From f6ed3cbd6dfd3456a93ad80c42d7e18948b197ef Mon Sep 17 00:00:00 2001
From: Haileyesus <something@gmail.com>
Date: Thu, 12 Feb 2026 20:53:34 +0300
Subject: [PATCH] refactor(calculateDiff): optimize LCS algorithm for improved
 diff calculation

---
 .../chat/utils/messageTransforms.ts           | 47 +++++++++++++------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/src/components/chat/utils/messageTransforms.ts b/src/components/chat/utils/messageTransforms.ts
index 2aee588..7093081 100644
--- a/src/components/chat/utils/messageTransforms.ts
+++ b/src/components/chat/utils/messageTransforms.ts
@@ -29,35 +29,54 @@ export const calculateDiff = (oldStr: string, newStr: string): DiffLine[] => {
   const oldLines = oldStr.split('\n');
   const newLines = newStr.split('\n');
 
+  // Use LCS alignment so insertions/deletions don't cascade into a full-file "changed" diff.
+  const lcsTable: number[][] = Array.from({ length: oldLines.length + 1 }, () =>
+    new Array<number>(newLines.length + 1).fill(0),
+  );
+  for (let oldIndex = oldLines.length - 1; oldIndex >= 0; oldIndex -= 1) {
+    for (let newIndex = newLines.length - 1; newIndex >= 0; newIndex -= 1) {
+      if (oldLines[oldIndex] === newLines[newIndex]) {
+        lcsTable[oldIndex][newIndex] = lcsTable[oldIndex + 1][newIndex + 1] + 1;
+      } else {
+        lcsTable[oldIndex][newIndex] = Math.max(
+          lcsTable[oldIndex + 1][newIndex],
+          lcsTable[oldIndex][newIndex + 1],
+        );
+      }
+    }
+  }
+
   const diffLines: DiffLine[] = [];
   let oldIndex = 0;
   let newIndex = 0;
 
-  while (oldIndex < oldLines.length || newIndex < newLines.length) {
+  while (oldIndex < oldLines.length && newIndex < newLines.length) {
     const oldLine = oldLines[oldIndex];
     const newLine = newLines[newIndex];
 
-    if (oldIndex >= oldLines.length) {
-      diffLines.push({ type: 'added', content: newLine, lineNum: newIndex + 1 });
-      newIndex += 1;
-      continue;
-    }
-
-    if (newIndex >= newLines.length) {
-      diffLines.push({ type: 'removed', content: oldLine, lineNum: oldIndex + 1 });
-      oldIndex += 1;
-      continue;
-    }
-
     if (oldLine === newLine) {
       oldIndex += 1;
       newIndex += 1;
       continue;
     }
 
-    diffLines.push({ type: 'removed', content: oldLine, lineNum: oldIndex + 1 });
+    if (lcsTable[oldIndex + 1][newIndex] >= lcsTable[oldIndex][newIndex + 1]) {
+      diffLines.push({ type: 'removed', content: oldLine, lineNum: oldIndex + 1 });
+      oldIndex += 1;
+      continue;
+    }
+
     diffLines.push({ type: 'added', content: newLine, lineNum: newIndex + 1 });
+    newIndex += 1;
+  }
+
+  while (oldIndex < oldLines.length) {
+    diffLines.push({ type: 'removed', content: oldLines[oldIndex], lineNum: oldIndex + 1 });
     oldIndex += 1;
+  }
+
+  while (newIndex < newLines.length) {
+    diffLines.push({ type: 'added', content: newLines[newIndex], lineNum: newIndex + 1 });
     newIndex += 1;
   }