ring.erl

%% Create N processes in a ring.
%% Send a message round the ring M times so that a total of N * M messages get sent.

-module(ring).
-export([start/0, start/2]).

start() ->
    start(10, 10).

start(N, M) ->
    start_child(N - 1, M, [self()]).

start_child(0, M, Pids) ->
    [LastPid|_Pids] = Pids,
    io:format("last pid ~p~n", [LastPid]),
    io:format("counting ~p~n", [M]),
    loop_first(M, LastPid);
start_child(N, M, Pids) ->
    [PrevPid|_Pids] = Pids,
    io:format("starting ~p with prev ~p~n", [N + 1, PrevPid]),
    Pid = spawn(fun() -> loop(N, PrevPid) end),
    start_child(N - 1, M, [Pid|Pids]).

loop_first(0, _Pid) ->
    io:format("done~n");
loop_first(M, Pid) ->
    Pid ! {next, M},
    receive
        {next, M} ->
            io:format("counting ~p~n", [M - 1]),
            loop_first(M - 1, Pid)
    end.

loop(N, PrevPid) ->
    receive
        {next, M} ->
            io:format("forward on #~p~n", [N + 1]),
            PrevPid ! {next, M},
            loop(N, PrevPid)
    end.