"""
Timezone-related classes and functions.
"""

import functools
from contextlib import ContextDecorator
from datetime import datetime, timedelta, timezone, tzinfo

import pytz
from asgiref.local import Local

from django.conf import settings

__all__ = [
    'utc', 'get_fixed_timezone',
    'get_default_timezone', 'get_default_timezone_name',
    'get_current_timezone', 'get_current_timezone_name',
    'activate', 'deactivate', 'override',
    'localtime', 'now',
    'is_aware', 'is_naive', 'make_aware', 'make_naive',
]


# UTC time zone as a tzinfo instance.
utc = pytz.utc


def get_fixed_timezone(offset):
    """Return a tzinfo instance with a fixed offset from UTC."""
    if isinstance(offset, timedelta):
        offset = offset.total_seconds() // 60
    sign = '-' if offset < 0 else '+'
    hhmm = '%02d%02d' % divmod(abs(offset), 60)
    name = sign + hhmm
    return timezone(timedelta(minutes=offset), name)


# In order to avoid accessing settings at compile time,
# wrap the logic in a function and cache the result.
@functools.lru_cache()
def get_default_timezone():
    """
    Return the default time zone as a tzinfo instance.

    This is the time zone defined by settings.TIME_ZONE.
    """
    return pytz.timezone(settings.TIME_ZONE)


# This function exists for consistency with get_current_timezone_name
def get_default_timezone_name():
    """Return the name of the default time zone."""
    return _get_timezone_name(get_default_timezone())


_active = Local()


def get_current_timezone():
    """Return the currently active time zone as a tzinfo instance."""
    return getattr(_active, "value", get_default_timezone())


def get_current_timezone_name():
    """Return the name of the currently active time zone."""
    return _get_timezone_name(get_current_timezone())


def _get_timezone_name(timezone):
    """Return the name of ``timezone``."""
    return timezone.tzname(None)

# Timezone selection functions.

# These functions don't change os.environ['TZ'] and call time.tzset()
# because it isn't thread safe.


def activate(timezone):
    """
    Set the time zone for the current thread.

    The ``timezone`` argument must be an instance of a tzinfo subclass or a
    time zone name.
    """
    if isinstance(timezone, tzinfo):
        _active.value = timezone
    elif isinstance(timezone, str):
        _active.value = pytz.timezone(timezone)
    else:
        raise ValueError("Invalid timezone: %r" % timezone)


def deactivate():
    """
    Unset the time zone for the current thread.

    Django will then use the time zone defined by settings.TIME_ZONE.
    """
    if hasattr(_active, "value"):
        del _active.value


class override(ContextDecorator):
    """
    Temporarily set the time zone for the current thread.

    This is a context manager that uses django.utils.timezone.activate()
    to set the timezone on entry and restores the previously active timezone
    on exit.

    The ``timezone`` argument must be an instance of a ``tzinfo`` subclass, a
    time zone name, or ``None``. If it is ``None``, Django enables the default
    time zone.
    """
    def __init__(self, timezone):
        self.timezone = timezone

    def __enter__(self):
        self.old_timezone = getattr(_active, 'value', None)
        if self.timezone is None:
            deactivate()
        else:
            activate(self.timezone)

    def __exit__(self, exc_type, exc_value, traceback):
        if self.old_timezone is None:
            deactivate()
        else:
            _active.value = self.old_timezone


# Templates

def template_localtime(value, use_tz=None):
    """
    Check if value is a datetime and converts it to local time if necessary.

    If use_tz is provided and is not None, that will force the value to
    be converted (or not), overriding the value of settings.USE_TZ.

    This function is designed for use by the template engine.
    """
    should_convert = (
        isinstance(value, datetime) and
        (settings.USE_TZ if use_tz is None else use_tz) and
        not is_naive(value) and
        getattr(value, 'convert_to_local_time', True)
    )
    return localtime(value) if should_convert else value


# Utilities

def localtime(value=None, timezone=None):
    """
    Convert an aware datetime.datetime to local time.

    Only aware datetimes are allowed. When value is omitted, it defaults to
    now().

    Local time is defined by the current time zone, unless another time zone
    is specified.
    """
    if value is None:
        value = now()
    if timezone is None:
        timezone = get_current_timezone()
    # Emulate the behavior of astimezone() on Python < 3.6.
    if is_naive(value):
        raise ValueError("localtime() cannot be applied to a naive datetime")
    return value.astimezone(timezone)


def localdate(value=None, timezone=None):
    """
    Convert an aware datetime to local time and return the value's date.

    Only aware datetimes are allowed. When value is omitted, it defaults to
    now().

    Local time is defined by the current time zone, unless another time zone is
    specified.
    """
    return localtime(value, timezone).date()


def now():
    """
    Return an aware or naive datetime.datetime, depending on settings.USE_TZ.
    """
    if settings.USE_TZ:
        # timeit shows that datetime.now(tz=utc) is 24% slower
        return datetime.utcnow().replace(tzinfo=utc)
    else:
        return datetime.now()


# By design, these four functions don't perform any checks on their arguments.
# The caller should ensure that they don't receive an invalid value like None.

def is_aware(value):
    """
    Determine if a given datetime.datetime is aware.

    The concept is defined in Python's docs:
    https://docs.python.org/library/datetime.html#datetime.tzinfo

    Assuming value.tzinfo is either None or a proper datetime.tzinfo,
    value.utcoffset() implements the appropriate logic.
    """
    return value.utcoffset() is not None


def is_naive(value):
    """
    Determine if a given datetime.datetime is naive.

    The concept is defined in Python's docs:
    https://docs.python.org/library/datetime.html#datetime.tzinfo

    Assuming value.tzinfo is either None or a proper datetime.tzinfo,
    value.utcoffset() implements the appropriate logic.
    """
    return value.utcoffset() is None


def make_aware(value, timezone=None, is_dst=None):
    """Make a naive datetime.datetime in a given time zone aware."""
    if timezone is None:
        timezone = get_current_timezone()
    if hasattr(timezone, 'localize'):
        # This method is available for pytz time zones.
        return timezone.localize(value, is_dst=is_dst)
    else:
        # Check that we won't overwrite the timezone of an aware datetime.
        if is_aware(value):
            raise ValueError(
                "make_aware expects a naive datetime, got %s" % value)
        # This may be wrong around DST changes!
        return value.replace(tzinfo=timezone)


def make_naive(value, timezone=None):
    """Make an aware datetime.datetime naive in a given time zone."""
    if timezone is None:
        timezone = get_current_timezone()
    # Emulate the behavior of astimezone() on Python < 3.6.
    if is_naive(value):
        raise ValueError("make_naive() cannot be applied to a naive datetime")
    return value.astimezone(timezone).replace(tzinfo=None)
