feat: add BackstagePassStrategy

python/gilded_rose.py

@@ -43,6 +43,34 @@ class AgedBrieStrategy:
                 item.quality = min(50, item.quality + 1)
 
 
+class BackstagePassStrategy:
+    """
+    Quality strategy for backstage passes.
+
+    Quality increases as the concert approaches:
+      - +1/day when more than 10 days remain
+      - +2/day when 10 or fewer days remain
+      - +3/day when 5 or fewer days remain
+    Quality drops to 0 on concert day (sell_in == 0 at start of day)
+    and stays 0 afterwards. Quality never exceeds 50.
+    """
+
+    def update(self, item: "Item", days: int) -> None:
+        for _ in range(days):
+            if item.sell_in <= 0:
+                # Concert day or past — pass is worthless
+                item.quality = 0
+                item.sell_in -= 1
+                continue
+            if item.sell_in <= 5:
+                item.quality = min(50, item.quality + 3)
+            elif item.sell_in <= 10:
+                item.quality = min(50, item.quality + 2)
+            else:
+                item.quality = min(50, item.quality + 1)
+            item.sell_in -= 1
+
+
 class GildedRose(object):
 
     def __init__(self, items):

python/tests/test_gilded_rose.py

@@ -7,7 +7,10 @@ _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, NormalStrategy, AgedBrieStrategy
+from gilded_rose import (
+    Item, GildedRose, NormalStrategy, AgedBrieStrategy,
+    BackstagePassStrategy,
+)
 
 
 class GildedRoseTest(unittest.TestCase):
@@ -92,5 +95,56 @@ class TestAgedBrieStrategy(unittest.TestCase):
         self.assertEqual(-2, item.sell_in)
 
 
+class TestBackstagePassStrategy(unittest.TestCase):
+    """Tests for BackstagePassStrategy — tiered increase, zeroes after concert."""
+
+    def setUp(self):
+        self.strategy = BackstagePassStrategy()
+
+    def test_quality_increments_by_1_when_more_than_10_days(self):
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=15, quality=20)
+        self.strategy.update(item, days=1)
+        self.assertEqual(21, item.quality)
+        self.assertEqual(14, item.sell_in)
+
+    def test_quality_increments_by_2_when_10_or_fewer_days(self):
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=10, quality=20)
+        self.strategy.update(item, days=1)
+        self.assertEqual(22, item.quality)
+        self.assertEqual(9, item.sell_in)
+
+    def test_quality_increments_by_3_when_5_or_fewer_days(self):
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=5, quality=20)
+        self.strategy.update(item, days=1)
+        self.assertEqual(23, item.quality)
+        self.assertEqual(4, item.sell_in)
+
+    def test_quality_drops_to_zero_on_concert_day(self):
+        # sell_in=0 means the concert is today — quality zeroes out
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=0, quality=30)
+        self.strategy.update(item, days=1)
+        self.assertEqual(0, item.quality)
+        self.assertEqual(-1, item.sell_in)
+
+    def test_quality_stays_zero_after_concert(self):
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=-1, quality=0)
+        self.strategy.update(item, days=1)
+        self.assertEqual(0, item.quality)
+
+    def test_quality_caps_at_50(self):
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=5, quality=49)
+        self.strategy.update(item, days=1)
+        self.assertEqual(50, item.quality)
+
+    def test_multi_day_crosses_all_thresholds(self):
+        # sell_in=12, quality=20, days=4
+        # Day1(si=12): +1 q=21 si=11 | Day2(si=11): +1 q=22 si=10
+        # Day3(si=10): +2 q=24 si=9  | Day4(si=9):  +2 q=26 si=8
+        item = Item("Backstage passes to a TAFKAL80ETC concert", sell_in=12, quality=20)
+        self.strategy.update(item, days=4)
+        self.assertEqual(26, item.quality)
+        self.assertEqual(8, item.sell_in)
+
+
 if __name__ == '__main__':
     unittest.main()