⚡️ Speed up method _Stack.forward by 55%

[codeflash-ai]

📄 55% (0.55x) speedup for _Stack.forward in lib/matplotlib/cbook.py

⏱️ Runtime : 1.46 milliseconds → 942 microseconds (best of 134 runs)

📝 Explanation and details

The optimized version achieves a 55% speedup by eliminating an expensive indirect method call and replacing it with direct list access.

Key optimizations:

1. Eliminated self() call overhead: The original code calls return self() which invokes the __call__ method, adding function call overhead and attribute lookups. The optimized version directly accesses elems[pos], avoiding this indirection.

2. Reduced attribute access: Local variables elems, pos, and n cache frequently accessed attributes, reducing repeated self._elements and self._pos lookups.

3. Simplified bounds checking: Replaced min(self._pos + 1, len(self._elements) - 1) with a more explicit conditional that's easier for Python to optimize.

Why this works: Python method calls are expensive due to dynamic dispatch, attribute resolution, and function call overhead. Direct list indexing (elems[pos]) is a highly optimized operation in CPython, while self() requires method lookup, frame creation, and additional bounds checking inside __call__.

Performance characteristics: The optimization shows consistent gains across all test scenarios:

• Empty stacks: 74-80% faster (eliminates unnecessary __call__ overhead)
• Single/multiple elements: 35-90% faster depending on position
• Large stacks (1000 elements): 52-56% faster, showing the optimization scales well
• Edge cases (invalid positions, boundary conditions): 20-90% faster

The optimization is particularly effective for workloads that frequently navigate through stack elements, as it removes a significant per-call overhead while maintaining identical functionality.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 3716 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

import pytest  # used for our unit tests
from matplotlib.cbook import _Stack

# unit tests

# -------- BASIC TEST CASES --------


def test_forward_empty_stack():
    """Test forward on an empty stack should return None and not error."""
    s = _Stack()
    codeflash_output = s.forward()  # 1.75μs -> 973ns (80.0% faster)


def test_forward_single_element_stack():
    """Test forward on a stack with one element."""
    s = _Stack()
    s._elements.append("A")
    # Initial position is -1, so forward should move to 0
    codeflash_output = s.forward()  # 1.76μs -> 926ns (90.3% faster)
    # Further forward should stay at last element
    codeflash_output = s.forward()  # 731ns -> 531ns (37.7% faster)


def test_forward_two_elements_stack():
    """Test forward on a stack with two elements."""
    s = _Stack()
    s._elements.extend(["A", "B"])
    # Initial position is -1, so forward should move to 0
    codeflash_output = s.forward()  # 1.76μs -> 948ns (85.5% faster)
    # Forward again should move to 1
    codeflash_output = s.forward()  # 684ns -> 443ns (54.4% faster)
    # Further forward should stay at last element
    codeflash_output = s.forward()  # 610ns -> 449ns (35.9% faster)


def test_forward_multiple_elements_stack():
    """Test forward on a stack with multiple elements."""
    s = _Stack()
    s._elements.extend(["A", "B", "C"])
    # Initial position is -1
    codeflash_output = s.forward()  # 1.72μs -> 913ns (88.1% faster)
    codeflash_output = s.forward()  # 656ns -> 455ns (44.2% faster)
    codeflash_output = s.forward()  # 462ns -> 290ns (59.3% faster)
    # Further forward should stay at last element
    codeflash_output = s.forward()  # 483ns -> 398ns (21.4% faster)


# -------- EDGE TEST CASES --------


def test_forward_already_at_last_element():
    """Test forward when already at the last element."""
    s = _Stack()
    s._elements.extend(["A", "B"])
    s._pos = 1  # Already at last element
    codeflash_output = s.forward()  # 1.73μs -> 1.01μs (71.8% faster)


def test_forward_negative_position():
    """Test forward when position is negative but stack is non-empty."""
    s = _Stack()
    s._elements.extend(["A", "B"])
    s._pos = -2  # Invalid negative position
    # Should move to position -1 + 1 = 0 (but from -2, min(-2+1, 1) = -1)
    codeflash_output = s.forward()  # 1.78μs -> 949ns (88.0% faster)


def test_forward_position_beyond_bounds():
    """Test forward when position is beyond the last index."""
    s = _Stack()
    s._elements.extend(["A", "B", "C"])
    s._pos = 10  # Invalid position
    # Should clamp position to last index
    codeflash_output = s.forward()  # 1.74μs -> 947ns (83.4% faster)


def test_forward_after_removing_elements():
    """Test forward after elements are removed from stack."""
    s = _Stack()
    s._elements.extend(["A", "B", "C"])
    s._pos = 1
    # Remove last element
    s._elements.pop()
    # Now last index is 1
    codeflash_output = s.forward()  # 1.73μs -> 997ns (73.5% faster)


def test_forward_with_non_string_elements():
    """Test forward with elements of different types."""
    s = _Stack()
    s._elements.extend([1, None, [3, 4], {"a": 5}])
    codeflash_output = s.forward()  # 1.77μs -> 935ns (88.9% faster)
    codeflash_output = s.forward()  # 647ns -> 469ns (38.0% faster)
    codeflash_output = s.forward()  # 440ns -> 285ns (54.4% faster)
    codeflash_output = s.forward()  # 423ns -> 236ns (79.2% faster)
    # Further forward should stay at last element
    codeflash_output = s.forward()  # 487ns -> 384ns (26.8% faster)


# -------- LARGE SCALE TEST CASES --------


def test_forward_large_stack():
    """Test forward on a large stack (1000 elements)."""
    n = 1000
    s = _Stack()
    s._elements = list(range(n))
    # Move forward through the stack
    for i in range(n):
        codeflash_output = s.forward()  # 383μs -> 248μs (54.3% faster)
    # Further forward should stay at last element
    for _ in range(10):
        codeflash_output = s.forward()  # 3.90μs -> 2.87μs (36.1% faster)


def test_forward_large_stack_starting_at_end():
    """Test forward on a large stack when already at the end."""
    n = 1000
    s = _Stack()
    s._elements = list(range(n))
    s._pos = n - 1
    codeflash_output = s.forward()  # 1.94μs -> 1.11μs (75.0% faster)


def test_forward_large_stack_starting_at_middle():
    """Test forward on a large stack starting at the middle."""
    n = 1000
    s = _Stack()
    s._elements = list(range(n))
    s._pos = 500
    # Should move to 501 on forward
    codeflash_output = s.forward()  # 1.88μs -> 1.04μs (80.4% faster)


def test_forward_large_stack_empty():
    """Test forward on a large stack that is empty."""
    s = _Stack()
    s._elements = []
    codeflash_output = s.forward()  # 1.72μs -> 988ns (74.4% faster)


# -------- DETERMINISM TEST CASES --------


def test_forward_determinism():
    """Test that repeated calls to forward produce deterministic results."""
    s = _Stack()
    s._elements.extend(["A", "B", "C"])
    results = []
    for _ in range(10):
        results.append(s.forward())  # 5.56μs -> 3.54μs (57.0% faster)


# -------- ERROR HANDLING TEST CASES --------


def test_forward_no_raise_on_empty():
    """Ensure forward does not raise on empty stack."""
    s = _Stack()
    try:
        codeflash_output = s.forward()
        res = codeflash_output
    except Exception as e:
        pytest.fail(f"forward raised {e} on empty stack")


def test_forward_no_raise_on_invalid_position():
    """Ensure forward does not raise on invalid position."""
    s = _Stack()
    s._elements.extend(["A", "B"])
    s._pos = 100
    try:
        codeflash_output = s.forward()
        res = codeflash_output
    except Exception as e:
        pytest.fail(f"forward raised {e} on invalid position")


# -------- FUNCTIONAL INTEGRITY TEST CASES --------


def test_forward_does_not_modify_elements():
    """Ensure forward does not modify stack elements."""
    s = _Stack()
    s._elements.extend(["A", "B", "C"])
    orig = list(s._elements)
    s.forward()  # 1.78μs -> 955ns (86.4% faster)
    s.forward()  # 644ns -> 431ns (49.4% faster)
    s.forward()  # 495ns -> 284ns (74.3% faster)


# codeflash_output is used to check that the output of the original code is the same as that of the optimized code.

from matplotlib.cbook import _Stack

# unit tests

# ----------- Basic Test Cases -----------


def test_forward_empty_stack_returns_none():
    # Test that forward on an empty stack returns None
    stack = _Stack()
    codeflash_output = stack.forward()  # 1.78μs -> 1.01μs (76.9% faster)


def test_forward_single_element_stack():
    # Test that forward on a stack with one element returns that element
    stack = _Stack()
    stack._elements.append("A")
    # Initially, cursor is at -1 (before first element)
    codeflash_output = stack.forward()  # 1.74μs -> 930ns (87.1% faster)
    # Further forward should stay at 'A'
    codeflash_output = stack.forward()  # 717ns -> 524ns (36.8% faster)


def test_forward_multiple_elements_basic():
    # Test moving forward through a stack with multiple elements
    stack = _Stack()
    stack._elements.extend(["A", "B", "C"])
    # Initial position is -1, so first forward moves to 0
    codeflash_output = stack.forward()  # 1.69μs -> 926ns (82.4% faster)
    # Next forward moves to 1
    codeflash_output = stack.forward()  # 671ns -> 453ns (48.1% faster)
    # Next forward moves to 2
    codeflash_output = stack.forward()  # 464ns -> 311ns (49.2% faster)
    # Further forward stays at last element
    codeflash_output = stack.forward()  # 485ns -> 395ns (22.8% faster)


def test_forward_does_not_exceed_bounds():
    # Test that forward does not move beyond the last element
    stack = _Stack()
    stack._elements.extend([1, 2])
    stack.forward()  # 1.73μs -> 903ns (92.0% faster)
    stack.forward()  # 646ns -> 449ns (43.9% faster)
    stack.forward()  # 572ns -> 461ns (24.1% faster)
    codeflash_output = stack.forward()  # 394ns -> 269ns (46.5% faster)


# ----------- Edge Test Cases -----------


def test_forward_after_manual_positioning():
    # Test forward after manually setting _pos to a valid index
    stack = _Stack()
    stack._elements.extend(["X", "Y", "Z"])
    stack._pos = 1  # Manually set to middle
    codeflash_output = stack.forward()  # 1.69μs -> 887ns (90.9% faster)
    # Should stay at last element
    codeflash_output = stack.forward()  # 641ns -> 469ns (36.7% faster)


def test_forward_after_manual_positioning_to_end():
    # Test forward after manually setting _pos to last index
    stack = _Stack()
    stack._elements.extend(["A", "B"])
    stack._pos = 1
    codeflash_output = stack.forward()  # 1.76μs -> 973ns (81.2% faster)


def test_forward_negative_position():
    # Test forward when _pos is less than -1 (invalid, but possible)
    stack = _Stack()
    stack._elements.extend(["A", "B"])
    stack._pos = -2
    codeflash_output = stack.forward()  # 1.78μs -> 940ns (89.9% faster)


def test_forward_with_non_string_elements():
    # Test forward with elements of various types
    stack = _Stack()
    stack._elements.extend([None, 42, 3.14, True])
    # Move forward through all elements
    codeflash_output = stack.forward()  # 1.77μs -> 913ns (94.1% faster)
    codeflash_output = stack.forward()  # 683ns -> 466ns (46.6% faster)
    codeflash_output = stack.forward()  # 430ns -> 298ns (44.3% faster)
    codeflash_output = stack.forward()  # 415ns -> 232ns (78.9% faster)
    # Further forward stays at last element
    codeflash_output = stack.forward()  # 482ns -> 387ns (24.5% faster)


def test_forward_with_duplicate_elements():
    # Test forward with duplicate elements
    stack = _Stack()
    stack._elements.extend(["A", "A", "B", "B"])
    codeflash_output = stack.forward()  # 1.75μs -> 924ns (89.2% faster)
    codeflash_output = stack.forward()  # 677ns -> 460ns (47.2% faster)
    codeflash_output = stack.forward()  # 454ns -> 278ns (63.3% faster)
    codeflash_output = stack.forward()  # 411ns -> 235ns (74.9% faster)
    # Further forward stays at last element
    codeflash_output = stack.forward()  # 484ns -> 403ns (20.1% faster)


def test_forward_on_stack_with_removed_elements():
    # Test forward after elements have been removed
    stack = _Stack()
    stack._elements.extend(["A", "B", "C"])
    stack.forward()  # 1.70μs -> 906ns (88.0% faster)
    stack.forward()  # 668ns -> 444ns (50.5% faster)
    stack._elements.pop()  # Remove 'C'
    # Now, forward should stay at last valid element
    codeflash_output = stack.forward()  # 561ns -> 457ns (22.8% faster)


def test_forward_on_stack_with_no_elements_after_removal():
    # Test forward after all elements have been removed
    stack = _Stack()
    stack._elements.extend(["A"])
    stack.forward()  # 1.62μs -> 918ns (76.6% faster)
    stack._elements.clear()
    codeflash_output = stack.forward()  # 785ns -> 518ns (51.5% faster)


# ----------- Large Scale Test Cases -----------


def test_forward_large_stack():
    # Test forward on a large stack (1000 elements)
    stack = _Stack()
    large_list = list(range(1000))
    stack._elements.extend(large_list)
    # Move forward through all elements
    for i in range(1000):
        codeflash_output = stack.forward()  # 385μs -> 248μs (55.4% faster)
    codeflash_output = stack.forward()  # 525ns -> 460ns (14.1% faster)


def test_forward_large_stack_multiple_calls():
    # Test forward called more times than stack size
    stack = _Stack()
    stack._elements.extend(range(500))
    # Call forward 600 times
    for i in range(600):
        expected = stack._elements[min(i, 499)]
        codeflash_output = stack.forward()  # 229μs -> 150μs (52.9% faster)


def test_forward_performance_large_stack():
    # Test performance (no assertion, but ensures no crash/hang)
    stack = _Stack()
    stack._elements.extend(range(1000))
    for _ in range(1000):
        stack.forward()  # 386μs -> 248μs (55.8% faster)
    codeflash_output = stack.forward()  # 560ns -> 456ns (22.8% faster)


def test_forward_with_large_stack_and_manual_position():
    # Test forward after manually setting position near end
    stack = _Stack()
    stack._elements.extend(range(1000))
    stack._pos = 995
    codeflash_output = stack.forward()  # 1.78μs -> 1.03μs (72.7% faster)
    codeflash_output = stack.forward()  # 666ns -> 395ns (68.6% faster)
    codeflash_output = stack.forward()  # 428ns -> 259ns (65.3% faster)
    codeflash_output = stack.forward()  # 457ns -> 245ns (86.5% faster)
    # Further forward stays at last
    codeflash_output = stack.forward()  # 515ns -> 416ns (23.8% faster)


# codeflash_output is used to check that the output of the original code is the same as that of the optimized code.

To edit these changes git checkout codeflash/optimize-_Stack.forward-miscwqad and push.