const { MongoClient } = require("mongodb")
const { database_url, database_name } = require('../../config')
const crypto = require("crypto")
let max_database_connection_timeout = 1000

const get_db = async () => {
    const client = new MongoClient(database_url, {
        serverSelectionTimeoutMS: max_database_connection_timeout, 
        useUnifiedTopology: true
    })
    try {
        await client.connect()
    } catch (err) {
        console.log('can not connect to database')
        return null
    }
    return client.db(database_name)
}

const get_titles = async (start, count) => {
    var db = await get_db()
    if (!db) {
        return null
    }
    var titles = await db.collection('title').find().skip(parseInt(start)).limit(parseInt(count)).toArray()
    return (!titles.length) ? null : titles
}

const get_free_session_servers = async (title) => {
    var db = await get_db()
    if (!db) {
        return null
    }

    var servers = await db.collection('session_server').find({title:title}).toArray()

    if (!servers.length) {
        return null
    }

    var active_session = db.collection('active_session')
    
    var free_servers = []
    var servers_length = servers.length
    for (var i = 0; i < servers_length; ++i) {
        var sessions = await active_session.find({url:servers[i].url}).toArray()
        if (servers[i].limit > sessions.length) {
            free_servers.push(servers[i])
        }
    }
    return free_servers
}

const add_active_session = async (server_url, session_id, websocket_url) => {
    var db = await get_db()
    if (!db) {
        return false
    }

    var active_session = db.collection('active_session')

    await active_session.insertOne({
        server_url: server_url,
        session_id: session_id,
        websocket_url: websocket_url
    })
    return true
}

const generate_session_id = async () => {
    var db = await get_db()
    if (!db) {
        return null
    }

    var active_session = db.collection('active_session')
    var session_id = {}

    var max_id_len = 6
    var sessions_with_id = 0
    while(true) {
        session_id = crypto.randomBytes(max_id_len/2).toString('hex').toUpperCase()
        sessions_with_id = await active_session.find({session_id:session_id}).toArray().length
        if (!sessions_with_id) {
            return session_id
        }
    }
}

const remove_active_session = async (session_id) => {
    var db = await get_db()
    if (!db) {
        return false
    }
    
    await db.collection('active_session').deleteOne({session_id:{$in:[session_id]}})
    return true
}

const get_session_websocket_url = async (session_id) => {
    var db = await get_db()
    if (!db) {
        return null
    }
    var active_session = db.collection('active_session')
    var session = await active_session.findOne({session_id:session_id})
    if (!session) {
        return null
    }
    return session.websocket_url
}

module.exports = {
    get_titles,
    get_free_session_servers,
    add_active_session,
    generate_session_id,
    remove_active_session,
    get_session_websocket_url
}