using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {       

        var articles = new[]{ 

                                new Article()

                                {

                                    Quantity = 3,

                                    UnitPrice = 10,

                                },

                                new Article()

                                {

                                    Quantity = 3,

                                    UnitPrice = 20,

                                },

                                new Article()

                                {

                                    Quantity = 1,

                                    UnitPrice = 30,

                                },

​

                            };

        var discounts = CalculateDiscounts(articles, 3, 2);

        DisplayDiscounts(discounts, 3, 2);

        discounts = CalculateDiscounts(articles, 3, 1);

        DisplayDiscounts(discounts, 3, 1);

    }

    static IEnumerable<Discount> CalculateDiscounts(IEnumerable<Article> items, int minPurchaseQuantity, int payFor)

    {

        var batchedItems = BatchItemsEligibleForDiscount(items, minPurchaseQuantity);

        return batchedItems.Select(batch => batch.Skip(payFor)) 

                           .SelectMany(batch => batch) // flatten nested IEnumerable to an IEnumerable<Artible>

                           .Select(article => new Discount() { Value = article.UnitPrice });;

    }

    static IEnumerable<IEnumerable<Article>> BatchItemsEligibleForDiscount(IEnumerable<Article> items, int minPurchaseQuantity)

    {

        return items.OrderByDescending(article => article.UnitPrice)

                    .Select(article => Enumerable.Range(1, article.Quantity).Select(n => new Article() { Quantity = 1, UnitPrice = article.UnitPrice })) // "explode" articles 

                    .SelectMany(item => item) // flatten to an IEnumerable<Article>

                    .Select((article, index) => new { article, index })

                    .GroupBy(x => x.index / minPurchaseQuantity)

                    .Where(group => group.Count() == 3) // only take batches elegible for discount

                    .Select(group => group.Select(x => x.article));

    }

    static void DisplayDiscounts(IEnumerable<Discount> discounts, int minPurchaseQuantity, int payFor)

    {

        Console.WriteLine("Take " + minPurchaseQuantity + " pay for " + payFor);

        foreach(var discount in discounts)

        {

            Console.WriteLine(discount.Value);

        }

    }

    class Article

    {

        public double UnitPrice { get; set; }

        public int Quantity { get; set; }

        public Article()

        {

        }

    }

    class Discount

    {

        public double Value { get; set; }

    }

}