⚡️ Speed up function fibonacci by 10,650%

[codeflash-ai]

📄 10,650% (106.50x) speedup for fibonacci in code_to_optimize_js/fibonacci.js

⏱️ Runtime : 1.88 milliseconds → 17.5 microseconds (best of 250 runs)

📝 Explanation and details

The optimized code achieves a 10,649% speedup (from 1.88ms to 17.5μs) by replacing the exponential-time recursive algorithm with a linear-time iterative solution.

Key Optimizations

1. Algorithmic Transformation: O(2^n) → O(n)

• The original recursive implementation recalculates the same Fibonacci values exponentially many times. For example, fibonacci(5) calls fibonacci(3) twice, fibonacci(2) three times, etc., creating a binary tree of recursive calls.
• The optimized version uses dynamic programming via iteration, computing each Fibonacci number exactly once by maintaining only the two previous values (prevPrev and prev) and building up to n in a simple loop.

2. Memory Efficiency: O(n) stack → O(1)

• Recursion depth reaches n, consuming stack frames proportional to input size.
• The iterative approach uses only a constant number of variables regardless of n, eliminating stack overhead entirely.

3. Cache-Friendly Sequential Access

• The loop performs predictable, sequential arithmetic operations that CPUs can pipeline effectively, versus the scattered memory access pattern of recursive calls.

Performance Impact by Test Case

The speedup is most dramatic for larger inputs where exponential growth becomes prohibitive:

• fibonacci(22): 35,761% faster (149μs → 417ns)
• fibonacci(25): 116,859% faster (632μs → 541ns)
• Sequence verification (n≤24): 9,644% faster (1.03ms → 10.6μs)

Even moderate values like fibonacci(18) show 4,722% improvement. The optimization maintains exact correctness across all test cases including edge cases (negative inputs, floats, base cases).

Why This Matters

This function likely appears in computational hotpaths based on the comprehensive performance test suite. Any code calling fibonacci repeatedly—especially in loops or with moderately large values (n>20)—will see substantial throughput improvements. The sub-microsecond execution time for typical inputs (n≤30) makes this practical for real-time or high-frequency scenarios where the original implementation would be prohibitively slow.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 77 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 be a function and return numbers for numeric inputs', () => {
            // Ensure the imported symbol is a function
            expect(typeof fibonacci).toBe('function');

            // Basic known Fibonacci values
            expect(fibonacci(0)).toBe(0); // F0 = 0
            expect(fibonacci(1)).toBe(1); // F1 = 1
            expect(fibonacci(2)).toBe(1); // F2 = 1
            expect(fibonacci(3)).toBe(2); // F3 = 2
            expect(fibonacci(4)).toBe(3); // F4 = 3
            expect(fibonacci(5)).toBe(5); // F5 = 5

            // Type checks
            expect(typeof fibonacci(7)).toBe('number');
            expect(Number.isFinite(fibonacci(7))).toBe(true);
        });

        test('should produce correct higher Fibonacci values (sanity check)', () => {
            // Common larger value to ensure recursion works across several levels
            expect(fibonacci(10)).toBe(55);   // F10 = 55
            expect(fibonacci(12)).toBe(144);  // F12 = 144
            expect(fibonacci(15)).toBe(610);  // F15 = 610
        });

        test('multiple calls produce consistent results (pure function property)', () => {
            // Call multiple times and expect same result each time
            const a = fibonacci(8);
            const b = fibonacci(8);
            expect(a).toBe(21);
            expect(b).toBe(21);
            expect(a).toBe(b);
        });
    });

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should handle n = 0 and n = 1 correctly (base cases)', () => {
            // Explicitly test the base-case behavior
            expect(fibonacci(0)).toBe(0);
            expect(fibonacci(1)).toBe(1);
        });

        test('should return the input for negative integers (current implementation behaviour)', () => {
            // The provided implementation returns n when n <= 1.
            // For negative integers that meets that condition, it simply returns the input.
            expect(fibonacci(-1)).toBe(-1);
            expect(fibonacci(-5)).toBe(-5);
        });

        test('should handle positive non-integer inputs (floating point) deterministically', () => {
            // The implementation uses arithmetic and comparisons which coerce floats naturally.
            // We assert the concrete behaviour observed for some fractional inputs.
            // e.g., fibonacci(2.5) => fibonacci(1.5) + fibonacci(0.5)
            // fibonacci(0.5) returns 0.5, fibonacci(1.5) resolves to 0. (see reasoning in test)
            expect(fibonacci(0.5)).toBeCloseTo(0.5);
            expect(fibonacci(1.5)).toBeCloseTo(0);   // fibonacci(1.5) => fibonacci(0.5)+fibonacci(-0.5) => 0.5 + (-0.5) = 0
            expect(fibonacci(2.5)).toBeCloseTo(0.5); // fibonacci(2.5) => fibonacci(1.5)+fibonacci(0.5) => 0 + 0.5
        });

        test('should behave deterministically for string numeric inputs that coerce to numbers', () => {
            // Strings that are numeric will coerce to numbers in comparisons/arithmetic.
            // "6" coerces to 6 so result should equal fibonacci(6).
            expect(fibonacci("6")).toBe(fibonacci(6));
            expect(fibonacci("6")).toBe(8); // F6 = 8
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should compute larger Fibonacci values correctly (reasonable performance)', () => {
            // This verifies correctness for a larger n while keeping the runtime reasonable.
            // F(30) = 832040. The naive recursive implementation is exponential but for n=30
            // it is typically practical in test environments. We allow a generous time budget.
            const n = 30;
            const expected = 832040;

            const start = Date.now();
            const result = fibonacci(n);
            const durationMs = Date.now() - start;

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

            // Basic performance assertion: ensure computation completes within a practical limit.
            // Use a generous upper bound to avoid flakiness on CI machines. If environment is very slow,
            // this assertion can be relaxed; however it encourages reasonable efficiency.
            expect(durationMs).toBeLessThanOrEqual(2000);
        });

        test('should produce the first 21 Fibonacci numbers correctly (sequence sanity)', () => {
            // Generate expected first 21 Fibonacci numbers hard-coded for accuracy.
            const expectedSequence = [
                0, 1, 1, 2, 3, 5, 8, 13, 21, 34,
                55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765
            ];

            // Query the function for n = 0..20 and compare to expected sequence.
            for (let i = 0; i < expectedSequence.length; i++) {
                expect(fibonacci(i)).toBe(expectedSequence[i]);
            }
        });
    });
});

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 correct fibonacci value for n=2', () => {
            expect(fibonacci(2)).toBe(1);
        });

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

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

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

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

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

        test('should return correct fibonacci value for n=15', () => {
            expect(fibonacci(15)).toBe(610);
        });

        test('should return correct fibonacci value for n=20', () => {
            expect(fibonacci(20)).toBe(6765);
        });
    });

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should handle negative input by returning the input value', () => {
            expect(fibonacci(-1)).toBe(-1);
        });

        test('should handle negative input -5', () => {
            expect(fibonacci(-5)).toBe(-5);
        });

        test('should return exact type (number) for valid inputs', () => {
            const result = fibonacci(5);
            expect(typeof result).toBe('number');
            expect(Number.isInteger(result)).toBe(true);
        });

        test('should handle input where n is exactly at boundary (n=1)', () => {
            expect(fibonacci(1)).toBe(1);
        });

        test('should handle input where n is exactly at boundary (n=0)', () => {
            expect(fibonacci(0)).toBe(0);
        });

        test('should maintain precision for moderate values', () => {
            const fib25 = fibonacci(25);
            expect(fib25).toBe(75025);  // 632μs -> 541ns (116859% faster)
        });

        test('should return a positive number for positive inputs', () => {
            const result = fibonacci(12);
            expect(result).toBeGreaterThan(0);  // 1.50μs -> 458ns (228% faster)
        });

        test('should return a non-negative number for non-negative inputs', () => {
            expect(fibonacci(0)).toBeGreaterThanOrEqual(0);
            expect(fibonacci(7)).toBeGreaterThanOrEqual(0);
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should compute fibonacci(25) within reasonable time', () => {
            const startTime = performance.now();
            const result = fibonacci(25);
            const endTime = performance.now();
            
            // Verify correctness first
            expect(result).toBe(75025);
            
            // Ensure it completes in a reasonable time (should be quick even for recursive)
            // Allow up to 2 seconds for execution
            expect(endTime - startTime).toBeLessThan(2000);
        });

        test('should compute fibonacci(28) and return correct value', () => {
            const result = fibonacci(28);
            expect(result).toBe(317811);
        });

        test('should compute fibonacci(30) and return correct value', () => {
            const result = fibonacci(30);
            expect(result).toBe(832040);
        });

        test('should handle consecutive calls with increasing values', () => {
            // Test that multiple calls work correctly without state issues
            const results = [];
            const expectedSequence = [0, 1, 1, 2, 3, 5, 8, 13, 21, 34];
            
            for (let i = 0; i < 10; i++) {
                results.push(fibonacci(i));
            }
            
            expect(results).toEqual(expectedSequence);  // 3.66μs -> 4.16μs (12.0% slower)
        });

        test('should verify fibonacci sequence properties (f(n) = f(n-1) + f(n-2))', () => {
            // Verify mathematical property: each fibonacci number is sum of previous two
            // Test for values up to 20 to keep performance reasonable
            for (let n = 2; n <= 20; n++) {
                const current = fibonacci(n);
                const prev1 = fibonacci(n - 1);
                const prev2 = fibonacci(n - 2);
                expect(current).toBe(prev1 + prev2);
            }
        });

        test('should handle boundary stress test with n=22', () => {
            const result = fibonacci(22);
            expect(result).toBe(17711);  // 149μs -> 417ns (35761% faster)
        });

        test('should maintain output consistency across multiple invocations', () => {
            // Call the function multiple times with same input
            const result1 = fibonacci(18);
            const result2 = fibonacci(18);
            const result3 = fibonacci(18);
            
            expect(result1).toBe(result2);  // 66.2μs -> 1.37μs (4722% faster)
            expect(result2).toBe(result3);
            expect(result1).toBe(2584);
        });

        test('should verify monotonic increase in sequence for positive integers', () => {
            // Fibonacci sequence should be monotonically increasing for n >= 0
            let previous = fibonacci(0);
            for (let n = 1; n <= 24; n++) {
                const current = fibonacci(n);
                expect(current).toBeGreaterThanOrEqual(previous);  // 1.03ms -> 10.6μs (9644% faster)
                previous = current;
            }
        });
    });
});

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