/*
message Pagination {
  string pageToken = 1;
  int32 pageSize = 2;
}
*/

  public static class Extensions
  {
    public static async Task<PagedResponse<T>> ToPagedResponseAsync<T>(this IQueryable<T> source, Pagination pagination)
    {
      if (source == null)
      {
        throw new ArgumentNullException(nameof(source));
      }

      pagination ??= new Pagination {PageSize = 10};

      var totalCount = await source.CountAsync();
      var skip = GetSkip();

      var items = await source
        .Skip(skip)
        .Take(pagination.PageSize)
        .ToListAsync();

      return new PagedResponse<T>
      {
        Items = items,
        NextPageToken = GetNextPageToken()
      };

      int GetSkip()
      {
        if (string.IsNullOrEmpty(pagination.PageToken))
        {
          return 0;
        }

        try
        {
          var bytes = Convert.FromBase64String(pagination.PageToken);
          return BitConverter.ToInt32(bytes);
        }
        catch (Exception e)
        {
          throw new ArgumentException("Could not decode page token", e);
        }
      }

      string GetNextPageToken()
      {
        var nextSkip = skip + pagination.PageSize;

        if (nextSkip >= totalCount)
        {
          return string.Empty;
        }

        var bytes = BitConverter.GetBytes(nextSkip);
        return Convert.ToBase64String(bytes);
      }
    }
  }