#!/usr/bin/env python3
"""Minimal BEP-5 get_peers responder for the trackerless magnet gate."""
import socket
import struct
import sys

peer_port = int(sys.argv[1])
sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind(("127.0.0.1", 0))
print("DHT_PORT %d" % sock.getsockname()[1], flush=True)

node_id = bytes(range(20))
compact_peer = socket.inet_aton("127.0.0.1") + struct.pack("!H", peer_port)

while True:
    packet, address = sock.recvfrom(65535)
    marker = b"1:t"
    start = packet.find(marker)
    if start < 0:
        continue
    length_start = start + len(marker)
    colon = packet.find(b":", length_start)
    if colon < 0:
        continue
    try:
        tx_len = int(packet[length_start:colon])
    except ValueError:
        continue
    tx = packet[colon + 1:colon + 1 + tx_len]
    if len(tx) != tx_len:
        continue
    response = (
        b"d1:rd2:id20:" + node_id +
        b"6:valuesl6:" + compact_peer +
        b"ee1:t" + str(tx_len).encode() + b":" + tx +
        b"1:y1:re"
    )
    sock.sendto(response, address)