try:
    import _winreg as winreg
except ImportError:
    try:
        import winreg
    except ImportError:
        winreg = None

from babel.core import get_global
import pytz


# When building the cldr data on windows this module gets imported.
# Because at that point there is no global.dat yet this call will
# fail.  We want to catch it down in that case then and just assume
# the mapping was empty.
try:
    tz_names = get_global('windows_zone_mapping')
except RuntimeError:
    tz_names = {}


def valuestodict(key):
    """Convert a registry key's values to a dictionary."""
    dict = {}
    size = winreg.QueryInfoKey(key)[1]
    for i in range(size):
        data = winreg.EnumValue(key, i)
        dict[data[0]] = data[1]
    return dict


def get_localzone_name():
    # Windows is special. It has unique time zone names (in several
    # meanings of the word) available, but unfortunately, they can be
    # translated to the language of the operating system, so we need to
    # do a backwards lookup, by going through all time zones and see which
    # one matches.
    handle = winreg.ConnectRegistry(None, winreg.HKEY_LOCAL_MACHINE)

    TZLOCALKEYNAME = r'SYSTEM\CurrentControlSet\Control\TimeZoneInformation'
    localtz = winreg.OpenKey(handle, TZLOCALKEYNAME)
    keyvalues = valuestodict(localtz)
    localtz.Close()
    if 'TimeZoneKeyName' in keyvalues:
        # Windows 7 (and Vista?)

        # For some reason this returns a string with loads of NUL bytes at
        # least on some systems. I don't know if this is a bug somewhere, I
        # just work around it.
        tzkeyname = keyvalues['TimeZoneKeyName'].split('\x00', 1)[0]
    else:
        # Windows 2000 or XP

        # This is the localized name:
        tzwin = keyvalues['StandardName']

        # Open the list of timezones to look up the real name:
        TZKEYNAME = r'SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones'
        tzkey = winreg.OpenKey(handle, TZKEYNAME)

        # Now, match this value to Time Zone information
        tzkeyname = None
        for i in range(winreg.QueryInfoKey(tzkey)[0]):
            subkey = winreg.EnumKey(tzkey, i)
            sub = winreg.OpenKey(tzkey, subkey)
            data = valuestodict(sub)
            sub.Close()
            if data['Std'] == tzwin:
                tzkeyname = subkey
                break

        tzkey.Close()
        handle.Close()

    if tzkeyname is None:
        raise LookupError('Can not find Windows timezone configuration')

    timezone = tz_names.get(tzkeyname)
    if timezone is None:
        # Nope, that didn't work. Try adding 'Standard Time',
        # it seems to work a lot of times:
        timezone = tz_names.get(tzkeyname + ' Standard Time')

    # Return what we have.
    if timezone is None:
        raise pytz.UnknownTimeZoneError('Can not find timezone ' + tzkeyname)

    return timezone


def _get_localzone():
    if winreg is None:
        raise pytz.UnknownTimeZoneError(
            'Runtime support not available')
    return pytz.timezone(get_localzone_name())
