using System;
using System.Threading.Tasks;

namespace Config.Net.Core
{
   /// <summary>
   /// Implements a lazy value i.e. that can expire in future
   /// </summary>
   /// <typeparam name="T"></typeparam>
   class LazyVar<T> where T : class
   {
      private readonly Func<Task<T?>>? _renewFuncAsync;
      private readonly Func<T?>? _renewFunc;
      private DateTime _lastRenewed = DateTime.MinValue;
      private readonly TimeSpan _timeToLive;
      private T? _value;

      /// <summary>
      /// Creates an instance of a lazy variable with time-to-live value
      /// </summary>
      /// <param name="timeToLive">Time to live. Setting to <see cref="TimeSpan.Zero"/> disables caching completely</param>
      /// <param name="renewFunc"></param>
      public LazyVar(TimeSpan timeToLive, Func<Task<T?>> renewFunc)
      {
         _timeToLive = timeToLive;
         _renewFuncAsync = renewFunc ?? throw new ArgumentNullException(nameof(renewFunc));
         _renewFunc = null;
      }

      /// <summary>
      /// Creates an instance of a lazy variable with time-to-live value
      /// </summary>
      /// <param name="timeToLive">Time to live. Setting to <see cref="TimeSpan.Zero"/> disables caching completely</param>
      /// <param name="renewFunc"></param>
      public LazyVar(TimeSpan timeToLive, Func<T?> renewFunc)
      {
         _timeToLive = timeToLive;
         _renewFuncAsync = null;
         _renewFunc = renewFunc ?? throw new ArgumentNullException(nameof(renewFunc));
      }

      /// <summary>
      /// Gets the values, renewing it if necessary
      /// </summary>
      /// <returns>Value</returns>
      public async Task<T?> GetValueAsync()
      {
         if (_renewFuncAsync == null)
         {
            throw new InvalidOperationException("cannot renew value, async delegate is not specified");
         }

         if (_timeToLive == TimeSpan.Zero)
         {
            return await _renewFuncAsync();
         }

         bool expired = (DateTime.UtcNow - _lastRenewed) > _timeToLive;

         if (expired)
         {
            _value = await _renewFuncAsync();
            _lastRenewed = DateTime.UtcNow;
         }

         return _value;
      }

      /// <summary>
      /// Gets the values, renewing it if necessary
      /// </summary>
      /// <returns>Value</returns>
      public T? GetValue()
      {
         if (_renewFunc == null)
         {
            throw new InvalidOperationException("cannot renew value, synchronous delegate is not specified");
         }

         if (_timeToLive == TimeSpan.Zero)
         {
            return _renewFunc();
         }

         bool expired = (DateTime.UtcNow - _lastRenewed) > _timeToLive;

         if (expired)
         {
            _value = _renewFunc();
            _lastRenewed = DateTime.UtcNow;
         }

         return _value;
      }
   }
}