/*
 * Copyright (c) 2023 Proton AG
 *
 * This file is part of ProtonVPN.
 *
 * ProtonVPN is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * ProtonVPN 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 for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with ProtonVPN.  If not, see <https://www.gnu.org/licenses/>.
 */
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using ProtonVPN.Client.Settings.Contracts;
using ProtonVPN.Common.Core.Extensions;
using ProtonVPN.Configurations.Contracts;
using ProtonVPN.Dns.Caching;
using ProtonVPN.Dns.Contracts;
using ProtonVPN.Dns.Contracts.Resolvers;
using ProtonVPN.Logging.Contracts;
using ProtonVPN.Logging.Contracts.Events.DnsLogs;

namespace ProtonVPN.Dns;

public class AlternativeHostsManager : IAlternativeHostsManager
{
    private readonly IDnsOverHttpsTxtRecordsResolver _dnsOverHttpsTxtRecordsResolver;
    private readonly ISettings _settings;
    private readonly ILogger _logger;
    private readonly IDnsCacheManager _dnsCacheManager;
    private readonly SemaphoreSlim _semaphore = new(1, 1);
    private readonly ConcurrentDictionary<string, DateTime> _failedRequestsCache = new();
    private readonly TimeSpan _failedDnsRequestTimeout;
    private readonly TimeSpan _newCacheTimeToLiveOnResolveError;

    public AlternativeHostsManager(IDnsOverHttpsTxtRecordsResolver dnsOverHttpsTxtRecordsResolver,
        ISettings settings, IConfiguration config, ILogger logger, IDnsCacheManager dnsCacheManager)
    {
        _dnsOverHttpsTxtRecordsResolver = dnsOverHttpsTxtRecordsResolver;
        _settings = settings;
        _logger = logger;
        _dnsCacheManager = dnsCacheManager;
        _failedDnsRequestTimeout = config.FailedDnsRequestTimeout;
        _newCacheTimeToLiveOnResolveError = config.NewCacheTimeToLiveOnResolveError;
    }

    public async Task<IList<string>> GetAsync(string host, CancellationToken cancellationToken)
    {
        IList<string> alternativeHosts = GetFreshAlternativeHostsFromCache(host);
        if (alternativeHosts.Count == 0)
        {
            alternativeHosts = await ResolveOrGetFromCacheAsync(host, cancellationToken);
        }

        return alternativeHosts;
    }

    private async Task<IList<string>> ResolveOrGetFromCacheAsync(string host, CancellationToken cancellationToken)
    {
        try
        {
            await _semaphore.WaitAsync(cancellationToken);
        }
        catch
        {
            _logger.Warn<DnsErrorLog>($"Alternative hosts resolve of host '{host}' was cancelled while waiting.");
            return new List<string>();
        }
        IList<string> alternativeHosts;
        try
        {
            alternativeHosts = GetFreshAlternativeHostsFromCache(host);
            if (alternativeHosts.Count == 0)
            {
                if (_failedRequestsCache.TryGetValue(host, out DateTime timeoutEndDateUtc) && timeoutEndDateUtc > DateTime.UtcNow)
                {
                    _logger.Debug<DnsLog>($"Skipping alternative hosts resolve of host '{host}' because its under timeout.");
                    alternativeHosts = GetAlternativeHostsFromCache(host);
                }
                else
                {
                    _logger.Info<DnsLog>($"No fresh alternative hosts for host '{host}' were found in the cache. " +
                        $"Triggering a refresh.");
                    alternativeHosts = await ResolveHostAsync(host, cancellationToken);
                    if (alternativeHosts.Count == 0)
                    {
                        alternativeHosts = await GetAlternativeHostsFromCacheAndSetNewTtlAsync(host);
                    }
                }
            }
            else
            {
                _logger.Debug<DnsLog>($"Locked re-check for a fresh alternative " +
                    $"hosts cache of host '{host}' was successful.");
            }
        }
        finally
        {
            _semaphore.Release();
            _logger.Debug<DnsLog>($"Released semaphore of alternative hosts DNS resolve of host '{host}'.");
        }

        return alternativeHosts;
    }

    private IList<string> GetFreshAlternativeHostsFromCache(string host)
    {
        IList<string> alternativeHosts = new List<string>();
        DateTime currentDateTimeUtc = DateTime.UtcNow;

        if (_settings.DnsCache.TryGetValueIfDictionaryIsNotNull(host, out DnsResponse dnsResponse) &&
            dnsResponse.ExpirationDateTimeUtc > currentDateTimeUtc)
        {
            alternativeHosts = dnsResponse.AlternativeHosts;
        }

        return alternativeHosts;
    }

    private IList<string> GetAlternativeHostsFromCache(string host)
    {
        IList<string> alternativeHosts = new List<string>();

        if (_settings.DnsCache.TryGetValueIfDictionaryIsNotNull(host, out DnsResponse dnsResponse))
        {
            alternativeHosts = dnsResponse.AlternativeHosts;
        }

        return alternativeHosts ?? new List<string>();
    }

    private async Task<IList<string>> GetAlternativeHostsFromCacheAndSetNewTtlAsync(string host)
    {
        IList<string> alternativeHosts = GetAlternativeHostsFromCache(host);

        if (alternativeHosts.Any())
        {
            DnsResponse newDnsResponse = await _dnsCacheManager.UpdateAsync(host, SetDatesAndTimeToLiveFactory);
            _logger.Info<DnsLog>($"Returning cached alternative hosts for host '{host}'. " +
                $"New TTL of {newDnsResponse.TimeToLive} resulting in a " +
                $"new expiration date of {newDnsResponse.ExpirationDateTimeUtc}.");
        }
        else
        {
            DateTime timeoutEndDateUtc = DateTime.UtcNow + _failedDnsRequestTimeout;
            _failedRequestsCache.AddOrUpdate(host, timeoutEndDateUtc, (_, _) => timeoutEndDateUtc);
            _logger.Warn<DnsErrorLog>($"No cached alternative hosts exist for host '{host}'. " +
                $"Next resolve can only be made after '{timeoutEndDateUtc}'.");
        }

        return alternativeHosts;
    }

    private DnsResponse SetDatesAndTimeToLiveFactory(DnsResponse dnsResponse)
    {
        dnsResponse.SetDatesAndTimeToLive(_newCacheTimeToLiveOnResolveError);
        return dnsResponse;
    }

    private async Task<IList<string>> ResolveHostAsync(string host, CancellationToken cancellationToken)
    {
        try
        {
            _logger.Info<DnsLog>($"Attempting a HTTPS DNS request for TXT records of host '{host}'.");
            DnsResponse dnsResponse = await _dnsOverHttpsTxtRecordsResolver.ResolveAsync(host, cancellationToken);

            if (dnsResponse != null && dnsResponse.AlternativeHosts.Any())
            {
                _logger.Info<DnsLog>($"The HTTPS DNS request for TXT records of host '{host}' was successful. " +
                    "Saving to cache.");
                IList<string> alternativeHosts = dnsResponse.AlternativeHosts;
                await _dnsCacheManager.AddOrReplaceAsync(host, dnsResponse);
                return alternativeHosts;
            }

            _logger.Error<DnsErrorLog>($"The HTTPS DNS request for TXT records of host '{host}' was unsuccessful.");
        }
        catch (Exception e)
        {
            _logger.Error<DnsErrorLog>($"An unexpected error as occurred when resolving " +
                $"HTTPS DNS for TXT records of host '{host}'.", e);
        }

        return new List<string>();
    }
}