(Key)),
            Msg;
       true ->
            erlang:error(badarg)
    end;
send1(_, _) ->
    ?THROW_GPROC_ERROR(badarg).

%% @spec (Key::key(), Msg::any()) -> Msg
%%
%% @equiv bcast(nodes(), Key, Msg)
%% @end
%%
bcast(Key, Msg) ->
    bcast(nodes(), Key, Msg).

%% @spec (Nodes::[atom()], Key::key(), Msg::any()) -> Msg
%%
%% @doc Sends a message to processes corresponding to Key on Nodes.
%%
%% This function complements `send/2' and works on locally registered resources
%% that `send/2' supports. Messages are routed via a special broadcast server
%% on each node to ensure that ordering is preserved. Distributed delivery
%% is asynchronous and carries the same guarantees as normal message passing
%% (with the added proviso that the broadcast server also needs to be available).
%% @see send/2
%% @end
%%
bcast(Ns, Key, Msg) ->
    ?CATCH_GPROC_ERROR(bcast1(Ns, Key, Msg), [Key, Msg]).

bcast1(Ns, {T,l,_} = Key, Msg) when T==p; T==a; T==c; T==n; T==p ->
    send1(Key, Msg),
    gen_server:abcast(Ns -- [node()], gproc_bcast, {send, Key, Msg}),
    Msg.


%% @spec (Context :: context()) -> key() | '$end_of_table'
%%
%% @doc Behaves as ets:first(Tab) for a given type of registration object.
%%
%% See [http://www.erlang.org/doc/man/ets.html#first-1].
%%  The registry behaves as an ordered_set table.
%% @end
%%
first(Context) ->
    {S, T} = get_s_t(Context),
    {HeadPat,_} = headpat({S, T}, '_', '_', '_'),
    case ets:select(?TAB, [{HeadPat,[],[{element,1,'$_'}]}], 1) of
        {[First], _} ->
            First;
        _ ->
            '$end_of_table'
    end.

%% @spec (Context :: context()) -> key() | '$end_of_table'
%%
%% @doc Behaves as ets:last(Tab) for a given type of registration object.
%%
%% See [http://www.erlang.org/doc/man/ets.html#last-1].
%% The registry behaves as an ordered_set table.
%% @end
%%
last(Context) ->
    {S, T} = get_s_t(Context),
    S1 = if S == '_'; S == l -> m;              % 'm' comes after 'l'
            S == g -> h                         % 'h' comes between 'g' & 'l'
         end,
    Beyond = {{T,S1,[]},[]},
    step(ets:prev(?TAB, Beyond), S, T).


%% @spec (Context::context(), Key::key()) -> key() | '$end_of_table'
%%
%% @doc Behaves as ets:next(Tab,Key) for a given type of registration object.
%%
%% See [http://www.erlang.org/doc/man/ets.html#next-2].
%% The registry behaves as an ordered_set table.
%% @end
%%
next(Context, K) ->
    {S,T} = get_s_t(Context),
    step(ets:next(?TAB,K), S, T).

%% @spec (Context::context(), Key::key()) -> key() | '$end_of_table'
%%
%% @doc Behaves as ets:prev(Tab,Key) for a given type of registration object.
%%
%% See [http://www.erlang.org/doc/man/ets.html#prev-2].
%% The registry behaves as an ordered_set table.
%% @end
%%
prev(Context, K) ->
    {S, T} = get_s_t(Context),
    step(ets:prev(?TAB, K), S, T).

step(Key, '_', '_') ->
    case Key of
        {{_,_,_},_} -> Key;
        _ -> '$end_of_table'
    end;
step(Key, '_', T) ->
    case Key of
        {{T,_,_},_} -> Key;
        _ -> '$end_of_table'
    end;
step(Key, S, '_') ->
    case Key of
        {{_, S, _}, _} -> Key;
        _ -> '$end_of_table'
    end;
step(Key, S, T) ->
    case Key of
        {{T, S, _}, _} -> Key;
        _ -> '$end_of_table'
    end.


%% @spec (Pid::pid()) -> ProcessInfo
%% ProcessInfo = [{gproc, [{Key,Value}]} | ProcessInfo]
%%
%% @doc Similar to `process_info(Pid)' but with additional gproc info.
%%
%% Returns the same information as process_info(Pid), but with the
%% addition of a `gproc' information item, containing the `{Key,Value}'
%% pairs registered to the process.
%% @end
info(Pid) when is_pid(Pid) ->
    Items = [?MODULE | [ I || {I,_} <- process_info(self())]],
    [info(Pid,I) || I <- Items].

%% @spec (Pid::pid(), Item::atom()) -> {Item, Info}
%%
%% @doc Similar to process_info(Pid, Item), but with additional gproc info.
%%
%% For `Item = gproc', this function returns a list of `{Key, Value}' pairs
%% registered to the process Pid. For other values of Item, it returns the
%% same as [http://www.erlang.org/doc/man/erlang.html#process_info-2].
%% @end
info(Pid, gproc) ->
    gproc_info(Pid, '_');
info(Pid, {gproc, Pat}) ->
    gproc_info(Pid, Pat);
info(Pid, current_function) ->
    {_, T} = process_info(Pid, backtrace),
    info_cur_f(T, process_info(Pid, current_function));
info(Pid, I) ->
    process_info(Pid, I).

%% We don't want to return the internal gproc:info() function as current
%% function, so we grab the 'backtrace' and extract the call stack from it,
%% filtering out the functions gproc:info/_ and gproc:'-info/1-lc...' entries.
%%
%% This is really an indication that wrapping the process_info() BIF was a
%% bad idea to begin with... :P
%%
info_cur_f(T, Default) ->
    {match, Matches} = re:run(T,<<"\\(([^\\)]+):(.+)/([0-9]+)">>,
			      [global,{capture,[1,2,3],list}]),
    case lists:dropwhile(fun(["gproc","info",_]) -> true;
			    (["gproc","'-info/1-lc" ++ _, _]) -> true;
			    (_) -> false
			 end, Matches) of
	[] ->
	    Default;
	[[M,F,A]|_] ->
	    {current_function,
	     {to_atom(M), to_atom(F), list_to_integer(A)}}
    end.

to_atom(S) ->
    case erl_scan:string(S) of
	{ok, [{atom,_,A}|_],_} ->
	    A;
	_ ->
	    list_to_atom(S)
    end.

gproc_info(Pid, Pat) ->
    Keys = ets:select(?TAB, [{ {{Pid,Pat}, '_'}, [], [{element,2,
						       {element,1,'$_'}}] }]),
    {?MODULE, lists:zf(
                fun(K) ->
                        try V = get_value(K, Pid),
			      {true, {K,V}}
                        catch
                            error:_ ->
                                false
                        end
                end, Keys)}.

%% @spec () -> ok
%%
%% @doc Similar to the built-in shell command `i()' but inserts information
%% about names and properties registered in Gproc, where applicable.
%% @end
i() ->
    gproc_info:i().

%%% ==========================================================

%% @hidden
handle_cast({monitor_me, Pid}, S) ->
    erlang:monitor(process, Pid),
    {noreply, S};
handle_cast({audit_process, Pid}, S) ->
    case is_process_alive(Pid) of
	false ->
	    process_is_down(Pid);
	true ->
	    ignore
    end,
    {noreply, S};
handle_cast({cancel_wait, Pid, {T,_,_} = Key, Ref}, S) ->
     _ = case ets:lookup(?TAB, {Key,T}) of
	     [{_, Waiters}] ->
		 gproc_lib:remove_wait(Key, Pid, Ref, Waiters);
	     _ ->
		 ignore
	 end,
    {noreply, S};
handle_cast({cancel_wait_or_monitor, Pid, {T,_,_} = Key}, S) ->
    _ = case ets:lookup(?TAB, {Key, T}) of
	    [{_, Waiters}] ->
		gproc_lib:remove_wait(Key, Pid, all, Waiters);
	    [{_, OtherPid, _}] ->
		gproc_lib:remove_monitors(Key, OtherPid, Pid);
	    _ ->
		ok
	end,
    {noreply, S}.

%% @hidden
handle_call({reg, {_T,l,_} = Key, Val}, {Pid,_}, S) ->
    case try_insert_reg(Key, Val, Pid) of
        true ->
            _ = gproc_lib:ensure_monitor(Pid,l),
            {reply, true, S};
        false ->
            {reply, badarg, S}
    end;
handle_call({reg_or_locate, {T,l,_} = Key, Val, P}, _, S) ->
    case ets:lookup(?TAB, {Key, T}) of
	[] ->
	    Pid = if is_function(P, 0) ->
			  spawn(P);
		     is_pid(P) ->
			  P
		  end,
	    true = gproc_lib:insert_reg(Key, Val, Pid, l),
	    _ = gproc_lib:ensure_monitor(Pid, l),
	    {reply, {Pid, Val}, S};
	[{_, OtherPid, OtherValue}] ->
	    {reply, {OtherPid, OtherValue}, S}
    end;
handle_call({monitor, {T,l,_} = Key, Pid}, _From, S)
  when T==n; T==a ->
    Ref = make_ref(),
    _ = case where(Key) of
	    undefined ->
		Pid ! {gproc, unreg, Ref, Key};
	    RegPid ->
		case ets:lookup(?TAB, {RegPid, Key}) of
		    [{K,r}] ->
			ets:insert(?TAB, {K, [{monitor, [{Pid,Ref}]}]});
		    [{K, Opts}] ->
			ets:insert(?TAB, {K, gproc_lib:add_monitor(Opts, Pid, Ref)})
		end
	end,
    {reply, Ref, S};
handle_call({demonitor, {T,l,_} = Key, Ref, Pid}, _From, S)
  when T==n; T==a ->
    _ = case where(Key) of
	    undefined ->
		ok;  % be nice
	    RegPid ->
		case ets:lookup(?TAB, {RegPid, Key}) of
		    [{_K,r}] ->
			ok;   % be nice
		    [{K, Opts}] ->
			ets:insert(?TAB, {K, gproc_lib:remove_monitor(
					       Opts, Pid, Ref)})
		end
	end,
    {reply, ok, S};
handle_call({reg_shared, {_T,l,_} = Key, Val}, _From, S) ->
    case try_insert_reg(Key, Val, shared) of
    %% case try_insert_shared(Key, Val) of
	true ->
	    {reply, true, S};
	false ->
	    {reply, badarg, S}
    end;
handle_call({unreg, {_,l,_} = Key}, {Pid,_}, S) ->
    case ets:lookup(?TAB, {Pid,Key}) of
        [{_, r}] ->
            _ = gproc_lib:remove_reg(Key, Pid, unreg),
            {reply, true, S};
        [{_, Opts}] when is_list(Opts) ->
            _ = gproc_lib:remove_reg(Key, Pid, unreg, Opts),
            {reply, true, S};
        [] ->
            {reply, badarg, S}
    end;
handle_call({unreg_shared, {_,l,_} = Key}, _, S) ->
    _ = case ets:lookup(?TAB, {shared, Key}) of
	    [{_, r}] ->
		_ = gproc_lib:remove_reg(Key, shared, unreg);
	    [{_, Opts}] ->
		_ = gproc_lib:remove_reg(Key, shared, unreg, Opts);
	    [] ->
		%% don't crash if shared key already unregged.
		ok
	end,
    {reply, true, S};
handle_call({await, {_,l,_} = Key, Pid}, From, S) ->
    %% Passing the pid explicitly is needed when leader_call is used,
    %% since the Pid given as From in the leader is the local gen_leader
    %% instance on the calling node.
    case gproc_lib:await(Key, Pid, From) of
        noreply ->
            {noreply, S};
        {reply, Reply, _} ->
            {reply, Reply, S}
    end;
handle_call({mreg, T, l, L}, {Pid,_}, S) ->
    try gproc_lib:insert_many(T, l, L, Pid) of
        {true,_} -> {reply, true, S};
        false    -> {reply, badarg, S}
    catch
        error:_  -> {reply, badarg, S}
    end;
handle_call({munreg, T, l, L}, {Pid,_}, S) ->
    _ = gproc_lib:remove_many(T, l, L, Pid),
    {reply, true, S};
handle_call({set, {_,l,_} = Key, Value}, {Pid,_}, S) ->
    case gproc_lib:do_set_value(Key, Value, Pid) of
        true ->
            {reply, true, S};
        false ->
            {reply, badarg, S}
    end;
handle_call({audit_process, Pid}, _, S) ->
    _ = case is_process_alive(Pid) of
	    false ->
		process_is_down(Pid);
	    true ->
		ignore
	end,
    {reply, ok, S};
handle_call({give_away, Key, To}, {Pid,_}, S) ->
    Reply = do_give_away(Key, To, Pid),
    {reply, Reply, S};
handle_call(_, _, S) ->
    {reply, badarg, S}.

%% @hidden
handle_info({'DOWN', _MRef, process, Pid, _}, S) ->
    _ = process_is_down(Pid),
    {noreply, S};
handle_info(_, S) ->
    {noreply, S}.


%% @hidden
code_change(_FromVsn, S, _Extra) ->
    %% We have changed local monitor markers from {Pid} to {Pid,l}.
    _ = case ets:select(?TAB, [{{'$1'},[],['$1']}]) of
            [] ->
                ok;
            Pids ->
                ets:insert(?TAB, [{P,l} || P <- Pids]),
                ets:select_delete(?TAB, [{{'_'},[],[true]}])
        end,
    {ok, S}.


%% @hidden
terminate(_Reason, _S) ->
    ok.


call(Req) ->
    call(Req, l).

call(Req, l) ->
    chk_reply(gen_server:call(?MODULE, Req));
call(Req, g) ->
    chk_reply(gproc_dist:leader_call(Req)).

call(N, Req, l) ->
    chk_reply(gen_server:call({?MODULE, N}, Req));
call(undefined, Req, g) ->
    %% we always call the leader
    chk_reply(gproc_dist:leader_call(Req)).


chk_reply(Reply) ->
    case Reply of
        badarg -> ?THROW_GPROC_ERROR(badarg);
        _  -> Reply
    end.


cast(Msg) ->
    cast(Msg, l).

cast(Msg, l) ->
    gen_server:cast(?MODULE, Msg);
cast(Msg, g) ->
    gproc_dist:leader_cast(Msg).

cast(N, Msg, l) ->
    gen_server:cast({?MODULE, N}, Msg).

try_insert_reg({T,l,_} = Key, Val, Pid) ->
    case gproc_lib:insert_reg(Key, Val, Pid, l) of
        false ->
            case ets:lookup(?TAB, {Key,T}) of
                %% In this particular case, the lookup cannot result in
                %% [{_, Waiters}], since the insert_reg/4 function would
                %% have succeeded then.
                [{_, OtherPid, _}] ->
                    case is_process_alive(OtherPid) of
                        true ->
                            false;
                        false ->
                            process_is_down(OtherPid),
                            true = gproc_lib:insert_reg(Key, Val, Pid, l)
                    end;
                [] ->
                    false
            end;
        true ->
            true
    end.


%% try_insert_shared({c,l,_} = Key, Val) ->
%%     ets:insert_new(?TAB, [{{Key,shared}, shared, Val}, {{shared, Key}, []}]);
%% try_insert_shared({a,l,_} = Key, Val) ->
%%     ets:insert_new(?TAB, [{{Key, a}, shared, Val}, {{shared, Key}, []}]).

-spec audit_process(pid()) -> ok.

audit_process(Pid) when is_pid(Pid) ->
    ok = gen_server:call(gproc, {audit_process, Pid}, infinity).

nb_audit_process(Pid) when is_pid(Pid) ->
    ok = gen_server:cast(gproc, {audit_process, Pid}).

-spec process_is_down(pid()) -> ok.

process_is_down(Pid) when is_pid(Pid) ->
    %% delete the monitor marker
    %% io:fwrite(user, "process_is_down(~p) - ~p~n", [Pid,ets:tab2list(?TAB)]),
    Marker = {Pid,l},
    case ets:member(?TAB, Marker) of
        false ->
            ok;
        true ->
            Revs = ets:select(?TAB, [{{{Pid,'$1'}, '$2'},
                                      [{'==',{element,2,'$1'},l}],
				      [{{'$1','$2'}}]}]),
            lists:foreach(
              fun({{n,l,_}=K, R}) ->
                      Key = {K,n},
                      case ets:lookup(?TAB, Key) of
                          [{_, Pid, _}] ->
                              ets:delete(?TAB, Key),
			      opt_notify(R, K);
                          [{_, Waiters}] ->
                              case [W || {P,_} = W <- Waiters,
                                         P =/= Pid] of
                                  [] ->
                                      ets:delete(?TAB, Key);
                                  Waiters1 ->
                                      ets:insert(?TAB, {Key, Waiters1})
                              end;
                          [] ->
                              true
                      end;
                 ({{c,l,C} = K, _}) ->
                      Key = {K, Pid},
                      [{_, _, Value}] = ets:lookup(?TAB, Key),
                      ets:delete(?TAB, Key),
                      gproc_lib:update_aggr_counter(l, C, -Value);
                 ({{a,l,_} = K, R}) ->
                      ets:delete(?TAB, {K,a}),
		      opt_notify(R, K);
                 ({{p,_,_} = K, _}) ->
                      ets:delete(?TAB, {K, Pid})
              end, Revs),
            ets:select_delete(?TAB, [{{{Pid,{'_',l,'_'}},'_'}, [], [true]}]),
            ets:delete(?TAB, Marker),
            ok
    end.

opt_notify(r, _) ->
    ok;
opt_notify(Opts, Key) ->
    gproc_lib:notify(Key, Opts).


do_give_away({T,l,_} = K, To, Pid) when T==n; T==a ->
    Key = {K, T},
    case ets:lookup(?TAB, Key) of
        [{_, Pid, Value}] ->
            %% Pid owns the reg; allowed to give_away
            case pid_to_give_away_to(To) of
                Pid ->
                    %% Give away to ourselves? Why not? We'll allow it,
                    %% but nothing needs to be done.
                    Pid;
                ToPid when is_pid(ToPid) ->
                    ets:insert(?TAB, [{Key, ToPid, Value},
                                      {{ToPid, K}, []}]),
		    _ = gproc_lib:remove_reverse_mapping({migrated,ToPid}, Pid, K),
                    _ = gproc_lib:ensure_monitor(ToPid, l),
                    ToPid;
                undefined ->
                    _ = gproc_lib:remove_reg(K, Pid, unreg),
                    undefined
            end;
        _ ->
            badarg
    end;
do_give_away({T,l,_} = K, To, Pid) when T==c; T==p ->
    Key = {K, Pid},
    case ets:lookup(?TAB, Key) of
        [{_, Pid, Value}] ->
            case pid_to_give_away_to(To) of
                ToPid when is_pid(ToPid) ->
                    ToKey = {K, ToPid},
                    case ets:member(?TAB, ToKey) of
                        true ->
                            badarg;
                        false ->
                            ets:insert(?TAB, [{ToKey, ToPid, Value},
                                              {{ToPid, K}, []}]),
                            ets:delete(?TAB, {Pid, K}),
                            ets:delete(?TAB, Key),
                            _ = gproc_lib:ensure_monitor(ToPid, l),
                            ToPid
                    end;
                undefined ->
                    _ = gproc_lib:remove_reg(K, Pid, {migrated, undefined}),
                    undefined
            end;
        _ ->
            badarg
    end.


pid_to_give_away_to(P) when is_pid(P), node(P) == node() ->
    P;
pid_to_give_away_to({T,l,_} = Key) when T==n; T==a ->
    case ets:lookup(?TAB, {Key, T}) of
        [{_, Pid, _}] ->
            Pid;
        _ ->
            undefined
    end.

create_tabs() ->
    Opts = gproc_lib:valid_opts(ets_options, [{write_concurrency,true},
					      {read_concurrency, true}]),
    case ets:info(?TAB, name) of
        undefined ->
            ets:new(?TAB, [ordered_set, public, named_table | Opts]);
        _ ->
            ok
    end.

%% @hidden
init([]) ->
    set_monitors(),
    {ok, #state{}}.


set_monitors() ->
    set_monitors(ets:select(?TAB, [{{{'$1',l}},[],['$1']}], 100)).


set_monitors('$end_of_table') ->
    ok;
set_monitors({Pids, Cont}) ->
    _ = [erlang:monitor(process,Pid) || Pid <- Pids],
    set_monitors(ets:select(Cont)).

monitor_me() ->
    case ets:insert_new(?TAB, {{self(),l}}) of
        false -> true;
        true  ->
            cast({monitor_me,self()}),
            true
    end.




pattern([{'_', Gs, As}], T) ->
    ?l,
    {HeadPat, Vs} = headpat(T, '$1', '$2', '$3'),
    [{HeadPat, rewrite(Gs,Vs), rewrite(As,Vs)}];
pattern([{{A,B,C},Gs,As}], Scope) ->
    ?l,
    {HeadPat, Vars} = headpat(Scope, A,B,C),
    [{HeadPat, rewrite(Gs,Vars), rewrite(As,Vars)}];
pattern([{Head, Gs, As}], Scope) ->
    ?l,
    {S, T} = get_s_t(Scope),
    case is_var(Head) of
        {true,_N} ->
            HeadPat = {{{T,S,'_'},'_'},'_','_'},
            Vs = [{Head, obj_prod()}],
            %% the headpat function should somehow verify that Head is
            %% consistent with Scope (or should we add a guard?)
            [{HeadPat, rewrite(Gs, Vs), rewrite(As, Vs)}];
        false ->
            erlang:error(badarg)
    end.

%% This is the expression to use in guards and the RHS to address the whole
%% object, in its logical representation.
obj_prod() ->
    {{ {element,1,{element,1,'$_'}},
       {element,2,'$_'},
       {element,3,'$_'} }}.

obj_prod_l() ->
    [ {element,1,{element,1,'$_'}},
      {element,2,'$_'},
      {element,3,'$_'} ].


headpat({S, T}, V1,V2,V3) ->
    headpat(type(T), scope(S), V1,V2,V3);
headpat(T, V1, V2, V3) when is_atom(T) ->
    headpat(type(T), l, V1, V2, V3);
headpat(_, _, _, _) -> erlang:error(badarg).

headpat(T, C, V1,V2,V3) ->
    Rf = fun(Pos) ->
                 {element,Pos,{element,1,{element,1,'$_'}}}
         end,
    K2 = if T==n orelse T==a -> T;
            true -> '_'
         end,
    {Kp,Vars} = case V1 of
                    {Vt,Vc,Vn} ->
                        ?l,
                        {T1,Vs1} = subst(T,Vt,fun() -> Rf(1) end,[]),
                        {C1,Vs2} = subst(C,Vc,fun() -> Rf(2) end,Vs1),
                        {{T1,C1,Vn}, Vs2};
                    '_' ->
                        ?l,
                        {{T,C,'_'}, []};
                    _ ->
                        ?l,
                        case is_var(V1) of
                            {true,_} ->
                                {{T,C,V1}, [{V1, {element,1,
                                                  {element,1,'$_'}}}]};
                            false ->
                                erlang:error(badarg)
                        end
                end,
    {{{Kp,K2},V2,V3}, Vars}.

%% l(L) -> L.



subst(X, '_', _F, Vs) ->
    {X, Vs};
subst(X, V, F, Vs) ->
    case is_var(V) of
        {true,_} ->
            {X, [{V,F()}|Vs]};
        false ->
            {V, Vs}
    end.

scope('_')    -> '_';
scope(all)    -> '_';
scope(global) -> g;
scope(local)  -> l;
scope(S) when S==l; S==g -> S.

type('_')   -> '_';
type(all)   -> '_';
type(T) when T==n; T==p; T==c; T==a -> T;
type(names) -> n;
type(props) -> p;
type(counters) -> c;
type(aggr_counters) -> a.

rev_keypat(Context) ->
    {S,T} = get_s_t(Context),
    {T,S,'_'}.

get_s_t({S,T}) -> {scope(S), type(T)};
get_s_t(T) when is_atom(T) ->
    {scope(all), type(T)}.

is_var('$1') -> {true,1};
is_var('$2') -> {true,2};
is_var('$3') -> {true,3};
is_var('$4') -> {true,4};
is_var('$5') -> {true,5};
is_var('$6') -> {true,6};
is_var('$7') -> {true,7};
is_var('$8') -> {true,8};
is_var('$9') -> {true,9};
is_var(X) when is_atom(X) ->
    case atom_to_list(X) of
        "\$" ++ Tl ->
            try N = list_to_integer(Tl),
                {true,N}
            catch
                error:_ ->
                    false
            end;
        _ ->
            false
    end;
is_var(_) -> false.


rewrite(Gs, R) ->
    [rewrite1(G, R) || G <- Gs].

rewrite1('$_', _) ->
    obj_prod();
rewrite1('$$', _) ->
    obj_prod_l();
rewrite1(Guard, R) when is_tuple(Guard) ->
    list_to_tuple([rewrite1(G, R) || G <- tuple_to_list(Guard)]);
rewrite1(Exprs, R) when is_list(Exprs) ->
    [rewrite1(E, R) || E <- Exprs];
rewrite1(V, R) when is_atom(V) ->
    case is_var(V) of
        {true,_N} ->
            case lists:keysearch(V, 1, R) of
                {value, {_, V1}} ->
                    V1;
                false ->
                    V
            end;
        false ->
            V
    end;
rewrite1(Expr, _) ->
    Expr.


%% @spec () -> any()
%%
%% @doc
%% @equiv table({all, all})
%% @end
table() ->
    table({all, all}).

%% @spec (Context::context()) -> any()
%%
%% @doc
%% @equiv table(Context, [])
%% @end
%%
table(Context) ->
    table(Context, []).

%% @spec (Context::context(), Opts) -> any()
%%
%% @doc QLC table generator for the gproc registry.
%% Context specifies which subset of the registry should be queried.
%% See [http://www.erlang.org/doc/man/qlc.html].
%%
%% NOTE: By default, the gproc table generator will not filter out entries
%% belonging to processes that have just died, but which have yet to be cleared
%% out of the registry. Use the option `check_pids' (or `{check_pids, true}')
%% if you want to filter out dead entries already in the query. There will be
%% some overhead associated with doing so, and given that the process monitoring
%% is asynchronous, there can never be any guarantee that there are no dead
%% entries in the list by the time your program processes it.
%%
%% @end
table(Context, Opts) ->
    Ctxt = {_, Type} = get_s_t(Context),
    [Traverse, NObjs] = [proplists:get_value(K,Opts,Def) ||
                            {K,Def} <- [{traverse,select}, {n_objects,100}]],
    CheckPids = proplists:get_bool(check_pids, Opts),
    TF = case Traverse of
             first_next ->
                 fun() -> qlc_next(Ctxt, first(Ctxt), CheckPids) end;
             last_prev -> fun() -> qlc_prev(Ctxt, last(Ctxt), CheckPids) end;
             select ->
                 fun(MS) -> qlc_select(
			      CheckPids,
			      select(Ctxt, wrap_qlc_ms_prod(CheckPids, MS),
				     NObjs))
		 end;
             {select,MS} ->
                 fun() -> qlc_select(
			    CheckPids,
			    select(Ctxt, wrap_qlc_ms_prod(CheckPids, MS),
				   NObjs))
		 end;
             _ ->
                 erlang:error(badarg, [Ctxt,Opts])
         end,
    InfoFun = fun(indices) -> [2];
                 (is_unique_objects) -> is_unique(Type);
                 (keypos) -> 1;
                 (is_sorted_key) -> true;
                 (num_of_objects) ->
                      %% this is just a guesstimate.
                      trunc(ets:info(?TAB,size) / 2.5)
              end,
    LookupFun =
        case Traverse of
            {select, _MS} -> undefined;
            _ -> fun(Pos, Ks) -> qlc_lookup(Ctxt, Pos, Ks, CheckPids) end
        end,
    qlc:table(TF, [{info_fun, InfoFun},
                   {lookup_fun, LookupFun}] ++ [{K,V} || {K,V} <- Opts,
                                                         K =/= traverse,
                                                         K =/= n_objects]).
wrap_qlc_ms_prod(false, Pats) ->
    Pats;
wrap_qlc_ms_prod(true, Pats) ->
    [ wrap_qlc_ms_prod_(P) || P <- Pats ].

wrap_qlc_ms_prod_({H, Gs, [P]}) ->
    {H, Gs, [{{ {element, 2, '$_'}, P }}]}.

qlc_lookup(_Scope, 1, Keys, Check) ->
    lists:flatmap(
      fun(Key) ->
              remove_dead(
		Check,
		ets:select(?TAB, [{ {{Key,'_'},'_','_'}, [],
				    [{{ {element,1,{element,1,'$_'}},
					{element,2,'$_'},
					{element,3,'$_'} }}] }]))
      end, Keys);
qlc_lookup(Scope, 2, Pids, Check) ->
    lists:flatmap(fun(Pid) ->
			  qlc_lookup_pid(Pid, Scope, Check)
		  end, Pids).

remove_dead(false, Objs) ->
    Objs;
remove_dead(true, Objs) ->
    [ Reg || {_, Pid, _} = Reg <- Objs,
	     not ?PID_IS_DEAD(Pid) ].

%% While it may seem obsessive not to do the sensible pid liveness check here
%% every time, we make it optional for consistency; this way, we can devise
%% a test case that verifies the difference between having the option and not.
qlc_lookup_pid(Pid, Scope, Check) ->
    case Check andalso ?PID_IS_DEAD(Pid) of
	true ->
	    [];
	false ->
	    Found =
		ets:select(?TAB, [{{{Pid, rev_keypat(Scope)}, '_'},
				   [], ['$_']}]),
	    lists:flatmap(
	      fun({{_,{T,_,_}=K}, _}) ->
		      K2 = if T==n orelse T==a -> T;
			      true -> Pid
			   end,
		      case ets:lookup(?TAB, {K,K2}) of
			  [{{Key,_},_,Value}] ->
			      [{Key, Pid, Value}];
			  [] ->
			      []
		      end
	      end, Found)
    end.


qlc_next(_, '$end_of_table', _) -> [];
qlc_next(Scope, K, Check) ->
    case ets:lookup(?TAB, K) of
        [{{Key,_}, Pid, V}] ->
	    case Check andalso ?PID_IS_DEAD(Pid) of
		true ->
		    qlc_next(Scope, next(Scope, K), Check);
		false ->
		    [{Key,Pid,V}] ++ fun() ->
					     qlc_next(Scope, next(Scope, K),
						      Check)
				     end
	    end;
        [] ->
            qlc_next(Scope, next(Scope, K), Check)
    end.

qlc_prev(_, '$end_of_table', _) -> [];
qlc_prev(Scope, K, Check) ->
    case ets:lookup(?TAB, K) of
        [{{Key,_},Pid,V}] ->
	    case Check andalso ?PID_IS_DEAD(Pid) of
		true ->
		    qlc_prev(Scope, prev(Scope, K), Check);
		false ->
		    [{Key,Pid,V}] ++ fun() ->
					     qlc_prev(Scope, prev(Scope, K),
						      Check)
				     end
	    end;
        [] ->
            qlc_prev(Scope, prev(Scope, K), Check)
    end.

qlc_select(_, '$end_of_table') ->
    [];
qlc_select(true, {Objects, Cont}) ->
    case [O || {Pid,O} <- Objects,
	       not ?PID_IS_DEAD(Pid)] of
	[] ->
	    %% re-run search
	    qlc_select(true, ets:select(Cont));
	Found ->
	    Found ++ fun() -> qlc_select(true, ets:select(Cont)) end
    end;
qlc_select(false, {Objects, Cont}) ->
    Objects ++ fun() -> qlc_select(false, ets:select(Cont)) end.


is_unique(n) -> true;
is_unique(a) -> true;
is_unique(_) -> false.