/*
* Copyright (c) 2024 Vaughn Nugent
* 
* Library: VNLib
* Package: VNLib.Plugins.Extensions.VNCache
* File: EntityResultCache.cs 
*
* EntityResultCache.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.Threading;
using System.Threading.Tasks;

namespace VNLib.Plugins.Extensions.VNCache.DataModel
{
    /// <summary>
    /// Represents a cache that can store entities by their unique key
    /// using a user-provided backing store and custom request state.
    /// </summary>
    /// <typeparam name="TEntity"></typeparam>
    /// <param name="cache">The cache backing store</param>
    /// <param name="taskPolicy">Specifies how background cache tasks are handled</param>
    /// <param name="expirationPolicy">The result expiration policy</param>
    public class EntityResultCache<TEntity>(
        IEntityCache<TEntity> cache, 
        ICacheTaskPolicy taskPolicy, 
        ICacheExpirationPolicy<TEntity> expirationPolicy
    )
        where TEntity : class
    {

        /// <summary>
        /// The backing entity cache store
        /// </summary>
        public IEntityCache<TEntity> Cache => cache;

        /// <summary>
        /// The task policy for which this result cache will 
        /// respect
        /// </summary>
        public ICacheTaskPolicy TaskPolicy => taskPolicy;

        /// <summary>
        /// The expiration policy for which this result cache will
        /// respect for entity expiration and refreshing
        /// </summary>
        public ICacheExpirationPolicy<TEntity> ExpirationPolicy => expirationPolicy;


        /// <summary>
        /// Fetchs a result by it's request entity
        /// </summary>
        /// <param name="request">The fetch request state object</param>
        /// <param name="cancellation">A token to canel the operation</param>
        /// <param name="resultFactory">A callback generator function</param>
        /// <returns>A task the returns the result of the requested entity, or null if it was not found or provided by the backing store</returns>
        public Task<TEntity?> FetchAsync<TRequest>(
            TRequest request, 
            Func<TRequest, CancellationToken, Task<TEntity?>> resultFactory, 
            CancellationToken cancellation = default
        ) where TRequest : IEntityCacheKey
        {
            ArgumentNullException.ThrowIfNull(request);
            ArgumentNullException.ThrowIfNull(resultFactory);

            return FetchAsync(
                key: request.GetKey(),
                state: (resultFactory, request), 
                resultFactory: static (rf, c) => rf.resultFactory(rf.request, c), 
                cancellation
            );
        }

        /// <summary>
        /// Fetchs a result by it's request entity
        /// </summary>
        /// <param name="key">The fetch request state object</param>
        /// <param name="cancellation">A token to canel the operation</param>
        /// <param name="resultFactory">A callback generator function</param>
        /// <returns>A task the returns the result of the requested entity, or null if it was not found or provided by the backing store</returns>
        public Task<TEntity?> FetchAsync(
            string key,
            Func<CancellationToken, Task<TEntity?>> resultFactory,
            CancellationToken cancellation = default
        )
        {
            ArgumentNullException.ThrowIfNull(resultFactory);

            return FetchAsync(
                key, 
                state: resultFactory, 
                resultFactory: static (rf, c) => rf(c), 
                cancellation
            );
        }

        private async Task<TEntity?> FetchAsync<TState>(
            string key,
            TState state,
            Func<TState, CancellationToken, Task<TEntity?>> resultFactory,
            CancellationToken cancellation = default
        )
        {
            ArgumentException.ThrowIfNullOrWhiteSpace(key);
            ArgumentNullException.ThrowIfNull(resultFactory);
            cancellation.ThrowIfCancellationRequested();

            //try to fetch from cache
            TEntity? entity = await cache.GetAsync(key, cancellation);

            if (entity is not null)
            {
                //Check if the entity is expired
                if (expirationPolicy.IsExpired(entity))
                {
                    //Setting to null will force a cache miss
                    entity = null;
                }
            }

            if (entity is null)
            {
                //Cache miss, load from factory
                entity = await resultFactory(state, cancellation);

                if (entity is not null)
                {
                    //Notify the expiration policy that the entity was refreshed before writing back to cache
                    expirationPolicy.OnRefreshed(entity);

                    //Fresh entity was fetched from the factory so write to cache
                    Task upsert = cache.UpsertAsync(key, entity, cancellation);

                    //Allow task policy to determine how completions are observed
                    await taskPolicy.ObserveOperationAsync(upsert);
                }
            }

            return entity;
        }

        /// <summary>
        /// Removes an entity from the cache by it's request entity
        /// </summary>
        /// <param name="request">The request entity to retrieve the entity key from</param>
        /// <param name="cancellation">A token to cancel the async operation</param>
        /// <returns>A task that completes when the key is removed, based on the task policy</returns>
        public Task RemoveAsync<TRequest>(TRequest request, CancellationToken cancellation = default) 
            where TRequest : IEntityCacheKey
        {
            ArgumentNullException.ThrowIfNull(request);

            string key = request.GetKey();

            return cache.RemoveAsync(key, cancellation);
        }

        /// <summary>
        /// Removes an entity from the cache by it's request entity
        /// </summary>
        /// <param name="key">The entities unique key</param>
        /// <param name="cancellation">A token to cancel the async operation</param>
        /// <returns>A task that completes when the key is removed, based on the task policy</returns>
        public Task RemoveAsync(string key, CancellationToken cancellation = default)
        {
            ArgumentException.ThrowIfNullOrWhiteSpace(key);

            Task remove = cache.RemoveAsync(key, cancellation);

            return taskPolicy.ObserveOperationAsync(remove);
        }

        /// <summary>
        /// Performs a cache replacement operation. That is substitutes an exiting
        /// value with a new one, or inserts a new value if the key does not exist.
        /// </summary>
        /// <param name="request">The operation request state object</param>
        /// <param name="entity">The entity object to store</param>
        /// <param name="action">A generic callback function to invoke in parallel with the upsert operation</param>
        /// <param name="cancellation">A token to cancel the async operation</param>
        /// <returns>
        /// A task that completes when the upsert operation has completed according to the 
        /// <see cref="TaskPolicy"/>
        /// </returns>
        /// <exception cref="ArgumentException"></exception>
        /// <exception cref="ArgumentNullException"></exception>
        public Task UpsertAsync<TRequest>(
            TRequest request,
            TEntity entity,
            Func<TRequest, TEntity, CancellationToken, Task> action,
            CancellationToken cancellation = default
        ) where TRequest : IEntityCacheKey
        {
            ArgumentNullException.ThrowIfNull(request);
            ArgumentNullException.ThrowIfNull(action);

            return UpsertAsync(
                key: request.GetKey(),
                entity: entity,
                state: (action, request),
                callback: static (cb, e, c) => cb.action.Invoke(cb.request, e, c),
                cancellation
            );
        }


        /// <summary>
        /// Performs a cache replacement operation. That is substitutes an exiting
        /// value with a new one, or inserts a new value if the key does not exist.
        /// </summary>
        /// <param name="key">The entity's unique id within the cache store</param>
        /// <param name="entity">The entity object to store</param>
        /// <param name="action">A generic callback function to invoke in parallel with the upsert operation</param>
        /// <param name="cancellation">A token to cancel the async operation</param>
        /// <returns>
        /// A task that completes when the upsert operation has completed according to the 
        /// <see cref="TaskPolicy"/>
        /// </returns>
        /// <exception cref="ArgumentException"></exception>
        /// <exception cref="ArgumentNullException"></exception>
        public Task UpsertAsync(
            string key,
            TEntity entity,
            Func<TEntity, CancellationToken, Task> action,
            CancellationToken cancellation = default
        )
        {
            ArgumentNullException.ThrowIfNull(action);

            return UpsertAsync(
                key, 
                entity, 
                state: action, 
                callback: static (cb, e, c) => cb.Invoke(e, c), 
                cancellation
            );
        }

        /// <summary>
        /// Performs a cache replacement operation. That is substitutes an exiting
        /// value with a new one, or inserts a new value if the key does not exist.
        /// </summary>
        /// <param name="key">The entity's unique id within the cache store</param>
        /// <param name="entity">The entity object to store</param>
        /// <param name="cancellation">A token to cancel the async operation</param>
        /// <returns>
        /// A task that completes when the upsert operation has completed according to the 
        /// <see cref="TaskPolicy"/>
        /// </returns>
        /// <exception cref="ArgumentNullException"></exception>
        public Task UpsertAsync(string key, TEntity entity, CancellationToken cancellation = default)
        {
            return UpsertAsync<object?>(
                key,
                entity,
                state: null,
                callback: static (_, _, _) => Task.CompletedTask,
                cancellation
            );
        }

        private Task UpsertAsync<TState>(
            string key, 
            TEntity entity, 
            TState state, 
            Func<TState, TEntity, CancellationToken, Task> callback,
            CancellationToken cancellation = default
        )
        {
            ArgumentException.ThrowIfNullOrWhiteSpace(key);
            ArgumentNullException.ThrowIfNull(callback);
            cancellation.ThrowIfCancellationRequested();

            //Call refresh before storing the entity incase any setup needs to be performed
            expirationPolicy.OnRefreshed(entity);

            //Cache task must be observed by the task policy
            Task upsert = taskPolicy.ObserveOperationAsync(
                 operation: cache.UpsertAsync(key, entity, cancellation)
            );

            Task cbResult = callback(state, entity, cancellation);

            //Combine the observed task and the callback function
            return Task.WhenAll(cbResult, upsert);
        }
    }
}