/*
* Copyright (c) 2023 Vaughn Nugent
* 
* Library: VNLib
* Package: VNLib.Plugins.Extensions.VNCache
* File: VNCacheExtensions.cs 
*
* VNCacheExtensions.cs is part of VNLib.Plugins.Extensions.VNCache which is part of the larger 
* VNLib collection of libraries and utilities.
*
* VNLib.Plugins.Extensions.VNCache is free software: you can redistribute it and/or modify 
* it under the terms of the GNU Affero General Public License as 
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* VNLib.Plugins.Extensions.VNCache 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 Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program.  If not, see https://www.gnu.org/licenses/.
*/

using System;
using System.Runtime.CompilerServices;

using VNLib.Hashing;
using VNLib.Utils.Memory;
using VNLib.Utils.Extensions;
using VNLib.Data.Caching;
using VNLib.Plugins.Extensions.VNCache.DataModel;

namespace VNLib.Plugins.Extensions.VNCache
{
    /// <summary>
    /// Contains extension methods for aquiring a Plugin managed 
    /// global cache provider.
    /// </summary>
    public static class VNCacheExtensions
    {
        internal const string CACHE_CONFIG_KEY = "vncache";
        internal const string MEMORY_CACHE_CONFIG_KEY = "memory_cache";
        internal const string MEMORY_CACHE_ONLY_KEY = "memory_only";
      

        /// <summary>
        /// Gets a simple scoped cache based on an entity prefix. The prefix is appended
        /// to the object id on each cache operation
        /// </summary>
        /// <param name="cache"></param>
        /// <param name="prefix">The simple prefix string to append to object ids before computing hashes</param>
        /// <param name="digest">The algorithm used to hash the combined object-ids</param>
        /// <param name="encoding">The string encoding method used to encode the hash output</param>
        /// <returns>The <see cref="ScopedCache"/> instance that will use the prefix to compute object ids</returns>
        /// <exception cref="ArgumentNullException"></exception>
        public static ScopedCache GetPrefixedCache(this IGlobalCacheProvider cache, string prefix, HashAlg digest = HashAlg.SHA1, HashEncodingMode encoding = HashEncodingMode.Base64)
        {
            _ = cache ?? throw new ArgumentNullException(nameof(cache));
            _ = prefix ?? throw new ArgumentNullException(nameof(prefix));
            //Create simple cache key generator
            SimpleCacheKeyImpl keyProv = new(prefix, digest, encoding);
            //Create the scoped cache from the simple provider
            return cache.GetScopedCache(keyProv);
        }

        private sealed class SimpleCacheKeyImpl : ICacheKeyGenerator
        {
            private readonly string Prefix;
            private readonly HashAlg Digest;
            private readonly HashEncodingMode Encoding;

            public SimpleCacheKeyImpl(string prefix, HashAlg digest, HashEncodingMode encoding)
            {
                Prefix = prefix;
                Digest = digest;
                Encoding = encoding;
            }

            [MethodImpl(MethodImplOptions.AggressiveInlining)]
            private int ComputeBufferSize(string id) => id.Length + Prefix.Length;


            string ICacheKeyGenerator.ComputedKey(string entityId)
            {
                //Compute the required character buffer size
                int bufferSize = ComputeBufferSize(entityId);

                if(bufferSize < 128)
                {
                    //Stack alloc a buffer
                    Span<char> buffer = stackalloc char[bufferSize];

                    //Writer to accumulate data
                    ForwardOnlyWriter<char> writer = new(buffer);

                    //Append prefix and entity id
                    writer.Append(Prefix);
                    writer.Append(entityId);

                    //Compute the simple hash of the combined values
                    return ManagedHash.ComputeHash(writer.AsSpan(), Digest, Encoding);
                }
                else
                {
                    //Alloc heap buffer for string concatination
                    using UnsafeMemoryHandle<char> buffer = MemoryUtil.UnsafeAlloc<char>(bufferSize, true);

                    //Writer to accumulate data
                    ForwardOnlyWriter<char> writer = new(buffer);

                    //Append prefix and entity id
                    writer.Append(Prefix);
                    writer.Append(entityId);

                    //Compute the simple hash of the combined values
                    return ManagedHash.ComputeHash(writer.AsSpan(), Digest, Encoding);
                }
            }
        }
    }
}