"""
This file contains the logic for cohorts, as exposed internally to the
forums, and to the cohort admin views.
"""

import logging
import random

from django.db import transaction
from django.db.models.signals import post_save, m2m_changed
from django.dispatch import receiver
from django.http import Http404
from django.utils.translation import ugettext as _

from courseware import courses
from eventtracking import tracker
from request_cache.middleware import RequestCache
from student.models import get_user_by_username_or_email

from .models import CourseUserGroup, CourseCohort, CourseCohortsSettings, CourseUserGroupPartitionGroup


log = logging.getLogger(__name__)


@receiver(post_save, sender=CourseUserGroup)
def _cohort_added(sender, **kwargs):
    """Emits a tracking log event each time a cohort is created"""
    instance = kwargs["instance"]
    if kwargs["created"] and instance.group_type == CourseUserGroup.COHORT:
        tracker.emit(
            "edx.cohort.created",
            {"cohort_id": instance.id, "cohort_name": instance.name}
        )


@receiver(m2m_changed, sender=CourseUserGroup.users.through)
def _cohort_membership_changed(sender, **kwargs):
    """Emits a tracking log event each time cohort membership is modified"""
    def get_event_iter(user_id_iter, cohort_iter):
        return (
            {"cohort_id": cohort.id, "cohort_name": cohort.name, "user_id": user_id}
            for user_id in user_id_iter
            for cohort in cohort_iter
        )

    action = kwargs["action"]
    instance = kwargs["instance"]
    pk_set = kwargs["pk_set"]
    reverse = kwargs["reverse"]

    if action == "post_add":
        event_name = "edx.cohort.user_added"
    elif action in ["post_remove", "pre_clear"]:
        event_name = "edx.cohort.user_removed"
    else:
        return

    if reverse:
        user_id_iter = [instance.id]
        if action == "pre_clear":
            cohort_iter = instance.course_groups.filter(group_type=CourseUserGroup.COHORT)
        else:
            cohort_iter = CourseUserGroup.objects.filter(pk__in=pk_set, group_type=CourseUserGroup.COHORT)
    else:
        cohort_iter = [instance] if instance.group_type == CourseUserGroup.COHORT else []
        if action == "pre_clear":
            user_id_iter = (user.id for user in instance.users.all())
        else:
            user_id_iter = pk_set

    for event in get_event_iter(user_id_iter, cohort_iter):
        tracker.emit(event_name, event)


# A 'default cohort' is an auto-cohort that is automatically created for a course if no cohort with automatic
# assignment have been specified. It is intended to be used in a cohorted-course for users who have yet to be assigned
# to a cohort.
# Translation Note: We are NOT translating this string since it is the constant identifier for the "default group"
#                   and needed across product boundaries.
DEFAULT_COHORT_NAME = "Default Group"


# tl;dr: global state is bad.  capa reseeds random every time a problem is loaded.  Even
# if and when that's fixed, it's a good idea to have a local generator to avoid any other
# code that messes with the global random module.
_local_random = None


def local_random():
    """
    Get the local random number generator.  In a function so that we don't run
    random.Random() at import time.
    """
    # ironic, isn't it?
    global _local_random

    if _local_random is None:
        _local_random = random.Random()

    return _local_random


def is_course_cohorted(course_key):
    """
    Given a course key, return a boolean for whether or not the course is
    cohorted.

    Raises:
       Http404 if the course doesn't exist.
    """
    return get_course_cohort_settings(course_key).is_cohorted


def get_cohort_id(user, course_key, use_cached=False):
    """
    Given a course key and a user, return the id of the cohort that user is
    assigned to in that course.  If they don't have a cohort, return None.
    """
    cohort = get_cohort(user, course_key, use_cached=use_cached)
    return None if cohort is None else cohort.id


def is_commentable_cohorted(course_key, commentable_id):
    """
    Args:
        course_key: CourseKey
        commentable_id: string

    Returns:
        Bool: is this commentable cohorted?

    Raises:
        Http404 if the course doesn't exist.
    """
    course = courses.get_course_by_id(course_key)
    course_cohort_settings = get_course_cohort_settings(course_key)

    if not course_cohort_settings.is_cohorted:
        # this is the easy case :)
        ans = False
    elif (
            commentable_id in course.top_level_discussion_topic_ids or
            course_cohort_settings.always_cohort_inline_discussions is False
    ):
        # top level discussions have to be manually configured as cohorted
        # (default is not).
        # Same thing for inline discussions if the default is explicitly set to False in settings
        ans = commentable_id in course_cohort_settings.cohorted_discussions
    else:
        # inline discussions are cohorted by default
        ans = True

    log.debug(u"is_commentable_cohorted({0}, {1}) = {2}".format(
        course_key, commentable_id, ans
    ))
    return ans


def get_cohorted_commentables(course_key):
    """
    Given a course_key return a set of strings representing cohorted commentables.
    """

    course_cohort_settings = get_course_cohort_settings(course_key)

    if not course_cohort_settings.is_cohorted:
        # this is the easy case :)
        ans = set()
    else:
        ans = set(course_cohort_settings.cohorted_discussions)

    return ans


@transaction.commit_on_success
def get_cohort(user, course_key, assign=True, use_cached=False):
    """Returns the user's cohort for the specified course.

    The cohort for the user is cached for the duration of a request. Pass
    use_cached=True to use the cached value instead of fetching from the
    database.

    Arguments:
        user: a Django User object.
        course_key: CourseKey
        assign (bool): if False then we don't assign a group to user
        use_cached (bool): Whether to use the cached value or fetch from database.

    Returns:
        A CourseUserGroup object if the course is cohorted and the User has a
        cohort, else None.

    Raises:
       ValueError if the CourseKey doesn't exist.
    """
    request_cache = RequestCache.get_request_cache()
    cache_key = u"cohorts.get_cohort.{}.{}".format(user.id, course_key)

    if use_cached and cache_key in request_cache.data:
        return request_cache.data[cache_key]

    request_cache.data.pop(cache_key, None)

    # First check whether the course is cohorted (users shouldn't be in a cohort
    # in non-cohorted courses, but settings can change after course starts)
    course_cohort_settings = get_course_cohort_settings(course_key)
    if not course_cohort_settings.is_cohorted:
        return request_cache.data.setdefault(cache_key, None)

    # If course is cohorted, check if the user already has a cohort.
    try:
        cohort = CourseUserGroup.objects.get(
            course_id=course_key,
            group_type=CourseUserGroup.COHORT,
            users__id=user.id,
        )
        return request_cache.data.setdefault(cache_key, cohort)
    except CourseUserGroup.DoesNotExist:
        # Didn't find the group. If we do not want to assign, return here.
        if not assign:
            # Do not cache the cohort here, because in the next call assign
            # may be True, and we will have to assign the user a cohort.
            return None

    # Otherwise assign the user a cohort.
    course = courses.get_course(course_key)
    cohorts = get_course_cohorts(course, assignment_type=CourseCohort.RANDOM)
    if cohorts:
        cohort = local_random().choice(cohorts)
    else:
        cohort = CourseCohort.create(
            cohort_name=DEFAULT_COHORT_NAME,
            course_id=course_key,
            assignment_type=CourseCohort.RANDOM
        ).course_user_group

    user.course_groups.add(cohort)

    return request_cache.data.setdefault(cache_key, cohort)


def migrate_cohort_settings(course):
    """
    Migrate all the cohort settings associated with this course from modulestore to mysql.
    After that we will never touch modulestore for any cohort related settings.
    """
    cohort_settings, created = CourseCohortsSettings.objects.get_or_create(
        course_id=course.id,
        defaults={
            'is_cohorted': course.is_cohorted,
            'cohorted_discussions': list(course.cohorted_discussions),
            'always_cohort_inline_discussions': course.always_cohort_inline_discussions
        }
    )

    # Add the new and update the existing cohorts
    if created:
        # Update the manual cohorts already present in CourseUserGroup
        manual_cohorts = CourseUserGroup.objects.filter(
            course_id=course.id,
            group_type=CourseUserGroup.COHORT
        ).exclude(name__in=course.auto_cohort_groups)
        for cohort in manual_cohorts:
            CourseCohort.create(course_user_group=cohort)

        for group_name in course.auto_cohort_groups:
            CourseCohort.create(cohort_name=group_name, course_id=course.id, assignment_type=CourseCohort.RANDOM)

    return cohort_settings


def get_course_cohorts(course, assignment_type=None):
    """
    Get a list of all the cohorts in the given course. This will include auto cohorts,
    regardless of whether or not the auto cohorts include any users.

    Arguments:
        course: the course for which cohorts should be returned
        assignment_type: cohort assignment type

    Returns:
        A list of CourseUserGroup objects. Empty if there are no cohorts. Does
        not check whether the course is cohorted.
    """
    # Migrate cohort settings for this course
    migrate_cohort_settings(course)

    query_set = CourseUserGroup.objects.filter(
        course_id=course.location.course_key,
        group_type=CourseUserGroup.COHORT
    )
    query_set = query_set.filter(cohort__assignment_type=assignment_type) if assignment_type else query_set
    return list(query_set)

### Helpers for cohort management views


def get_cohort_by_name(course_key, name):
    """
    Return the CourseUserGroup object for the given cohort.  Raises DoesNotExist
    it isn't present.
    """
    return CourseUserGroup.objects.get(
        course_id=course_key,
        group_type=CourseUserGroup.COHORT,
        name=name
    )


def get_cohort_by_id(course_key, cohort_id):
    """
    Return the CourseUserGroup object for the given cohort.  Raises DoesNotExist
    it isn't present.  Uses the course_key for extra validation.
    """
    return CourseUserGroup.objects.get(
        course_id=course_key,
        group_type=CourseUserGroup.COHORT,
        id=cohort_id
    )


def add_cohort(course_key, name, assignment_type):
    """
    Add a cohort to a course.  Raises ValueError if a cohort of the same name already
    exists.
    """
    log.debug("Adding cohort %s to %s", name, course_key)
    if is_cohort_exists(course_key, name):
        raise ValueError(_("You cannot create two cohorts with the same name"))

    try:
        course = courses.get_course_by_id(course_key)
    except Http404:
        raise ValueError("Invalid course_key")

    cohort = CourseCohort.create(
        cohort_name=name,
        course_id=course.id,
        assignment_type=assignment_type
    ).course_user_group

    tracker.emit(
        "edx.cohort.creation_requested",
        {"cohort_name": cohort.name, "cohort_id": cohort.id}
    )
    return cohort


def is_cohort_exists(course_key, name):
    """
    Check if a cohort already exists.
    """
    return CourseUserGroup.objects.filter(course_id=course_key, group_type=CourseUserGroup.COHORT, name=name).exists()


def add_user_to_cohort(cohort, username_or_email):
    """
    Look up the given user, and if successful, add them to the specified cohort.

    Arguments:
        cohort: CourseUserGroup
        username_or_email: string.  Treated as email if has '@'

    Returns:
        Tuple of User object and string (or None) indicating previous cohort

    Raises:
        User.DoesNotExist if can't find user.
        ValueError if user already present in this cohort.
    """
    user = get_user_by_username_or_email(username_or_email)
    previous_cohort_name = None
    previous_cohort_id = None

    course_cohorts = CourseUserGroup.objects.filter(
        course_id=cohort.course_id,
        users__id=user.id,
        group_type=CourseUserGroup.COHORT
    )
    if course_cohorts.exists():
        if course_cohorts[0] == cohort:
            raise ValueError("User {user_name} already present in cohort {cohort_name}".format(
                user_name=user.username,
                cohort_name=cohort.name
            ))
        else:
            previous_cohort = course_cohorts[0]
            previous_cohort.users.remove(user)
            previous_cohort_name = previous_cohort.name
            previous_cohort_id = previous_cohort.id

    tracker.emit(
        "edx.cohort.user_add_requested",
        {
            "user_id": user.id,
            "cohort_id": cohort.id,
            "cohort_name": cohort.name,
            "previous_cohort_id": previous_cohort_id,
            "previous_cohort_name": previous_cohort_name,
        }
    )
    cohort.users.add(user)
    return (user, previous_cohort_name)


def get_group_info_for_cohort(cohort, use_cached=False):
    """
    Get the ids of the group and partition to which this cohort has been linked
    as a tuple of (int, int).

    If the cohort has not been linked to any group/partition, both values in the
    tuple will be None.

    The partition group info is cached for the duration of a request. Pass
    use_cached=True to use the cached value instead of fetching from the
    database.
    """
    request_cache = RequestCache.get_request_cache()
    cache_key = u"cohorts.get_group_info_for_cohort.{}".format(cohort.id)

    if use_cached and cache_key in request_cache.data:
        return request_cache.data[cache_key]

    request_cache.data.pop(cache_key, None)

    try:
        partition_group = CourseUserGroupPartitionGroup.objects.get(course_user_group=cohort)
        return request_cache.data.setdefault(cache_key, (partition_group.group_id, partition_group.partition_id))
    except CourseUserGroupPartitionGroup.DoesNotExist:
        pass

    return request_cache.data.setdefault(cache_key, (None, None))


def set_assignment_type(user_group, assignment_type):
    """
    Set assignment type for cohort.
    """
    course_cohort = user_group.cohort

    if is_default_cohort(user_group) and course_cohort.assignment_type != assignment_type:
        raise ValueError(_("There must be one cohort to which students can automatically be assigned."))

    course_cohort.assignment_type = assignment_type
    course_cohort.save()


def get_assignment_type(user_group):
    """
    Get assignment type for cohort.
    """
    course_cohort = user_group.cohort
    return course_cohort.assignment_type


def is_default_cohort(user_group):
    """
    Check if a cohort is default.
    """
    random_cohorts = CourseUserGroup.objects.filter(
        course_id=user_group.course_id,
        group_type=CourseUserGroup.COHORT,
        cohort__assignment_type=CourseCohort.RANDOM
    )

    return len(random_cohorts) == 1 and random_cohorts[0].name == user_group.name


def set_course_cohort_settings(course_key, **kwargs):
    """
    Set cohort settings for a course.

    Arguments:
        course_key: CourseKey
        is_cohorted (bool): If the course should be cohorted.
        always_cohort_inline_discussions (bool): If inline discussions should always be cohorted.
        cohorted_discussions (list): List of discussion ids.

    Returns:
        A CourseCohortSettings object.

    Raises:
        Http404 if course_key is invalid.
    """
    fields = {'is_cohorted': bool, 'always_cohort_inline_discussions': bool, 'cohorted_discussions': list}
    course_cohort_settings = get_course_cohort_settings(course_key)
    for field, field_type in fields.items():
        if field in kwargs:
            if not isinstance(kwargs[field], field_type):
                raise ValueError("Incorrect field type for `{}`. Type must be `{}`".format(field, field_type.__name__))
            setattr(course_cohort_settings, field, kwargs[field])
    course_cohort_settings.save()
    return course_cohort_settings


def get_course_cohort_settings(course_key):
    """
    Return cohort settings for a course.

    Arguments:
        course_key: CourseKey

    Returns:
        A CourseCohortSettings object.

    Raises:
        Http404 if course_key is invalid.
    """
    try:
        course_cohort_settings = CourseCohortsSettings.objects.get(course_id=course_key)
    except CourseCohortsSettings.DoesNotExist:
        course = courses.get_course_by_id(course_key)
        course_cohort_settings = migrate_cohort_settings(course)
    return course_cohort_settings
