with surviving mutants

GildedRoseTest.cs

using Xunit;

namespace Gilded_rose.Test
{
    public class GildedRoseTest
    {
        private const int MinQuality = 0;
        private const int MaxQuality = 50;

        [Fact]
        public void regular_items_quality_decreases_every_day()
        {
            Item item = RegularItem(1, 3);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(2, item.Quality);
            Assert.Equal(0, item.SellIn);
        }

        [Fact]
        public void expired_regular_items_quality_decreases_twice_as_fast_once_sellIn_has_passed()
        {
            Item item = RegularItem(0, 3);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(1, item.Quality);
            Assert.Equal( -1, item.SellIn);
        }

        [Fact]
        public void regular_items_quality_never_decreases_below_the_minimum_quality()
        {
            Item item = RegularItem(5, 0);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(MinQuality, item.Quality);
            Assert.Equal(4, item.SellIn);
        }

        [Fact]
        public void aged_brie_increases_quality_by_1()
        {
            Item item = AgedBrie(5, 8);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(9, item.Quality);
            Assert.Equal(4, item.SellIn);
        }

        [Fact]
        public void aged_brie_quality_never_increases_over_the_maximun_quality()
        {
            Item item = AgedBrie(5, MaxQuality);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(MaxQuality, item.Quality);
            Assert.Equal(4, item.SellIn);
        }

        [Fact]
        public void expired_aged_brie_increases_quality_by_2()
        {
            Item item = AgedBrie(0, 8);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(10, item.Quality);
            Assert.Equal(-1, item.SellIn);
        }

        [Fact]
        public void sulfuras_never_changes()
        {
            Item item = Sulfuras(1, 80);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(80, item.Quality);
            Assert.Equal(1, item.SellIn);
        }
        
        [Fact]
        public void expired_sulfuras_never_changes()
        {
            Item item = Sulfuras(-1, 80);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(80, item.Quality);
            Assert.Equal(-1, item.SellIn);
        }

        [Fact]
        public void backstage_increase_quality_by_1_when_sellIn_is_greater_than_10()
        {
            Item item = BackstagePasses(11, 6);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(7, item.Quality);
            Assert.Equal(10, item.SellIn);
        }

        [Fact]
        public void backstage_increase_quality_by_2_when_sellIn_is_between_10_and_6()
        {
            Item item1 = BackstagePasses(8, 6);
            GildedRose shop = new GildedRose(new[] { item1 });

            shop.UpdateQuality();

            Assert.Equal(8, item1.Quality);
            Assert.Equal(7, item1.SellIn);
        }

        [Fact]
        public void backstage_increase_quality_by_3_when_sellIn_is_between_5_and_1()
        {
            Item item1 = BackstagePasses(4, 6);
            GildedRose shop = new GildedRose(new[] { item1 });

            shop.UpdateQuality();

            Assert.Equal(9, item1.Quality);
            Assert.Equal(3, item1.SellIn);
        }

        [Fact]
        public void backstage_quality_drops_to_0_after_the_concert()
        {
            Item item = BackstagePasses(0, 6);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(MinQuality, item.Quality);
            Assert.Equal(-1, item.SellIn);
        }

        [Fact]
        public void backstage_quality_never_increases_over_the_maximun_quality()
        {
            Item item = BackstagePasses(11, MaxQuality);
            GildedRose shop = new GildedRose(new[] { item });

            shop.UpdateQuality();

            Assert.Equal(MaxQuality, item.Quality);
            Assert.Equal(10, item.SellIn);
        }

        private Item RegularItem(int sellIn, int quality)
        {
            return new Item("foo", sellIn, quality);
        }

        private Item AgedBrie(int sellIn, int quality)
        {
            return new Item("Aged Brie", sellIn, quality);
        }

        private Item Sulfuras(int sellIn, int quality)
        {
            return new Item("Sulfuras, Hand of Ragnaros", sellIn, quality);
        }

        private Item BackstagePasses(int sellIn, int quality)
        {
            return new Item("Backstage passes to a TAFKAL80ETC concert", sellIn, quality);
        }
    }
}