// Copyright (c) 2019, 2020, Oracle and/or its affiliates. All rights reserved.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 2.0, as
// published by the Free Software Foundation.
//
// This program is also distributed with certain software (including
// but not limited to OpenSSL) that is licensed under separate terms,
// as designated in a particular file or component or in included license
// documentation.  The authors of MySQL hereby grant you an
// additional permission to link the program and your derivative works
// with the separately licensed software that they have included with
// MySQL.
//
// Without limiting anything contained in the foregoing, this file,
// which is part of MySQL Connector/NET, is also subject to the
// Universal FOSS Exception, version 1.0, a copy of which can be found at
// http://oss.oracle.com/licenses/universal-foss-exception.
//
// This program is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
// See the GNU General Public License, version 2.0, for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA

using Ubiety.Dns.Core;
using System;
using System.Collections.Generic;
using System.Linq;
using Ubiety.Dns.Core.Common;
using MySql.Data.MySqlClient;
using Sog.Properties;

namespace MySql.Data.Common
{
    /// <summary>
    /// DNS resolver that runs queries against a server.
    /// </summary>
    internal static class DnsResolver
    {
        // Resolver object that looks up for DNS SRV records.
        private static Resolver _resolver;

        // DNS domain.
        internal static string ServiceName { get; private set; }

        /// <summary>
        /// Initializes a new instance of the <see cref="Ubiety.Dns.Core.Resolver"/> class.
        /// </summary>
        internal static void CreateResolver(string serviceName)
        {
            _resolver = new Resolver
            {
                Recursion = true,
                UseCache = true,
                Retries = 3,
                TransportType = TransportType.Udp
            };

            ServiceName = serviceName;
        }

        /// <summary>
        /// Gets the DNS SVR records of the service name that is provided.
        /// </summary>
        /// <returns>A list of <see cref="DnsSrvRecord"/>s sorted as described in RFC2782.</returns>
        internal static List<DnsSrvRecord> GetDnsSrvRecords(string serviceName)
        {
            if (_resolver == null)
            {
                CreateResolver(serviceName);
            }

            List<DnsSrvRecord> records = new List<DnsSrvRecord>();
            const QuestionType qType = QuestionType.SRV;
            const QuestionClass qClass = QuestionClass.IN;

            Response response = _resolver.Query(ServiceName, qType, qClass);

#pragma warning disable CS0618 // “Response.RecordSrv”已过时:“Use GetRecords<T> instead.”
            foreach (var record in response.RecordSrv)
#pragma warning restore CS0618 // “Response.RecordSrv”已过时:“Use GetRecords<T> instead.”
            {
                records.Add(record);
            }

            if (records.Count > 0)
            {
                Reset();
                return SortSrvRecords(records);
            }
            else
            {
                throw new MySqlException(string.Format(Resources.DnsSrvNoHostsAvailable, ServiceName));
            }
        }

        /// <summary>
        /// Sorts a list of DNS SRV records according to the sorting rules described in RFC2782.
        /// </summary>
        /// <param name="srvRecords">List of <see cref="DnsSrvRecord"/>s to sort.</param>
        /// <returns>A new list of sorted <see cref="DnsSrvRecord"/>s.</returns>
        internal static List<DnsSrvRecord> SortSrvRecords(List<DnsSrvRecord> srvRecords)
        {
            srvRecords.Sort(new DnsSrvRecord());

            Random random = new Random();
            List<DnsSrvRecord> srvRecordsSortedRfc2782 = new List<DnsSrvRecord>();

            List<int> priorities = srvRecords.Select(s => s.Priority).Distinct().ToList();
            foreach (int priority in priorities)
            {
                List<DnsSrvRecord> srvRecordsSamePriority = srvRecords.Where(r => r.Priority == priority).ToList();
                while (srvRecordsSamePriority.Count > 1)
                {
                    int recCount = srvRecordsSamePriority.Count;
                    int sumOfWeights = 0;
                    int[] weights = new int[recCount];
                    for (int i = 0; i < recCount; i++)
                    {
                        sumOfWeights += srvRecordsSamePriority[i].Weight;
                        weights[i] = sumOfWeights;
                    }

                    int selection = random.Next(sumOfWeights + 1);
                    int pos = 0;
                    for (; pos < recCount && weights[pos] < selection; pos++) { }
                    srvRecordsSortedRfc2782.Add(srvRecordsSamePriority[pos]);
                    srvRecordsSamePriority.RemoveAt(pos);
                }

                srvRecordsSortedRfc2782.Add(srvRecordsSamePriority[0]);
            }

            return srvRecordsSortedRfc2782;
        }

        /// <summary>
        /// Resets the DnsSrvResolver
        /// </summary>
        private static void Reset()
        {
            if (_resolver != null)
            {
                _resolver = null;
            }
        }
    }
}