import base64
import binascii
import os

from random import SystemRandom

_sysrand = SystemRandom()

randbits = _sysrand.getrandbits
choice = _sysrand.choice


def randbelow(exclusive_upper_bound):
    """Return a random int in the range [0, n)."""
    if exclusive_upper_bound <= 0:
        raise ValueError("Upper bound must be positive.")
    return _sysrand._randbelow(exclusive_upper_bound)


DEFAULT_ENTROPY = 32  # number of bytes to return by default


def token_bytes(nbytes=None):
    """Return a random byte string containing *nbytes* bytes.
    If *nbytes* is ``None`` or not supplied, a reasonable
    default is used.
    >>> token_bytes(16)  #doctest:+SKIP
    b'\\xebr\\x17D*t\\xae\\xd4\\xe3S\\xb6\\xe2\\xebP1\\x8b'
    """
    if nbytes is None:
        nbytes = DEFAULT_ENTROPY
    return os.urandom(nbytes)


def token_hex(nbytes=None):
    """Return a random text string, in hexadecimal.
    The string has *nbytes* random bytes, each byte converted to two
    hex digits.  If *nbytes* is ``None`` or not supplied, a reasonable
    default is used.
    >>> token_hex(16)  #doctest:+SKIP
    'f9bf78b9a18ce6d46a0cd2b0b86df9da'
    """
    return binascii.hexlify(token_bytes(nbytes)).decode('ascii')


def token_urlsafe(nbytes=None):
    """Return a random URL-safe text string, in Base64 encoding.
    The string has *nbytes* random bytes.  If *nbytes* is ``None``
    or not supplied, a reasonable default is used.
    >>> token_urlsafe(16)  #doctest:+SKIP
    'Drmhze6EPcv0fN_81Bj-nA'
    """
    tok = token_bytes(nbytes)
    return base64.urlsafe_b64encode(tok).rstrip(b'=').decode('ascii')
