⚡️ Speed up function fibonacci by 114,050%

[codeflash-ai]

📄 114,050% (1,140.50x) speedup for fibonacci in code_to_optimize_js/fibonacci.js

⏱️ Runtime : 33.8 milliseconds → 29.6 microseconds (best of 250 runs)

📝 Explanation and details

The optimized code achieves a 1140x speedup (from 33.8ms to 29.6μs) by replacing the exponential-time recursive algorithm with a linear-time iterative approach for the most common case: non-negative integers.

Key Changes:

1. Fast Path for Integers (O(n) vs O(2^n)): The optimization adds a check for Number.isInteger(n) && n >= 0 and computes Fibonacci iteratively using just two variables (a and b). This eliminates the exponential explosion of recursive calls that occurs with the naive implementation—for example, fibonacci(20) requires ~21,891 recursive calls in the original but only 19 iterations in the optimized version.

2. Preserves Original Semantics: For edge cases like negative numbers, floats, or string coercions, the code falls back to the original recursive implementation (slow()), ensuring behavioral compatibility.

Why This Works:

• Eliminates Redundant Computation: The naive recursion recomputes the same Fibonacci values exponentially many times (e.g., fibonacci(2) is called thousands of times when computing fibonacci(20)). The iterative approach computes each value exactly once.

• Minimal Memory Overhead: Uses O(1) space instead of O(n) call stack depth, preventing stack overflow on larger inputs.

Test Results Show:

• Large inputs benefit massively: Performance tests computing fibonacci(0) through fibonacci(30) now complete in microseconds instead of seconds. The test expecting completion under 3000ms would pass trivially with the optimized version.

• Small inputs remain fast: Basic cases (0-10) already execute quickly but still benefit from eliminating function call overhead.

• Edge cases preserved: Tests with negative numbers, floats (e.g., fibonacci(1.5)), and string coercion (fibonacci('6')) still pass because the fallback path maintains exact original behavior.

Impact:
For typical workloads using integer inputs (which represent 100% of canonical Fibonacci use cases), this optimization delivers transformative performance gains while maintaining full backward compatibility for unusual inputs.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 129 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Click to see Generated Regression Tests

// imports
const { fibonacci } = require('../fibonacci');

// unit tests
describe('fibonacci', () => {
    // Basic Test Cases
    describe('Basic functionality', () => {
        test('should handle normal input', () => {
            // Fundamental integer cases: 0, 1 and small positive integers
            // These are the canonical Fibonacci base cases and some small values.
            expect(fibonacci(0)).toBe(0);
            expect(fibonacci(1)).toBe(1);
            expect(fibonacci(2)).toBe(1);
            expect(fibonacci(3)).toBe(2);
            expect(fibonacci(4)).toBe(3);
            expect(fibonacci(5)).toBe(5);
            // Larger but reasonable input to ensure recursion composes correctly
            expect(fibonacci(10)).toBe(55);
            expect(fibonacci(20)).toBe(6765);
        });
    });

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should handle edge case', () => {
            // Negative inputs:
            // According to the provided implementation, any n <= 1 is returned as-is.
            // Therefore negative numbers return the same negative number.
            expect(fibonacci(-1)).toBe(-1);
            expect(fibonacci(-5)).toBe(-5);

            // Non-integer numeric inputs:
            // The implementation does not restrict to integers. JavaScript numeric
            // operations will be applied, and the recursion will terminate when n <= 1.
            // Verify some representative fractional cases to exercise that behavior.
            // Example: fibonacci(1.5) -> fibonacci(0.5) + fibonacci(-0.5) => 0.5 + (-0.5) === 0
            expect(fibonacci(1.5)).toBe(0);
            // fibonacci(2.5) -> fibonacci(1.5) + fibonacci(0.5) => 0 + 0.5 === 0.5
            expect(fibonacci(2.5)).toBe(0.5);

            // String input that is numeric-like should be coerced to number by JS operators:
            // fibonacci('6') -> coerced and computed as fibonacci(6) === 8
            expect(fibonacci('6')).toBe(8);

            // Ensure function behaves deterministically and is pure for same primitive inputs:
            const firstCall = fibonacci(12);
            const secondCall = fibonacci(12);
            expect(firstCall).toBe(secondCall);
            expect(firstCall).toBe(144);

            // Inputs that are not easily coercible to numbers (e.g., plain objects) can
            // lead to non-terminating recursion or runtime errors in this implementation.
            // We assert that passing non-primitive non-numeric values behaves in a way
            // consistent with the implementation by observing that it throws (stack overflow
            // or similar). We do NOT mock or alter the function under test.
            expect(() => fibonacci({})).toThrow();
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should handle large inputs efficiently', () => {
            // This test exercises larger inputs to validate correctness and observe runtime.
            // We compute Fibonacci numbers for the range 0..30 inclusive (31 items),
            // which keeps memory/time bounded and stays well under the limits specified.
            // Precomputed sequence for 0..30
            const expected = [
                0, 1, 1, 2, 3, 5, 8, 13, 21, 34,
                55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181,
                6765, 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229,
                832040
            ];

            // Measure time taken to compute the sequence (keeps the loop < 1000 iterations).
            const start = Date.now();
            const results = [];
            for (let i = 0; i <= 30; i++) {
                // compute each using the real fibonacci implementation (no mocking)
                results.push(fibonacci(i));
            }
            const durationMs = Date.now() - start;

            // Validate the sequence exactly matches expected values.
            expect(results).toHaveLength(expected.length);
            for (let i = 0; i < expected.length; i++) {
                expect(results[i]).toBe(expected[i]);
            }

            // Performance assertion:
            // The naive recursive implementation is exponential, but calculating up to 30
            // should complete in a reasonable amount of time on typical CI machines.
            // We use a generous threshold to avoid flakiness while still catching
            // pathological regressions. If this environment is extremely slow, adjust threshold.
            const thresholdMs = 3000; // 3 seconds
            expect(durationMs).toBeLessThan(thresholdMs);
        });
    });
});

const { fibonacci } = require('../fibonacci');

describe('fibonacci', () => {
    // Basic Test Cases
    describe('Basic functionality', () => {
        test('should return 0 for input 0', () => {
            expect(fibonacci(0)).toBe(0);
        });

        test('should return 1 for input 1', () => {
            expect(fibonacci(1)).toBe(1);
        });

        test('should return 1 for input 2', () => {
            expect(fibonacci(2)).toBe(1);
        });

        test('should return 2 for input 3', () => {
            expect(fibonacci(3)).toBe(2);
        });

        test('should return 3 for input 4', () => {
            expect(fibonacci(4)).toBe(3);
        });

        test('should return 5 for input 5', () => {
            expect(fibonacci(5)).toBe(5);
        });

        test('should return 8 for input 6', () => {
            expect(fibonacci(6)).toBe(8);
        });

        test('should return 13 for input 7', () => {
            expect(fibonacci(7)).toBe(13);
        });

        test('should return 21 for input 8', () => {
            expect(fibonacci(8)).toBe(21);
        });

        test('should return 55 for input 10', () => {
            expect(fibonacci(10)).toBe(55);
        });

        test('should return correct sequence for consecutive numbers', () => {
            const sequence = [0, 1, 1, 2, 3, 5, 8, 13, 21, 34];
            for (let i = 0; i < sequence.length; i++) {
                expect(fibonacci(i)).toBe(sequence[i]);
            }
        });
    });

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should handle negative input and return negative value', () => {
            // For negative inputs, recursive nature may cause issues
            // but function should still execute
            const result = fibonacci(-1);
            expect(typeof result).toBe('number');
        });

        test('should handle floating point input by treating as integer', () => {
            // JavaScript will coerce float to behavior, should work with integer-like behavior
            const result = fibonacci(5.9);
            expect(typeof result).toBe('number');
        });

        test('should return exact values for critical small integers', () => {
            expect(fibonacci(0)).toBe(0);
            expect(fibonacci(1)).toBe(1);
        });

        test('should maintain type consistency (returns number)', () => {
            const result = fibonacci(5);
            expect(typeof result).toBe('number');
            expect(Number.isInteger(result)).toBe(true);
        });

        test('should handle single digit inputs correctly', () => {
            const singleDigitResults = {
                0: 0,
                1: 1,
                2: 1,
                3: 2,
                4: 3,
                5: 5,
                6: 8,
                7: 13,
                8: 21,
                9: 34
            };
            Object.entries(singleDigitResults).forEach(([n, expected]) => {
                expect(fibonacci(parseInt(n))).toBe(expected);
            });
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should compute fibonacci(15) within reasonable time', () => {
            const start = Date.now();
            const result = fibonacci(15);
            const elapsed = Date.now() - start;
            
            expect(result).toBe(610);
            // Should complete in less than 1000ms even with naive recursion
            expect(elapsed).toBeLessThan(1000);
        });

        test('should compute fibonacci(20) correctly', () => {
            const start = Date.now();
            const result = fibonacci(20);
            const elapsed = Date.now() - start;
            
            expect(result).toBe(6765);
            expect(elapsed).toBeLessThan(5000);
        });

        test('should maintain numeric precision for larger valid inputs', () => {
            // Testing up to input where computation is still feasible with naive recursion
            const result = fibonacci(25);
            expect(result).toBe(75025);
            expect(typeof result).toBe('number');
        });

        test('should handle computed results without overflow for valid range', () => {
            // Test that numbers remain valid JavaScript numbers
            const result = fibonacci(30);
            expect(Number.isFinite(result)).toBe(true);
            expect(result).toBe(832040);
        });

        test('should process multiple sequential calls efficiently', () => {
            const start = Date.now();
            
            // Call fibonacci multiple times with increasing arguments
            for (let i = 0; i <= 15; i++) {
                fibonacci(i);
            }
            
            const elapsed = Date.now() - start;
            // Multiple calls should complete in reasonable time
            expect(elapsed).toBeLessThan(2000);
        });

        test('should return consistent results across repeated calls', () => {
            const input = 12;
            const result1 = fibonacci(input);
            const result2 = fibonacci(input);
            
            expect(result1).toBe(result2);
            expect(result1).toBe(144);
        });

        test('should compute valid values for all integers 0-25', () => {
            // Known Fibonacci sequence values
            const expectedValues = [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368, 75025];
            
            for (let i = 0; i < expectedValues.length; i++) {
                expect(fibonacci(i)).toBe(expectedValues[i]);
            }
        });
    });
});

To edit these changes git checkout codeflash/optimize-fibonacci-mkhe09u9 and push.