docs: document gilded rose kata
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README.md
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README.md
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@ -102,13 +102,120 @@ Tests for simple cases (gutter game, one spare, one strike) drove an algorithm t
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---
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### 3. [Third Kata Name] 🚧
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### 3. Gilded Rose ✅
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**Status:** Not yet started
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**Implementation:** `lib/[third_kata].dart`
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**Tests:** `test/[third_kata]_test.dart`
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**Implementation:** [`lib/gilded_rose.dart`](lib/gilded_rose.dart)
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**Tests:** [`test/gilded_rose_test.dart`](test/gilded_rose_test.dart)
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*Coming soon...*
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A legacy code refactoring kata demonstrating how to safely transform deeply nested conditionals into clean, extensible code using characterization tests and the Strategy pattern.
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#### Domain Context
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An inn's inventory system that updates item quality daily based on complex business rules:
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**Item Types:**
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1. **Normal Items:** Quality decreases by 1/day, 2/day after sell-by date
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2. **Aged Brie:** Quality increases by 1/day, 2/day after expiration (improves with age!)
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3. **Sulfuras (Legendary):** Never changes, quality always 80, never "expires"
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4. **Backstage Passes:** Complex appreciation:
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- More than 10 days: +1 quality/day
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- 10 days or less: +2 quality/day
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- 5 days or less: +3 quality/day
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- After concert (sellIn < 0): Quality drops to 0
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5. **Conjured Items:** Degrade twice as fast as normal items (2/day, 4/day after expiration)
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**Domain Constraints:**
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- Quality never negative (≥ 0)
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- Quality never exceeds 50 (except Sulfuras at 80)
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- Cannot modify the `Item` class (goblin constraint!)
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#### Key Design Decisions
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**Characterization Testing:** Before touching legacy code, created 17 tests to capture existing behavior as a "safety net." Includes a Golden Master test simulating 30 days.
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**Strategy Pattern:** Each item type gets its own updater class implementing `ItemUpdater` interface. Eliminates conditional branching and enables Open-Closed Principle.
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**Helper Methods & Constants:** `_degradeQuality()` and `_improveQuality()` with automatic clamping eliminate scattered boundary checks. Domain constants (`_minQuality`, `_maxQuality`) remove magic numbers.
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**Factory Pattern:** `_selectUpdater()` method chooses the appropriate strategy based on item name, enabling polymorphic dispatch.
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#### The Refactoring Journey
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**Phase 1: Understand Legacy Code**
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```dart
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// 60+ lines of 7-8 level nested conditionals
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// Nearly incomprehensible logic mixing all item types
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```
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**Phase 2: Characterization Tests (GREEN Phase)**
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- 14 comprehensive tests covering all item types
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- Golden Master test: 30-day simulation baseline
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- Result: **Safety net established** ✅
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**Phase 3: Refactor with Confidence (3 steps)**
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**Step 1 - Extract Methods (REFACTOR):**
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```dart
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// Before: 60 lines of nested hell
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// After: Clean if-else delegating to 4 private methods
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_updateNormalItem(), _updateAgedBrie(),
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_updateBackstagePasses(), _updateSulfuras()
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```
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**Step 2 - Strategy Pattern (REFACTOR):**
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```dart
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abstract class ItemUpdater {
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void update(Item item);
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}
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class NormalItemUpdater implements ItemUpdater { ... }
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class AgedBrieUpdater implements ItemUpdater { ... }
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// ... 4 concrete strategies
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```
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**Step 3 - Domain Helpers (REFACTOR):**
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```dart
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const int _minQuality = 0;
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const int _maxQuality = 50;
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void _degradeQuality(Item item, int amount) {
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item.quality = (item.quality - amount).clamp(_minQuality, _maxQuality);
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}
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```
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**Phase 4: Add Conjured Items (RED-GREEN)**
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**RED:** Added 3 failing tests for Conjured items behavior
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**GREEN:** Created `ConjuredItemUpdater` class—**one class, one condition**
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**Result:** Feature added in minutes thanks to refactoring!
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#### Usage
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```dart
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import 'package:tdd_katas/gilded_rose.dart';
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final items = [
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Item('Normal Sword', 10, 20),
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Item('Aged Brie', 2, 0),
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Item('Sulfuras, Hand of Ragnaros', 0, 80),
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Item('Backstage passes to a TAFKAL80ETC concert', 15, 20),
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Item('Conjured Mana Cake', 3, 6),
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];
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final gildedRose = GildedRose(items);
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gildedRose.updateQuality(); // Updates all items per domain rules
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```
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#### The "Aha!" Moments
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1. **Characterization Tests = Freedom:** With tests in place, aggressive refactoring felt safe. Every change validated instantly.
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2. **Strategy Pattern = Extensibility:** Adding Conjured items took **5 minutes**. Before refactoring, it would have meant diving into nested conditionals and risking bugs.
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3. **Small Steps = Big Wins:** Three refactoring commits transformed spaghetti into clean code. Each step kept tests GREEN, proving behavior preservation.
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4. **Open-Closed Principle in Action:** New item types don't modify existing code—they just add new updater classes. The system is "open for extension, closed for modification."
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---
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@ -121,6 +228,7 @@ dart test
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# Run specific test file
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dart test test/roman_numerals_test.dart
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dart test test/bowling_game_test.dart
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dart test test/gilded_rose_test.dart
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# Run with coverage
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dart test --coverage
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@ -244,19 +352,203 @@ Tests are organized by **scoring complexity**, mirroring how the domain rules bu
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---
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## Gilded Rose: Detailed Journey
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### The Legacy Code Challenge
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Unlike the previous two katas (greenfield TDD), Gilded Rose simulates **real-world legacy code refactoring**. You inherit messy, working code with no tests and must:
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1. Understand what it does (without breaking it)
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2. Add tests to capture behavior
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3. Refactor safely
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4. Add new features
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### Test Strategy
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Tests are organized by **item type behavior** and include a **Golden Master**:
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**Normal Items:**
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- Quality degradation (1/day, 2/day after expiration)
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- Quality never negative
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**Aged Brie:**
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- Quality appreciation (improves with age)
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- Respects quality cap (≤ 50)
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**Sulfuras (Legendary Items):**
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- Never changes (quality, sellIn)
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- Always quality 80
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**Backstage Passes:**
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- Threshold-based appreciation (10 days, 5 days)
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- Drops to 0 after concert
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**Conjured Items (new feature):**
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- Degrades 2x faster than normal items
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**Golden Master Test:**
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- 30-day simulation with all item types
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- Captures baseline output before refactoring
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- Detects any behavioral regression
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### Refactoring Timeline
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**Phase 1: Create Legacy Code**
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- Intentionally nested 7-8 levels deep
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- Mixed concerns (all item types in one method)
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- Magic numbers scattered throughout
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- Result: Represents realistic legacy code
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**Phase 2: Characterization Tests (GREEN)**
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- 14 comprehensive tests written before any refactoring
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- Golden Master baseline captured
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- Commit: `"GREEN: Add characterization tests"`
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- Result: **Safety net established** ✅
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**Phase 3: Refactor in Small Steps (3 REFACTOR commits)**
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**Step 1 - Extract Methods:**
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```dart
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// Before: 60 lines, items[i] everywhere, deeply nested
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for (var i = 0; i < items.length; i++) {
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if (items[i].name != 'Aged Brie' && ...) {
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if (items[i].quality > 0) {
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if (items[i].name != 'Sulfuras...') {
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// ... 5 more levels ...
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// After: Clean delegation, readable
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for (final item in items) {
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if (item.name == 'Sulfuras, Hand of Ragnaros') {
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_updateSulfuras(item);
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} else if (item.name == 'Aged Brie') {
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_updateAgedBrie(item);
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// ...
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}
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```
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Commit: `"REFACTOR: Extract item type methods from nested conditionals"`
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**Step 2 - Introduce Strategy Pattern:**
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```dart
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abstract class ItemUpdater {
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void update(Item item);
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}
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class NormalItemUpdater implements ItemUpdater {
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@override
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void update(Item item) {
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_degradeQuality(item, 1);
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item.sellIn -= 1;
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if (item.sellIn < 0) {
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_degradeQuality(item, 1);
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}
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}
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}
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// ... 4 concrete strategies
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ItemUpdater _selectUpdater(Item item) { ... }
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```
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Commit: `"REFACTOR: Introduce Strategy pattern for item types"`
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**Step 3 - Extract Domain Helpers:**
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```dart
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const int _minQuality = 0;
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const int _maxQuality = 50;
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void _degradeQuality(Item item, int amount) {
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item.quality = (item.quality - amount).clamp(_minQuality, _maxQuality);
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}
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void _improveQuality(Item item, int amount) {
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item.quality = (item.quality + amount).clamp(_minQuality, _maxQuality);
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}
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```
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Commit: `"REFACTOR: Extract helper methods and domain constants"`
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All 14 tests stayed **GREEN** throughout! 🟢
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**Phase 4: Add Conjured Items (RED-GREEN-REFACTOR)**
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**RED:** Write failing tests
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```dart
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test('degrade in quality twice as fast as normal items', () {
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final items = [Item('Conjured Mana Cake', 10, 20)];
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GildedRose(items).updateQuality();
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expect(items[0].quality, equals(18)); // -2 instead of -1
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});
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```
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Result: 2 tests **FAILING** ❌ (Expected: 18, Actual: 19)
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Commit: `"RED: Add failing tests for Conjured items"`
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**GREEN:** Implement minimal solution
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```dart
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class ConjuredItemUpdater implements ItemUpdater {
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@override
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void update(Item item) {
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_degradeQuality(item, 2); // 2x normal rate
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item.sellIn -= 1;
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if (item.sellIn < 0) {
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_degradeQuality(item, 2); // 4x total after expiration
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}
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}
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}
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ItemUpdater _selectUpdater(Item item) {
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// ... existing checks ...
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} else if (item.name.startsWith('Conjured')) {
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return ConjuredItemUpdater();
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} else {
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return NormalItemUpdater();
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}
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}
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```
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Result: All 17 tests **PASSING** ✅
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Commit: `"GREEN: Implement Conjured items degrading twice as fast"`
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**REFACTOR:** Already clean! No duplication, clear names, reusing helpers.
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Decision: Skip refactor commit—code is already excellent.
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### Development Metrics
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| Metric | Before Refactoring | After Refactoring |
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|--------|-------------------|-------------------|
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| **Cyclomatic Complexity** | ~25 (very high) | ~3 per class (low) |
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| **Lines per Method** | 60+ | 5-10 |
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| **Max Nesting** | 7-8 levels | 2 levels |
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| **Time to Add Feature** | Hours (risky) | Minutes (safe) |
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| **Test Coverage** | 0% → 100% | 100% (maintained) |
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### Key Insights
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**Characterization Tests Are Your Lifeline:** Without tests, refactoring is guesswork. With tests, it's engineering. Every change validated in milliseconds.
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**Small Steps = Low Risk:** Three refactoring commits, each preserving behavior. No "big bang" rewrite—steady, safe progress.
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**Strategy Pattern = Future-Proofing:** Adding Conjured items demonstrated the payoff:
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- **Before refactoring:** Would require diving into nested conditionals, risking bugs
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- **After refactoring:** One new class, one condition, done in 5 minutes
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**Golden Master Testing:** The 30-day simulation test caught edge cases that individual unit tests missed. It serves as a comprehensive regression detector.
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**Open-Closed Principle Validated:** New item types extend the system without modifying existing updater classes. The design is "open for extension, closed for modification."
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**Refactoring ≠ Rewriting:** We never changed what the code does, only how it's structured. Tests prove behavioral equivalence at every step.
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---
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## Comparing the Katas
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### Roman Numerals vs. Bowling Game
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### All Three Katas at a Glance
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| Aspect | Roman Numerals | Bowling Game |
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|--------|----------------|--------------|
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| **Complexity** | Beginner | Intermediate |
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| **State** | Stateless transformation | Stateful (rolls accumulate) |
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| **Algorithm** | Table-driven lookup | Frame iteration with look-ahead |
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| **Key Challenge** | Recognizing the pattern | Managing state and bonuses |
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| **Design Pattern** | Value Object | Strategy-like (frame types) |
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| **Lines of Code** | ~45 production | ~30 production |
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| **Aha! Moment** | Table eliminates duplication | Simple tests → complex games work |
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| Aspect | Roman Numerals | Bowling Game | Gilded Rose |
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|--------|----------------|--------------|-------------|
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| **Complexity** | Beginner | Intermediate | Advanced |
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| **Approach** | Greenfield TDD | Greenfield TDD | Legacy refactoring |
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| **State** | Stateless | Stateful | Stateful |
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| **Algorithm** | Table-driven lookup | Frame iteration | Strategy pattern |
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| **Key Challenge** | Pattern recognition | State & bonuses | Refactoring safely |
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| **Design Pattern** | Value Object | Implicit strategy | Explicit strategy |
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| **Lines of Code** | ~45 production | ~30 production | ~120 production |
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| **Test Count** | ~15 tests | ~10 tests | ~17 tests |
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| **Aha! Moment** | Table = data structure | Simple → complex works | Refactoring = safety |
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### What Each Kata Teaches
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@ -269,14 +561,21 @@ Tests are organized by **scoring complexity**, mirroring how the domain rules bu
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- State management without over-engineering
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- Look-ahead logic in sequential data
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- How correct abstractions scale beyond test cases
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**Gilded Rose:**
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- Safely refactoring legacy code with characterization tests
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- Strategy pattern for eliminating conditional complexity
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- Open-Closed Principle for extensibility
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- Working effectively with code you didn't write
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### Progressive Learning Path
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1. **Roman Numerals first:** Learn TDD fundamentals without state complexity
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2. **Bowling Game second:** Apply TDD to stateful problems
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3. **Next kata:** Choose based on what you want to practice:
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3. **Gilded Rose third:** Master refactoring legacy code with tests as safety net
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4. **Next kata:** Choose based on what you want to practice:
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- **String Calculator:** Parsing, validation, error handling
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- **Gilded Rose:** Refactoring legacy code without tests
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- **Mars Rover:** Command pattern, multiple behaviors
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- **Prime Factors:** Mathematical decomposition, algorithmic thinkingsts
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- **Mars Rover:** Command pattern, multiple behaviors
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---
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@ -284,6 +583,7 @@ Tests are organized by **scoring complexity**, mirroring how the domain rules bu
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## References
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### General TDD & Clean Code
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- [Gilded Rose Kata](https://github.com/emilybache/GildedRose-Refactoring-Kata)
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- [Clean Code by Robert C. Martin](https://www.oreilly.com/library/view/clean-code-a/9780136083238/)
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- [Domain-Driven Design by Eric Evans](https://www.domainlanguage.com/ddd/)
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- [Test-Driven Development by Kent Beck](https://www.amazon.com/Test-Driven-Development-Kent-Beck/dp/0321146530)
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