进程环

编写一个程序，它生成N个进程并相连成环，。一旦启动，这些进程会绕环发送M个消息，然后当收到推出消息的时候正常终止。你可以调用rings:start(N,M,Msg)来启动环。

有两种策略可以完成这个练习，第一种是通过一个中央进程，它设置环并启动发送消息。第二种方法是换里面的新进程产生下一个进程。在编写程序的时候，请确保你的代码在每一个循环迭代中都有很多io:format声明，以检测到底什么发生了，什么没有发生。
    
http://www.sics.se/~joe/ericsson/du98024.html

-module(zog).

%% This is a test program that first creates N processes (that are
%% "connected" in a ring) and then sends M messages in that ring.
%%
%% - September 1998
%% - roland


-export([start/0, start/1, start/2]).

-export([run/2, process/1]).			% Local exports - ouch

start() -> start(16000).

start(N) -> start(N, 1000000).

start(N, M) -> spawn(?MODULE, run, [N, M]).


run(N, M) when N < 1 ->
    io:format("Must be at least 1 process~n", []),
    0.0;
run(N, M) ->
    statistics(wall_clock),

    Pid = setup(N-1, self()),

    {_,T1} = statistics(wall_clock),
    io:format("Setup : ~w s", [T1/1000]),
    case N of
	1 -> io:format(" (0 spawns)~n", []);
	_ -> io:format(" (~w us per spawn) (~w spawns)~n",
		       [1000*T1/(N-1), N-1])
    end,
    statistics(wall_clock),

    Pid ! M,
    K = process(Pid),

    {_,T2} = statistics(wall_clock),
    Time = 1000*T2/(M+K),
    io:format("Run   : ~w s (~w us per msg) (~w msgs)~n",
	      [T2/1000, Time, (M+K)]),

    Time.

setup(0, OldPid) ->
    OldPid;
setup(N, OldPid) ->
    NewPid = spawn(?MODULE, process, [OldPid]),
    setup(N-1, NewPid).


process(Pid) ->
    receive
	M ->
	    Pid ! M-1,
	    if
		M < 0  -> -M;
		true   -> process(Pid)
	    end
    end.

不用使用中央管理来标记各个进程，不用保存，看上去好看很多