using System;
using System.Collections.Generic;

namespace Chernobyl.Creation
{
    /// <summary>
    /// An <see cref="IFactory{T1, TResult}"/> that caches the results from another
    /// <see cref="IFactory{T1, TResult}"/> and returns the cache result if a request matches a
    /// cached item.
    /// </summary>
    public class CacheFactory<TKey, T1, TResult> : Builder<TResult>, IFactory<T1, TResult>
    {
        /// <param name="keyCreator">The instance that creates keys for items
        /// that are to be cached or found.</param>
        /// <param name="factory">The instance used to create items if a cached
        /// item cannot be found and returned.</param>
        public CacheFactory(Func<T1, TKey> keyCreator, IFactory<T1, TResult> factory)
            : this(keyCreator, factory, new Dictionary<TKey, TResult>())
        {}

        /// <summary>
        /// Initializes a new instance of the <see cref="CacheFactory{TKey, T1, TResult}"/> class.
        /// </summary>
        /// <param name="keyCreator">The instance that creates keys for items
        /// that are to be cached or found.</param>
        /// <param name="factory">The instance used to create items if a cached
        /// item cannot be found and returned.</param>
        /// <param name="cache">The instance where cached items are stored.</param>
        public CacheFactory(Func<T1, TKey> keyCreator, IFactory<T1, TResult> factory, IDictionary<TKey, TResult> cache)
        {
            keyCreator.ThrowIfNull("keyCreator");
            _keyCreator = keyCreator;

            factory.ThrowIfNull("factory");
            _factory = factory;

            cache.ThrowIfNull("cache");
            _cache = cache;
        }

        /// <inheritdoc />
        public TResult Create(T1 arg1)
        {
            TKey key = _keyCreator(arg1);
            if (!_cache.TryGetValue(key, out var result))
            {
                // We do not have this instance cached. Have the factory create a new instance and report it.
                result = _factory.Create(arg1);
                _cache[key] = result;
            }

            ReportCreation(new CreationEventArgs<TResult>(result, arg1));
            return result;
        }

        /// <summary>
        /// The instance that creates keys for items that are to be cached or
        /// found.
        /// </summary>
        readonly Func<T1, TKey> _keyCreator;

        /// <summary>
        /// The instance used to create items if a cached item cannot be found
        /// and returned.
        /// </summary>
        readonly IFactory<T1, TResult> _factory;

        /// <summary>
        /// The instance where cached items are stored.
        /// </summary>
        readonly IDictionary<TKey, TResult> _cache;
    }
}