import argparse
import heapq
from collections import defaultdict
from concurrent.futures import ThreadPoolExecutor, wait
from functools import partial
from pathlib import Path
from queue import Queue


def reader(queue, path):
    with path.open("r") as f:
        for line in f:
            queue.put(line)
    queue.put(None)


def writer(queue, path):
    with path.open("w") as f:
        for line in iter_queue(queue):
            f.write(line)


def iter_queue(queue):
    while True:
        value = queue.get()
        if value is None:
            break
        yield value


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("paths", nargs="+", type=Path)
    parser.add_argument("-o", "--output", type=Path, required=True)
    ns = parser.parse_args()
    queues = defaultdict(partial(Queue, maxsize=100))
    output_queue = queues[ns.output]
    with ThreadPoolExecutor(max_workers=len(ns.paths)) as pool:
        fs = [pool.submit(reader, queues[path], path) for path in ns.paths]
        fs.append(pool.submit(writer, output_queue, ns.output))
        iterables = [iter_queue(queues[path]) for path in ns.paths]
        for line in heapq.merge(*iterables):
            output_queue.put(line)
        output_queue.put(None)
        wait(fs)


if __name__ == "__main__":
    main()