From fb82cfdf71a2839d3a1d463784e32dc5a87b5d8f Mon Sep 17 00:00:00 2001
From: "codeflash-ai[bot]"
 <148906541+codeflash-ai[bot]@users.noreply.github.com>
Date: Fri, 23 Jan 2026 16:11:01 +0000
Subject: [PATCH] Optimize fibonacci
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The optimized code replaces the exponential-time recursive algorithm with a linear-time iterative approach, achieving a **111% speedup** (from 45.5μs to 21.5μs).

**Key optimization:**
- **Original:** Naive recursion with O(2^n) time complexity. Each call spawns two more recursive calls, creating an exponential tree of redundant calculations. For example, `fibonacci(5)` recalculates `fibonacci(3)` twice and `fibonacci(2)` three times.
- **Optimized:** Iterative bottom-up computation with O(n) time complexity and O(1) space. Uses two variables (`prev`, `curr`) to track consecutive Fibonacci numbers, computing each value exactly once in a simple loop.

**Why this is faster:**
The recursive approach makes approximately 2^n function calls for input n, each with call stack overhead. The iterative version performs exactly n-1 loop iterations with minimal overhead—no function calls, no stack frames, just basic arithmetic operations. This eliminates both the exponential branching factor and the per-call overhead.

**Test performance characteristics:**
- Small inputs (n ≤ 10): Both implementations are fast, but the 2x speedup is already visible
- Moderate inputs (n = 15-25): The optimization becomes critical—naive recursion slows dramatically while the iterative version remains nearly instant
- The annotated tests confirm correctness is preserved across all cases, including edge cases (negative numbers, n ≤ 1) and sequential values

**Impact:**
This optimization is particularly valuable if `fibonacci()` is called with values above n=20 or invoked frequently in loops/hot paths, where the exponential cost of recursion would compound significantly.
---
 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;
 }
 
 /**