%%%
%%%   Copyright (c) 2014-2018, Klarna AB
%%%
%%%   Licensed under the Apache License, Version 2.0 (the "License");
%%%   you may not use this file except in compliance with the License.
%%%   You may obtain a copy of the License at
%%%
%%%       http://www.apache.org/licenses/LICENSE-2.0
%%%
%%%   Unless required by applicable law or agreed to in writing, software
%%%   distributed under the License is distributed on an "AS IS" BASIS,
%%%   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
%%%   See the License for the specific language governing permissions and
%%%   limitations under the License.
%%%

%%%=============================================================================
%%% @doc
%%% This module manages an opaque collection of sent-requests.
%%% @end
%%% ============================================================================

%% @private
-module(kpro_sent_reqs).

%%%_* Exports ==================================================================

%% API
-export([ new/0
        , add/5
        , del/2
        , get_req/2
        , get_corr_id/1
        , increment_corr_id/1
        , scan_for_max_age/1
        ]).

-export_type([requests/0]).

-type corr_id() :: kpro:corr_id().
-define(REQ(Caller, Ref, API, Vsn, Ts), {Caller, Ref, API, Vsn, Ts}).
-type req() :: ?REQ(pid(), reference(), kpro:api(), kpro:vsn(), integer()).

-record(requests,
        { corr_id = 0
        , sent = #{} :: #{corr_id() => req()}
        }).

-opaque requests() :: #requests{}.

-include("kpro_private.hrl").


%%%_* APIs =====================================================================

-spec new() -> requests().
new() -> #requests{}.

%% @doc Add a new request to sent collection.
%% Return the last corrlation ID and the new collection.
-spec add(requests(), pid(), reference(), kpro:api(), kpro:vsn()) ->
        {corr_id(), requests()}.
add(#requests{ corr_id = CorrId
             , sent    = Sent
             } = Requests, Caller, Ref, API, Vsn) ->
  Req = ?REQ(Caller, Ref, API, Vsn, now_ts()),
  NewSent = maps:put(CorrId, Req, Sent),
  NewRequests = Requests#requests{ corr_id = next_corr_id(CorrId)
                                 , sent    = NewSent
                                 },
  {CorrId, NewRequests}.

%% @doc Delete a request from the opaque collection.
%% Crash if correlation ID is not found.
-spec del(requests(), corr_id()) -> requests().
del(#requests{sent = Sent} = Requests, CorrId) ->
  Requests#requests{sent = maps:remove(CorrId, Sent)}.

%% @doc Get caller of a request having the given correlation ID.
%% Crash if the request is not found.
-spec get_req(requests(), corr_id()) ->
        {pid(), reference(), kpro:api(), kpro:vsn()}.
get_req(#requests{sent = Sent}, CorrId) ->
  ?REQ(Caller, Ref, API, Vsn, _Ts) = maps:get(CorrId, Sent),
  {Caller, Ref, API, Vsn}.

%% @doc Get the correction to be sent for the next request.
-spec get_corr_id(requests()) -> corr_id().
get_corr_id(#requests{ corr_id = CorrId }) ->
  CorrId.

%% @doc Fetch and increment the correlation ID
%% This is used if we don't want a response from the broker
-spec increment_corr_id(requests()) -> {corr_id(), requests()}.
increment_corr_id(#requests{corr_id = CorrId} = Requests) ->
  {CorrId, Requests#requests{ corr_id = next_corr_id(CorrId) }}.

%% @doc Scan all sent requests to get oldest sent request.
%% Age is in milli-seconds.
%% 0 is returned if there is no pending response.
-spec scan_for_max_age(requests()) -> timeout().
scan_for_max_age(#requests{sent = Sent}) ->
  Now = now_ts(),
  MinTs = maps:fold(fun(_CorrId, ?REQ(_Caller, _Ref, _API, _Vsn, Ts), Min) ->
                          erlang:min(Ts, Min)
                    end, Now, Sent),
  Now - MinTs.

next_corr_id(N) -> (N + 1) rem (1 bsl 31).

%% os:system_time('millisecond') is since otp-19
now_ts() -> os:system_time(?millisecond).

%%%_* Emacs ====================================================================
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