From 321b82c64301817c20cafb9bbc6e73216f3602bc Mon Sep 17 00:00:00 2001
From: "codeflash-ai[bot]"
 <148906541+codeflash-ai[bot]@users.noreply.github.com>
Date: Fri, 23 Jan 2026 15:53:49 +0000
Subject: [PATCH] Optimize fibonacci
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The optimization transforms the Fibonacci implementation from **exponential recursive** to **linear iterative**, achieving a **408% speedup** (5x faster).

**What Changed:**
- Replaced recursive calls with a simple loop that iterates from 2 to n
- Uses two variables (`prev`, `curr`) to track the last two Fibonacci numbers
- Eliminates all function call overhead

**Why It's Faster:**
The original recursive approach has **O(2^n) time complexity** because it recalculates the same values exponentially many times. For example, `fibonacci(5)` calls `fibonacci(3)` twice, `fibonacci(2)` three times, etc. The optimized version computes each Fibonacci number exactly once in a forward pass, achieving **O(n) time complexity**.

The speedup is dramatic for larger inputs:
- For n=10: minimal improvement (already fast)
- For n=20-30: **orders of magnitude faster** (as shown in annotated tests where n=30 completes instantly vs. taking noticeable time)
- For n=35+: the recursive version becomes impractically slow while the iterative version remains near-instant

**Impact on Workloads:**
Based on the test suite, this optimization particularly benefits:
- **Moderate-to-large inputs (n ≥ 20)**: Tests show n=30, n=35 now complete instantly instead of taking seconds
- **Repeated calculations**: The batch tests with multiple sequential calls benefit from consistent fast performance
- **No stack overflow risk**: The iterative approach eliminates the "maximum call stack" errors that occur with very large n (e.g., n=10000 test)

**Trade-offs:**
The optimization changes behavior for edge cases involving non-integer inputs (fractional numbers, null, undefined) since the loop-based approach doesn't naturally handle these. However, for the primary use case (non-negative integers), correctness is preserved and performance is vastly superior.
---
 code_to_optimize_js_esm/fibonacci.js | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/code_to_optimize_js_esm/fibonacci.js b/code_to_optimize_js_esm/fibonacci.js
index 0ee526315..1a299d4b4 100644
--- a/code_to_optimize_js_esm/fibonacci.js
+++ b/code_to_optimize_js_esm/fibonacci.js
@@ -13,7 +13,17 @@ export function fibonacci(n) {
     if (n <= 1) {
         return n;
     }
-    return fibonacci(n - 1) + fibonacci(n - 2);
+    
+    let prev = 0;
+    let curr = 1;
+    
+    for (let i = 2; i <= n; i++) {
+        const next = prev + curr;
+        prev = curr;
+        curr = next;
+    }
+    
+    return curr;
 }
 
 /**