⚡️ Speed up function string_concat by 41%

[codeflash-ai]

📄 41% (0.41x) speedup for string_concat in src/dsa/various.py

⏱️ Runtime : 611 microseconds → 433 microseconds (best of 617 runs)

📝 Explanation and details

Let's break down the line profiler results.

Profiling Bottleneck Analysis

Line	Time (%)	Analysis
s = ""	1.5%	negligible
for i in range(n):	40.3%	iteration not itself expensive; it's the work done in each iteration
s += str(i)	57.2%	bottleneck: string concatenation in a loop is O(N²) in Python
return s	0.9%	negligible

Main bottleneck: s += str(i) — repeatedly concatenating immutable strings, which causes the data to copy each time.

---

Suggested Optimization

• Instead of string concatenation in the loop (O(N²)), use a list to collect substrings then ''.join(...) at the end (O(N)).

---

Optimized Code

You asked to keep the function signature and logic return intact, but rewrite internals for speed.

• This approach is vastly faster for large n.
• It also uses minimal extra memory and is more Pythonic.

---

Installed Libraries

Since there are no installed libraries, this solution remains standard-library-only and maximally compatible.

---

Summary Table

Suggestion	Justification
Use list, then ''.join to concat strings	Reduces time complexity from O(N²) to O(N)
No external libraries used	Matches current environment, no dependencies needed

---

Final Notes

• This change will dramatically improve runtime for large values of n.
• The memory impact is negligible compared to the huge runtime savings.
• For further speedup, consider using PyPy instead of CPython, but this solution is universal and effective as written.

No other optimizations are necessary based on the profiler data and environment.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 54 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

import pytest
from src.dsa.various import string_concat

# unit tests

# 1. Basic Test Cases

def test_concat_zero():
    # n = 0 should return an empty string
    codeflash_output = string_concat(0) # 417ns -> 541ns (22.9% slower)

def test_concat_one():
    # n = 1 should return "0"
    codeflash_output = string_concat(1) # 542ns -> 625ns (13.3% slower)

def test_concat_small_number():
    # n = 5 should return "01234"
    codeflash_output = string_concat(5) # 917ns -> 1.00μs (8.30% slower)

def test_concat_typical_number():
    # n = 10 should return "0123456789"
    codeflash_output = string_concat(10) # 1.33μs -> 1.38μs (2.98% slower)

def test_concat_double_digits():
    # n = 15 should return "01234567891011121314"
    codeflash_output = string_concat(15) # 1.75μs -> 1.75μs (0.000% faster)

# 2. Edge Test Cases

def test_concat_negative():
    # n < 0 should return empty string (since range(n) is empty)
    codeflash_output = string_concat(-1) # 250ns -> 292ns (14.4% slower)
    codeflash_output = string_concat(-100) # 250ns -> 292ns (14.4% slower)

def test_concat_large_single_digit():
    # n = 9 should return "012345678"
    codeflash_output = string_concat(9) # 1.29μs -> 1.38μs (6.04% slower)

def test_concat_large_transition_to_double_digits():
    # n = 10 should include the first two-digit number, "9"
    codeflash_output = string_concat(10) # 1.33μs -> 1.38μs (3.05% slower)

def test_concat_large_transition_to_triple_digits():
    # n = 100 should end with "99"
    codeflash_output = string_concat(100); result = codeflash_output # 8.50μs -> 7.83μs (8.52% faster)
    # The length should be sum of lengths of string representations of 0..99
    expected_length = sum(len(str(i)) for i in range(100))

def test_concat_non_integer_input():
    # Should raise TypeError if input is not an integer
    with pytest.raises(TypeError):
        string_concat("10")
    with pytest.raises(TypeError):
        string_concat(3.5)
    with pytest.raises(TypeError):
        string_concat(None)
    with pytest.raises(TypeError):
        string_concat([5])

def test_concat_boolean_input():
    # Booleans are subclasses of int: True == 1, False == 0
    # Should return "0" for True, "" for False
    codeflash_output = string_concat(True) # 208ns -> 250ns (16.8% slower)
    codeflash_output = string_concat(False) # 208ns -> 250ns (16.8% slower)

def test_concat_boundary_values():
    # n = 2 should return "01"
    codeflash_output = string_concat(2) # 708ns -> 833ns (15.0% slower)
    # n = 11 should return "012345678910"
    codeflash_output = string_concat(11) # 708ns -> 833ns (15.0% slower)

# 3. Large Scale Test Cases

def test_concat_large_scale_100():
    # n = 100, check length and a few substrings
    codeflash_output = string_concat(100); result = codeflash_output # 8.46μs -> 7.75μs (9.15% faster)
    # Check that the length matches expected
    expected_length = sum(len(str(i)) for i in range(100))

def test_concat_large_scale_999():
    # n = 999, check only length and start/end due to size
    codeflash_output = string_concat(999); result = codeflash_output # 116μs -> 75.7μs (53.4% faster)
    expected_length = sum(len(str(i)) for i in range(999))

def test_concat_performance_medium():
    # n = 500, should not take too long and length should be correct
    codeflash_output = string_concat(500); result = codeflash_output # 56.1μs -> 37.1μs (51.2% faster)
    expected_length = sum(len(str(i)) for i in range(500))

def test_concat_all_digits_present():
    # For n=100, all digits 0-9 should be present in the result
    codeflash_output = string_concat(100); result = codeflash_output # 8.46μs -> 7.62μs (10.9% faster)
    for digit in "0123456789":
        pass

# 4. Miscellaneous/Mutation-Resistant Tests

def test_concat_mutation_resistant():
    # Ensure that omitting any number or changing order will fail
    n = 20
    codeflash_output = string_concat(n); result = codeflash_output # 2.17μs -> 2.08μs (3.93% faster)
    # Check that each number appears in order at the correct position
    pos = 0
    for i in range(n):
        s = str(i)
        idx = result.find(s, pos)
        pos += len(s)
    # The result should be exactly the concatenation of str(i) for i in range(n)
    expected = ''.join(str(i) for i in range(n))

def test_concat_no_extra_characters():
    # The result should contain only digits, no spaces or separators
    codeflash_output = string_concat(50); result = codeflash_output # 4.50μs -> 4.04μs (11.4% faster)
# codeflash_output is used to check that the output of the original code is the same as that of the optimized code.

import pytest  # used for our unit tests
from src.dsa.various import string_concat

# unit tests

# 1. Basic Test Cases

def test_concat_zero():
    # n = 0 should return an empty string
    codeflash_output = string_concat(0) # 375ns -> 500ns (25.0% slower)

def test_concat_one():
    # n = 1 should return "0"
    codeflash_output = string_concat(1) # 542ns -> 625ns (13.3% slower)

def test_concat_small_number():
    # n = 5 should return "01234"
    codeflash_output = string_concat(5) # 958ns -> 1.04μs (8.06% slower)

def test_concat_two_digits():
    # n = 12 should return "01234567891011"
    codeflash_output = string_concat(12) # 1.54μs -> 1.54μs (0.000% faster)

def test_concat_typical_number():
    # n = 10 should return "0123456789"
    codeflash_output = string_concat(10) # 1.33μs -> 1.42μs (5.86% slower)

# 2. Edge Test Cases

def test_concat_negative():
    # Negative n should return an empty string (since range(n) is empty)
    codeflash_output = string_concat(-5) # 417ns -> 500ns (16.6% slower)

def test_concat_large_single_digit_boundary():
    # n = 10, checks transition from single to double digit
    codeflash_output = string_concat(10) # 1.33μs -> 1.33μs (0.000% faster)

def test_concat_double_digit_boundary():
    # n = 100, checks transition from double to triple digit
    codeflash_output = string_concat(100); result = codeflash_output # 8.46μs -> 7.67μs (10.3% faster)
    # Length should be sum of digits in 0..99
    expected_length = sum(len(str(i)) for i in range(100))

def test_concat_non_integer_input():
    # Should raise TypeError for non-integer input (e.g., float)
    with pytest.raises(TypeError):
        string_concat(5.5)
    with pytest.raises(TypeError):
        string_concat("10")
    with pytest.raises(TypeError):
        string_concat(None)

def test_concat_boolean_input():
    # Booleans are subclasses of int, so True -> 1, False -> 0
    codeflash_output = string_concat(True) # 250ns -> 291ns (14.1% slower)
    codeflash_output = string_concat(False) # 250ns -> 291ns (14.1% slower)

def test_concat_large_negative():
    # Large negative n should still return empty string
    codeflash_output = string_concat(-1000) # 458ns -> 542ns (15.5% slower)

def test_concat_mutation_resistance():
    # Ensure that mutation (e.g., off-by-one errors) would fail
    # n = 3 should never return "0123" or "123" or "01"
    codeflash_output = string_concat(3); result = codeflash_output # 792ns -> 917ns (13.6% slower)

# 3. Large Scale Test Cases

def test_concat_large_n_100():
    # n = 100, check correctness and performance within reasonable limits
    codeflash_output = string_concat(100); result = codeflash_output # 8.42μs -> 7.67μs (9.78% faster)

def test_concat_large_n_999():
    # n = 999, check start, end, and length
    codeflash_output = string_concat(999); result = codeflash_output # 117μs -> 80.9μs (45.3% faster)

def test_concat_performance_under_1000():
    # n = 1000, ensure function completes and result is correct
    codeflash_output = string_concat(1000); result = codeflash_output # 116μs -> 74.6μs (55.7% faster)

def test_concat_no_side_effects():
    # Ensure function does not modify any global state or rely on external variables
    n = 10
    before = n
    codeflash_output = string_concat(n); result = codeflash_output # 1.29μs -> 1.38μs (6.11% slower)

# 4. Miscellaneous Robustness

@pytest.mark.parametrize("invalid_input", [None, "string", 5.5, [], {}, object()])
def test_concat_various_invalid_types(invalid_input):
    # All non-integer, non-bool types should raise TypeError
    with pytest.raises(TypeError):
        string_concat(invalid_input)

@pytest.mark.parametrize("n", [0, 1, 10, 100, 999])
def test_concat_is_deterministic(n):
    # Multiple calls with same input should return same output
    codeflash_output = string_concat(n) # 459ns -> 542ns (15.3% slower)
# 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-string_concat-mc8rzh1o and push.