-module(pingpong).
-export([start/1, ppR/1, ppS/2]).

% A program that starts two processes and then sends a message back and 
% forth between them N times.  Note that ppR and ppS also have to be 
% exported because they are used in the spawn calls.

start(N) ->
	B = spawn(pingpong, ppR, [N]),
	A = spawn(pingpong, ppS, [N, B]),
	io:format("Started A:~w, B:~w~n", [A, B]).

% the sender process
ppS(0, _) ->
	true;
ppS(N, B) ->
	pp_send({B, N}),
	pp_receive(),
	ppS(N-1, B).

% the receiver process
ppR(0) ->
	true;
ppR(N) ->
	Msg = pp_receive(),
	pp_send(Msg),
	ppR(N - 1).

% send a message
pp_send({Pid, N}) ->
	Pid ! {self(), N}.

% receive a message and return it
pp_receive() ->
        receive
                {From, Msg} ->
                        io:format("~w Got message ~w from ~w~n",
                                [self(), Msg, From])
        end,
	{From, Msg}.