Merge pull request GH-4 from gh/string-calculator

string calculator

README.md

@@ -219,6 +219,99 @@ gildedRose.updateQuality(); // Updates all items per domain rules
 
 ---
 
+### 4. String Calculator ✅
+
+**Implementation:** [`lib/string_calculator.dart`](lib/string_calculator.dart)  
+**Tests:** [`test/string_calculator_test.dart`](test/string_calculator_test.dart)
+
+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.
+
+#### Domain Context
+
+A simple calculator that sums numbers from a string input with various delimiter support:
+
+**Features:**
+
+1. **Empty String:** Returns 0
+2. **Single Number:** Returns that number (`"5"` → 5)
+3. **Comma Delimiter:** Sums comma-separated numbers (`"1,2,3"` → 6)
+4. **Custom Delimiters:** Supports format `"//[delimiter]\n[numbers]"` (`"//;\n1;2"` → 3)
+5. **Ignore Large Numbers:** Numbers > 1000 are ignored (`"2,1001"` → 2)
+
+#### The Bug Hunt Approach
+
+**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.
+
+**Bug Hunt Process:**
+1. **RED:** Write test exposing a specific bug
+2. **GREEN:** Fix only that bug
+3. **Commit:** Document the bug found and fixed
+4. **Repeat:** Move to next bug
+
+#### Bugs Found & Fixed
+
+**Bug #1: Empty String Returns Wrong Value**
+- **Bug:** Returned 1 instead of 0
+- **Test:** `expect(calculator.add(''), equals(0))`
+- **Fix:** Changed return value from 1 to 0
+- **Commits:** RED → GREEN
+
+**Bug #2: Single Number Off-By-One**
+- **Bug:** Added +1 to parsed number
+- **Test:** `expect(calculator.add('5'), equals(5))`
+- **Expected:** 5, **Actual:** 6
+- **Fix:** Removed `+ 1` from parsing
+- **Commits:** RED → GREEN
+
+**Bug #3: Summation Loop Misses Last Element**
+- **Bug:** Loop condition `i < length - 1` skipped last item
+- **Test:** `expect(calculator.add('1,2'), equals(3))`
+- **Expected:** 3, **Actual:** 1
+- **Fix:** Changed to `i < length`
+- **Commits:** RED → GREEN
+
+**Bug #4: Custom Delimiter Not Extracted**
+- **Bug:** Delimiter extraction line was commented out
+- **Test:** `expect(calculator.add('//;\n1;2'), equals(3))`
+- **Error:** FormatException trying to parse '1;2'
+- **Fix:** Uncommented `delimiter = parts[0].substring(2)`
+- **Commits:** RED → GREEN
+
+**Bug #5: Missing Feature - Ignore Numbers > 1000**
+- **Bug:** All numbers included in sum
+- **Test:** `expect(calculator.add('2,1001'), equals(2))`
+- **Expected:** 2, **Actual:** 1003
+- **Fix:** Added `.where((n) => n <= 1000)` filter
+- **Commits:** RED → GREEN
+
+#### Usage
+
+```dart
+import 'package:tdd_katas/string_calculator.dart';
+
+final calculator = StringCalculator();
+
+calculator.add('');              // Returns: 0
+calculator.add('5');             // Returns: 5
+calculator.add('1,2,3');         // Returns: 6
+calculator.add('//;\n1;2');      // Returns: 3
+calculator.add('2,1001');        // Returns: 2 (1001 ignored)
+```
+
+#### The "Aha!" Moments
+
+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.
+
+2. **RED-GREEN Still Works:** Even when fixing bugs (not adding features), the RED-GREEN rhythm provides safety. You're never fixing blind.
+
+3. **Regression Prevention:** After fixing each bug, ALL previous tests stay green. This proves you didn't break something while fixing something else.
+
+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.
+
+5. **Real-World Skill:** This mirrors production work—most code you touch is existing code with bugs, not greenfield TDD.
+
+---
+
 ## Running Tests
 
 ```bash
@@ -229,6 +322,7 @@ dart test
 dart test test/roman_numerals_test.dart
 dart test test/bowling_game_test.dart
 dart test test/gilded_rose_test.dart
+dart test test/string_calculator_test.dart
 
 # Run with coverage
 dart test --coverage
@@ -536,22 +630,23 @@ Decision: Skip refactor commit—code is already excellent.
 
 ## Comparing the Katas
 
-### All Three Katas at a Glance
+### All Four Katas at a Glance
 
-| Aspect | Roman Numerals | Bowling Game | Gilded Rose |
-|--------|----------------|--------------|-------------|
-| **Complexity** | Beginner | Intermediate | Advanced |
-| **Approach** | Greenfield TDD | Greenfield TDD | Legacy refactoring |
-| **State** | Stateless | Stateful | Stateful |
-| **Algorithm** | Table-driven lookup | Frame iteration | Strategy pattern |
-| **Key Challenge** | Pattern recognition | State & bonuses | Refactoring safely |
-| **Design Pattern** | Value Object | Implicit strategy | Explicit strategy |
-| **Lines of Code** | ~45 production | ~30 production | ~120 production |
-| **Test Count** | ~15 tests | ~10 tests | ~17 tests |
-| **Aha! Moment** | Table = data structure | Simple → complex works | Refactoring = safety |
+| Aspect | Roman Numerals | Bowling Game | Gilded Rose | String Calculator |
+|--------|----------------|--------------|-------------|-------------------|
+| **Complexity** | Beginner | Intermediate | Advanced | Beginner |
+| **Approach** | Greenfield TDD | Greenfield TDD | Legacy refactoring | Bug hunting |
+| **State** | Stateless | Stateful | Stateful | Stateless |
+| **Algorithm** | Table-driven | Frame iteration | Strategy pattern | String parsing |
+| **Key Challenge** | Pattern recognition | State & bonuses | Refactoring safely | Finding bugs |
+| **Design Pattern** | Value Object | Implicit strategy | Explicit strategy | Filters & pipes |
+| **Lines of Code** | ~45 production | ~30 production | ~120 production | ~25 production |
+| **Test Count** | ~15 tests | ~10 tests | ~17 tests | 6 tests |
+| **Aha! Moment** | Table = data | Simple → complex | Refactor = safe | Tests find bugs |
 
 ### What Each Kata Teaches
 
+**Roman Numerals:**
 **Roman Numerals:**
 - Converting domain rules into data structures
 - Value Objects for enforcing constraints
@@ -561,22 +656,29 @@ Decision: Skip refactor commit—code is already excellent.
 - State management without over-engineering
 - Look-ahead logic in sequential data
 - How correct abstractions scale beyond test cases
+
 **Gilded Rose:**
 - Safely refactoring legacy code with characterization tests
 - Strategy pattern for eliminating conditional complexity
 - Open-Closed Principle for extensibility
 - Working effectively with code you didn't write
 
+**String Calculator:**
+- Using tests to expose bugs in existing code
+- Bug hunting with RED-GREEN discipline
+- Incremental debugging with clear commits
+- Regression prevention through test accumulation
+
 ### Progressive Learning Path
 
 1. **Roman Numerals first:** Learn TDD fundamentals without state complexity
 2. **Bowling Game second:** Apply TDD to stateful problems
 3. **Gilded Rose third:** Master refactoring legacy code with tests as safety net
-4. **Next kata:** Choose based on what you want to practice:
-   - **String Calculator:** Parsing, validation, error handling
-   - **Mars Rover:** Command pattern, multiple behaviors
-   - **Prime Factors:** Mathematical decomposition, algorithmic thinkingsts
+4. **String Calculator fourth:** Practice bug hunting and fixing with TDD
+5. **Next kata:** Choose based on what you want to practice:
    - **Mars Rover:** Command pattern, multiple behaviors
+   - **Prime Factors:** Mathematical decomposition, algorithmic thinking
+   - **Tennis Scoring:** State machines, domain language
 
 ---
 

bin/tdd_katas.dart

@@ -1,5 +1,3 @@
-import 'package:tdd_katas/roman_numerals.dart' as roman_numerals;
-
 void main(List<String> arguments) {
-  print('Hello world: ${roman_numerals.integerToRoman(1)}');
+  print('TDD Katas exercises, please read the README.md file.');
 }

lib/string_calculator.dart

@@ -0,0 +1,55 @@
+/// String Calculator - Buggy Implementation
+/// This code has intentional bugs for the Bug Hunt Kata exercise
+///
+/// Requirements:
+/// 1. Empty string returns 0
+/// 2. Single number returns that number
+/// 3. Two numbers comma-delimited returns sum
+/// 4. Handle newlines as delimiters
+/// 5. Support custom delimiters: "//[delimiter]\n[numbers]"
+library;
+
+class StringCalculator {
+  int add(String numbers) {
+    // Bug 1: Empty string handling
+    if (numbers.isEmpty) {
+      return 0; // Fixed: Return 0 for empty string
+    }
+
+    // Bug 2: Single number parsing
+    if (!numbers.contains(',') &&
+        !numbers.contains('\n') &&
+        !numbers.startsWith('//')) {
+      return int.parse(numbers); // Fixed: Removed off-by-one error
+    }
+
+    String delimiter = ',';
+    String numbersToProcess = numbers;
+
+    // Custom delimiter support
+    if (numbers.startsWith('//')) {
+      // Bug 4: Custom delimiter not actually used
+      final parts = numbers.split('\n');
+      delimiter = parts[0].substring(2); // Fixed: Extract custom delimiter
+      numbersToProcess = parts.skip(1).join('\n');
+    }
+
+    // Bug 3 & 4: Delimiter handling issues
+    final numList = numbersToProcess
+        .replaceAll('\n', delimiter)
+        .split(delimiter)
+        .where((s) => s.isNotEmpty)
+        .map((s) => int.parse(s))
+        .where((n) => n <= 1000) // Filter out numbers > 1000
+        .toList();
+
+    // Bug 3: Off-by-one in summation
+    int sum = 0;
+    for (int i = 0; i < numList.length; i++) {
+      // Fixed: Include last element
+      sum += numList[i];
+    }
+
+    return sum;
+  }
+}

test/string_calculator_test.dart

@@ -0,0 +1,41 @@
+import 'package:tdd_katas/string_calculator.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('String Calculator - Bug Hunt', () {
+    late StringCalculator calculator;
+
+    setUp(() {
+      calculator = StringCalculator();
+    });
+
+    test('empty string returns 0', () {
+      expect(calculator.add(''), equals(0));
+    });
+
+    test('single number returns that number', () {
+      expect(calculator.add('5'), equals(5));
+      expect(calculator.add('42'), equals(42));
+    });
+
+    test('two comma-delimited numbers return sum', () {
+      expect(calculator.add('1,2'), equals(3));
+      expect(calculator.add('10,20'), equals(30));
+    });
+
+    test('multiple comma-delimited numbers return sum', () {
+      expect(calculator.add('1,2,3'), equals(6));
+      expect(calculator.add('5,10,15,20'), equals(50));
+    });
+
+    test('custom delimiter works correctly', () {
+      expect(calculator.add('//;\n1;2'), equals(3));
+      expect(calculator.add('//|\n10|20|30'), equals(60));
+    });
+
+    test('numbers greater than 1000 are ignored', () {
+      expect(calculator.add('2,1001'), equals(2));
+      expect(calculator.add('1000,1001,2'), equals(1002));
+    });
+  });
+}