Add conjured items

Conjured items degrade twice as fast as normal ones. Matched by name
prefix so any "Conjured ..." item works, not just the Mana Cake.
Wrote the tests first, then the updater. Only the Mana Cake lines
changed in the approved output.

python/gilded_rose.py

@@ -3,6 +3,7 @@
 AGED_BRIE = "Aged Brie"
 BACKSTAGE_PASS = "Backstage passes to a TAFKAL80ETC concert"
 SULFURAS = "Sulfuras, Hand of Ragnaros"
+CONJURED_PREFIX = "Conjured"
 
 MIN_QUALITY = 0
 MAX_QUALITY = 50
@@ -71,17 +72,29 @@ class SulfurasUpdater(ItemUpdater):
         pass
 
 
+class ConjuredUpdater(ItemUpdater):
+    """Degrades twice as fast as an ordinary item."""
+
+    def _quality_delta(self, item):
+        return 2 * super()._quality_delta(item)
+
+
 _UPDATERS_BY_NAME = {
     AGED_BRIE: AgedBrieUpdater(),
     BACKSTAGE_PASS: BackstagePassUpdater(),
     SULFURAS: SulfurasUpdater(),
 }
+_CONJURED_UPDATER = ConjuredUpdater()
 _DEFAULT_UPDATER = ItemUpdater()
 
 
 def updater_for(item):
     """Pick the rule set that applies to an item. Updaters are stateless."""
-    return _UPDATERS_BY_NAME.get(item.name, _DEFAULT_UPDATER)
+    if item.name in _UPDATERS_BY_NAME:
+        return _UPDATERS_BY_NAME[item.name]
+    if item.name.startswith(CONJURED_PREFIX):
+        return _CONJURED_UPDATER
+    return _DEFAULT_UPDATER
 
 
 class Item:

python/tests/approved_files/test_gilded_rose_approvals.test_gilded_rose_approvals.approved.txt

@@ -21,7 +21,7 @@ Sulfuras, Hand of Ragnaros, -1, 80
 Backstage passes to a TAFKAL80ETC concert, 14, 21
 Backstage passes to a TAFKAL80ETC concert, 9, 50
 Backstage passes to a TAFKAL80ETC concert, 4, 50
-Conjured Mana Cake, 2, 5
+Conjured Mana Cake, 2, 4
 
 -------- day 2 --------
 name, sellIn, quality
@@ -33,7 +33,7 @@ Sulfuras, Hand of Ragnaros, -1, 80
 Backstage passes to a TAFKAL80ETC concert, 13, 22
 Backstage passes to a TAFKAL80ETC concert, 8, 50
 Backstage passes to a TAFKAL80ETC concert, 3, 50
-Conjured Mana Cake, 1, 4
+Conjured Mana Cake, 1, 2
 
 -------- day 3 --------
 name, sellIn, quality
@@ -45,7 +45,7 @@ Sulfuras, Hand of Ragnaros, -1, 80
 Backstage passes to a TAFKAL80ETC concert, 12, 23
 Backstage passes to a TAFKAL80ETC concert, 7, 50
 Backstage passes to a TAFKAL80ETC concert, 2, 50
-Conjured Mana Cake, 0, 3
+Conjured Mana Cake, 0, 0
 
 -------- day 4 --------
 name, sellIn, quality
@@ -57,7 +57,7 @@ Sulfuras, Hand of Ragnaros, -1, 80
 Backstage passes to a TAFKAL80ETC concert, 11, 24
 Backstage passes to a TAFKAL80ETC concert, 6, 50
 Backstage passes to a TAFKAL80ETC concert, 1, 50
-Conjured Mana Cake, -1, 1
+Conjured Mana Cake, -1, 0
 
 -------- day 5 --------
 name, sellIn, quality

python/tests/test_gilded_rose.py

@@ -12,6 +12,7 @@ from gilded_rose import GildedRose, Item
 AGED_BRIE = "Aged Brie"
 SULFURAS = "Sulfuras, Hand of Ragnaros"
 BACKSTAGE_PASS = "Backstage passes to a TAFKAL80ETC concert"
+CONJURED_CAKE = "Conjured Mana Cake"
 
 
 def update(name, sell_in, quality):
@@ -81,12 +82,29 @@ class TestBackstagePasses:
         assert quality == 0
 
 
+class TestConjuredItem:
+    def test_quality_degrades_twice_as_fast_as_a_normal_item(self):
+        assert update(CONJURED_CAKE, 3, 6) == (2, 4)
+
+    def test_quality_degrades_four_per_day_once_sell_by_date_has_passed(self):
+        assert update(CONJURED_CAKE, 0, 6) == (-1, 2)
+
+    @pytest.mark.parametrize("sell_in, quality", [(3, 1), (0, 3), (-2, 0)])
+    def test_quality_is_never_negative(self, sell_in, quality):
+        _, new_quality = update(CONJURED_CAKE, sell_in, quality)
+        assert new_quality == 0
+
+    def test_any_item_whose_name_starts_with_conjured_is_conjured(self):
+        assert update("Conjured +5 Dexterity Vest", 10, 20) == (9, 18)
+
+
 def test_all_items_in_the_inventory_are_updated():
     items = [
         Item("+5 Dexterity Vest", 10, 20),
         Item(AGED_BRIE, 2, 0),
         Item(SULFURAS, 0, 80),
         Item(BACKSTAGE_PASS, 5, 20),
+        Item(CONJURED_CAKE, 3, 6),
     ]
     GildedRose(items).update_quality()
     assert [(i.sell_in, i.quality) for i in items] == [
@@ -94,6 +112,7 @@ def test_all_items_in_the_inventory_are_updated():
         (1, 1),
         (0, 80),
         (4, 23),
+        (2, 4),
     ]