shq_atomics

shq_atomics.erl

-module(shq_atomics).

-export([init/1,
         in/2,
         in_r/2,
         out/1,
         out_r/1,
         size/1,
         test_in_out/1]).

-define(FRONT, 1).
-define(REAR, 2).

init(Tab) ->
    Ref = ets:new(Tab, [public, set]),
    ARef = atomics:new(2,[]), %% front is index 1, rear is index 2
    {ok, {Ref, ARef}}.

in({Ref, ARef}, Value) ->
    true = ets:insert(Ref, {atomics:add_get(ARef, ?REAR, 1) - 1, Value}),
    ok.

in_r({Ref, ARef}, Value) ->
    true = ets:insert(Ref, {atomics:add_get(ARef, ?FRONT, -1), Value}),
    ok.
    
out({Ref, ARef}) ->
    KF = atomics:add_get(ARef, ?FRONT, 1) - 1,
    case ets:take(Ref, KF) of
      [{_, V}] ->
          {ok, V};
      [] ->
        %% reset counter
        atomics:add_get(ARef, ?FRONT, -1),
        empty
    end.

out_r({Ref, ARef}) ->
    KR1 = atomics:add_get(ARef, ?REAR, -1),
    case ets:take(Ref, KR1) of
      [{_, V}] ->
          {ok, V};
      [] ->
        %% reset counter
        atomics:add_get(ARef, ?REAR, 1),
        empty
    end.

size({_Ref, ARef}) ->
    atomic:get(ARef, ?REAR) - atomics:get(ARef, ?FRONT).

test_in_out(Ref) ->
    in(Ref, 0),
    out(Ref).