#!/usr/bin/env python3
"""
aggregate_to_16.py
-----------------
Group a list of IPv4 addresses into /16 subnets and report the count per subnet.

Usage:
    python3 aggregate_to_16.py [path/to/file.txt]

If no file is given, the script reads from STDIN, so you can also do:
    cat ips.txt | python3 aggregate_to_16.py
"""

import sys
from collections import Counter
from ipaddress import IPv4Address, IPv4Network

def ip_to_16net(ip_str: str) -> IPv4Network:
    """
    Convert a single IPv4 string to its containing /16 network.
    Example: 10.3.45.78 → IPv4Network('10.3.0.0/16')
    """
    # IPv4Network with strict=False treats the address as a host and
    # expands it to the requested prefix length.
    return IPv4Network(f"{IPv4Address(ip_str)}/16", strict=False)


def main():
    # ------------------------------------------------------------------
    # 1️⃣  Get an iterator over the input lines (file or stdin)
    # ------------------------------------------------------------------
    if len(sys.argv) > 1:
        source = open(sys.argv[1], "r")
    else:
        source = sys.stdin

    # ------------------------------------------------------------------
    # 2️⃣  Build a Counter keyed by the /16 network string
    # ------------------------------------------------------------------
    counter = Counter()
    for raw_line in source:
        line = raw_line.strip()
        if not line:                 # skip empty lines
            continue
        try:
            net = ip_to_16net(line)
            counter[str(net)] += 1
        except ValueError:
            # If the line isn’t a valid IPv4 address we simply ignore it.
            # (You could also collect errors here if you wish.)
            continue

    # Close the file if we opened one
    if source is not sys.stdin:
        source.close()

    # ------------------------------------------------------------------
    # 3️⃣  Print the summary sorted by network address
    # ------------------------------------------------------------------
    for net in sorted(counter, key=lambda x: tuple(map(int, x.split('/')[0].split('.')))):
        #print(f"{net}\t{counter[net]} addresses")
        print(f"{net}")


if __name__ == "__main__":
    main()