-module(pingpong).
-export([main/0]).

% this runs two processes and has them talk to each other
% output:
%
% 2> pingpong:main().
% <0.96.0> received {pong, <0.97.0>, 10}
% <0.97.0> received {ping, <0.96.0>, 9}
% <0.96.0> received {pong, <0.97.0>, 8}
% <0.97.0> received {ping, <0.96.0>, 7}
% <0.96.0> received {pong, <0.97.0>, 6}
% <0.97.0> received {ping, <0.96.0>, 5}
% <0.96.0> received {pong, <0.97.0>, 4}
% <0.97.0> received {ping, <0.96.0>, 3}
% <0.96.0> received {pong, <0.97.0>, 2}
% <0.97.0> received {ping, <0.96.0>, 1}
% <0.96.0> received {pong, <0.97.0>, 0}


main() ->
    PingerPID = spawn(fun pinger/0),
    PongerPID = spawn(fun ponger/0),
    PingerPID ! {pong, PongerPID, 10}.

pinger() ->
    receive
        {pong, PID, N} ->
            io:format("~p received {pong, ~p, ~p}~n", [self(), PID, N]),
            % ignore once we get to 0 or lower
            case N > 0 of
                true  -> PID ! {ping, self(), N-1};
                false -> ok
            end,
            % once we're done, go back to the top
            pinger()
    end.


ponger() ->
    receive
        {ping, PID, N} ->
            io:format("~p received {ping, ~p, ~p}~n", [self(), PID, N]),
            % ignore once we get to 0 or lower
            case N > 0 of
                true  -> PID ! {pong, self(), N-1};
                false -> ok
            end,
            % once we're done, go back to the top
            ponger()
    end.