/*
* Copyright (c) 2022 Vaughn Nugent
* 
* Library: VNLib
* Package: VNLib.Plugins.Extensions.Data
* File: LWStorageManager.cs 
*
* LWStorageManager.cs is part of VNLib.Plugins.Extensions.Data which is part of the larger 
* VNLib collection of libraries and utilities.
*
* VNLib.Plugins.Extensions.Data 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.Data 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.IO;
using System.Data;
using System.Linq;
using System.Threading;
using System.IO.Compression;
using System.Threading.Tasks;

using Microsoft.EntityFrameworkCore;

using VNLib.Utils;
using VNLib.Utils.IO;
using VNLib.Utils.Memory;

namespace VNLib.Plugins.Extensions.Data.Storage
{

    /// <summary>
    /// Provides single table database object storage services
    /// </summary>
    public sealed class LWStorageManager
    { 
        /// <summary>
        /// The generator function that is invoked when a new <see cref="LWStorageDescriptor"/> is to 
        /// be created without an explicit id
        /// </summary>
        public Func<string> NewDescriptorIdGenerator { get; init; } = static () => Guid.NewGuid().ToString("N");

        private readonly DbContextOptions DbOptions;
        private readonly string TableName;

        private LWStorageContext GetContext() => new(DbOptions, TableName);

        /// <summary>
        /// Creates a new <see cref="LWStorageManager"/> with 
        /// </summary>
        /// <param name="options">The db context options to create database connections with</param>
        /// <param name="tableName">The name of the table to operate on</param>
        /// <exception cref="ArgumentNullException"></exception>
        public LWStorageManager(DbContextOptions options, string tableName)
        {
            DbOptions = options ?? throw new ArgumentNullException(nameof(options));
            TableName = tableName ?? throw new ArgumentNullException(nameof(tableName));
        }

        /// <summary>
        /// Creates a new <see cref="LWStorageDescriptor"/> fror a given user
        /// </summary>
        /// <param name="userId">Id of user</param>
        /// <param name="descriptorIdOverride">An override to specify the new descriptor's id</param>
        /// <param name="cancellation">A token to cancel the operation</param>
        /// <returns>A new <see cref="LWStorageDescriptor"/> if successfully created, null otherwise</returns>
        /// <exception cref="ArgumentNullException"></exception>
        /// <exception cref="LWDescriptorCreationException"></exception>
        public async Task<LWStorageDescriptor> CreateDescriptorAsync(string userId, string? descriptorIdOverride = null, CancellationToken cancellation = default)
        {
            if (string.IsNullOrWhiteSpace(userId))
            {
                throw new ArgumentNullException(nameof(userId));
            }
            
            //If no override id was specified, generate a new one
            descriptorIdOverride ??= NewDescriptorIdGenerator();

            DateTime createdOrModifedTime = DateTime.UtcNow;

            await using LWStorageContext ctx = GetContext();
            await ctx.OpenTransactionAsync(cancellation);

            //Make sure the descriptor doesnt exist only by its descriptor id
            if (await ctx.Descriptors.AnyAsync(d => d.Id == descriptorIdOverride, cancellation))
            {
                throw new LWDescriptorCreationException($"A descriptor with id {descriptorIdOverride} already exists");
            }

            //Cache time
            DateTime now = DateTime.UtcNow;

            //Create the new descriptor
            LWStorageEntry entry = new()
            {
                Created = now,
                LastModified = now,
                Id = descriptorIdOverride,
                UserId = userId,
            };

            //Add and save changes
            ctx.Descriptors.Add(entry);

            ERRNO result = await ctx.SaveChangesAsync(cancellation);

            if (!result)
            {
                //Rollback and raise exception
                await ctx.RollbackTransctionAsync(cancellation);
                throw new LWDescriptorCreationException("Failed to create descriptor, because changes could not be saved");
            }
            else
            {
                //Commit transaction and return the new descriptor
                await ctx.CommitTransactionAsync(cancellation);
                return new LWStorageDescriptor(this, entry);
            }
        }

        /// <summary>
        /// Attempts to retrieve <see cref="LWStorageDescriptor"/> for a given user-id. The caller is responsible for 
        /// consitancy state of the descriptor
        /// </summary>
        /// <param name="userid">User's id</param>
        /// <param name="cancellation">A token to cancel the operation</param>
        /// <returns>The descriptor belonging to the user, or null if not found or error occurs</returns>
        /// <exception cref="ArgumentNullException"></exception>
        public async Task<LWStorageDescriptor?> GetDescriptorFromUIDAsync(string userid, CancellationToken cancellation = default)
        {
            //Allow null/empty entrys to just return null
            if (string.IsNullOrWhiteSpace(userid))
            {
                throw new ArgumentNullException(nameof(userid));
            }

            //Init db
            await using LWStorageContext db = GetContext();
            //Begin transaction
            await db.OpenTransactionAsync(cancellation);
            //Get entry
            LWStorageEntry? entry = await (from s in db.Descriptors
                                           where s.UserId == userid
                                           select s)
                                           .SingleOrDefaultAsync(cancellation);

            //Close transactions and return
            if (entry == null)
            {
                await db.RollbackTransctionAsync(cancellation);
                return null;
            }
            else
            {
                await db.CommitTransactionAsync(cancellation);
                return new (this, entry);
            }
        }
        
        /// <summary>
        /// Attempts to retrieve the <see cref="LWStorageDescriptor"/> for the given descriptor id. The caller is responsible for 
        /// consitancy state of the descriptor
        /// </summary>
        /// <param name="descriptorId">Unique identifier for the descriptor</param>
        /// <param name="cancellation">A token to cancel the opreeaiton</param>
        /// <returns>The descriptor belonging to the user, or null if not found or error occurs</returns>
        /// <exception cref="ArgumentNullException"></exception>
        public async Task<LWStorageDescriptor?> GetDescriptorFromIDAsync(string descriptorId, CancellationToken cancellation = default)
        {
            //Allow null/empty entrys to just return null
            if (string.IsNullOrWhiteSpace(descriptorId))
            {
                throw new ArgumentNullException(nameof(descriptorId));
            }
           
            //Init db
            await using LWStorageContext db = GetContext();
            //Begin transaction
            await db.OpenTransactionAsync(cancellation);
            //Get entry
            LWStorageEntry? entry = await (from s in db.Descriptors
                                           where s.Id == descriptorId
                                           select s)
                                           .SingleOrDefaultAsync(cancellation);

            //Close transactions and return
            if (entry == null)
            {
                await db.RollbackTransctionAsync(cancellation);
                return null;
            }
            else
            {
                await db.CommitTransactionAsync(cancellation);
                return new (this, entry);
            }
        }
       
        /// <summary>
        /// Cleanup entries before the specified <see cref="TimeSpan"/>. Entires are store in UTC time
        /// </summary>
        /// <param name="compareTime">Time before <see cref="DateTime.UtcNow"/> to compare against</param>
        /// <param name="cancellation">A token to cancel the operation</param>
        /// <returns>The number of entires cleaned</returns>S
        public Task<ERRNO> CleanupTableAsync(TimeSpan compareTime, CancellationToken cancellation = default) => CleanupTableAsync(DateTime.UtcNow.Subtract(compareTime), cancellation);
        
        /// <summary>
        /// Cleanup entries before the specified <see cref="DateTime"/>. Entires are store in UTC time
        /// </summary>
        /// <param name="compareTime">UTC time to compare entires against</param>
        /// <param name="cancellation">A token to cancel the operation</param>
        /// <returns>The number of entires cleaned</returns>
        public async Task<ERRNO> CleanupTableAsync(DateTime compareTime, CancellationToken cancellation = default)
        {
            //Init db
            await using LWStorageContext db = GetContext();
            //Begin transaction
            await db.OpenTransactionAsync(cancellation);

            //Get all expired entires
            LWStorageEntry[] expired = await (from s in db.Descriptors
                                              where s.Created < compareTime
                                              select s)
                                              .ToArrayAsync(cancellation);

            //Delete
            db.Descriptors.RemoveRange(expired);

            //Save changes
            ERRNO count = await db.SaveChangesAsync(cancellation);

            //Commit transaction
            await db.CommitTransactionAsync(cancellation);

            return count;
        }

        /// <summary>
        /// Updates a descriptor's data field
        /// </summary>
        /// <param name="descriptorObj">Descriptor to update</param>
        /// <param name="data">Data string to store to descriptor record</param>
        /// <exception cref="LWStorageUpdateFailedException"></exception>
        internal async Task UpdateDescriptorAsync(object descriptorObj, Stream data)
        {
            LWStorageEntry entry = (descriptorObj as LWStorageDescriptor)!.Entry;
            ERRNO result = 0;
            try
            {
                await using LWStorageContext ctx = GetContext();
                await ctx.OpenTransactionAsync(CancellationToken.None);

                //Begin tracking
                ctx.Descriptors.Attach(entry);

                //Convert stream to vnstream
                VnMemoryStream vms = (VnMemoryStream)data;
                using (IMemoryHandle<byte> encBuffer = MemoryUtil.SafeAlloc<byte>((int)vms.Length))
                {
                    //try to compress
                    if(!BrotliEncoder.TryCompress(vms.AsSpan(), encBuffer.Span, out int compressed))
                    {
                        throw new InvalidDataException("Failed to compress the descriptor data");
                    }
                    //Set the data 
                    entry.Data = encBuffer.Span.ToArray();
                }
                //Update modified time
                entry.LastModified = DateTime.UtcNow;

                //Save changes
                result = await ctx.SaveChangesAsync(CancellationToken.None);

                //Commit or rollback
                if (result)
                {
                    await ctx.CommitTransactionAsync(CancellationToken.None);
                }
                else
                {
                    await ctx.RollbackTransctionAsync(CancellationToken.None);
                }
            }
            catch (Exception ex)
            {
                throw new LWStorageUpdateFailedException("", ex);
            }
            //If the result is 0 then the update failed
            if (!result)
            {
                throw new LWStorageUpdateFailedException($"Descriptor {entry.Id} failed to update");
            }
        }
        
        /// <summary>
        /// Function to remove the specified descriptor 
        /// </summary>
        /// <param name="descriptorObj">The active descriptor to remove from the database</param>
        /// <exception cref="LWStorageRemoveFailedException"></exception>
        internal async Task RemoveDescriptorAsync(object descriptorObj)
        {
            LWStorageEntry descriptor = (descriptorObj as LWStorageDescriptor)!.Entry;
            ERRNO result;
            try
            {
                //Init db
                await using LWStorageContext db = GetContext();
                //Begin transaction
                await db.OpenTransactionAsync();

                //Delete the user from the database
                db.Descriptors.Remove(descriptor);

                //Save changes and commit if successful
                result = await db.SaveChangesAsync();

                if (result)
                {
                    await db.CommitTransactionAsync();
                }
                else
                {
                    await db.RollbackTransctionAsync();
                }
            }
            catch (Exception ex)
            {
                throw new LWStorageRemoveFailedException("", ex);
            }
            if (!result)
            {
                throw new LWStorageRemoveFailedException("Failed to delete the user account because of a database failure, the user may already be deleted");
            }
        }
        
    }
}