"""
escape/unescape routines available for backends which need
alphanumeric usernames, services, or other values
"""

import re
import string
import sys

# True if we are running on Python 3.
# taken from six.py
PY3 = sys.version_info[0] == 3

# allow use of unicode literals
_unichr = chr if PY3 else unichr  # noqa: F821

LEGAL_CHARS = (
    getattr(string, 'letters', None)  # Python 2
    or getattr(string, 'ascii_letters')  # Python 3
) + string.digits

ESCAPE_FMT = "_%02X"


def _escape_char(c):
    "Single char escape. Return the char, escaped if not already legal"
    if isinstance(c, int):
        c = _unichr(c)
    return c if c in LEGAL_CHARS else ESCAPE_FMT % ord(c)


def escape(value):
    """
    Escapes given string so the result consists of alphanumeric chars and
    underscore only.
    """
    return "".join(_escape_char(c) for c in value.encode('utf-8'))


def _unescape_code(regex_match):
    ordinal = int(regex_match.group('code'), 16)
    return bytes([ordinal]) if PY3 else chr(ordinal)


def unescape(value):
    """
    Inverse of escape.
    """
    pattern = ESCAPE_FMT.replace('%02X', '(?P<code>[0-9A-Fa-f]{2})')
    # the pattern must be bytes to operate on bytes
    pattern_bytes = pattern.encode('ascii')
    re_esc = re.compile(pattern_bytes)
    return re_esc.sub(_unescape_code, value.encode('ascii')).decode('utf-8')
