"""
Functions for accessing and displaying courses within the
courseware.
"""
import logging
from collections import defaultdict
from datetime import datetime

import branding
import pytz
from courseware.access import has_access
from courseware.access_response import StartDateError, MilestoneAccessError
from courseware.date_summary import (
    CourseEndDate,
    CourseStartDate,
    TodaysDate,
    VerificationDeadlineDate,
    VerifiedUpgradeDeadlineDate,
    CertificateAvailableDate
)
from courseware.model_data import FieldDataCache
from courseware.module_render import get_module
from django.conf import settings
from django.core.urlresolvers import reverse
from django.http import Http404, QueryDict
from enrollment.api import get_course_enrollment_details
from edxmako.shortcuts import render_to_string
from fs.errors import ResourceNotFound
from lms.djangoapps.courseware.courseware_access_exception import CoursewareAccessException
from lms.djangoapps.courseware.exceptions import CourseAccessRedirect
from opaque_keys.edx.keys import UsageKey
from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
from openedx.core.djangoapps.site_configuration import helpers as configuration_helpers
from path import Path as path
from six import text_type
from static_replace import replace_static_urls
from student.models import CourseEnrollment
from survey.utils import is_survey_required_and_unanswered
from util.date_utils import strftime_localized
from xmodule.modulestore.django import modulestore
from xmodule.modulestore.exceptions import ItemNotFoundError
from xmodule.x_module import STUDENT_VIEW

log = logging.getLogger(__name__)


def get_course(course_id, depth=0):
    """
    Given a course id, return the corresponding course descriptor.

    If the course does not exist, raises a ValueError.  This is appropriate
    for internal use.

    depth: The number of levels of children for the modulestore to cache.
    None means infinite depth.  Default is to fetch no children.
    """
    course = modulestore().get_course(course_id, depth=depth)
    if course is None:
        raise ValueError(u"Course not found: {0}".format(course_id))
    return course


def get_course_by_id(course_key, depth=0):
    """
    Given a course id, return the corresponding course descriptor.

    If such a course does not exist, raises a 404.

    depth: The number of levels of children for the modulestore to cache. None means infinite depth
    """
    with modulestore().bulk_operations(course_key):
        course = modulestore().get_course(course_key, depth=depth)
    if course:
        return course
    else:
        raise Http404("Course not found: {}.".format(unicode(course_key)))


def get_course_with_access(user, action, course_key, depth=0, check_if_enrolled=False, check_survey_complete=True):
    """
    Given a course_key, look up the corresponding course descriptor,
    check that the user has the access to perform the specified action
    on the course, and return the descriptor.

    Raises a 404 if the course_key is invalid, or the user doesn't have access.

    depth: The number of levels of children for the modulestore to cache. None means infinite depth

    check_if_enrolled: If true, additionally verifies that the user is either enrolled in the course
      or has staff access.
    check_survey_complete: If true, additionally verifies that the user has either completed the course survey
      or has staff access.
      Note: We do not want to continually add these optional booleans.  Ideally,
      these special cases could not only be handled inside has_access, but could
      be plugged in as additional callback checks for different actions.
    """
    course = get_course_by_id(course_key, depth)
    check_course_access(course, user, action, check_if_enrolled, check_survey_complete)
    return course


def get_course_overview_with_access(user, action, course_key, check_if_enrolled=False):
    """
    Given a course_key, look up the corresponding course overview,
    check that the user has the access to perform the specified action
    on the course, and return the course overview.

    Raises a 404 if the course_key is invalid, or the user doesn't have access.

    check_if_enrolled: If true, additionally verifies that the user is either enrolled in the course
      or has staff access.
    """
    try:
        course_overview = CourseOverview.get_from_id(course_key)
    except CourseOverview.DoesNotExist:
        raise Http404("Course not found.")
    check_course_access(course_overview, user, action, check_if_enrolled)
    return course_overview


def check_course_access(course, user, action, check_if_enrolled=False, check_survey_complete=True):
    """
    Check that the user has the access to perform the specified action
    on the course (CourseDescriptor|CourseOverview).

    check_if_enrolled: If true, additionally verifies that the user is enrolled.
    check_survey_complete: If true, additionally verifies that the user has completed the survey.
    """
    # Allow staff full access to the course even if not enrolled
    if has_access(user, 'staff', course.id):
        return

    access_response = has_access(user, action, course, course.id)
    if not access_response:
        # Redirect if StartDateError
        if isinstance(access_response, StartDateError):
            start_date = strftime_localized(course.start, 'SHORT_DATE')
            params = QueryDict(mutable=True)
            params['notlive'] = start_date
            raise CourseAccessRedirect('{dashboard_url}?{params}'.format(
                dashboard_url=reverse('dashboard'),
                params=params.urlencode()
            ), access_response)

        # Redirect if the user must answer a survey before entering the course.
        if isinstance(access_response, MilestoneAccessError):
            raise CourseAccessRedirect('{dashboard_url}'.format(
                dashboard_url=reverse('dashboard'),
            ), access_response)

        # Deliberately return a non-specific error message to avoid
        # leaking info about access control settings
        raise CoursewareAccessException(access_response)

    if check_if_enrolled:
        # If the user is not enrolled, redirect them to the about page
        if not CourseEnrollment.is_enrolled(user, course.id):
            raise CourseAccessRedirect(reverse('about_course', args=[unicode(course.id)]))

    # Redirect if the user must answer a survey before entering the course.
    if check_survey_complete and action == 'load':
        if is_survey_required_and_unanswered(user, course):
            raise CourseAccessRedirect(reverse('course_survey', args=[unicode(course.id)]))


def can_self_enroll_in_course(course_key):
    """
    Returns True if the user can enroll themselves in a course.

    Note: an example of a course that a user cannot enroll in directly
    is a CCX course. For such courses, a user can only be enrolled by
    a CCX coach.
    """
    if hasattr(course_key, 'ccx'):
        return False
    return True


def course_open_for_self_enrollment(course_key):
    """
    For a given course_key, determine if the course is available for enrollment
    """
    # Check to see if learners can enroll themselves.
    if not can_self_enroll_in_course(course_key):
        return False

    # Check the enrollment start and end dates.
    course_details = get_course_enrollment_details(unicode(course_key))
    now = datetime.now().replace(tzinfo=pytz.UTC)
    start = course_details['enrollment_start']
    end = course_details['enrollment_end']

    start = start if start is not None else now
    end = end if end is not None else now

    # If we are not within the start and end date for enrollment.
    if now < start or end < now:
        return False

    return True


def find_file(filesystem, dirs, filename):
    """
    Looks for a filename in a list of dirs on a filesystem, in the specified order.

    filesystem: an OSFS filesystem
    dirs: a list of path objects
    filename: a string

    Returns d / filename if found in dir d, else raises ResourceNotFound.
    """
    for directory in dirs:
        filepath = path(directory) / filename
        if filesystem.exists(filepath):
            return filepath
    raise ResourceNotFound(u"Could not find {0}".format(filename))


def get_course_about_section(request, course, section_key):
    """
    This returns the snippet of html to be rendered on the course about page,
    given the key for the section.

    Valid keys:
    - overview
    - short_description
    - description
    - key_dates (includes start, end, exams, etc)
    - video
    - course_staff_short
    - course_staff_extended
    - requirements
    - syllabus
    - textbook
    - faq
    - effort
    - more_info
    - ocw_links
    """

    # Many of these are stored as html files instead of some semantic
    # markup. This can change without effecting this interface when we find a
    # good format for defining so many snippets of text/html.

    html_sections = {
        'short_description',
        'description',
        'key_dates',
        'video',
        'course_staff_short',
        'course_staff_extended',
        'requirements',
        'syllabus',
        'textbook',
        'faq',
        'more_info',
        'overview',
        'effort',
        'end_date',
        'prerequisites',
        'ocw_links'
    }

    if section_key in html_sections:
        try:
            loc = course.location.replace(category='about', name=section_key)

            # Use an empty cache
            field_data_cache = FieldDataCache([], course.id, request.user)
            about_module = get_module(
                request.user,
                request,
                loc,
                field_data_cache,
                log_if_not_found=False,
                wrap_xmodule_display=False,
                static_asset_path=course.static_asset_path,
                course=course
            )

            html = ''

            if about_module is not None:
                try:
                    html = about_module.render(STUDENT_VIEW).content
                except Exception:  # pylint: disable=broad-except
                    html = render_to_string('courseware/error-message.html', None)
                    log.exception(
                        u"Error rendering course=%s, section_key=%s",
                        course, section_key
                    )
            return html

        except ItemNotFoundError:
            log.warning(
                u"Missing about section %s in course %s",
                section_key, text_type(course.location)
            )
            return None

    raise KeyError("Invalid about key " + str(section_key))


def get_course_info_usage_key(course, section_key):
    """
    Returns the usage key for the specified section's course info module.
    """
    return course.id.make_usage_key('course_info', section_key)


def get_course_info_section_module(request, user, course, section_key):
    """
    This returns the course info module for a given section_key.

    Valid keys:
    - handouts
    - guest_handouts
    - updates
    - guest_updates
    """
    usage_key = get_course_info_usage_key(course, section_key)

    # Use an empty cache
    field_data_cache = FieldDataCache([], course.id, user)

    return get_module(
        user,
        request,
        usage_key,
        field_data_cache,
        log_if_not_found=False,
        wrap_xmodule_display=False,
        static_asset_path=course.static_asset_path,
        course=course
    )


def get_course_info_section(request, user, course, section_key):
    """
    This returns the snippet of html to be rendered on the course info page,
    given the key for the section.

    Valid keys:
    - handouts
    - guest_handouts
    - updates
    - guest_updates
    """
    info_module = get_course_info_section_module(request, user, course, section_key)

    html = ''
    if info_module is not None:
        try:
            html = info_module.render(STUDENT_VIEW).content.strip()
        except Exception:  # pylint: disable=broad-except
            html = render_to_string('courseware/error-message.html', None)
            log.exception(
                u"Error rendering course_id=%s, section_key=%s",
                unicode(course.id), section_key
            )

    return html


def get_course_date_blocks(course, user):
    """
    Return the list of blocks to display on the course info page,
    sorted by date.
    """
    block_classes = (
        CertificateAvailableDate,
        CourseEndDate,
        CourseStartDate,
        TodaysDate,
        VerificationDeadlineDate,
        VerifiedUpgradeDeadlineDate,
    )

    blocks = (cls(course, user) for cls in block_classes)

    def block_key_fn(block):
        """
        If the block's date is None, return the maximum datetime in order
        to force it to the end of the list of displayed blocks.
        """
        if block.date is None:
            return datetime.max.replace(tzinfo=pytz.UTC)
        return block.date
    return sorted((b for b in blocks if b.is_enabled), key=block_key_fn)


# TODO: Fix this such that these are pulled in as extra course-specific tabs.
#       arjun will address this by the end of October if no one does so prior to
#       then.
def get_course_syllabus_section(course, section_key):
    """
    This returns the snippet of html to be rendered on the syllabus page,
    given the key for the section.

    Valid keys:
    - syllabus
    - guest_syllabus
    """

    # Many of these are stored as html files instead of some semantic
    # markup. This can change without effecting this interface when we find a
    # good format for defining so many snippets of text/html.

    if section_key in ['syllabus', 'guest_syllabus']:
        try:
            filesys = course.system.resources_fs
            # first look for a run-specific version
            dirs = [path("syllabus") / course.url_name, path("syllabus")]
            filepath = find_file(filesys, dirs, section_key + ".html")
            with filesys.open(filepath) as html_file:
                return replace_static_urls(
                    html_file.read().decode('utf-8'),
                    getattr(course, 'data_dir', None),
                    course_id=course.id,
                    static_asset_path=course.static_asset_path,
                )
        except ResourceNotFound:
            log.exception(
                u"Missing syllabus section %s in course %s",
                section_key, text_type(course.location)
            )
            return "! Syllabus missing !"

    raise KeyError("Invalid about key " + str(section_key))


def get_courses(user, org=None, filter_=None):
    """
    Returns a list of courses available, sorted by course.number and optionally
    filtered by org code (case-insensitive).
    """
    courses = branding.get_visible_courses(org=org, filter_=filter_)

    permission_name = configuration_helpers.get_value(
        'COURSE_CATALOG_VISIBILITY_PERMISSION',
        settings.COURSE_CATALOG_VISIBILITY_PERMISSION
    )

    courses = [c for c in courses if has_access(user, permission_name, c)]

    return courses


def get_permission_for_course_about():
    """
    Returns the CourseOverview object for the course after checking for access.
    """
    return configuration_helpers.get_value(
        'COURSE_ABOUT_VISIBILITY_PERMISSION',
        settings.COURSE_ABOUT_VISIBILITY_PERMISSION
    )


def sort_by_announcement(courses):
    """
    Sorts a list of courses by their announcement date. If the date is
    not available, sort them by their start date.
    """

    # Sort courses by how far are they from they start day
    key = lambda course: course.sorting_score
    courses = sorted(courses, key=key)

    return courses


def sort_by_start_date(courses):
    """
    Returns a list of courses sorted by their start date, latest first.
    """
    courses = sorted(
        courses,
        key=lambda course: (course.has_ended(), course.start is None, course.start),
        reverse=False
    )

    return courses


def get_cms_course_link(course, page='course'):
    """
    Returns a link to course_index for editing the course in cms,
    assuming that the course is actually cms-backed.
    """
    # This is fragile, but unfortunately the problem is that within the LMS we
    # can't use the reverse calls from the CMS
    return u"//{}/{}/{}".format(settings.CMS_BASE, page, unicode(course.id))


def get_cms_block_link(block, page):
    """
    Returns a link to block_index for editing the course in cms,
    assuming that the block is actually cms-backed.
    """
    # This is fragile, but unfortunately the problem is that within the LMS we
    # can't use the reverse calls from the CMS
    return u"//{}/{}/{}".format(settings.CMS_BASE, page, block.location)


def get_studio_url(course, page):
    """
    Get the Studio URL of the page that is passed in.

    Args:
        course (CourseDescriptor)
    """
    studio_link = None
    if course.course_edit_method == "Studio":
        studio_link = get_cms_course_link(course, page)
    return studio_link


def get_problems_in_section(section):
    """
    This returns a dict having problems in a section.
    Returning dict has problem location as keys and problem
    descriptor as values.
    """

    problem_descriptors = defaultdict()
    if not isinstance(section, UsageKey):
        section_key = UsageKey.from_string(section)
    else:
        section_key = section
    # it will be a Mongo performance boost, if you pass in a depth=3 argument here
    # as it will optimize round trips to the database to fetch all children for the current node
    section_descriptor = modulestore().get_item(section_key, depth=3)

    # iterate over section, sub-section, vertical
    for subsection in section_descriptor.get_children():
        for vertical in subsection.get_children():
            for component in vertical.get_children():
                if component.location.block_type == 'problem' and getattr(component, 'has_score', False):
                    problem_descriptors[unicode(component.location)] = component

    return problem_descriptors


def get_current_child(xmodule, min_depth=None, requested_child=None):
    """
    Get the xmodule.position's display item of an xmodule that has a position and
    children.  If xmodule has no position or is out of bounds, return the first
    child with children of min_depth.

    For example, if chapter_one has no position set, with two child sections,
    section-A having no children and section-B having a discussion unit,
    `get_current_child(chapter, min_depth=1)`  will return section-B.

    Returns None only if there are no children at all.
    """
    # TODO: convert this method to use the Course Blocks API
    def _get_child(children):
        """
        Returns either the first or last child based on the value of
        the requested_child parameter.  If requested_child is None,
        returns the first child.
        """
        if requested_child == 'first':
            return children[0]
        elif requested_child == 'last':
            return children[-1]
        else:
            return children[0]

    def _get_default_child_module(child_modules):
        """Returns the first child of xmodule, subject to min_depth."""
        if min_depth <= 0:
            return _get_child(child_modules)
        else:
            content_children = [
                child for child in child_modules
                if child.has_children_at_depth(min_depth - 1) and child.get_display_items()
            ]
            return _get_child(content_children) if content_children else None

    child = None
    if hasattr(xmodule, 'position'):
        children = xmodule.get_display_items()
        if len(children) > 0:
            if xmodule.position is not None and not requested_child:
                pos = xmodule.position - 1  # position is 1-indexed
                if 0 <= pos < len(children):
                    child = children[pos]
                    if min_depth > 0 and not child.has_children_at_depth(min_depth - 1):
                        child = None
            if child is None:
                child = _get_default_child_module(children)

    return child