⚡️ Speed up function string_concat by 32%

[codeflash-ai]

📄 32% (0.32x) speedup for string_concat in src/dsa/various.py

⏱️ Runtime : 342 microseconds → 258 microseconds (best of 347 runs)

📝 Explanation and details

The optimization replaces the inefficient string concatenation pattern s += str(i) with a list comprehension followed by "".join().

Key Changes:

• Eliminated quadratic behavior: The original code creates a new string object on each iteration since strings are immutable in Python. This leads to O(n²) time complexity as each concatenation copies all previous characters.
• Used list-then-join pattern: The optimized version builds a list of string components first, then joins them in a single operation, achieving O(n) time complexity.

Why it's faster:

• String concatenation with += has to allocate new memory and copy existing content for each iteration
• List appends are O(1) amortized, and "".join() performs the concatenation in one efficient C-level operation
• The line profiler shows the bottleneck moved from the repeated string operations (54.3% of time) to the single list comprehension (85.8% of time)

Performance characteristics:

• Small inputs (n < 20): The optimization shows minimal gains or slight overhead due to list creation costs
• Medium to large inputs (n ≥ 100): Significant speedups of 11-38% as the quadratic penalty of string concatenation becomes dominant
• Best suited for cases where n is reasonably large, as evidenced by the test results showing increasing performance gains with larger n values

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 25 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
🔮 Hypothesis Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

from __future__ import annotations

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

# unit tests

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

def test_concat_zero():
    # Test with n=0, should return empty string
    codeflash_output = string_concat(0) # 375ns -> 542ns (30.8% slower)

def test_concat_one():
    # Test with n=1, should return '0'
    codeflash_output = string_concat(1) # 541ns -> 708ns (23.6% slower)

def test_concat_small():
    # Test with n=5, should return '01234'
    codeflash_output = string_concat(5) # 875ns -> 1.04μs (16.0% slower)

def test_concat_ten():
    # Test with n=10, should return '0123456789'
    codeflash_output = string_concat(10) # 1.21μs -> 1.38μs (12.1% slower)

def test_concat_typical():
    # Test with a typical medium value
    codeflash_output = string_concat(15) # 1.58μs -> 1.71μs (7.32% slower)

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

def test_concat_negative():
    # Test with negative n, should return empty string (no iterations)
    codeflash_output = string_concat(-1) # 417ns -> 583ns (28.5% slower)

def test_concat_large_digit_boundary():
    # Test with n=10, boundary between single and double digits
    codeflash_output = string_concat(10) # 1.25μs -> 1.33μs (6.30% slower)
    # Test with n=11, should include '10'
    codeflash_output = string_concat(11) # 958ns -> 959ns (0.104% slower)

def test_concat_non_integer_input():
    # Should raise TypeError for non-integer input
    with pytest.raises(TypeError):
        string_concat("5") # 625ns -> 625ns (0.000% faster)
    with pytest.raises(TypeError):
        string_concat(5.5) # 458ns -> 500ns (8.40% slower)
    with pytest.raises(TypeError):
        string_concat(None) # 417ns -> 458ns (8.95% slower)
    with pytest.raises(TypeError):
        string_concat([5]) # 375ns -> 417ns (10.1% slower)

def test_concat_boolean_input():
    # Booleans are subclasses of int in Python: True==1, False==0
    # Should treat True as 1, so output should be '0'
    codeflash_output = string_concat(True) # 542ns -> 708ns (23.4% slower)
    # Should treat False as 0, so output should be ''
    codeflash_output = string_concat(False) # 166ns -> 250ns (33.6% slower)

def test_concat_huge_negative():
    # Large negative n should still return empty string
    codeflash_output = string_concat(-999) # 416ns -> 583ns (28.6% slower)

def test_concat_boundary_just_below_zero():
    # n=-1, should return empty string
    codeflash_output = string_concat(-1) # 375ns -> 583ns (35.7% slower)
    # n=0, should return empty string
    codeflash_output = string_concat(0) # 166ns -> 208ns (20.2% slower)

def test_concat_mutation_resistance():
    # Ensure function does not skip or repeat indices
    n = 20
    codeflash_output = string_concat(n); result = codeflash_output # 1.96μs -> 2.04μs (4.07% slower)
    # Should contain all numbers from 0 to n-1 in order, with no repeats or skips
    expected = ''.join(str(i) for i in range(n))

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

def test_concat_large_n_100():
    # Test with n=100 (double digits)
    expected = ''.join(str(i) for i in range(100))
    codeflash_output = string_concat(100) # 7.42μs -> 6.67μs (11.2% faster)

def test_concat_large_n_999():
    # Test with n=999, close to limit, triple digits
    expected = ''.join(str(i) for i in range(999))
    codeflash_output = string_concat(999) # 88.8μs -> 65.2μs (36.3% faster)

def test_concat_performance_upper_bound():
    # Test with n=1000, upper reasonable bound for this function
    expected = ''.join(str(i) for i in range(1000))
    codeflash_output = string_concat(1000); result = codeflash_output # 88.9μs -> 64.4μs (38.0% faster)

def test_concat_large_n_random():
    # Test with a random large n within the allowed range
    n = 543
    expected = ''.join(str(i) for i in range(n))
    codeflash_output = string_concat(n) # 46.9μs -> 35.0μs (33.9% faster)

def test_concat_large_n_all_digits():
    # Test with n=1000, ensure all digit transitions are handled
    codeflash_output = string_concat(1000); result = codeflash_output # 88.9μs -> 64.8μs (37.3% faster)

def test_concat_mutation_no_extra_characters():
    # Ensure the result contains only digits, no spaces or separators
    codeflash_output = string_concat(50); result = codeflash_output # 4.08μs -> 3.75μs (8.88% faster)

def test_concat_mutation_order():
    # Ensure the order of concatenation is strictly increasing
    n = 50
    codeflash_output = string_concat(n); result = codeflash_output # 4.08μs -> 3.75μs (8.88% faster)
    # Split result into substrings at each index
    idx = 0
    for i in range(n):
        s = str(i)
        idx += len(s)
# 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-mha5ukd5 and push.