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%% The contents of this file are subject to the Mozilla Public License
%% Version 1.1 (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.mozilla.org/MPL/
%%
%% 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 Original Code is RabbitMQ.
%%
%% The Initial Developer of the Original Code is GoPivotal, Inc.
%% Copyright (c) 2007-2014 GoPivotal, Inc. All rights reserved.
%%
%% Priority queues have essentially the same interface as ordinary
%% queues, except that a) there is an in/3 that takes a priority, and
%% b) we have only implemented the core API we need.
%%
%% Priorities should be integers - the higher the value the higher the
%% priority - but we don't actually check that.
%%
%% in/2 inserts items with priority 0.
%%
%% We optimise the case where a priority queue is being used just like
%% an ordinary queue. When that is the case we represent the priority
%% queue as an ordinary queue. We could just call into the 'queue'
%% module for that, but for efficiency we implement the relevant
%% functions directly in here, thus saving on inter-module calls and
%% eliminating a level of boxing.
%%
%% When the queue contains items with non-zero priorities, it is
%% represented as a sorted kv list with the inverted Priority as the
%% key and an ordinary queue as the value. Here again we use our own
%% ordinary queue implemention for efficiency, often making recursive
%% calls into the same function knowing that ordinary queues represent
%% a base case.
-module(priority_queue).
-export([new/0, is_queue/1, is_empty/1, len/1, to_list/1, from_list/1,
in/2, in/3, out/1, out_p/1, join/2, filter/2, fold/3, highest/1]).
%%----------------------------------------------------------------------------
-ifdef(use_specs).
-export_type([q/0]).
-type(q() :: pqueue()).
-type(priority() :: integer() | 'infinity').
-type(squeue() :: {queue, [any()], [any()], non_neg_integer()}).
-type(pqueue() :: squeue() | {pqueue, [{priority(), squeue()}]}).
-spec(new/0 :: () -> pqueue()).
-spec(is_queue/1 :: (any()) -> boolean()).
-spec(is_empty/1 :: (pqueue()) -> boolean()).
-spec(len/1 :: (pqueue()) -> non_neg_integer()).
-spec(to_list/1 :: (pqueue()) -> [{priority(), any()}]).
-spec(from_list/1 :: ([{priority(), any()}]) -> pqueue()).
-spec(in/2 :: (any(), pqueue()) -> pqueue()).
-spec(in/3 :: (any(), priority(), pqueue()) -> pqueue()).
-spec(out/1 :: (pqueue()) -> {empty | {value, any()}, pqueue()}).
-spec(out_p/1 :: (pqueue()) -> {empty | {value, any(), priority()}, pqueue()}).
-spec(join/2 :: (pqueue(), pqueue()) -> pqueue()).
-spec(filter/2 :: (fun ((any()) -> boolean()), pqueue()) -> pqueue()).
-spec(fold/3 ::
(fun ((any(), priority(), A) -> A), A, pqueue()) -> A).
-spec(highest/1 :: (pqueue()) -> priority() | 'empty').
-endif.
%%----------------------------------------------------------------------------
new() ->
{queue, [], [], 0}.
is_queue({queue, R, F, L}) when is_list(R), is_list(F), is_integer(L) ->
true;
is_queue({pqueue, Queues}) when is_list(Queues) ->
lists:all(fun ({infinity, Q}) -> is_queue(Q);
({P, Q}) -> is_integer(P) andalso is_queue(Q)
end, Queues);
is_queue(_) ->
false.
is_empty({queue, [], [], 0}) ->
true;
is_empty(_) ->
false.
len({queue, _R, _F, L}) ->
L;
len({pqueue, Queues}) ->
lists:sum([len(Q) || {_, Q} <- Queues]).
to_list({queue, In, Out, _Len}) when is_list(In), is_list(Out) ->
[{0, V} || V <- Out ++ lists:reverse(In, [])];
to_list({pqueue, Queues}) ->
[{maybe_negate_priority(P), V} || {P, Q} <- Queues,
{0, V} <- to_list(Q)].
from_list(L) ->
lists:foldl(fun ({P, E}, Q) -> in(E, P, Q) end, new(), L).
in(Item, Q) ->
in(Item, 0, Q).
in(X, 0, {queue, [_] = In, [], 1}) ->
{queue, [X], In, 2};
in(X, 0, {queue, In, Out, Len}) when is_list(In), is_list(Out) ->
{queue, [X|In], Out, Len + 1};
in(X, Priority, _Q = {queue, [], [], 0}) ->
in(X, Priority, {pqueue, []});
in(X, Priority, Q = {queue, _, _, _}) ->
in(X, Priority, {pqueue, [{0, Q}]});
in(X, Priority, {pqueue, Queues}) ->
P = maybe_negate_priority(Priority),
{pqueue, case lists:keysearch(P, 1, Queues) of
{value, {_, Q}} ->
lists:keyreplace(P, 1, Queues, {P, in(X, Q)});
false when P == infinity ->
[{P, {queue, [X], [], 1}} | Queues];
false ->
case Queues of
[{infinity, InfQueue} | Queues1] ->
[{infinity, InfQueue} |
lists:keysort(1, [{P, {queue, [X], [], 1}} | Queues1])];
_ ->
lists:keysort(1, [{P, {queue, [X], [], 1}} | Queues])
end
end}.
out({queue, [], [], 0} = Q) ->
{empty, Q};
out({queue, [V], [], 1}) ->
{{value, V}, {queue, [], [], 0}};
out({queue, [Y|In], [], Len}) ->
[V|Out] = lists:reverse(In, []),
{{value, V}, {queue, [Y], Out, Len - 1}};
out({queue, In, [V], Len}) when is_list(In) ->
{{value,V}, r2f(In, Len - 1)};
out({queue, In,[V|Out], Len}) when is_list(In) ->
{{value, V}, {queue, In, Out, Len - 1}};
out({pqueue, [{P, Q} | Queues]}) ->
{R, Q1} = out(Q),
NewQ = case is_empty(Q1) of
true -> case Queues of
[] -> {queue, [], [], 0};
[{0, OnlyQ}] -> OnlyQ;
[_|_] -> {pqueue, Queues}
end;
false -> {pqueue, [{P, Q1} | Queues]}
end,
{R, NewQ}.
out_p({queue, _, _, _} = Q) -> add_p(out(Q), 0);
out_p({pqueue, [{P, _} | _]} = Q) -> add_p(out(Q), maybe_negate_priority(P)).
add_p(R, P) -> case R of
{empty, Q} -> {empty, Q};
{{value, V}, Q} -> {{value, V, P}, Q}
end.
join(A, {queue, [], [], 0}) ->
A;
join({queue, [], [], 0}, B) ->
B;
join({queue, AIn, AOut, ALen}, {queue, BIn, BOut, BLen}) ->
{queue, BIn, AOut ++ lists:reverse(AIn, BOut), ALen + BLen};
join(A = {queue, _, _, _}, {pqueue, BPQ}) ->
{Pre, Post} =
lists:splitwith(fun ({P, _}) -> P < 0 orelse P == infinity end, BPQ),
Post1 = case Post of
[] -> [ {0, A} ];
[ {0, ZeroQueue} | Rest ] -> [ {0, join(A, ZeroQueue)} | Rest ];
_ -> [ {0, A} | Post ]
end,
{pqueue, Pre ++ Post1};
join({pqueue, APQ}, B = {queue, _, _, _}) ->
{Pre, Post} =
lists:splitwith(fun ({P, _}) -> P < 0 orelse P == infinity end, APQ),
Post1 = case Post of
[] -> [ {0, B} ];
[ {0, ZeroQueue} | Rest ] -> [ {0, join(ZeroQueue, B)} | Rest ];
_ -> [ {0, B} | Post ]
end,
{pqueue, Pre ++ Post1};
join({pqueue, APQ}, {pqueue, BPQ}) ->
{pqueue, merge(APQ, BPQ, [])}.
merge([], BPQ, Acc) ->
lists:reverse(Acc, BPQ);
merge(APQ, [], Acc) ->
lists:reverse(Acc, APQ);
merge([{P, A}|As], [{P, B}|Bs], Acc) ->
merge(As, Bs, [ {P, join(A, B)} | Acc ]);
merge([{PA, A}|As], Bs = [{PB, _}|_], Acc) when PA < PB orelse PA == infinity ->
merge(As, Bs, [ {PA, A} | Acc ]);
merge(As = [{_, _}|_], [{PB, B}|Bs], Acc) ->
merge(As, Bs, [ {PB, B} | Acc ]).
filter(Pred, Q) -> fold(fun(V, P, Acc) ->
case Pred(V) of
true -> in(V, P, Acc);
false -> Acc
end
end, new(), Q).
fold(Fun, Init, Q) -> case out_p(Q) of
{empty, _Q} -> Init;
{{value, V, P}, Q1} -> fold(Fun, Fun(V, P, Init), Q1)
end.
highest({queue, [], [], 0}) -> empty;
highest({queue, _, _, _}) -> 0;
highest({pqueue, [{P, _} | _]}) -> maybe_negate_priority(P).
r2f([], 0) -> {queue, [], [], 0};
r2f([_] = R, 1) -> {queue, [], R, 1};
r2f([X,Y], 2) -> {queue, [X], [Y], 2};
r2f([X,Y|R], L) -> {queue, [X,Y], lists:reverse(R, []), L}.
maybe_negate_priority(infinity) -> infinity;
maybe_negate_priority(P) -> -P.
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