erlflake

%%%---------------------------------------------------------------------------
%%% Copyright (C) 2021, Fix Solution All Rights Reserved
%%% Unauthorized copy of this file is through any medium strictly not allowed.
%%%
%%% Authors : Alpha Shaw <shawalpha5@gmail.com>
%%% Created :  20 Apr 2021  by <shawalpha5@gmail.com>
%%% Purpose :
%%%
%%%
%%% 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.
%%%---------------------------------------------------------------------------

-module(timestamp).
-author("Alpha Umaru Shaw").

-behaviour(gen_server).

%% API
-export([start_link/0]).
-export([since_epoch/0, time_units/0, last_timestamp/0]).

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

-define(SERVER, ?MODULE).
-define(TS_TABLE, ts_table).
-define(LAST_TS, last_ts).
-define(TIMESTAMP_SAVE_INTERVAL, 100).


-record(state, {
  dets_table,
  dets_file
}).


%%%===================================================================
%%% API
%%%===================================================================

time_units() ->
  Ms = ew_time:micros(),
  {Ms, erlang:round(Ms / 1000)}.

start_link() ->
  gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).


last_timestamp() ->
  gen_server:call(?SERVER, read_last_ts, 5000).

%%%===================================================================
%%% gen_server callbacks
%%%===================================================================

init([]) ->
  process_flag(trap_exit, true),
  File = "/tmp/" ++ atom_to_list(node())
                    ++ "/" ++ "flake-timestamp",
  case touch_dirs(File) of
    {error, Reason} ->
      {error, Reason};
    _ ->
      {ok, Table} =
      dets:open_file(?TS_TABLE, [
        {estimated_no_objects, 10},
        {type, set},
        {file, File}
      ]),
      case read_last_timestamp(Table) of
        {ok, LTS} ->
          CurrentTs = ew_time:millis(),
          if LTS > CurrentTs ->
            {stop, clock_is_backward};
            true ->
              erlang:send_after(?TIMESTAMP_SAVE_INTERVAL, self(), write_ts),
              {ok, #state{dets_table = Table, dets_file = File}}
          end
      end
  end.


handle_call(read_last_ts, _From, #state{dets_table = Table} = State) ->
  {reply, read_last_timestamp(Table), State};


handle_call(_Request, _From, State) ->
  {reply, ok, State}.


handle_cast(_Request, State) ->
  {noreply, State}.


handle_info(write_ts, #state{dets_table = Table} = State) ->
  {ok, _} = write_last_ts(Table),
  erlang:send_after(?TIMESTAMP_SAVE_INTERVAL, self(), write_ts),
  {noreply, State};


handle_info(_Info, State) ->
  {noreply, State}.


terminate(_Reason, #state{dets_table = Table}) ->
  catch dets:close(Table),
  ok.


code_change(_OldVsn, State, _Extra) ->
  {ok, State}.


%%%===================================================================
%%% Internal functions
%%%===================================================================

read_last_timestamp(Table) ->
  case dets:lookup(Table, ?LAST_TS) of
    [{?LAST_TS, TS}] when is_integer(TS) ->
      {ok, TS};
    _ -> write_last_ts(Table)
  end.


write_last_ts(Table) ->
  TS = ew_time:millis(),
  ok = dets:insert(Table, {?LAST_TS, TS}),
  {ok, TS}.

touch_dirs(File) when is_list(File) ->
  case filelib:ensure_dir(File) of
    ok ->
      ok;
    {error, Reason} ->
      {error, Reason}
  end.

since_epoch() ->
  erlang:error(not_implemented).