dining_simple_bug.erl

-module(dining_simple_bug).
-export([main/0, waiter/0, fork/1, philo/2]).

main() ->
  spawn(?MODULE, waiter, []).

spawn_forks(0, Dict) ->
  Dict;
spawn_forks(N, Dict) ->
  Pair = {N, spawn(?MODULE, fork, [free])},
  spawn_forks(N-1, [Pair] ++ Dict).

spawn_philos(0, Dict, _) ->
  Dict;
spawn_philos(N, Dict, Pid) ->
  Pair = {spawn(?MODULE, philo, [Pid, N]), N},
  spawn_philos(N-1, [Pair] ++ Dict, Pid).

waiter() ->
  ForkDict = spawn_forks(5, []),
  PhiloDict = spawn_philos(5, [], self()),
  waiter_1(ForkDict, PhiloDict).

waiter_1(ForkDict, PhiloDict) ->
    receive
      {eaten, PhiloPid} ->
        PhiloId = proplists:get_value(PhiloPid, PhiloDict),
        LeftForkId = PhiloId,
        RightForkId =  1 + (LeftForkId rem 5),
        LeftPid = proplists:get_value(LeftForkId, ForkDict),
        RightPid = proplists:get_value(RightForkId, ForkDict),
        set_state(LeftPid, free),
        set_state(RightPid, free);
      {hungry, PhiloPid} ->
        PhiloId = proplists:get_value(PhiloPid, PhiloDict),
        LeftForkId = PhiloId,
        RightForkId =  1 + (LeftForkId rem 5),
        LeftPid = proplists:get_value(LeftForkId, ForkDict),
        RightPid = proplists:get_value(RightForkId, ForkDict),
        LeftForkState = ask_state(LeftPid),
        RightForkState = ask_state(RightPid),
        case {LeftForkState, RightForkState} of
          {free, free} ->
            set_state(LeftPid, used),
            set_state(RightPid, used),
            PhiloPid ! eat;
          _ ->
            PhiloPid ! think
        end
    end,
    waiter_1(ForkDict, PhiloDict).

ask_state(Pid) ->
  Pid ! {get_state, self()},
  receive
    % {state, State, _} -> State % Correct version
    State -> State               % Buggy version
  end.

set_state(Pid, State) ->
  Pid ! {set_state, State, self()},
  receive
    % {been_set, _} -> ok % Correct version
    been_set -> ok        % Buggy version
  end.

philo(WaiterPid, PhiloId) ->
  think(PhiloId),
  request_until_eaten(WaiterPid, PhiloId),
  philo(WaiterPid, PhiloId).

think(PhiloId) ->
  io:fwrite("Philo " ++ integer_to_list(PhiloId) ++ " is thinking~n"),
  ThinkTime = rand:uniform(1000),
  timer:sleep(ThinkTime),
  ok.

eat(PhiloId) ->
  io:fwrite("Philo " ++ integer_to_list(PhiloId) ++ " has eaten~n"),
  timer:sleep(1000),
  ok.

request_until_eaten(WaiterPid, PhiloId) ->
    io:fwrite("Philo " ++ integer_to_list(PhiloId) ++ " is hungry~n"),
    WaiterPid ! {hungry, self()},
    receive
      think ->
        think(PhiloId),
        request_until_eaten(WaiterPid, PhiloId);
      eat ->
        eat(PhiloId),
        WaiterPid ! {eaten, self()}
    end.

fork(State) ->
  receive
    {get_state, WaiterPid} ->
      % WaiterPid ! {state, State, self()}, % Correct version
      WaiterPid ! State,                    % Buggy version
      fork(State);
    {set_state, NewState, WaiterPid} ->
      % WaiterPid ! {been_set, self()}, % Correct version
      WaiterPid ! been_set,             % Buggy version
      fork(NewState)
  end.