ByteDecoder/RoyalLibrary

using System;
using System.Collections.Generic;
using System.Linq;
using ByteDecoder.Common.GuardClauses;

namespace ByteDecoder.RoyalLibrary
{
    /// <summary>
    /// 
    /// </summary>
    public static class ParityMathExtensions
    {
        /// <summary>
        ///  Even expression evaluation delegate type
        /// </summary>
        public static readonly Func<int, bool> EvenPredicate = value => value % 2 == 0;

        /// <summary>
        /// Odd expression evaluation delegate type
        /// </summary>
        public static readonly Func<int, bool> OddPredicate = value => value % 2 != 0;

        /// <summary>
        /// Return an array of evens integers from an integer source collection
        /// </summary>
        /// <param name="numbers">Integer source collection</param>
        /// <returns></returns>
        public static IEnumerable<int> Evens(this IEnumerable<int> numbers) => numbers.Where(EvenPredicate);

        /// <summary>
        /// Deferred execution
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="source"></param>
        /// <param name="selector"></param>
        /// <returns></returns>
        public static IEnumerable<T> Evens<T>(this IEnumerable<T> source, Func<T, int> selector) =>
          source.ParityEvaluator(EvenPredicate, selector);

        /// <summary>
        /// Return an array of odds integers from an integer source collection
        /// </summary>
        /// <param name="numbers">Integer source collection</param>
        /// <returns></returns>
        public static IEnumerable<int> Odds(this IEnumerable<int> numbers) => numbers.Where(OddPredicate);

        /// <summary>
        /// Deferred Execution
        /// </summary>
        /// <param name="source"></param>
        /// <param name="selector"></param>
        /// <returns></returns>
        public static IEnumerable<T> Odds<T>(this IEnumerable<T> source, Func<T, int> selector) =>
          source.ParityEvaluator(OddPredicate, selector);

        /// <summary>
        /// Return a sum of all evens integers from an integer source collection
        /// </summary>
        /// <param name="numbers">Integer source collection</param>
        /// <returns></returns>
        public static long TotalAllEvens(this IEnumerable<int> numbers) => numbers.Evens().LongSum();

        /// <summary>
        /// Return a sum of all odds integers from an integer source collection
        /// </summary>
        /// <param name="numbers">Integer source collection</param>
        /// <returns></returns>
        public static long TotalAllOdds(this IEnumerable<int> numbers) => numbers.Odds().LongSum();

        /// <summary>
        /// Deferred execution
        /// </summary>
        /// <param name="source"></param>
        /// <param name="evaluatorPredicate"></param>
        /// <param name="selector"></param>
        /// <typeparam name="T"></typeparam>
        /// <returns></returns>
        public static IEnumerable<T> ParityEvaluator<T>(this IEnumerable<T> source, Func<int, bool> evaluatorPredicate, Func<T, int> selector)
        {
            Guard.Break.IfArgumentIsNull(source, nameof(source));
            Guard.Break.IfArgumentIsNull(evaluatorPredicate, nameof(evaluatorPredicate));
            Guard.Break.IfArgumentIsNull(selector, nameof(selector));

            foreach (var item in source)
            {
                if (evaluatorPredicate(selector(item)))
                    yield return item;
            }
        }
    }
}