using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Shared.Extensions
{
    public static class EnumerableExtensions
    {
        /// <summary>
        /// Determines whether the collection is null or contains no elements.
        /// </summary>
        /// <typeparam name="T">The IEnumerable type.</typeparam>
        /// <param name="enumerable">The enumerable, which may be null or empty.</param>
        /// <returns>
        ///     <c>true</c> if the IEnumerable is null or empty; otherwise, <c>false</c>.
        /// </returns>
        /// <source>https://stackoverflow.com/a/8582374/2030635</source>
        public static bool IsNullOrEmpty<T>(this IEnumerable<T> enumerable)
        {
            if (enumerable == null)
            {
                return true;
            }
            /* If this is a list, use the Count property for efficiency.
             * The Count property is O(1) while IEnumerable.Count() is O(N). */
            if (enumerable is ICollection<T> collection)
            {
                return collection.Count < 1;
            }
            return !enumerable.Any();
        }

        /// <summary>
        /// Calculate the product of a collection of decimals.
        /// </summary>
        /// <param name="enumerable"></param>
        /// <returns></returns>
        public static decimal Product(this IEnumerable<decimal> enumerable)
        {
            //Product of empty collection is 1: https://en.wikipedia.org/wiki/Empty_product
            if (!enumerable.Any()) return 1M;

            return enumerable.Aggregate((d1, d2) => d1 * d2);
        }

        /// <summary>
        /// Calculate the product of a collection with a selector to translate its items to decimals.
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="enumerable"></param>
        /// <param name="selector"></param>
        /// <returns></returns>
        public static decimal Product<T>(this IEnumerable<T> enumerable, Func<T, decimal> selector)
        {
            //Product of empty collection is 1: https://en.wikipedia.org/wiki/Empty_product
            if (!enumerable.Any()) return 1M;

            return enumerable.Aggregate(1M, (s, i2) => s * selector(i2));
        }
    }
}