% places.erl

-module(places).
-author('Alan G. Labouseur').
-define(else, true).  % -- This is to make the if statements (somewhat) readable.

%--------
% Public
%--------
-export([start/0, p1/0]).  % Note that p2/0 is not exported.

start() ->
   io:fwrite("~nstart() [pid ~w] has... started.~n~n", [self()]),

   P1pid = spawn(places, p1, []),   % Note the different syntax compared to ag_server. Requires p1/0 to be public.
   % We want to publish this process in Erlang's Process Dictionary.
   % Before we do that, we need to un-register it if it's already been registered.
   SomePlace = whereis(place1),
   if (SomePlace /= undefined) ->  % "not undefined" = "is defined"  (This is horrible, I know.)
      unregister(place1);
   ?else ->
      % The proccess was NOT already published/registered, so we don't really want to do anything here.
      true    % This is dumb, but I couldn't think of a better "no-op".
   end,

   % Publish this process in Erlang's  Process Dictionary.
   register(place1, P1pid),

   % Now we can refer to it by atom place1 rather than it's pid when sending.
   place1 ! {self(), someRequest},
   receive
      {P1pid, Response1} ->            % This waits for a response from P1pid, which is place1. Can we use place1 somehow?
         io:fwrite("start() [pid ~w] received a response from pid ~w:~n", [self(), P1pid]),
         io:fwrite("   ~s~n", [Response1])
   end,

   OtherPlace = whereis(place2),
   if (OtherPlace /= undefined) ->  % "not undefined" = "is defined"  (This is horrible, I know.)
      unregister(place2);
   ?else ->
      % The proccess was NOT already published/registered, so we don't really want to do anything here.
      true % This is dumb, but I couldn't think of a better "no-op".
   end,
   % Spawn and register (like in our book on page 214) at the same time.
   register(place2, spawn(fun() -> p2() end)),  % This spawn syntax does NOT require p2/0 to be exported.

   % We can refer to it by the atom place2 rather than it's pid when sending as long as we're consistent about that.
   place2 ! {self(), anotherRequest},
   receive
      {place2, Response2} ->
         io:fwrite("start() [pid ~w] received a response from pid ~w:~n", [self(), place2]),  % (consistent)
         io:fwrite("   ~s~n", [Response2])
   end.


% This is spawned from start() in a manner that requires it to be public.
p1() ->
   receive
      {FromPid, Request} ->
         io:fwrite("p1() [pid ~w] received request [~w] from pid ~w.~n", [self(), Request, FromPid]),
         Response = io_lib:format("This text is p1()'s [pid ~w's] response to pid ~w.~n", [self(), FromPid]),
         FromPid ! {self(), Response},
         p1()
   end.



%--------
% Private
%--------

% This is spawned from start() in a manner that allows it to be private,
% but still "seen" when messages are sent to it.
p2() ->
   receive
      {FromPid, Request} ->
         io:fwrite("p2() [pid ~w] received request [~w] from pid ~w.~n", [self(), Request, FromPid]),
         Response = io_lib:format("This text is p2()'s [pid ~w's] response to pid ~w.~n", [self(), FromPid]),
         FromPid ! {place2, Response},
         p2()
   end.