// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections.Concurrent;
using System.Threading;
using System.Threading.Tasks;
using NuGet.Common;
using NuGet.Protocol.Core.Types;

namespace NuGet.Protocol
{
    public class ODataServiceDocumentResourceV2Provider : ResourceProvider
    {
        private static readonly TimeSpan _defaultCacheDuration = TimeSpan.FromMinutes(40);
        private readonly ConcurrentDictionary<string, ODataServiceDocumentCacheInfo> _cache;
        private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);

        /// <summary>
        /// Maximum amount of time to store index.json
        /// </summary>
        public TimeSpan MaxCacheDuration { get; protected set; }

        public ODataServiceDocumentResourceV2Provider()
            : base(typeof(ODataServiceDocumentResourceV2),
                  nameof(ODataServiceDocumentResourceV2Provider),
                  NuGetResourceProviderPositions.Last)
        {
            MaxCacheDuration = _defaultCacheDuration;
            _cache = new ConcurrentDictionary<string, ODataServiceDocumentCacheInfo>(StringComparer.OrdinalIgnoreCase);
        }

        public override async Task<Tuple<bool, INuGetResource?>> TryCreate(SourceRepository source, CancellationToken token)
        {
            ODataServiceDocumentResourceV2? serviceDocument = null;
            ODataServiceDocumentCacheInfo? cacheInfo = null;
            var url = source.PackageSource.Source;

            var utcNow = DateTime.UtcNow;
            var entryValidCutoff = utcNow.Subtract(MaxCacheDuration);

            // check the cache before downloading the file
            if (!_cache.TryGetValue(url, out cacheInfo) || entryValidCutoff > cacheInfo.CachedTime)
            {
                // Track if the semaphore needs to be released
                var release = false;
                try
                {
                    await _semaphore.WaitAsync(token);
                    release = true;

                    token.ThrowIfCancellationRequested();

                    // check the cache again, another thread may have finished this one waited for the lock
                    if (!_cache.TryGetValue(url, out cacheInfo) || entryValidCutoff > cacheInfo.CachedTime)
                    {
                        var client = (await source.GetResourceAsync<HttpSourceResource>(token))!.HttpSource;
                        serviceDocument = await ODataServiceDocumentUtils.CreateODataServiceDocumentResourceV2(url, client, utcNow, NullLogger.Instance, token);

                        // cache the value even if it is null to avoid checking it again later
                        var cacheEntry = new ODataServiceDocumentCacheInfo
                        {
                            CachedTime = utcNow,
                            ServiceDocument = serviceDocument
                        };

                        // If the cache entry has expired it will already exist
                        _cache.AddOrUpdate(url, cacheEntry, (key, value) => cacheEntry);
                    }
                }
                finally
                {
                    if (release)
                    {
                        _semaphore.Release();
                    }
                }
            }

            if (serviceDocument == null && cacheInfo != null)
            {
                serviceDocument = cacheInfo.ServiceDocument;
            }

            return new Tuple<bool, INuGetResource?>(serviceDocument != null, serviceDocument);
        }

        protected class ODataServiceDocumentCacheInfo
        {
            public ODataServiceDocumentResourceV2? ServiceDocument { get; set; }

            public DateTime CachedTime { get; set; }
        }
    }
}