⚡️ Speed up function fibonacci by 217,795%

[codeflash-ai]

📄 217,795% (2,177.95x) speedup for fibonacci in code_to_optimize_js/fibonacci.js

⏱️ Runtime : 60.5 milliseconds → 27.8 microseconds (best of 250 runs)

📝 Explanation and details

The optimized code achieves a 2,177x speedup (from 60.5 ms to 27.8 μs) by replacing the exponential-time recursive algorithm with two optimized paths:

Key Optimizations

1. Integer Fast Path (O(n) iterative)

• For integer inputs, uses a simple iterative loop instead of recursion
• Maintains only two variables (a, b) rather than exponentially many stack frames
• The original recursive implementation has O(2^n) time complexity due to redundant recalculations—for example, fibonacci(30) makes over 2.6 million function calls
• The iterative version makes exactly n-1 iterations with constant memory

2. Memoized Recursion for Non-Integers

• For fractional/non-integer inputs (e.g., fibonacci(1.5)), preserves the original recursive semantics but caches intermediate results in a Map
• Prevents redundant recalculations while maintaining correct behavior for edge cases like fractional values

3. V8 Engine Optimization Hint

• Caches loop boundary (len = n) to help the JavaScript engine optimize the loop iteration

Why This Matters

The test results show this optimization excels across all categories:

• Basic functionality tests (fibonacci(0) through fibonacci(10)) now execute near-instantaneously
• Performance tests with larger inputs (n=15, 20, 25, 30) that would timeout or take seconds with the naive implementation now complete in microseconds
• Batch operations (computing fibonacci(0..30) in a loop) benefit enormously since each call is O(n) instead of O(2^n)

The massive speedup is because the original algorithm's exponential growth becomes catastrophic even for moderate values—fibonacci(30) has ~2^30 recursive calls vs. 30 iterations. All test cases pass because the mathematical results remain identical; only the computation strategy changed.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 117 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', () => {
            // Basic known values from the Fibonacci sequence
            expect(fibonacci(0)).toBe(0);   // F(0) = 0
            expect(fibonacci(1)).toBe(1);   // F(1) = 1
            expect(fibonacci(2)).toBe(1);   // F(2) = 1
            expect(fibonacci(3)).toBe(2);   // F(3) = 2
            expect(fibonacci(10)).toBe(55); // F(10) = 55
        });

        test('should coerce numeric strings and produce same result as numbers', () => {
            // JS will coerce numeric strings in arithmetic/comparison operations.
            // The implementation uses <= and subtraction which cause coercion.
            expect(fibonacci('7')).toBe(fibonacci(7));
            expect(fibonacci('7')).toBe(13); // F(7) = 13
        });
    });

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should return input for n <= 1 (including negatives and certain non-number falsy values)', () => {
            // Implementation immediately returns n when n <= 1.
            // For negative numbers the function will return the same negative input.
            expect(fibonacci(-5)).toBe(-5);
            expect(fibonacci(-1)).toBe(-1);

            // null is coerced to 0 in comparisons (null <= 1 is true),
            // so the function returns the original value (null).
            expect(fibonacci(null)).toBe(null);

            // booleans: true coerces to 1, false to 0. Because of the <= check,
            // the function returns the original boolean value.
            expect(fibonacci(false)).toBe(false); // false is 0
            expect(fibonacci(true)).toBe(true);   // true is 1
        });

        test('should handle non-integer (fractional) inputs consistently with the implementation', () => {
            // For fractional values the recursion still runs using subtraction.
            // Example: fibonacci(1.5) -> fibonacci(0.5) + fibonacci(-0.5)
            // fibonacci(0.5) returns 0.5 (base case), fibonacci(-0.5) returns -0.5 (base case)
            // So the result is 0.
            expect(fibonacci(1.5)).toBe(0);

            // Check that this property (recurrence) still holds for a fractional input
            // where recursive decomposition ends in base-case returns.
            const a = 2.5;
            const left = fibonacci(a);
            const right = fibonacci(a - 1) + fibonacci(a - 2);
            expect(left).toBe(right);
        });

        test('should throw (maximum call stack) for undefined input (avoids infinite recursion)', () => {
            // Passing undefined leads to NaN propagation and repeated recursion until
            // the stack overflows. The runtime typically throws a RangeError for
            // maximum call stack size exceeded.
            expect(() => fibonacci(undefined)).toThrow(RangeError);
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should satisfy the recurrence relation for a range of n (0..25)', () => {
            // Check Fibonacci recurrence for a number of values.
            // This both confirms correctness for a sequence of inputs and exercises
            // the implementation across a wider range. We keep the loop size well
            // under the 1000-iteration constraint.
            const maxN = 25;
            for (let n = 0; n <= maxN; n++) {
                // F(n+2) == F(n+1) + F(n)
                expect(fibonacci(n + 2)).toBe(fibonacci(n + 1) + fibonacci(n));
            }
        });

        test('should compute a reasonably large Fibonacci number (n = 30) within acceptable time', () => {
            // The naive recursive implementation is exponential, so choose a value
            // that is large enough to be meaningful but small enough to run quickly
            // in a unit-test environment.
            const n = 30;
            const expected = 832040; // F(30)
            const start = Date.now();
            const result = fibonacci(n);
            const durationMs = Date.now() - start;

            // Correctness
            expect(result).toBe(expected);

            // Performance: ensure it completes in a reasonable time window
            // to catch regressions that might make the implementation dramatically slower.
            // 2000ms is generous for this environment; adjust if CI environment is slower.
            expect(durationMs).toBeLessThan(2000);
        });

        test('should compute a batch of Fibonacci numbers without exhausting resources', () => {
            // Compute Fibonacci values for n = 0..30 (31 calls). This verifies the
            // function works repeatedly and that intermediate caching is not required.
            const inputs = Array.from({ length: 31 }, (_, i) => i); // 0..30
            const outputs = inputs.map((i) => fibonacci(i));
            // Sanity checks: length and a couple of known values
            expect(outputs.length).toBe(31);
            expect(outputs[0]).toBe(0);
            expect(outputs[1]).toBe(1);
            expect(outputs[10]).toBe(55);
            expect(outputs[30]).toBe(832040);
        });
    });
});

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

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

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

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

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

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

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

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

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

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

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

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should handle negative numbers correctly', () => {
            // For negative n, the function should still compute based on the recursive definition
            // fibonacci(-1) should return -1 based on the condition n <= 1
            expect(fibonacci(-1)).toBe(-1);
        });

        test('should handle negative numbers with n = -5', () => {
            // Negative numbers should return the negative number itself due to n <= 1 condition
            expect(fibonacci(-5)).toBe(-5);
        });

        test('should return correct value for n = 0 (boundary)', () => {
            // Zero is the boundary case where n <= 1 is true
            expect(fibonacci(0)).toBe(0);
        });

        test('should return correct value for n = 1 (boundary)', () => {
            // One is the boundary case where n <= 1 is true
            expect(fibonacci(1)).toBe(1);
        });

        test('should return correct value at n = 2 (first recursive call)', () => {
            // n = 2 is the first case that requires recursion
            expect(fibonacci(2)).toBe(1);
        });

        test('should maintain mathematical consistency for sequence', () => {
            // Verify that f(n) = f(n-1) + f(n-2) holds
            const f5 = fibonacci(5);
            const f4 = fibonacci(4);
            const f3 = fibonacci(3);
            expect(f5).toBe(f4 + f3);
        });

        test('should maintain mathematical consistency for larger sequence', () => {
            // Verify property for larger values
            const f9 = fibonacci(9);
            const f8 = fibonacci(8);
            const f7 = fibonacci(7);
            expect(f9).toBe(f8 + f7);
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should compute fibonacci(15) within reasonable time', () => {
            // Testing with n = 15 to verify the function works for moderately large inputs
            // Expected result: 610
            const startTime = performance.now();
            const result = fibonacci(15);
            const endTime = performance.now();
            
            expect(result).toBe(610);
            // Ensure computation completes in under 5 seconds
            expect(endTime - startTime).toBeLessThan(5000);
        });

        test('should compute fibonacci(20) with correct result', () => {
            // Testing with n = 20
            // Expected result: 6765
            const result = fibonacci(20);
            expect(result).toBe(6765);
        });

        test('should compute fibonacci(12) efficiently', () => {
            // Testing with n = 12
            // Expected result: 144
            const startTime = performance.now();
            const result = fibonacci(12);
            const endTime = performance.now();
            
            expect(result).toBe(144);
            // Should complete quickly for n = 12
            expect(endTime - startTime).toBeLessThan(1000);
        });

        test('should return valid integer for fibonacci(25)', () => {
            // Testing with n = 25 to verify it returns a valid result
            // Expected result: 75025
            const result = fibonacci(25);
            expect(result).toBe(75025);
            expect(Number.isInteger(result)).toBe(true);
        });

        test('should maintain consistency across multiple calls', () => {
            // Verify that multiple calls to the same input return the same result
            const result1 = fibonacci(10);
            const result2 = fibonacci(10);
            const result3 = fibonacci(10);
            
            expect(result1).toBe(result2);
            expect(result2).toBe(result3);
            expect(result1).toBe(55);
        });

        test('should produce increasing values for increasing inputs', () => {
            // Verify that fibonacci sequence is monotonically increasing for positive n
            const f5 = fibonacci(5);
            const f10 = fibonacci(10);
            const f15 = fibonacci(15);
            
            expect(f5).toBeLessThan(f10);
            expect(f10).toBeLessThan(f15);
        });

        test('should handle sequence of calculations', () => {
            // Verify multiple sequential calls work correctly
            const results = [];
            for (let i = 0; i <= 10; i++) {
                results.push(fibonacci(i));
            }
            
            // Expected sequence: [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55]
            const expected = [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55];
            expect(results).toEqual(expected);
        });
    });
});

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