docs: Document String Calculator Bug Hunt Kata
- Add comprehensive String Calculator section to README - Document Bug Hunt approach: test-driven bug finding - List all 5 bugs found and fixed with RED-GREEN commits - Include code examples and usage - Update comparison table to include all 4 katas - Add 'What Each Kata Teaches' for String Calculator - Update progressive learning path to include fourth kata
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README.md
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README.md
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@ -219,6 +219,99 @@ gildedRose.updateQuality(); // Updates all items per domain rules
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---
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### 4. String Calculator ✅
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**Implementation:** [`lib/string_calculator.dart`](lib/string_calculator.dart)
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**Tests:** [`test/string_calculator_test.dart`](test/string_calculator_test.dart)
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A Bug Hunt Kata demonstrating how to use TDD to discover and fix bugs in existing code. Each bug is exposed with a RED test, then fixed with GREEN implementation.
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#### Domain Context
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A simple calculator that sums numbers from a string input with various delimiter support:
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**Features:**
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1. **Empty String:** Returns 0
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2. **Single Number:** Returns that number (`"5"` → 5)
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3. **Comma Delimiter:** Sums comma-separated numbers (`"1,2,3"` → 6)
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4. **Custom Delimiters:** Supports format `"//[delimiter]\n[numbers]"` (`"//;\n1;2"` → 3)
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5. **Ignore Large Numbers:** Numbers > 1000 are ignored (`"2,1001"` → 2)
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#### The Bug Hunt Approach
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**Different from Previous Katas:** This wasn't built test-first. Instead, we started with **buggy working code** and used tests to expose and fix bugs one by one.
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**Bug Hunt Process:**
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1. **RED:** Write test exposing a specific bug
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2. **GREEN:** Fix only that bug
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3. **Commit:** Document the bug found and fixed
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4. **Repeat:** Move to next bug
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#### Bugs Found & Fixed
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**Bug #1: Empty String Returns Wrong Value**
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- **Bug:** Returned 1 instead of 0
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- **Test:** `expect(calculator.add(''), equals(0))`
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- **Fix:** Changed return value from 1 to 0
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- **Commits:** RED → GREEN
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**Bug #2: Single Number Off-By-One**
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- **Bug:** Added +1 to parsed number
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- **Test:** `expect(calculator.add('5'), equals(5))`
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- **Expected:** 5, **Actual:** 6
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- **Fix:** Removed `+ 1` from parsing
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- **Commits:** RED → GREEN
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**Bug #3: Summation Loop Misses Last Element**
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- **Bug:** Loop condition `i < length - 1` skipped last item
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- **Test:** `expect(calculator.add('1,2'), equals(3))`
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- **Expected:** 3, **Actual:** 1
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- **Fix:** Changed to `i < length`
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- **Commits:** RED → GREEN
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**Bug #4: Custom Delimiter Not Extracted**
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- **Bug:** Delimiter extraction line was commented out
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- **Test:** `expect(calculator.add('//;\n1;2'), equals(3))`
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- **Error:** FormatException trying to parse '1;2'
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- **Fix:** Uncommented `delimiter = parts[0].substring(2)`
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- **Commits:** RED → GREEN
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**Bug #5: Missing Feature - Ignore Numbers > 1000**
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- **Bug:** All numbers included in sum
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- **Test:** `expect(calculator.add('2,1001'), equals(2))`
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- **Expected:** 2, **Actual:** 1003
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- **Fix:** Added `.where((n) => n <= 1000)` filter
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- **Commits:** RED → GREEN
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#### Usage
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```dart
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import 'package:tdd_katas/string_calculator.dart';
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final calculator = StringCalculator();
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calculator.add(''); // Returns: 0
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calculator.add('5'); // Returns: 5
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calculator.add('1,2,3'); // Returns: 6
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calculator.add('//;\n1;2'); // Returns: 3
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calculator.add('2,1001'); // Returns: 2 (1001 ignored)
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```
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#### The "Aha!" Moments
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1. **Tests as Bug Detectors:** Each test acted like a spotlight, illuminating exactly ONE bug at a time. No guessing—the test tells you what's broken.
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2. **RED-GREEN Still Works:** Even when fixing bugs (not adding features), the RED-GREEN rhythm provides safety. You're never fixing blind.
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3. **Regression Prevention:** After fixing each bug, ALL previous tests stay green. This proves you didn't break something while fixing something else.
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4. **Incremental Debugging:** Fixing one bug at a time with commits creates a clear audit trail. You can see exactly what each bug was and how it was fixed.
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5. **Real-World Skill:** This mirrors production work—most code you touch is existing code with bugs, not greenfield TDD.
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---
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## Running Tests
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```bash
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@ -229,6 +322,7 @@ dart test
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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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dart test test/string_calculator_test.dart
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# Run with coverage
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dart test --coverage
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@ -536,22 +630,23 @@ Decision: Skip refactor commit—code is already excellent.
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## Comparing the Katas
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### All Three Katas at a Glance
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### All Four Katas at a Glance
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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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| Aspect | Roman Numerals | Bowling Game | Gilded Rose | String Calculator |
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|--------|----------------|--------------|-------------|-------------------|
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| **Complexity** | Beginner | Intermediate | Advanced | Beginner |
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| **Approach** | Greenfield TDD | Greenfield TDD | Legacy refactoring | Bug hunting |
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| **State** | Stateless | Stateful | Stateful | Stateless |
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| **Algorithm** | Table-driven | Frame iteration | Strategy pattern | String parsing |
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| **Key Challenge** | Pattern recognition | State & bonuses | Refactoring safely | Finding bugs |
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| **Design Pattern** | Value Object | Implicit strategy | Explicit strategy | Filters & pipes |
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| **Lines of Code** | ~45 production | ~30 production | ~120 production | ~25 production |
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| **Test Count** | ~15 tests | ~10 tests | ~17 tests | 6 tests |
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| **Aha! Moment** | Table = data | Simple → complex | Refactor = safe | Tests find bugs |
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### What Each Kata Teaches
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**Roman Numerals:**
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**Roman Numerals:**
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- Converting domain rules into data structures
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- Value Objects for enforcing constraints
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@ -561,22 +656,29 @@ Decision: Skip refactor commit—code is already excellent.
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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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**String Calculator:**
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- Using tests to expose bugs in existing code
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- Bug hunting with RED-GREEN discipline
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- Incremental debugging with clear commits
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- Regression prevention through test accumulation
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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. **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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- **Mars Rover:** Command pattern, multiple behaviors
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- **Prime Factors:** Mathematical decomposition, algorithmic thinkingsts
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4. **String Calculator fourth:** Practice bug hunting and fixing with TDD
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5. **Next kata:** Choose based on what you want to practice:
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- **Mars Rover:** Command pattern, multiple behaviors
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- **Prime Factors:** Mathematical decomposition, algorithmic thinking
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- **Tennis Scoring:** State machines, domain language
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---
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