using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var skus = new List<Sku>();

                skus.Add(new Sku(){ ID = 0, ProductID = 1 });

​

        skus.Add(new Sku(){ ID = 1, ProductID = 1, Availability = ProductAvailability.InStock });

        skus.Add(new Sku(){ ID = 2, ProductID = 2, Availability = ProductAvailability.InStock });

        skus.Add(new Sku(){ ID = 3, ProductID = 1 });

        skus.Add(new Sku(){ ID = 4, ProductID = 1 });

        skus.Add(new Sku(){ ID = 5, ProductID = 2 });

        skus.Add(new Sku(){ ID = 6, ProductID = 3 });

        skus.Add(new Sku(){ ID = 7, ProductID = 4, Availability = ProductAvailability.InStock });

        skus.Add(new Sku(){ ID = 8, ProductID = 3, Availability = ProductAvailability.InStock });

​

        var orderedSkus = skus.OrderByDescending(o => GetInStock().Contains(o.Availability)); // it is to check "real" Availability.

        var products = GetProducts(orderedSkus);

        Write(products);

    }

    public class Sku {

        public int ID {get;set;}

        public int ProductID {get;set;}

        public ProductAvailability Availability {get; set;} = ProductAvailability.OutOfStock;   

    }

    public class Product {

        public ProductAvailability Availability {get; set;} = ProductAvailability.OutOfStock;   

​

        public int SkuID{get; set;}

        public int ID {get; set;}

    }

    public static Func<IEnumerable<Sku>, IEnumerable<IGrouping<object, Sku>>> GroupSkus { get; set; } = sku => sku.GroupBy(o => (object)o.ProductID);

​

    public static IEnumerable<Product> GetProducts(IEnumerable<Sku> skus) { 

        return GroupSkus(skus)

            .Select(o => Map(o))

            .ToList();

    }

    public static void Write(Product product){

        Console.WriteLine(product.SkuID + " " + product.Availability);

    }

    public static void Write(IEnumerable<Product> products){

        foreach (var product in products){

            Write(product);

        }

    }

    public static Product Map(IEnumerable<Sku> skus) {

        var productVM = new Product();

​

        var sku = SelectSku(skus);

        productVM.Availability = sku.Availability;

        productVM.ID = sku.ProductID;

        productVM.SkuID = sku.ID;

​

        return productVM;

    }

​

    protected static Sku SelectSku(IEnumerable<Sku> skus) {

        var sku = skus

            .OrderByDescending(o => GetInStock().Contains(o.Availability)) // it is to check "real" Availability.

            .FirstOrDefault();

​

        if (sku == null) {

            // No suitable SKU available, let the first one fly.

            sku = skus.First();

        }

​

        return sku;

    }

​

​

​

​

    public static IEnumerable<ProductAvailability> GetInStock() {

        return new ProductAvailability[] {

            ProductAvailability.InStock,

            ProductAvailability.LastPieces,

            ProductAvailability.InExternalStock,

        };

    } 

​

​

​

​

    public enum ProductAvailability : int {

        /// <summary>

        /// Out of stock

        /// </summary>

​

        OutOfStock = 0,

​

        /// <summary>

        /// In stock

        /// </summary>

        InStock = 1,

​

​

        InExternalStock = 2,

​

​

        InProduction = 3,

​

        /// <summary>

        /// Discarded, discontinued.

        /// </summary>

        Discarded = 4,

​

        Checking = 5,

​

        Waiting = 6,

​

        Boarding = 7,

​

        LastPieces = 8,

​

        SaleFinished = 9,

    }

}