from youtube import util, yt_data_extract, channel
from youtube import yt_app
import settings

import sqlite3
import os
import time
import gevent
import json
import traceback
import contextlib
import defusedxml.ElementTree
import urllib
import math
import secrets

import flask
from flask import request


thumbnails_directory = os.path.join(settings.data_dir, "subscription_thumbnails")

# https://stackabuse.com/a-sqlite-tutorial-with-python/

database_path = os.path.join(settings.data_dir, "subscriptions.sqlite")

def open_database():
    if not os.path.exists(settings.data_dir):
        os.makedirs(settings.data_dir)
    connection = sqlite3.connect(database_path, check_same_thread=False)

    try:
        cursor = connection.cursor()
        cursor.execute('''PRAGMA foreign_keys = 1''')
        # Create tables if they don't exist
        cursor.execute('''CREATE TABLE IF NOT EXISTS subscribed_channels (
                              id integer PRIMARY KEY,
                              yt_channel_id text UNIQUE NOT NULL,
                              channel_name text NOT NULL,
                              time_last_checked integer,
                              next_check_time integer,
                              muted integer DEFAULT 0
                          )''')
        cursor.execute('''CREATE TABLE IF NOT EXISTS videos (
                              id integer PRIMARY KEY,
                              sql_channel_id integer NOT NULL REFERENCES subscribed_channels(id) ON UPDATE CASCADE ON DELETE CASCADE,
                              video_id text UNIQUE NOT NULL,
                              title text NOT NULL,
                              duration text,
                              time_published integer NOT NULL,
                              description text
                          )''')
        cursor.execute('''CREATE TABLE IF NOT EXISTS tag_associations (
                              id integer PRIMARY KEY,
                              tag text NOT NULL,
                              sql_channel_id integer NOT NULL REFERENCES subscribed_channels(id) ON UPDATE CASCADE ON DELETE CASCADE,
                              UNIQUE(tag, sql_channel_id)
                          )''')

        connection.commit()
    except:
        connection.rollback()
        connection.close()
        raise

    # https://stackoverflow.com/questions/19522505/using-sqlite3-in-python-with-with-keyword
    return contextlib.closing(connection)

def with_open_db(function, *args, **kwargs):
    with open_database() as connection:
        with connection as cursor:
            return function(cursor, *args, **kwargs)

def is_subscribed(channel_id):
    if not os.path.exists(database_path):
        return False

    with open_database() as connection:
        with connection as cursor:
            result = cursor.execute('''SELECT EXISTS(
                                           SELECT 1
                                           FROM subscribed_channels
                                           WHERE yt_channel_id=?
                                           LIMIT 1
                                       )''', [channel_id]).fetchone()
            return bool(result[0])


def _subscribe(cursor, channels):
    ''' channels is a list of (channel_id, channel_name) '''

    # set time_last_checked to 0 on all channels being subscribed to
    channels = ( (channel_id, channel_name, 0) for channel_id, channel_name in channels)

    cursor.executemany('''INSERT OR IGNORE INTO subscribed_channels (yt_channel_id, channel_name, time_last_checked)
                          VALUES (?, ?, ?)''', channels)


def delete_thumbnails(to_delete):
    for thumbnail in to_delete:
        try:
            video_id = thumbnail[0:-4]
            if video_id in existing_thumbnails:
                os.remove(os.path.join(thumbnails_directory, thumbnail))
                existing_thumbnails.remove(video_id)
        except Exception:
            print('Failed to delete thumbnail: ' + thumbnail)
            traceback.print_exc()

def _unsubscribe(cursor, channel_ids):
    ''' channel_ids is a list of channel_ids '''
    to_delete = []
    for channel_id in channel_ids:
        rows = cursor.execute('''SELECT video_id
                                 FROM videos
                                 WHERE sql_channel_id = (
                                     SELECT id
                                     FROM subscribed_channels
                                     WHERE yt_channel_id=?
                                 )''', (channel_id,)).fetchall()
        to_delete += [row[0] + '.jpg' for row in rows]

    gevent.spawn(delete_thumbnails, to_delete)
    cursor.executemany("DELETE FROM subscribed_channels WHERE yt_channel_id=?", ((channel_id, ) for channel_id in channel_ids))

def _get_videos(cursor, number_per_page, offset, tag = None):
    '''Returns a full page of videos with an offset, and a value good enough to be used as the total number of videos'''
    # We ask for the next 9 pages from the database
    # Then the actual length of the results tell us if there are more than 9 pages left, and if not, how many there actually are
    # This is done since there are only 9 page buttons on display at a time
    # If there are more than 9 pages left, we give a fake value in place of the real number of results if the entire database was queried without limit
    # This fake value is sufficient to get the page button generation macro to display 9 page buttons
    # If we wish to display more buttons this logic must change
    # We cannot use tricks with the sql id for the video since we frequently have filters and other restrictions in place on the results anyway
    # TODO: This is probably not the ideal solution
    if tag is not None:
        db_videos = cursor.execute('''SELECT video_id, title, duration, channel_name
                                      FROM videos
                                      INNER JOIN subscribed_channels on videos.sql_channel_id = subscribed_channels.id
                                      INNER JOIN tag_associations on videos.sql_channel_id = tag_associations.sql_channel_id
                                      WHERE tag = ?
                                      ORDER BY time_published DESC
                                      LIMIT ? OFFSET ?''', (tag, number_per_page*9, offset)).fetchall()
    else:
        db_videos = cursor.execute('''SELECT video_id, title, duration, channel_name
                                      FROM videos
                                      INNER JOIN subscribed_channels on videos.sql_channel_id = subscribed_channels.id
                                      ORDER BY time_published DESC
                                      LIMIT ? OFFSET ?''', (number_per_page*9, offset)).fetchall()

    pseudo_number_of_videos = offset + len(db_videos)

    videos = []
    for db_video in db_videos[0:number_per_page]:
        videos.append({
            'id':   db_video[0],
            'title':    db_video[1],
            'duration': db_video[2],
            'author':   db_video[3],
        })

    return videos, pseudo_number_of_videos




def _get_subscribed_channels(cursor):
    for item in cursor.execute('''SELECT channel_name, yt_channel_id, muted
                                  FROM subscribed_channels
                                  ORDER BY channel_name COLLATE NOCASE'''):
        yield item


def _add_tags(cursor, channel_ids, tags):
    pairs = [(tag, yt_channel_id) for tag in tags for yt_channel_id in channel_ids]
    cursor.executemany('''INSERT OR IGNORE INTO tag_associations (tag, sql_channel_id)
                          SELECT ?, id FROM subscribed_channels WHERE yt_channel_id = ? ''', pairs)


def _remove_tags(cursor, channel_ids, tags):
    pairs = [(tag, yt_channel_id) for tag in tags for yt_channel_id in channel_ids]
    cursor.executemany('''DELETE FROM tag_associations
                          WHERE tag = ? AND sql_channel_id = (
                              SELECT id FROM subscribed_channels WHERE yt_channel_id = ?
                           )''', pairs)



def _get_tags(cursor, channel_id):
    return [row[0] for row in cursor.execute('''SELECT tag
                                                FROM tag_associations
                                                WHERE sql_channel_id = (
                                                    SELECT id FROM subscribed_channels WHERE yt_channel_id = ?
                                                )''', (channel_id,))]

def _get_all_tags(cursor):
    return [row[0] for row in cursor.execute('''SELECT DISTINCT tag FROM tag_associations''')]

def _get_channel_names(cursor, channel_ids):
    ''' returns list of (channel_id, channel_name) '''
    result = []
    for channel_id in channel_ids:
        row = cursor.execute('''SELECT channel_name
                                FROM subscribed_channels
                                WHERE yt_channel_id = ?''', (channel_id,)).fetchone()
        result.append( (channel_id, row[0]) )
    return result


def _channels_with_tag(cursor, tag, order=False, exclude_muted=False, include_muted_status=False):
    ''' returns list of (channel_id, channel_name) '''

    statement = '''SELECT yt_channel_id, channel_name'''

    if include_muted_status:
        statement += ''', muted'''

    statement += '''
                   FROM subscribed_channels
                   WHERE subscribed_channels.id IN (
                       SELECT tag_associations.sql_channel_id FROM tag_associations WHERE tag=?
                   )
                '''
    if exclude_muted:
        statement += '''AND muted != 1\n'''
    if order:
        statement += '''ORDER BY channel_name COLLATE NOCASE'''

    return cursor.execute(statement, [tag]).fetchall()

def _schedule_checking(cursor, channel_id, next_check_time):
    cursor.execute('''UPDATE subscribed_channels SET next_check_time = ? WHERE yt_channel_id = ?''', [int(next_check_time), channel_id])

def _is_muted(cursor, channel_id):
    return bool(cursor.execute('''SELECT muted FROM subscribed_channels WHERE yt_channel_id=?''', [channel_id]).fetchone()[0])

units = {
    'year': 31536000,   # 365*24*3600
    'month': 2592000,   # 30*24*3600
    'week': 604800,     # 7*24*3600
    'day':  86400,      # 24*3600
    'hour': 3600,
    'minute': 60,
    'second': 1,
}
def youtube_timestamp_to_posix(dumb_timestamp):
    ''' Given a dumbed down timestamp such as 1 year ago, 3 hours ago,
         approximates the unix time (seconds since 1/1/1970) '''
    dumb_timestamp = dumb_timestamp.lower()
    now = time.time()
    if dumb_timestamp == "just now":
        return now
    split = dumb_timestamp.split(' ')
    number, unit = int(split[0]), split[1]
    if number > 1:
        unit = unit[:-1]    # remove s from end
    return now - number*units[unit]


try:
    existing_thumbnails = set(os.path.splitext(name)[0] for name in os.listdir(thumbnails_directory))
except FileNotFoundError:
    existing_thumbnails = set()


# --- Manual checking system. Rate limited in order to support very large numbers of channels to be checked ---
# Auto checking system plugs into this for convenience, though it doesn't really need the rate limiting

check_channels_queue = util.RateLimitedQueue()
checking_channels = set()

# Just to use for printing channel checking status to console without opening database
channel_names = dict()

def check_channel_worker():
    while True:
        channel_id = check_channels_queue.get()
        try:
            _get_upstream_videos(channel_id)
        finally:
            checking_channels.remove(channel_id)

for i in range(0,5):
    gevent.spawn(check_channel_worker)
# ----------------------------



# --- Auto checking system ---

if settings.autocheck_subscriptions:
    # job application format: dict with keys (channel_id, channel_name, next_check_time)
    autocheck_job_application = gevent.queue.Queue() # only really meant to hold 1 item, just reusing gevent's wait and timeout machinery

    autocheck_jobs = [] # list of dicts with the keys (channel_id, channel_name, next_check_time). Stores all the channels that need to be autochecked and when to check them
    with open_database() as connection:
        with connection as cursor:
            now = time.time()
            for row in cursor.execute('''SELECT yt_channel_id, channel_name, next_check_time FROM subscribed_channels WHERE next_check_time IS NOT NULL AND muted != 1''').fetchall():
                if row[2] < now:    # expired, check randomly within the 30 minutes
                    next_check_time = now + 3600*secrets.randbelow(60)/60
                    row = (row[0], row[1], next_check_time)
                    _schedule_checking(cursor, row[0], next_check_time)
                autocheck_jobs.append({'channel_id': row[0], 'channel_name': row[1], 'next_check_time': row[2]})



    def autocheck_dispatcher():
        '''Scans the auto_check_list. Sleeps until the earliest job is due, then adds that channel to the checking queue above. Can be sent a new job through autocheck_job_application'''
        while True:
            if len(autocheck_jobs) == 0:
                new_job = autocheck_job_application.get()
                autocheck_jobs.append(new_job)
            else:
                earliest_job_index = min(range(0, len(autocheck_jobs)), key=lambda index: autocheck_jobs[index]['next_check_time']) # https://stackoverflow.com/a/11825864
                earliest_job = autocheck_jobs[earliest_job_index]
                time_until_earliest_job = earliest_job['next_check_time'] - time.time()

                if time_until_earliest_job <= 0:
                    print('ERROR: autocheck_dispatcher got job scheduled in the past, skipping and rescheduling: ' + earliest_job['channel_id'] + ', ' + earliest_job['channel_name'] + ', ' + str(earliest_job['next_check_time']))
                    next_check_time = time.time() + 3600*secrets.randbelow(60)/60
                    with_open_db(_schedule_checking, earliest_job['channel_id'], next_check_time)
                    autocheck_jobs[earliest_job_index]['next_check_time'] = next_check_time
                    continue

                # make sure it's not muted
                if with_open_db(_is_muted, earliest_job['channel_id']):
                    del autocheck_jobs[earliest_job_index]
                    continue

                try:
                    new_job = autocheck_job_application.get(timeout = time_until_earliest_job)  # sleep for time_until_earliest_job time, but allow to be interrupted by new jobs
                except gevent.queue.Empty: # no new jobs, time to execute the earliest job
                    channel_names[earliest_job['channel_id']] = earliest_job['channel_name']
                    checking_channels.add(earliest_job['channel_id'])
                    check_channels_queue.put(earliest_job['channel_id'])
                    del autocheck_jobs[earliest_job_index]
                else: # new job, add it to the list
                    autocheck_jobs.append(new_job)


    gevent.spawn(autocheck_dispatcher)
# ----------------------------



def check_channels_if_necessary(channel_ids):
    for channel_id in channel_ids:
        if channel_id not in checking_channels:
            checking_channels.add(channel_id)
            check_channels_queue.put(channel_id)



def _get_upstream_videos(channel_id):
    try:
        print("Checking channel: " + channel_names[channel_id])
    except KeyError:
        print("Checking channel " + channel_id)

    videos = []

    channel_videos = channel.extract_info(json.loads(channel.get_channel_tab(channel_id)), 'videos')['items']
    for i, video_item in enumerate(channel_videos):
        if 'description' not in video_item:
            video_item['description'] = ''
        try:
            video_item['time_published'] = youtube_timestamp_to_posix(video_item['published']) - i  # subtract a few seconds off the videos so they will be in the right order
        except KeyError:
            print(video_item)
        videos.append((channel_id, video_item['id'], video_item['title'], video_item['duration'], video_item['time_published'], video_item['description']))


    if len(videos) == 0:
        average_upload_period = 4*7*24*3600 # assume 1 month for channel with no videos
    elif len(videos) < 5:
        average_upload_period = int((time.time() - videos[len(videos)-1][4])/len(videos))
    else:
        average_upload_period = int((time.time() - videos[4][4])/5) # equivalent to averaging the time between videos for the last 5 videos

    # calculate when to check next for auto checking
    # add some quantization and randomness to make pattern analysis by Youtube slightly harder
    quantized_upload_period = average_upload_period - (average_upload_period % (4*3600)) + 4*3600   # round up to nearest 4 hours
    randomized_upload_period = quantized_upload_period*(1 + secrets.randbelow(50)/50*0.5) # randomly between 1x and 1.5x
    next_check_delay = randomized_upload_period/10    # check at 10x the channel posting rate. might want to fine tune this number
    next_check_time = int(time.time() + next_check_delay)

    with open_database() as connection:
        with connection as cursor:
            cursor.executemany('''INSERT OR IGNORE INTO videos (sql_channel_id, video_id, title, duration, time_published, description)
                                  VALUES ((SELECT id FROM subscribed_channels WHERE yt_channel_id=?), ?, ?, ?, ?, ?)''', videos)
            cursor.execute('''UPDATE subscribed_channels
                              SET time_last_checked = ?, next_check_time = ?
                              WHERE yt_channel_id=?''', [int(time.time()), next_check_time, channel_id])

            if settings.autocheck_subscriptions:
                if not _is_muted(cursor, channel_id):
                    autocheck_job_application.put({'channel_id': channel_id, 'channel_name': channel_names[channel_id], 'next_check_time': next_check_time})


def check_all_channels():
    with open_database() as connection:
        with connection as cursor:
            channel_id_name_list = cursor.execute('''SELECT yt_channel_id, channel_name
                                                     FROM subscribed_channels
                                                     WHERE muted != 1''').fetchall()

    channel_names.update(channel_id_name_list)
    check_channels_if_necessary([item[0] for item in channel_id_name_list])


def check_tags(tags):
    channel_id_name_list = []
    with open_database() as connection:
        with connection as cursor:
            for tag in tags:
                channel_id_name_list += _channels_with_tag(cursor, tag, exclude_muted=True)

    channel_names.update(channel_id_name_list)
    check_channels_if_necessary([item[0] for item in channel_id_name_list])


def check_specific_channels(channel_ids):
    with open_database() as connection:
        with connection as cursor:
            channel_id_name_list = []
            for channel_id in channel_ids:
                channel_id_name_list += cursor.execute('''SELECT yt_channel_id, channel_name
                                                          FROM subscribed_channels
                                                          WHERE yt_channel_id=?''', [channel_id]).fetchall()
    channel_names.update(channel_id_name_list)
    check_channels_if_necessary(channel_ids)



@yt_app.route('/import_subscriptions', methods=['POST'])
def import_subscriptions():

    # check if the post request has the file part
    if 'subscriptions_file' not in request.files:
        #flash('No file part')
        return flask.redirect(util.URL_ORIGIN + request.full_path)
    file = request.files['subscriptions_file']
    # if user does not select file, browser also
    # submit an empty part without filename
    if file.filename == '':
        #flash('No selected file')
        return flask.redirect(util.URL_ORIGIN + request.full_path)


    mime_type = file.mimetype

    if mime_type == 'application/json':
        file = file.read().decode('utf-8')
        try:
            file = json.loads(file)
        except json.decoder.JSONDecodeError:
            traceback.print_exc()
            return '400 Bad Request: Invalid json file', 400

        try:
            channels = ( (item['snippet']['resourceId']['channelId'], item['snippet']['title']) for item in file)
        except (KeyError, IndexError):
            traceback.print_exc()
            return '400 Bad Request: Unknown json structure', 400
    elif mime_type in ('application/xml', 'text/xml', 'text/x-opml'):
        file = file.read().decode('utf-8')
        try:
            root = defusedxml.ElementTree.fromstring(file)
            assert root.tag == 'opml'
            channels = []
            for outline_element in root[0][0]:
                if (outline_element.tag != 'outline') or ('xmlUrl' not in outline_element.attrib):
                    continue


                channel_name = outline_element.attrib['text']
                channel_rss_url = outline_element.attrib['xmlUrl']
                channel_id = channel_rss_url[channel_rss_url.find('channel_id=')+11:].strip()
                channels.append( (channel_id, channel_name) )

        except (AssertionError, IndexError, defusedxml.ElementTree.ParseError) as e:
            return '400 Bad Request: Unable to read opml xml file, or the file is not the expected format', 400
    else:
            return '400 Bad Request: Unsupported file format: ' + mime_type + '. Only subscription.json files (from Google Takeouts) and XML OPML files exported from Youtube\'s subscription manager page are supported', 400

    with_open_db(_subscribe, channels)

    return flask.redirect(util.URL_ORIGIN + '/subscription_manager', 303)



@yt_app.route('/subscription_manager', methods=['GET'])
def get_subscription_manager_page():
    group_by_tags = request.args.get('group_by_tags', '0') == '1'
    with open_database() as connection:
        with connection as cursor:
            if group_by_tags:
                tag_groups = []

                for tag in _get_all_tags(cursor):
                    sub_list = []
                    for channel_id, channel_name, muted in _channels_with_tag(cursor, tag, order=True, include_muted_status=True):
                        sub_list.append({
                            'channel_url': util.URL_ORIGIN + '/channel/' + channel_id,
                            'channel_name': channel_name,
                            'channel_id': channel_id,
                            'muted': muted,
                            'tags': [t for t in _get_tags(cursor, channel_id) if t != tag],
                        })

                    tag_groups.append( (tag, sub_list) )

                # Channels with no tags
                channel_list = cursor.execute('''SELECT yt_channel_id, channel_name, muted
                                                 FROM subscribed_channels
                                                 WHERE id NOT IN (
                                                     SELECT sql_channel_id FROM tag_associations
                                                 )
                                                 ORDER BY channel_name COLLATE NOCASE''').fetchall()
                if channel_list:
                    sub_list = []
                    for channel_id, channel_name, muted in channel_list:
                        sub_list.append({
                            'channel_url': util.URL_ORIGIN + '/channel/' + channel_id,
                            'channel_name': channel_name,
                            'channel_id': channel_id,
                            'muted': muted,
                            'tags': [],
                        })

                    tag_groups.append( ('No tags', sub_list) )
            else:
                sub_list = []
                for channel_name, channel_id, muted in _get_subscribed_channels(cursor):
                    sub_list.append({
                        'channel_url': util.URL_ORIGIN + '/channel/' + channel_id,
                        'channel_name': channel_name,
                        'channel_id': channel_id,
                        'muted': muted,
                        'tags': _get_tags(cursor, channel_id),
                    })




    if group_by_tags:
        return flask.render_template('subscription_manager.html',
            group_by_tags = True,
            tag_groups = tag_groups,
        )
    else:
        return flask.render_template('subscription_manager.html',
            group_by_tags = False,
            sub_list = sub_list,
        )

def list_from_comma_separated_tags(string):
    return [tag.strip() for tag in string.split(',') if tag.strip()]


@yt_app.route('/subscription_manager', methods=['POST'])
def post_subscription_manager_page():
    action = request.values['action']

    with open_database() as connection:
        with connection as cursor:
            if action == 'add_tags':
                _add_tags(cursor, request.values.getlist('channel_ids'), [tag.lower() for tag in list_from_comma_separated_tags(request.values['tags'])])
            elif action == 'remove_tags':
                _remove_tags(cursor, request.values.getlist('channel_ids'), [tag.lower() for tag in list_from_comma_separated_tags(request.values['tags'])])
            elif action == 'unsubscribe':
                _unsubscribe(cursor, request.values.getlist('channel_ids'))
            elif action == 'unsubscribe_verify':
                unsubscribe_list = _get_channel_names(cursor, request.values.getlist('channel_ids'))
                return flask.render_template('unsubscribe_verify.html', unsubscribe_list = unsubscribe_list)

            elif action == 'mute':
                cursor.executemany('''UPDATE subscribed_channels
                                      SET muted = 1
                                      WHERE yt_channel_id = ?''', [(ci,) for ci in request.values.getlist('channel_ids')])
            elif action == 'unmute':
                cursor.executemany('''UPDATE subscribed_channels
                                      SET muted = 0
                                      WHERE yt_channel_id = ?''', [(ci,) for ci in request.values.getlist('channel_ids')])
            else:
                flask.abort(400)

    return flask.redirect(util.URL_ORIGIN + request.full_path, 303)

@yt_app.route('/subscriptions', methods=['GET'])
@yt_app.route('/feed/subscriptions', methods=['GET'])
def get_subscriptions_page():
    page = int(request.args.get('page', 1))
    with open_database() as connection:
        with connection as cursor:
            tag = request.args.get('tag', None)
            videos, number_of_videos_in_db = _get_videos(cursor, 60, (page - 1)*60, tag)
            for video in videos:
                video['thumbnail'] = util.URL_ORIGIN + '/data/subscription_thumbnails/' + video['id'] + '.jpg'
                video['type'] = 'video'
                video['item_size'] = 'small'
                yt_data_extract.add_extra_html_info(video)

            tags = _get_all_tags(cursor)


            subscription_list = []
            for channel_name, channel_id, muted in _get_subscribed_channels(cursor):
                subscription_list.append({
                    'channel_url': util.URL_ORIGIN + '/channel/' + channel_id,
                    'channel_name': channel_name,
                    'channel_id': channel_id,
                    'muted': muted,
                })

    return flask.render_template('subscriptions.html',
        videos = videos,
        num_pages = math.ceil(number_of_videos_in_db/60),
        parameters_dictionary = request.args,
        tags = tags,
        current_tag = tag,
        subscription_list = subscription_list,
    )

@yt_app.route('/subscriptions', methods=['POST'])
@yt_app.route('/feed/subscriptions', methods=['POST'])
def post_subscriptions_page():
    action = request.values['action']
    if action == 'subscribe':
        if len(request.values.getlist('channel_id')) != len(request.values.getlist('channel_name')):
            return '400 Bad Request, length of channel_id != length of channel_name', 400
        with_open_db(_subscribe, zip(request.values.getlist('channel_id'), request.values.getlist('channel_name')))

    elif action == 'unsubscribe':
        with_open_db(_unsubscribe, request.values.getlist('channel_id'))

    elif action == 'refresh':
        type = request.values['type']
        if type == 'all':
            check_all_channels()
        elif type == 'tag':
            check_tags(request.values.getlist('tag_name'))
        elif type == 'channel':
            check_specific_channels(request.values.getlist('channel_id'))
        else:
            flask.abort(400)
    else:
        flask.abort(400)

    return '', 204


@yt_app.route('/data/subscription_thumbnails/<thumbnail>')
def serve_subscription_thumbnail(thumbnail):
    '''Serves thumbnail from disk if it's been saved already. If not, downloads the thumbnail, saves to disk, and serves it.'''
    assert thumbnail[-4:] == '.jpg'
    video_id = thumbnail[0:-4]
    thumbnail_path = os.path.join(thumbnails_directory, thumbnail)

    if video_id in existing_thumbnails:
        try:
            f = open(thumbnail_path, 'rb')
        except FileNotFoundError:
            existing_thumbnails.remove(video_id)
        else:
            image = f.read()
            f.close()
            return flask.Response(image, mimetype='image/jpeg')

    url = "https://i.ytimg.com/vi/" + video_id + "/mqdefault.jpg"
    try:
        image = util.fetch_url(url, report_text="Saved thumbnail: " + video_id)
    except urllib.error.HTTPError as e:
        print("Failed to download thumbnail for " + video_id + ": " + str(e))
        abort(e.code)
    try:
        f = open(thumbnail_path, 'wb')
    except FileNotFoundError:
        os.makedirs(thumbnails_directory, exist_ok = True)
        f = open(thumbnail_path, 'wb')
    f.write(image)
    f.close()
    existing_thumbnails.add(video_id)

    return flask.Response(image, mimetype='image/jpeg')