Coverage Enhancer

def find_max(numbers):
    if not numbers:
        return None

    max_val = numbers[0]
    for num in numbers[1:]:
        if num > max_val:
            max_val = num
    return max_val

def test_find_max_normal():
    assert find_max([1, 5, 3, 2]) == 5

def test_find_max_empty():
    """Test that empty list returns None."""
    assert find_max([]) is None
    # Coverage: Tests the 'if not numbers' branch

def test_find_max_single_element():
    """Test with single element (zero loop iterations)."""
    assert find_max([42]) == 42
    # Coverage: Tests loop with zero iterations

def test_find_max_all_equal():
    """Test with all identical elements."""
    assert find_max([5, 5, 5, 5]) == 5
    # Coverage: Tests loop where condition never true

def test_find_max_negative_numbers():
    """Test with negative numbers."""
    assert find_max([-5, -1, -10, -3]) == -1
    # Coverage: Edge case for comparison logic

def load_config(filename):
    try:
        with open(filename) as f:
            return json.load(f)
    except FileNotFoundError:
        return {}
    except json.JSONDecodeError as e:
        raise ValueError(f"Invalid JSON in {filename}: {e}")
    except Exception as e:
        raise RuntimeError(f"Unexpected error loading {filename}: {e}")

def test_load_config_success(tmp_path):
    config_file = tmp_path / "config.json"
    config_file.write_text('{"key": "value"}')
    assert load_config(str(config_file)) == {"key": "value"}

def test_load_config_file_not_found():
    """Test that missing file returns empty dict."""
    assert load_config("nonexistent.json") == {}
    # Coverage: Tests FileNotFoundError exception path

def test_load_config_invalid_json(tmp_path):
    """Test that invalid JSON raises ValueError."""
    config_file = tmp_path / "bad.json"
    config_file.write_text("{invalid json}")

    with pytest.raises(ValueError, match="Invalid JSON"):
        load_config(str(config_file))
    # Coverage: Tests JSONDecodeError exception path

def test_load_config_permission_error(tmp_path):
    """Test that permission errors raise RuntimeError."""
    config_file = tmp_path / "protected.json"
    config_file.write_text('{"key": "value"}')
    config_file.chmod(0o000)

    with pytest.raises(RuntimeError, match="Unexpected error"):
        load_config(str(config_file))
    # Coverage: Tests generic Exception handler

class Connection:
    def __init__(self):
        self.state = "closed"

    def connect(self):
        if self.state == "connected":
            raise RuntimeError("Already connected")
        self.state = "connected"

    def disconnect(self):
        if self.state == "closed":
            raise RuntimeError("Not connected")
        self.state = "closed"

def test_connection_happy_path():
    conn = Connection()
    conn.connect()
    assert conn.state == "connected"

def test_connection_double_connect():
    """Test that connecting twice raises error."""
    conn = Connection()
    conn.connect()

    with pytest.raises(RuntimeError, match="Already connected"):
        conn.connect()
    # Coverage: Tests invalid state transition

def test_connection_disconnect_when_not_connected():
    """Test that disconnecting when closed raises error."""
    conn = Connection()

    with pytest.raises(RuntimeError, match="Not connected"):
        conn.disconnect()
    # Coverage: Tests disconnect precondition check

def test_connection_full_lifecycle():
    """Test complete connect-disconnect cycle."""
    conn = Connection()
    assert conn.state == "closed"

    conn.connect()
    assert conn.state == "connected"

    conn.disconnect()
    assert conn.state == "closed"
    # Coverage: Tests all valid state transitions

Total lines: 100
Covered lines: 75
Coverage: 75%

if condition:    # Branch 1
    do_something()
else:            # Branch 2
    do_other()

# 100% branch coverage requires testing both paths

def complex(a, b, c):
    if a:          # 2 branches
        if b:      # 2 branches
            if c:  # 2 branches
                return "all true"

pytest --cov=mymodule --cov-report=term-missing --cov-report=html

# Look for:
# - Missing lines (shown in report)
# - Uncovered branches (with --cov-branch)
# - Files with low coverage (<80%)

npm test -- --coverage --verbose

// coverage/lcov-report/index.html shows:
// - Uncovered lines (highlighted in red)
// - Uncovered branches
// - Function coverage

mvn test jacoco:report

target/site/jacoco/index.html

go test -coverprofile=coverage.out ./...
go tool cover -html=coverage.out

## Coverage Gap: [Description]

**Location:** [file:line or function name]
**Current Coverage:** [X%]
**Impact:** +[N] lines, +[M] branches

### Suggested Test:

[Complete test code]

**Explanation:**
[What this test covers and why it's important]

**Where to add:**
[In which test file, near which existing test]

## Coverage Gap: Error handling for invalid input

**Location:** src/validator.py:45-48
**Current Coverage:** 60% (missing exception path)
**Impact:** +3 lines, +1 branch

### Suggested Test:

```python
def test_validate_email_invalid_format():
    """Test that invalid email format raises ValueError."""
    with pytest.raises(ValueError, match="Invalid email format"):
        validate_email("not-an-email")

    # Additional invalid cases
    with pytest.raises(ValueError):
        validate_email("")
    with pytest.raises(ValueError):
        validate_email("@example.com")

## Best Practices

1. **Start with existing tests** - Always read current tests first to understand patterns
2. **Match the style** - Use same framework, naming, and structure as existing tests
3. **Focus on value** - Prioritize high-impact coverage gaps over achieving 100%
4. **Test behavior, not implementation** - Focus on what the code does, not how
5. **Keep tests isolated** - Each test should be independent
6. **Use descriptive names** - Test names should explain what's being verified
7. **Add explanations** - Comment why each test is needed for coverage
8. **Suggest coverage tools** - Help users measure and track coverage
9. **Consider mutation testing** - Suggest tests that would catch actual bugs
10. **Balance coverage and maintainability** - Don't over-test trivial code

## Common Coverage Gaps

### Gap 1: Error Cases Not Tested

```python
# Often only happy path is tested
def parse_int(s):
    return int(s)  # ValueError not tested

# Suggest:
def test_parse_int_invalid():
    with pytest.raises(ValueError):
        parse_int("not a number")

# Common values tested, boundaries ignored
def clamp(value, min_val, max_val):
    return max(min_val, min(max_val, value))

# Need tests for:
# - value == min_val
# - value == max_val
# - value < min_val
# - value > max_val

if condition:
    # Tested
    do_something()
else:
    # Never tested!
    do_other()

for item in collection:
    process(item)

# Need tests for:
# - Empty collection
# - Single item
# - Many items

try:
    risky_operation()
finally:
    cleanup()  # Often not verified

# Suggest test that verifies cleanup happens

def calculate_grade(score):
    """Calculate letter grade from numeric score."""
    if score < 0 or score > 100:
        raise ValueError("Score must be between 0 and 100")

    if score >= 90:
        return 'A'
    elif score >= 80:
        return 'B'
    elif score >= 70:
        return 'C'
    elif score >= 60:
        return 'D'
    else:
        return 'F'

def test_calculate_grade_a():
    assert calculate_grade(95) == 'A'

def test_calculate_grade_c():
    assert calculate_grade(75) == 'C'

def test_calculate_grade_invalid_negative():
    """Test that negative scores raise ValueError."""
    with pytest.raises(ValueError, match="between 0 and 100"):
        calculate_grade(-1)
    # Coverage: +2 lines (score < 0 branch)

def test_calculate_grade_invalid_too_high():
    """Test that scores above 100 raise ValueError."""
    with pytest.raises(ValueError, match="between 0 and 100"):
        calculate_grade(101)
    # Coverage: +0 lines (same as above, but tests second condition)

def test_calculate_grade_b():
    """Test B grade threshold."""
    assert calculate_grade(85) == 'B'
    # Coverage: +1 line (score >= 80 branch)

def test_calculate_grade_d():
    """Test D grade threshold."""
    assert calculate_grade(65) == 'D'
    # Coverage: +1 line (score >= 60 branch)

def test_calculate_grade_f():
    """Test F grade for failing scores."""
    assert calculate_grade(45) == 'F'
    # Coverage: +1 line (else branch)

def test_calculate_grade_boundaries():
    """Test exact boundary values."""
    assert calculate_grade(90) == 'A'  # Exact boundary
    assert calculate_grade(89) == 'B'  # Just below
    assert calculate_grade(80) == 'B'  # Exact boundary
    assert calculate_grade(79) == 'C'  # Just below
    assert calculate_grade(70) == 'C'  # Exact boundary
    assert calculate_grade(60) == 'D'  # Exact boundary
    assert calculate_grade(0) == 'F'   # Lower bound
    assert calculate_grade(100) == 'A' # Upper bound
    # Coverage: Ensures all boundaries work correctly