from django.conf import settings
from django.core.exceptions import ValidationError
from django.utils.encoding import smart_str
import datetime
import pytz


def localtime_for_timezone(value, timezone):
    """
    Given a ``datetime.datetime`` object in UTC and a timezone represented as
    a string, return the localized time for the timezone.
    """
    if not value:
        value = datetime.datetime.now()
    return adjust_datetime_to_timezone(value, settings.TIME_ZONE, timezone)


def adjust_datetime_to_timezone(value, from_tz, to_tz=None):
    """
    Given a ``datetime`` object adjust it according to the from_tz timezone
    string into the to_tz timezone string.
    """
    if to_tz is None:
        to_tz = settings.TIME_ZONE
    if value.tzinfo is None:
        if not hasattr(from_tz, "localize"):
            from_tz = pytz.timezone(smart_str(from_tz))
        value = from_tz.localize(value)
    return value.astimezone(pytz.timezone(smart_str(to_tz)))


def coerce_timezone_value(value):
    try:
        return pytz.timezone(value)
    except pytz.UnknownTimeZoneError:
        raise ValidationError("Unknown timezone")


def validate_timezone_max_length(max_length, zones):
    def reducer(x, y):
        return x and (len(y) <= max_length)
    if not reduce(reducer, zones, True):
        raise Exception("timezones.fields.TimeZoneField MAX_TIMEZONE_LENGTH is too small")