DD.WellWorkover.Cloud/AsbCloudInfrastructure/Services/Cache/EfCacheL2.cs

using System;
using System.Collections;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace AsbCloudInfrastructure.Services.Cache
{
#nullable enable
    public static class EfCacheL2
    {
        public static int RequestsToDb = 0;

        static readonly ConcurrentDictionary<string, Lazy<CacheItem>> caches = new(1, 16);
        
        private const int semaphoreTimeout = 25_000;
        private static readonly SemaphoreSlim semaphore = new(1);

        private struct CacheItem
        {
            public readonly IEnumerable Data;
            public readonly DateTime DateObsolete;
            
            public CacheItem(IEnumerable data, TimeSpan obsolescence)
            {
                DateObsolete = DateTime.Now + obsolescence;
                Data = data;
            }
        }

        private static CacheItem GetOrAddCache(string tag, Func<CacheItem> valueFactory)
        {
            Lazy<CacheItem>? lazyCache;
            while (!caches.TryGetValue(tag, out lazyCache))
            {
                if (semaphore.Wait(0))
                {
                    lazyCache = new Lazy<CacheItem>(valueFactory);
                    caches.TryAdd(tag, lazyCache);
                    _ = lazyCache.Value;
                    semaphore.Release();
                    break;
                }
                else
                {
                    if(semaphore.Wait(semaphoreTimeout))
                        semaphore.Release();
                    else
                    {
                        semaphore.Release();
                        throw new TimeoutException("EfCacheL2.GetOrAddCache. Can't wait too long while getting cache");
                    }
                }                    
            }

            if (lazyCache.Value.DateObsolete < DateTime.Now)
            {
                var isUpdated = false;
                if (semaphore.Wait(0))
                {
                    lazyCache = new Lazy<CacheItem>(valueFactory);
                    caches.Remove(tag, out _);
                    caches.TryAdd(tag, lazyCache);
                    _ = lazyCache.Value;
                    isUpdated = true;
                    semaphore.Release();
                }
                else
                {
                    if (semaphore.Wait(semaphoreTimeout))
                        semaphore.Release();
                    else
                    {
                        semaphore.Release();
                        throw new TimeoutException("EfCacheL2.GetOrAddCache. Can't wait too long while getting cache");
                    }
                }                

                if (isUpdated || caches.TryGetValue(tag, out lazyCache))
                    return lazyCache.Value;

                throw new Exception("EfCacheL2.GetOrAddCache it should never happens");
            }
            else
                return lazyCache.Value;
        }

        private static async Task<CacheItem> GetOrAddCacheAsync(string tag, Func<CacheItem> valueFactory, CancellationToken token)
        {
            Lazy<CacheItem>? lazyCache;
            while (!caches.TryGetValue(tag, out lazyCache))
            {
                if (semaphore.Wait(0, CancellationToken.None))
                {
                    lazyCache = new Lazy<CacheItem>(valueFactory);
                    caches.TryAdd(tag, lazyCache);
                    _ = lazyCache.Value;
                    semaphore.Release();
                    break;
                }
                else
                {
                    if (await semaphore.WaitAsync(semaphoreTimeout, token))
                        semaphore.Release();
                    else
                    {
                        semaphore.Release();
                        throw new TimeoutException("EfCacheL2.GetOrAddCache. Can't wait too long while getting cache");
                    }
                }
            }

            if (lazyCache.Value.DateObsolete < DateTime.Now)
            {
                var isUpdated = false;
                if (semaphore.Wait(0, CancellationToken.None))
                {
                    lazyCache = new Lazy<CacheItem>(valueFactory);
                    caches.Remove(tag, out _);
                    caches.TryAdd(tag, lazyCache);
                    _ = lazyCache.Value;
                    isUpdated = true;
                    semaphore.Release();
                }
                else
                {
                    if (await semaphore.WaitAsync(semaphoreTimeout, token))
                        semaphore.Release();
                    else
                    {
                        semaphore.Release();
                        throw new TimeoutException("EfCacheL2.GetOrAddCache. Can't wait too long while getting cache");
                    }
                }

                if (isUpdated || caches.TryGetValue(tag, out lazyCache))
                    return lazyCache.Value;

                throw new Exception("EfCacheL2.GetOrAddCache it should never happens");
            }
            else
                return lazyCache.Value;
        }

        private static IEnumerable<T> ConvertToIEnumerable<T>(IEnumerable data)
        {
            if (data is IEnumerable<T> list)
                return list;
            else if (data is IDictionary dictionary)
            {
                System.Diagnostics.Trace.TraceWarning($"ConvertToIEnumerable. Use keyless method on keyed cache. Type: {typeof(T).Name};");
                return (IEnumerable<T>)dictionary.Values;
            }
            else
                throw new NotSupportedException("cache.Data has wrong type.");
        }

        private static Dictionary<TKey, T> ConvertToDictionary<TKey, T>(IEnumerable data, Func<T, TKey> keySelector)
            where TKey : notnull
        {
            if (data is Dictionary<TKey, T> dictionary)
                return dictionary;
            else
            {
                System.Diagnostics.Trace.TraceWarning($"ConvertToDictionary. Use keyed method on keyless cache. Type: {typeof(T).Name};");
                return ((IEnumerable<T>)data).ToDictionary(keySelector);
            }
        }

        private static Func<CacheItem> MakeValueListFactory<T>(IQueryable<T> query, TimeSpan obsolescence)
        {
            CacheItem ValueFactory()
            {
                var list = query.ToList();
                RequestsToDb++;
                return new CacheItem(list, obsolescence);
            }
            return ValueFactory;
        }

        private static Func<CacheItem> MakeValueDictionaryFactory<TKey, T>(IQueryable<T> query, TimeSpan obsolescence, Func<T, TKey> keySelector)
            where TKey : notnull
        {
            CacheItem ValueFactory()
            {
                var dictionary = query.ToDictionary(keySelector);
                RequestsToDb++;
                return new CacheItem(dictionary, obsolescence);
            };
            return ValueFactory;
        }

        public static IEnumerable<T> FromCache<T>(this IQueryable<T> query, string tag, TimeSpan obsolescence)
        {
            var factory = MakeValueListFactory(query, obsolescence);
            var cache = GetOrAddCache(tag, factory);
            return ConvertToIEnumerable<T>(cache.Data);
        }

        public static async Task<IEnumerable<T>> FromCacheAsync<T>(this IQueryable<T> query, string tag, TimeSpan obsolescence, CancellationToken token = default)
        {
            var factory = MakeValueListFactory(query, obsolescence);
            var cache = await GetOrAddCacheAsync(tag, factory, token);
            return ConvertToIEnumerable<T>(cache.Data);
        }

        public static Dictionary<TKey, T> FromCache<TKey, T>(this IQueryable<T> query, string tag, TimeSpan obsolescence, Func<T, TKey> keySelector)
            where TKey: notnull
        {
            var factory = MakeValueDictionaryFactory(query, obsolescence, keySelector);
            var cache = GetOrAddCache(tag, factory);
            return ConvertToDictionary(cache.Data, keySelector);
        }

        public static async Task<Dictionary<TKey, T>> FromCacheAsync<TKey, T>(this IQueryable<T> query, string tag, TimeSpan obsolescence, Func<T, TKey> keySelector, CancellationToken token = default)
            where TKey : notnull
        {
            var factory = MakeValueDictionaryFactory(query, obsolescence, keySelector);
            var cache = await GetOrAddCacheAsync(tag, factory, token);
            return ConvertToDictionary(cache.Data, keySelector);
        }

        public static T? FromCacheGetValueOrDefault<TKey, T>(this IQueryable<T> query, string tag, TimeSpan obsolescence, Func<T, TKey> keySelector, TKey key)
            where TKey : notnull
        {
            var factory = MakeValueDictionaryFactory(query, obsolescence, keySelector);
            var cache = GetOrAddCache(tag, factory);

            if (cache.Data is Dictionary<TKey, T> dictionary)
                return dictionary.GetValueOrDefault(key);
            else
            {
                System.Diagnostics.Trace.TraceWarning($"Use keyed method on keyless cache. Tag: {tag}, type: {typeof(T).Name};");
                return ((IEnumerable<T>)cache.Data).FirstOrDefault(v => keySelector(v).Equals(key));
            }
        }

        public static void DropCache<T>(this IQueryable<T> query, string tag)
        {
            caches.Remove(tag, out var _);
        }
    }
#nullable disable
}