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+%% The contents of this file are subject to the Mozilla Public License
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+%% Version 1.1 (the "License"); you may not use this file except in
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+%% compliance with the License. You may obtain a copy of the License
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+%% at http://www.mozilla.org/MPL/
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+%%
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+%% Software distributed under the License is distributed on an "AS IS"
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+%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
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+%% the License for the specific language governing rights and
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+%% limitations under the License.
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+%%
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+%% The Original Code is RabbitMQ.
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+%%
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+%% The Initial Developer of the Original Code is GoPivotal, Inc.
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+%% Copyright (c) 2007-2015 Pivotal Software, Inc. All rights reserved.
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+%%
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+
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+%% Priority queues have essentially the same interface as ordinary
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+%% queues, except that a) there is an in/3 that takes a priority, and
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+%% b) we have only implemented the core API we need.
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+%%
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+%% Priorities should be integers - the higher the value the higher the
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+%% priority - but we don't actually check that.
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+%%
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+%% in/2 inserts items with priority 0.
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+%%
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+%% We optimise the case where a priority queue is being used just like
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+%% an ordinary queue. When that is the case we represent the priority
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+%% queue as an ordinary queue. We could just call into the 'queue'
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+%% module for that, but for efficiency we implement the relevant
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+%% functions directly in here, thus saving on inter-module calls and
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+%% eliminating a level of boxing.
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+%%
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+%% When the queue contains items with non-zero priorities, it is
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+%% represented as a sorted kv list with the inverted Priority as the
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+%% key and an ordinary queue as the value. Here again we use our own
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+%% ordinary queue implemention for efficiency, often making recursive
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+%% calls into the same function knowing that ordinary queues represent
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+%% a base case.
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+
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+
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+-module(priority_queue).
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+
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+-export([new/0, is_queue/1, is_empty/1, len/1, to_list/1, from_list/1,
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+ in/2, in/3, out/1, out_p/1, join/2, filter/2, fold/3, highest/1]).
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+
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+%%----------------------------------------------------------------------------
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+
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+-ifdef(use_specs).
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+
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+-export_type([q/0]).
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+
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+-type(q() :: pqueue()).
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+-type(priority() :: integer() | 'infinity').
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+-type(squeue() :: {queue, [any()], [any()], non_neg_integer()}).
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+-type(pqueue() :: squeue() | {pqueue, [{priority(), squeue()}]}).
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+
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+-spec(new/0 :: () -> pqueue()).
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+-spec(is_queue/1 :: (any()) -> boolean()).
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+-spec(is_empty/1 :: (pqueue()) -> boolean()).
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+-spec(len/1 :: (pqueue()) -> non_neg_integer()).
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+-spec(to_list/1 :: (pqueue()) -> [{priority(), any()}]).
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+-spec(from_list/1 :: ([{priority(), any()}]) -> pqueue()).
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+-spec(in/2 :: (any(), pqueue()) -> pqueue()).
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+-spec(in/3 :: (any(), priority(), pqueue()) -> pqueue()).
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+-spec(out/1 :: (pqueue()) -> {empty | {value, any()}, pqueue()}).
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+-spec(out_p/1 :: (pqueue()) -> {empty | {value, any(), priority()}, pqueue()}).
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+-spec(join/2 :: (pqueue(), pqueue()) -> pqueue()).
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+-spec(filter/2 :: (fun ((any()) -> boolean()), pqueue()) -> pqueue()).
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+-spec(fold/3 ::
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+ (fun ((any(), priority(), A) -> A), A, pqueue()) -> A).
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+-spec(highest/1 :: (pqueue()) -> priority() | 'empty').
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+
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+-endif.
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+
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+%%----------------------------------------------------------------------------
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+
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+new() ->
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+ {queue, [], [], 0}.
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+
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+is_queue({queue, R, F, L}) when is_list(R), is_list(F), is_integer(L) ->
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+ true;
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+is_queue({pqueue, Queues}) when is_list(Queues) ->
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+ lists:all(fun ({infinity, Q}) -> is_queue(Q);
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+ ({P, Q}) -> is_integer(P) andalso is_queue(Q)
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+ end, Queues);
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+is_queue(_) ->
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+ false.
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+
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+is_empty({queue, [], [], 0}) ->
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+ true;
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+is_empty(_) ->
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+ false.
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+
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+len({queue, _R, _F, L}) ->
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+ L;
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+len({pqueue, Queues}) ->
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+ lists:sum([len(Q) || {_, Q} <- Queues]).
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+
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+to_list({queue, In, Out, _Len}) when is_list(In), is_list(Out) ->
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+ [{0, V} || V <- Out ++ lists:reverse(In, [])];
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+to_list({pqueue, Queues}) ->
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+ [{maybe_negate_priority(P), V} || {P, Q} <- Queues,
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+ {0, V} <- to_list(Q)].
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+
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+from_list(L) ->
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+ lists:foldl(fun ({P, E}, Q) -> in(E, P, Q) end, new(), L).
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+
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+in(Item, Q) ->
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+ in(Item, 0, Q).
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+
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+in(X, 0, {queue, [_] = In, [], 1}) ->
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+ {queue, [X], In, 2};
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+in(X, 0, {queue, In, Out, Len}) when is_list(In), is_list(Out) ->
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+ {queue, [X|In], Out, Len + 1};
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+in(X, Priority, _Q = {queue, [], [], 0}) ->
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+ in(X, Priority, {pqueue, []});
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+in(X, Priority, Q = {queue, _, _, _}) ->
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+ in(X, Priority, {pqueue, [{0, Q}]});
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+in(X, Priority, {pqueue, Queues}) ->
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+ P = maybe_negate_priority(Priority),
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+ {pqueue, case lists:keysearch(P, 1, Queues) of
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+ {value, {_, Q}} ->
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+ lists:keyreplace(P, 1, Queues, {P, in(X, Q)});
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+ false when P == infinity ->
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+ [{P, {queue, [X], [], 1}} | Queues];
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+ false ->
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+ case Queues of
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+ [{infinity, InfQueue} | Queues1] ->
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+ [{infinity, InfQueue} |
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+ lists:keysort(1, [{P, {queue, [X], [], 1}} | Queues1])];
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+ _ ->
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+ lists:keysort(1, [{P, {queue, [X], [], 1}} | Queues])
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+ end
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+ end}.
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+
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+out({queue, [], [], 0} = Q) ->
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+ {empty, Q};
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+out({queue, [V], [], 1}) ->
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+ {{value, V}, {queue, [], [], 0}};
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+out({queue, [Y|In], [], Len}) ->
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+ [V|Out] = lists:reverse(In, []),
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+ {{value, V}, {queue, [Y], Out, Len - 1}};
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+out({queue, In, [V], Len}) when is_list(In) ->
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+ {{value,V}, r2f(In, Len - 1)};
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+out({queue, In,[V|Out], Len}) when is_list(In) ->
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+ {{value, V}, {queue, In, Out, Len - 1}};
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+out({pqueue, [{P, Q} | Queues]}) ->
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+ {R, Q1} = out(Q),
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+ NewQ = case is_empty(Q1) of
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+ true -> case Queues of
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+ [] -> {queue, [], [], 0};
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+ [{0, OnlyQ}] -> OnlyQ;
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+ [_|_] -> {pqueue, Queues}
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+ end;
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+ false -> {pqueue, [{P, Q1} | Queues]}
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+ end,
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+ {R, NewQ}.
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+
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+out_p({queue, _, _, _} = Q) -> add_p(out(Q), 0);
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+out_p({pqueue, [{P, _} | _]} = Q) -> add_p(out(Q), maybe_negate_priority(P)).
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+
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+add_p(R, P) -> case R of
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+ {empty, Q} -> {empty, Q};
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+ {{value, V}, Q} -> {{value, V, P}, Q}
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+ end.
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+
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+join(A, {queue, [], [], 0}) ->
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+ A;
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+join({queue, [], [], 0}, B) ->
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+ B;
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+join({queue, AIn, AOut, ALen}, {queue, BIn, BOut, BLen}) ->
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+ {queue, BIn, AOut ++ lists:reverse(AIn, BOut), ALen + BLen};
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+join(A = {queue, _, _, _}, {pqueue, BPQ}) ->
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+ {Pre, Post} =
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+ lists:splitwith(fun ({P, _}) -> P < 0 orelse P == infinity end, BPQ),
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+ Post1 = case Post of
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+ [] -> [ {0, A} ];
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+ [ {0, ZeroQueue} | Rest ] -> [ {0, join(A, ZeroQueue)} | Rest ];
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+ _ -> [ {0, A} | Post ]
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+ end,
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+ {pqueue, Pre ++ Post1};
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+join({pqueue, APQ}, B = {queue, _, _, _}) ->
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+ {Pre, Post} =
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+ lists:splitwith(fun ({P, _}) -> P < 0 orelse P == infinity end, APQ),
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+ Post1 = case Post of
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+ [] -> [ {0, B} ];
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+ [ {0, ZeroQueue} | Rest ] -> [ {0, join(ZeroQueue, B)} | Rest ];
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+ _ -> [ {0, B} | Post ]
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+ end,
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+ {pqueue, Pre ++ Post1};
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+join({pqueue, APQ}, {pqueue, BPQ}) ->
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+ {pqueue, merge(APQ, BPQ, [])}.
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+
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+merge([], BPQ, Acc) ->
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+ lists:reverse(Acc, BPQ);
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+merge(APQ, [], Acc) ->
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+ lists:reverse(Acc, APQ);
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+merge([{P, A}|As], [{P, B}|Bs], Acc) ->
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+ merge(As, Bs, [ {P, join(A, B)} | Acc ]);
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+merge([{PA, A}|As], Bs = [{PB, _}|_], Acc) when PA < PB orelse PA == infinity ->
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+ merge(As, Bs, [ {PA, A} | Acc ]);
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+merge(As = [{_, _}|_], [{PB, B}|Bs], Acc) ->
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+ merge(As, Bs, [ {PB, B} | Acc ]).
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+
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+filter(Pred, Q) -> fold(fun(V, P, Acc) ->
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+ case Pred(V) of
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+ true -> in(V, P, Acc);
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+ false -> Acc
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+ end
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+ end, new(), Q).
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+
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+fold(Fun, Init, Q) -> case out_p(Q) of
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+ {empty, _Q} -> Init;
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+ {{value, V, P}, Q1} -> fold(Fun, Fun(V, P, Init), Q1)
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+ end.
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+
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+highest({queue, [], [], 0}) -> empty;
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+highest({queue, _, _, _}) -> 0;
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+highest({pqueue, [{P, _} | _]}) -> maybe_negate_priority(P).
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+
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+r2f([], 0) -> {queue, [], [], 0};
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+r2f([_] = R, 1) -> {queue, [], R, 1};
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+r2f([X,Y], 2) -> {queue, [X], [Y], 2};
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+r2f([X,Y|R], L) -> {queue, [X,Y], lists:reverse(R, []), L}.
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+
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+maybe_negate_priority(infinity) -> infinity;
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+maybe_negate_priority(P) -> -P.
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Feng Lee