refactor Gilded Rose into per-item updaters and add Conjured items

- add characterization tests covering all existing item rules
- replace nested conditionals with per-item updater classes
- add Conjured items that degrade twice as fast as normal

python/gilded_rose.py

@@ -1,4 +1,100 @@
 # -*- coding: utf-8 -*-
+"""Gilded Rose inventory system.
+
+The quality-update rules differ per item category. Rather than branch on the
+item name inside one giant method, each category is modelled as an updater that
+knows how to age a single item by one day. ``GildedRose`` simply dispatches each
+item to the matching updater.
+
+NOTE: The ``Item`` class and its attributes are owned by the "goblin in the
+corner" and must not be modified, so all behaviour lives in the updaters.
+"""
+
+MIN_QUALITY = 0
+MAX_QUALITY = 50
+
+AGED_BRIE = "Aged Brie"
+BACKSTAGE_PASS = "Backstage passes to a TAFKAL80ETC concert"
+SULFURAS = "Sulfuras, Hand of Ragnaros"
+CONJURED_PREFIX = "Conjured"
+
+
+def _clamp(quality):
+    """Keep quality within the legal [0, 50] range."""
+    return max(MIN_QUALITY, min(MAX_QUALITY, quality))
+
+
+class ItemUpdater(object):
+    """Base updater: a normal item degrades by 1, then by 2 once expired."""
+
+    daily_degradation = 1
+
+    def __init__(self, item):
+        self.item = item
+
+    def update(self):
+        self._update_sell_in()
+        self._update_quality()
+
+    def _update_sell_in(self):
+        self.item.sell_in -= 1
+
+    def _update_quality(self):
+        amount = self.daily_degradation
+        if self.item.sell_in < 0:
+            amount *= 2  # degrades twice as fast after the sell-by date
+        self.item.quality = _clamp(self.item.quality - amount)
+
+
+class AgedBrieUpdater(ItemUpdater):
+    """Aged Brie increases in quality the older it gets, faster once expired."""
+
+    def _update_quality(self):
+        amount = 1
+        if self.item.sell_in < 0:
+            amount *= 2
+        self.item.quality = _clamp(self.item.quality + amount)
+
+
+class BackstagePassUpdater(ItemUpdater):
+    """Backstage passes gain value as the concert approaches, then drop to zero
+    once the concert has passed."""
+
+    def _update_quality(self):
+        if self.item.sell_in < 0:
+            self.item.quality = 0
+            return
+        amount = 1
+        if self.item.sell_in < 5:
+            amount = 3
+        elif self.item.sell_in < 10:
+            amount = 2
+        self.item.quality = _clamp(self.item.quality + amount)
+
+
+class SulfurasUpdater(ItemUpdater):
+    """Sulfuras is legendary: it never has to be sold and never changes."""
+
+    def update(self):
+        pass
+
+
+class ConjuredUpdater(ItemUpdater):
+    """Conjured items degrade twice as fast as normal items."""
+
+    daily_degradation = 2
+
+def updater_for(item):
+    """Pick the right updater for an item based on its name."""
+    if item.name == AGED_BRIE:
+        return AgedBrieUpdater(item)
+    if item.name == BACKSTAGE_PASS:
+        return BackstagePassUpdater(item)
+    if item.name == SULFURAS:
+        return SulfurasUpdater(item)
+    if item.name.startswith(CONJURED_PREFIX):
+        return ConjuredUpdater(item)
+    return ItemUpdater(item)
 
 class GildedRose(object):
 
@@ -7,33 +103,7 @@ class GildedRose(object):
 
     def update_quality(self):
         for item in self.items:
-            if item.name != "Aged Brie" and item.name != "Backstage passes to a TAFKAL80ETC concert":
-                if item.quality > 0:
-                    if item.name != "Sulfuras, Hand of Ragnaros":
-                        item.quality = item.quality - 1
-            else:
-                if item.quality < 50:
-                    item.quality = item.quality + 1
-                    if item.name == "Backstage passes to a TAFKAL80ETC concert":
-                        if item.sell_in < 11:
-                            if item.quality < 50:
-                                item.quality = item.quality + 1
-                        if item.sell_in < 6:
-                            if item.quality < 50:
-                                item.quality = item.quality + 1
-            if item.name != "Sulfuras, Hand of Ragnaros":
-                item.sell_in = item.sell_in - 1
-            if item.sell_in < 0:
-                if item.name != "Aged Brie":
-                    if item.name != "Backstage passes to a TAFKAL80ETC concert":
-                        if item.quality > 0:
-                            if item.name != "Sulfuras, Hand of Ragnaros":
-                                item.quality = item.quality - 1
-                    else:
-                        item.quality = item.quality - item.quality
-                else:
-                    if item.quality < 50:
-                        item.quality = item.quality + 1
+            updater_for(item).update()
 
 
 class Item:
@@ -43,4 +113,4 @@ class Item:
         self.quality = quality
 
     def __repr__(self):
-        return "%s, %s, %s" % (self.name, self.sell_in, self.quality)
+        return "%s, %s, %s" % (self.name, self.sell_in, self.quality)