import math
from typing import Callable, Iterable, Optional, TypeVar

_T = TypeVar("_T")


def first(
    iterable: Iterable[_T], condition: Callable[[_T], bool] = lambda x: True
) -> Optional[_T]:
    return next((x for x in iterable if condition(x)), None)


def percentile(
    N: Iterable[float], percent: float, key: Callable[[float], float] = lambda x: x
) -> Optional[float]:
    if not N:
        return None
    N = sorted(N)
    k = (len(N) - 1) * percent
    f = math.floor(k)
    c = math.ceil(k)
    if f == c:
        return key(N[int(k)])
    d0 = key(N[int(f)]) * (c - k)
    d1 = key(N[int(c)]) * (k - f)
    return d0 + d1