⚡ Bolt: Optimize lcsLength with Uint16Array

.jules/bolt.md

@@ -1,3 +1,7 @@
 ## 2024-07-01 - Optimizing React rendering with useMemo
 **Learning:** Found an O(n) array sorting operation and deep recursive countFiles inside the body of a React Function Component (`TreeNode`) which is called many times for deeply nested trees. React components that are pure should memoize their expensive operations instead of doing them synchronously during each render pass.
 **Action:** Always wrap `children` sorting and recursive tree traversal computations in `React.useMemo` to prevent deep performance degradation during re-renders.
+
+## 2024-05-19 - Fast LCS Calculation in JavaScript
+**Learning:** For dynamic programming algorithms like calculating Longest Common Subsequence (LCS) that allocate large multi-dimensional matrices, using standard arrays via `new Array(n).fill(0)` and `Math.max` calls per cell creates significant overhead. Using `Uint16Array` for flat integer sequences avoids hidden v8 array optimization deopts and reduces memory allocations. Furthermore, in tight nested loops over strings, `String.prototype.charCodeAt(i)` caches far better and avoids single-character string allocations compared to `str[i] === str2[j]`. Finally, avoiding `Math.max` using inline conditionals `a > b ? a : b` significantly reduces overhead.
+**Action:** Always favor typed arrays (`Uint16Array`, `Uint8Array`, etc.) over `Array.prototype.fill(0)` when running matrix-based DP in hot loops, and inline min/max evaluations. For string parsing, use `.charCodeAt()` over char extraction where equality is being checked.

src/lib/parser.js

@@ -761,26 +761,24 @@ const Parser={
     // Longest common subsequence length (optimized for similarity)
     lcsLength:function(s1,s2){
         // Use simplified approach for performance
-        if(s1.length>500||s2.length>500){
-            // For long strings, use sampling
-            s1=s1.substring(0,500);
-            s2=s2.substring(0,500);
-        }
+        if(s1.length>500) s1=s1.substring(0,500);
+        if(s2.length>500) s2=s2.substring(0,500);
 
         var m=s1.length,n=s2.length;
-        var prev=new Array(n+1).fill(0);
-        var curr=new Array(n+1).fill(0);
+        var prev=new Uint16Array(n+1);
+        var curr=new Uint16Array(n+1);
 
         for(var i=1;i<=m;i++){
+            var c1=s1.charCodeAt(i-1);
             for(var j=1;j<=n;j++){
-                if(s1[i-1]===s2[j-1]){
+                if(c1===s2.charCodeAt(j-1)){
                     curr[j]=prev[j-1]+1;
                 }else{
-                    curr[j]=Math.max(prev[j],curr[j-1]);
+                    var p=prev[j],c=curr[j-1];
+                    curr[j]=p>c?p:c;
                 }
             }
             var tmp=prev;prev=curr;curr=tmp;
-            curr.fill(0);
         }
         return prev[n];
     },