feat: add NormalStrategy with days injection

python/gilded_rose.py

@@ -1,4 +1,31 @@
 # -*- coding: utf-8 -*-
+from dataclasses import dataclass
+from typing import Protocol
+
+
+class UpdateStrategy(Protocol):
+    """Interface for per-item-type quality update logic."""
+
+    def update(self, item: "Item", days: int) -> None:
+        """Mutate item.quality and item.sell_in to reflect `days` passing."""
+        ...
+
+
+class NormalStrategy:
+    """
+    Default degradation strategy.
+
+    Quality decreases by 1 per day; by 2 per day once the sell date
+    has passed (sell_in < 0). Quality never falls below 0.
+    """
+
+    def update(self, item: "Item", days: int) -> None:
+        for _ in range(days):
+            item.quality = max(0, item.quality - 1)
+            item.sell_in -= 1
+            if item.sell_in < 0:
+                item.quality = max(0, item.quality - 1)
+
 
 class GildedRose(object):
 

python/tests/test_gilded_rose.py

@@ -7,7 +7,7 @@ _EXERCISE_ROOT = Path(__file__).resolve().parent.parent
 if str(_EXERCISE_ROOT) not in sys.path:
     sys.path.insert(0, str(_EXERCISE_ROOT))
 
-from gilded_rose import Item, GildedRose
+from gilded_rose import Item, GildedRose, NormalStrategy
 
 
 class GildedRoseTest(unittest.TestCase):
@@ -18,5 +18,46 @@ class GildedRoseTest(unittest.TestCase):
         self.assertEqual(0, items[0].quality)
 
         
+class TestNormalStrategy(unittest.TestCase):
+    """Tests for NormalStrategy — default degradation behaviour."""
+
+    def setUp(self):
+        self.strategy = NormalStrategy()
+
+    def test_quality_decrements_by_one_each_day(self):
+        item = Item("widget", sell_in=10, quality=20)
+        self.strategy.update(item, days=1)
+        self.assertEqual(19, item.quality)
+        self.assertEqual(9, item.sell_in)
+
+    def test_quality_floors_at_zero(self):
+        # Quality must never go negative
+        item = Item("widget", sell_in=5, quality=0)
+        self.strategy.update(item, days=1)
+        self.assertEqual(0, item.quality)
+
+    def test_quality_degrades_twice_after_sell_date(self):
+        # Once sell_in goes below 0, each day removes 2 quality
+        item = Item("widget", sell_in=0, quality=10)
+        self.strategy.update(item, days=1)
+        self.assertEqual(8, item.quality)
+        self.assertEqual(-1, item.sell_in)
+
+    def test_multi_day_crosses_sell_date(self):
+        # Days=5 from sell_in=2, quality=10
+        # Day1: q=9 si=1 | Day2: q=8 si=0 | Day3: q=6 si=-1
+        # Day4: q=4 si=-2 | Day5: q=2 si=-3
+        item = Item("widget", sell_in=2, quality=10)
+        self.strategy.update(item, days=5)
+        self.assertEqual(2, item.quality)
+        self.assertEqual(-3, item.sell_in)
+
+    def test_quality_floors_at_zero_when_crossing_sell_date(self):
+        # sell_in=1, quality=1: Day1 q=0 si=0 | Day2 post-sell but already 0
+        item = Item("widget", sell_in=1, quality=1)
+        self.strategy.update(item, days=2)
+        self.assertEqual(0, item.quality)
+
+
 if __name__ == '__main__':
     unittest.main()