from typing import Dict, Iterable, Tuple, Callable, TypeVar, Iterator

import os
import pty
import sys
import errno

JobData = TypeVar('JobData')

def run_parallel(job: Callable[[JobData], int],
                 data: Iterable[JobData],
                 threads: int=6) -> Iterator[Tuple[int, JobData]]:
    pids = {}  # type: Dict[int, JobData]

    def wait_for_one() -> Tuple[int, JobData]:
        while True:
            try:
                (pid, status) = os.wait()
                return status, pids.pop(pid)
            except KeyError:
                pass

    for item in data:
        pid = os.fork()
        if pid == 0:
            sys.stdin.close()
            try:
                os.close(pty.STDIN_FILENO)
            except OSError as e:
                if e.errno != errno.EBADF:
                    raise
            sys.stdin = open("/dev/null", "r")
            os._exit(job(item))

        pids[pid] = item
        threads = threads - 1

        if threads == 0:
            (status, item) = wait_for_one()
            threads += 1
            yield (status, item)
            if status != 0:
                # Stop if any error occurred
                break

    while True:
        try:
            (status, item) = wait_for_one()
            yield (status, item)
        except OSError as e:
            if e.errno == errno.ECHILD:
                break
            else:
                raise

if __name__ == "__main__":
    # run some unit tests
    import time
    jobs = [10, 19, 18, 6, 14, 12, 8, 2, 1, 13, 3, 17, 9, 11, 5, 16, 7, 15, 4]
    expected_output = [6, 10, 12, 2, 1, 14, 8, 3, 18, 19, 5, 9, 13, 11, 4, 7, 17, 16, 15]

    def wait_and_print(x: int) -> int:
        time.sleep(x * 0.1)
        return 0

    output = []
    for (status, job) in run_parallel(wait_and_print, jobs):
        output.append(job)
    if output == expected_output:
        print("Successfully passed test!")
    else:
        print("Failed test!")
        print(jobs)
        print(expected_output)
        print(output)