Erlang/OTP Forums

-module(ex2).
-export([start/2, proc/4]).

%%% helper function for add L to the end of the given list
append_list([H|T], L) -> [H|append_list(T, L)];
append_list([], L) -> [L].

%%% starts task N - number of processes, M - number of times message should fly through the ring
start(N, M) ->
   Pid = spawn(ex2, proc, [create, N - 1, [], nothing]),
   Pid ! {resend, hello, M*N},
   done.


%%% creates last N-th process
proc(create, 0, [H|_], _) ->
   Next = H,
   proc(waitmsg, 0, [], Next);

%%% creates 1,...,N-1 processes
proc(create, N, T, _) when N > 0 ->
   Next = spawn(ex2, proc, [create, N-1, append_list(T, self()), nothing]),
   proc(waitmsg, N, [], Next);

%%% wait messages for N-th process
proc(waitmsg, 0, _, Next) ->
   receive
      {resend, Mess, Counter} ->
         io:format("accepted ~w:~p~n", [self(), Mess]),
         Next ! {resend, Mess, Counter-1},
         proc(waitmsg, 0, [], Next);
      stop ->
         io:format("finished: ~w~n", [self()])
   end;

%%% wait messages for 1, ..., N-1 process
proc(waitmsg, N, _, Next) ->
   receive
      %%% stop resending and terminate processes ring
      {resend, _, 0} ->
         self() ! stop,
         proc(waitmsg, N, [], Next);
      %%% resend message to the next process
      {resend, Mess, Counter} ->
         io:format("accepted ~w:~p~n", [self(), Mess]),
         Next ! {resend, Mess, Counter-1},
         proc(waitmsg, N, [], Next);
      stop ->
         Next ! stop,
         io:format("finished: ~w~n", [self()])
   end.