#!/usr/bin/env python3
"""Real loopback TCP framing and UDP datagrams, Python standard library only."""
import json
import socket
import struct
from concurrent.futures import ThreadPoolExecutor


def read_exact(sock,count,allow_clean_eof=False):
    chunks=[];remaining=count
    while remaining:
        part=sock.recv(min(remaining,2))  # Force small API reads; not a claim about TCP packet size.
        if not part:
            if allow_clean_eof and remaining==count:return None
            raise EOFError(f'EOF with {remaining} bytes missing')
        chunks.append(part);remaining-=len(part)
    return b''.join(chunks)


def receive_message(sock):
    header=read_exact(sock,4,allow_clean_eof=True)
    if header is None:return None
    size=struct.unpack('!I',header)[0]
    if size>4096:raise ValueError('message exceeds lab limit')
    return read_exact(sock,size)


def demo():
    messages=[b'cat','網路'.encode('utf-8'),b'']
    with socket.socket() as server,ThreadPoolExecutor(max_workers=1) as pool:
        server.bind(('127.0.0.1',0));server.listen();server.settimeout(5)
        def serve():
            with server.accept()[0] as conn:
                conn.settimeout(5)
                result=[]
                while True:
                    message=receive_message(conn)
                    if message is None:return result
                    result.append(message.decode('utf-8'))
        future=pool.submit(serve)
        with socket.create_connection(server.getsockname(),timeout=5) as client:
            wire=b''.join(struct.pack('!I',len(m))+m for m in messages)
            client.sendall(wire);client.shutdown(socket.SHUT_WR)
        tcp=future.result(timeout=6)
    with socket.socket(type=socket.SOCK_DGRAM) as receiver,socket.socket(type=socket.SOCK_DGRAM) as sender:
        receiver.bind(('127.0.0.1',0));receiver.settimeout(5)
        for message in messages[:2]:sender.sendto(message,receiver.getsockname())
        udp=[receiver.recvfrom(4096)[0].decode('utf-8') for _ in range(2)]
    return {'kind':'real loopback sockets','tcp_messages':tcp,'udp_datagrams':udp,
            'note':'recv chunks are limited by application; TCP segment sizes are not measured; UDP loopback demonstration assumes no loss/reordering.'}


if __name__=='__main__':print(json.dumps(demo(),ensure_ascii=False,indent=2))