USAGE: ring:start(M,N,Msg).<br/>M: times to pass message, N: number of processes in ring, Msg: message

ring.erl

-module(ring).
-export([start_node/3, start_node/4]).
-export([start/3]).

start(M, N, Msg) when (M>0) and (N>0) -> spawn(?MODULE, start_node, [M, N, Msg]).
    

start_node(M, N, Msg) ->
    NextPid = spawn(?MODULE, start_node, [M, N-1, Msg, self()]),
    io:format("DEBUG: started first node. pid: ~p, npid: ~p, N: ~p~n", [self(), NextPid, N]),
    loop(NextPid).
    
start_node(M, 1, Msg, FirstPid) ->
    io:format("DEBUG: started last node. pid: ~p, fpid: ~p~n", [self(), FirstPid]),
    FirstPid ! {msg, Msg},
    io:format("DEBUG: sent first message to ~p~n", [FirstPid]),
    loop(M, FirstPid);
    
start_node(M, N, Msg, FirstPid) ->
    NextPid = spawn(?MODULE, start_node, [M, N-1, Msg, FirstPid]),
    io:format("DEBUG: started middle node. pid: ~p, npid: ~p, N: ~p~n", [self(), NextPid, N]),
    loop(NextPid).       
    

loop(1, FirstPid) -> 
    receive
        {msg, _Msg} ->
            io:format("DEBUG: last node ~p received last message~n", [self()]),
            io:format("DEBUG: ~p terminating ring~n", [self()]),
            FirstPid ! quit,
            loop(FirstPid)
    end;

loop(M, FirstPid) ->
    receive
        {msg, Msg} -> 
            io:format("DEBUG: last node ~p received message~n", [self()]),
            io:format("DEBUG: passing message one more time. M: ~p~n", [M]),
            FirstPid ! {msg, Msg},
            loop(M-1, FirstPid)
    end.
    
loop(NextPid) ->
    receive
        quit -> 
            NextPid ! quit,
            io:format("DEBUG: ~p quitting~n", [self()]);
        {msg, Msg}  -> 
            io:format("DEBUG: ~p received message~n", [self()]),
            NextPid ! {msg, Msg},
            io:format("DEBUG: ~p passed message to ~p~n", [self(), NextPid]),
            loop(NextPid)
    end.