⚡️ Speed up function fibonacci by 61%

[codeflash-ai]

📄 61% (0.61x) speedup for fibonacci in code_to_optimize_js_esm/fibonacci.js

⏱️ Runtime : 97.1 microseconds → 60.4 microseconds (best of 1 runs)

📝 Explanation and details

Runtime improvement (primary): the optimized version cuts median runtime from ~97.1μs to ~60.4μs (≈60% speedup). The change was accepted because it reduces execution time for the common integer-Fibonacci use case.

What changed (specific optimizations)

• Replaced exponential recursion for non-negative integer inputs with an iterative dynamic-programming approach that builds Fibonacci numbers bottom-up.
• Added a module-level cache (_fibArray = [0,1]) that stores computed Fibonacci values and is incrementally extended. Recalling a cached value is an O(1) read.
• Fast-path guard: only use the iterative/cached code for typeof n === 'number' && Number.isInteger(n) && n >= 0. For other inputs the function falls back to the original recursive behavior (preserving the prior semantics for strings, null, floating inputs, negatives).
• Micro-optimizations in the loop: two running variables (a, b) and a single push per iteration to extend the cache, avoiding repeated recursive calls or repeated full-array recomputation.

Why this speeds things up (mechanics)

• Algorithmic improvement: naive recursion is exponential in n (lots of repeated work). Iteration is linear O(n) to compute fib(n) once and O(1) to return cached results. That removes the huge call overhead and redundant recomputation that dominates runtime for moderate n.
• Lower call/stack overhead: loops and local variables are far cheaper than deeply recursive calls, which incur function-call overhead and work duplications.
• Amortized benefit across calls: the cache is module-level and persistent. If you compute fib(20) and then fib(21), only one extra loop iteration is needed. Repeated queries for the same n are a direct array lookup.
• Memory/locality: storing values in an array gives good locality and fast index access; push is efficient and compact compared to maintaining a recursive call graph or map lookups.

Behavioral changes and trade-offs

• The function preserves original behavior for non-number inputs and non-integer numbers by keeping a recursive fallback — so correctness/regressions are minimized.
• Small-input overhead: trivial inputs can be slightly slower due to the extra typeof/Number.isInteger checks and the cache logic (annotated test shows fibonacci(0) moved from 22.0μs to 29.2μs). This is an acceptable trade-off given the large overall runtime benefit for typical numeric calls.
• Module-level cache means state is persistent across calls: memory usage grows to O(max_n_seen) (very small for typical n) and repeated calls become faster. In multi-worker or isolated contexts this is usually fine; if truly stateless behavior is required you’d need to clear or avoid the cache.

Which workloads benefit most (based on tests)

• Integer, non-negative inputs and moderate/large n benefit the most (performance tests such as fibonacci(30) and the sequence checks up to 20). The iterative/cached path makes computing fib(30) and repeated sequence queries much faster.
• The optimization is less relevant for inputs that rely on the original coercion behaviors (strings, null) in their first step — those still work, but the first call may follow the recursive path until it hits numeric values.

Summary

• Primary win: dramatically lower runtime for the common integer use-case by replacing exponential recursion with an iterative, cached algorithm and reusing computed values across calls.
• Trade-off: small overhead for trivial inputs and a tiny persistent memory footprint for the cache — reasonable given the 60% runtime improvement for typical workloads.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	✅ 13 Passed
🌀 Generated Regression Tests	✅ 11 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

⚙️ Click to see Existing Unit Tests

Test File::Test Function	Original ⏱️	Optimized ⏱️	Speedup
fibonacci.test.js::fibonacci returns 0 for n=0	25.2μs	20.5μs	23.0%✅
fibonacci.test.js::fibonacci returns 1 for n=1	1.17μs	916ns	27.4%✅
fibonacci.test.js::fibonacci returns 1 for n=2	2.00μs	2.33μs	-14.3%⚠️
fibonacci.test.js::fibonacci returns 233 for n=13	36.0μs	2.21μs	1529%✅
fibonacci.test.js::fibonacci returns 5 for n=5	4.21μs	2.38μs	77.2%✅
fibonacci.test.js::fibonacci returns 55 for n=10	6.58μs	2.83μs	132%✅

🌀 Click to see Generated Regression Tests

// imports
import { fibonacci } from '../fibonacci.js';

// unit tests
describe('fibonacci', () => {
    // Basic Test Cases
    describe('Basic functionality', () => {
        test('should handle normal input', () => {
            // Basic positive integers and small inputs that define Fibonacci sequence
            // 0 -> 0, 1 -> 1, 2 -> 1, 3 -> 2, 5 -> 5, 10 -> 55
            expect(fibonacci(0)).toBe(0);  // 22.0μs -> 29.2μs (24.8% slower)
            expect(fibonacci(1)).toBe(1);
            expect(fibonacci(2)).toBe(1);
            expect(fibonacci(3)).toBe(2);
            expect(fibonacci(5)).toBe(5);
            expect(fibonacci(10)).toBe(55);
        });
    });

    // Edge Test Cases
    describe('Edge cases', () => {
        test('should handle non-positive integers and identity behavior for negative inputs', () => {
            // The provided implementation returns n when n <= 1.
            // For 0 and 1 it returns the expected Fibonacci base cases.
            // For negative numbers, because of the check (n <= 1) it will return the input directly.
            expect(fibonacci(0)).toBe(0);
            expect(fibonacci(1)).toBe(1);
            expect(fibonacci(-5)).toBe(-5); // preserves input per implementation
            expect(fibonacci(-1)).toBe(-1);
        });

        test('should handle null and numeric-like inputs (coercion and floats)', () => {
            // null is coerced: null <= 1 is true, so the function returns null as implemented.
            expect(fibonacci(null)).toBeNull();

            // Numeric strings are coerced through arithmetic operations, e.g. '6' -> 6
            // fibonacci('6') should behave like fibonacci(6) => 8
            expect(fibonacci('6')).toBe(8);

            // Floating point inputs: the implementation will recurse using arithmetic
            // For example:
            // fibonacci(1.5) -> fibonacci(0.5) + fibonacci(-0.5)
            // fibonacci(0.5) returns 0.5 (because 0.5 <= 1), fibonacci(-0.5) returns -0.5
            // so fibonacci(1.5) === 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);
        });

        test('should not use NaN/undefined (avoid calling with them in tests)', () => {
            // The implementation does not guard against NaN/undefined and passing them
            // would lead to infinite recursion (or max call stack). We assert that the
            // function behaves deterministically for valid inputs and avoid invoking it
            // with NaN/undefined here. This test exists to document that such inputs are not safe.
            expect(true).toBe(true);
        });
    });

    // Large Scale Test Cases
    describe('Performance tests', () => {
        test('should compute moderately large Fibonacci numbers correctly and reasonably fast', () => {
            // The provided implementation is a naive recursive algorithm (exponential time).
            // Choose a moderate input that verifies correctness and is unlikely to time out.
            // fibonacci(30) is 832040
            const start = Date.now();
            const result = fibonacci(30);
            const durationMs = Date.now() - start;

            expect(result).toBe(832040);

            // Expect it to complete within a reasonable time window for typical test environments.
            // We allow up to 2000ms to accommodate slower CI runners, but this is intentionally generous.
            // This is a performance assertion to ensure the implementation is usable for moderate inputs.
            expect(durationMs).toBeLessThanOrEqual(2000);
        });

        test('sequence property holds for the first 20 Fibonacci numbers when computed by the function', () => {
            // Verify the defining property fib(n) = fib(n-1) + fib(n-2) for 2 <= n <= 20.
            // Use the function under test to compute each term.
            const computed = [];
            for (let i = 0; i <= 20; i++) {
                // Keep calls bounded (<= 1000 iterations requirement is satisfied)
                computed[i] = fibonacci(i);
                // Basic sanity: results should be numbers for these integer inputs
                expect(typeof computed[i]).toBe('number');
            }

            for (let n = 2; n <= 20; n++) {
                expect(computed[n]).toBe(computed[n - 1] + computed[n - 2]);
            }
        });
    });
});

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