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Одна из самых маленьких P2P приложений. Написано на Python.
Код:
# simplep2p.py v1.1 (C) 2004, Richard Jones

# with litle touch from Leonardo Santagada

# A slightly more readable version of tinyp2p.py (C) 2004, E.W. Felten

# (also handles binary transmissions now too)

# license: http://creativecommons.org/licenses/by-nc-sa/2.0

# Usage:

#   python simplep2p.py password server hostname portnum [otherurl]

# or

#   python simplep2p.py password client serverurl pattern

import os, SimpleXMLRPCServer, re, hmac, sets, base64

from sys import argv

from xmlrpclib import ServerProxy



def gen_password(url):

   return hmac.new(argv[1], url).hexdigest()

def ls(pat=""):

    ''' List the files in the current working directory that optionall match

    a regular expression "pat". '''

    return [fn for fn in os.listdir(os.getcwd())

        if not pat or re.search(pat, fn)]



if argv[2] == "server":

    my_url = "http://"+argv[3]+":"+argv[4]



    # keep a list of servers we know about

    servers = sets.Set([my_url] + argv[5:])

    def update_servers(new_servers=[]):

        servers.union_update(sets.Set(new_servers))

        return list(servers)

    def discover(other_url):

        if other_url == my_url: return servers

        pw = gen_password(other_url)

        server = ServerProxy(other_url)

        return update_servers(server.list_servers(pw, update_servers()))



    # ask all our known servers about the servers *they* know about

    if servers: [discover(url) for url in list(servers)]



    # serve up the files

    def list_servers(password, arg=[]):

        if password == gen_password(my_url):

            return update_servers(arg)

    def list_files(password, arg):

        if password == gen_password(my_url):

            return ls(arg)

    def get_file(password, arg):

        if password == gen_password(my_url):

            f = file(arg)

            try:

                return base64.encodestring(f.read())

            finally:

                f.close()

    server = SimpleXMLRPCServer.SimpleXMLRPCServer((argv[3], int(argv[4])))

    server.register_function(list_servers)

    server.register_function(list_files)

    server.register_function(get_file)

    server.serve_forever()



# client - contact our server

for url in ServerProxy(argv[3]).list_servers(gen_password(argv[3])):

    # ask for the files we want, that we don't already have

    files = sets.Set(ServerProxy(url).list_files(gen_password(url), argv[4]))

    my_files = sets.Set(ls())

    for fn in files - my_files:

        # and fetch

        c = ServerProxy(url).get_file(gen_password(url), fn)

        f = file(fn, "wb")

        f.write(base64.decodestring(c))

        f.close()