rot13_mapping = {}
for a, b, c, d in zip("anAN05", "mzMZ49", "naNA50", "zmZM94"):
    rot13_mapping.update(dict((chr(k), chr(v))
                              for k, v in zip(range(ord(a), ord(b) + 1),
                                              range(ord(c), ord(d) + 1))))

def right_now():
    from datetime import datetime, timezone
    return datetime.now(timezone.utc)

def nonsense_consistent(domain):
    from random import Random
    from string import ascii_lowercase
    from zlib import crc32
    rng = Random(crc32(domain.encode("utf-8")))
    length = rng.choices((9, 10, 11, 12), (4, 5, 3, 2))[0]
    return "".join(rng.choice(ascii_lowercase) for i in range(length))

def rot13(s):
    return "".join(rot13_mapping.get(c, c) for c in s)

def concat_nonsense(domain):
    return nonsense_consistent(domain) + "." + domain

def head(n, it):
    res = []
    try:
        while len(res) < n:
            res.append(next(it))
    except StopIteration:
        pass
    return res

def taskize(item):
    from types import CoroutineType
    from asyncio import Task, create_task

    if isinstance(item, CoroutineType):
        assert not isinstance(item, Task)  # TODO: paranoid?
        item = create_task(item)
    return item

def make_pooler(pool_size, finisher=None):
    # TODO: write a less confusing interface that allows the code to be written more flatly.
    # maybe like: async for done in apply(doit, [tuple_of_args]):
    from asyncio import wait, FIRST_COMPLETED

    pending = set()
    async def pooler(item=None):
        nonlocal pending
        finish = item is None
        if not finish:
            pending.add(taskize(item))
        desired_size = 0 if finish else pool_size - 1
        while len(pending) > desired_size:
            done, pending = await wait(pending, return_when=FIRST_COMPLETED)
            finisher(done, pending)
    return pooler

class AttrCheck:
    """
    Inheriting AttrCheck prevents accidentally setting attributes
    that don't already exist.
    """
    def __setattr__(self, name, value):
        # NOTE: hasattr doesn't do what we want here. dir does.
        if name.startswith("_") or name in dir(self):
            super().__setattr__(name, value)
        else:
            raise AttributeError(name)