import warnings

from django.conf import settings
from django.core.cache.backends.base import InvalidCacheBackendError

from .compat import import_attribute, get_cache


def get_backend(alias='default'):
    return get_available_backends()[alias]


def get_available_backends():
    """ Returns a dictionary of defined backend classes. For example:
    {
        'default': 'django.core.mail.backends.smtp.EmailBackend',
        'locmem': 'django.core.mail.backends.locmem.EmailBackend',
    }
    """
    backends = get_config().get('BACKENDS', {})

    if backends:
        return backends

    # Try to get backend settings from old style
    # POST_OFFICE = {
    #     'EMAIL_BACKEND': 'mybackend'
    # }
    backend = get_config().get('EMAIL_BACKEND')
    if backend:
        warnings.warn('Please use the new POST_OFFICE["BACKENDS"] settings',
                      DeprecationWarning)

        backends['default'] = backend
        return backends

    # Fall back to Django's EMAIL_BACKEND definition
    backends['default'] = getattr(
        settings, 'EMAIL_BACKEND',
        'django.core.mail.backends.smtp.EmailBackend')

    # If EMAIL_BACKEND is set to use PostOfficeBackend
    # and POST_OFFICE_BACKEND is not set, fall back to SMTP
    if 'post_office.EmailBackend' in backends['default']:
        backends['default'] = 'django.core.mail.backends.smtp.EmailBackend'

    return backends


def get_cache_backend():
    if hasattr(settings, 'CACHES'):
        if "post_office" in settings.CACHES:
            return get_cache("post_office")
        else:
            # Sometimes this raises InvalidCacheBackendError, which is ok too
            try:
                return get_cache("default")
            except InvalidCacheBackendError:
                pass
    return None


def get_config():
    """
    Returns Post Office's configuration in dictionary format. e.g:
    POST_OFFICE = {
        'BATCH_SIZE': 1000
    }
    """
    return getattr(settings, 'POST_OFFICE', {})


def get_batch_size():
    return get_config().get('BATCH_SIZE', 100)


def get_threads_per_process():
    return get_config().get('THREADS_PER_PROCESS', 5)


def get_default_priority():
    return get_config().get('DEFAULT_PRIORITY', 'medium')


def get_log_level():
    return get_config().get('LOG_LEVEL', 2)


def get_sending_order():
    return get_config().get('SENDING_ORDER', ['-priority'])


CONTEXT_FIELD_CLASS = get_config().get('CONTEXT_FIELD_CLASS',
                                       'jsonfield.JSONField')
context_field_class = import_attribute(CONTEXT_FIELD_CLASS)