Implement text diffing for proper CRDT operations

adapters/vim/bridge.ts

@@ -61,19 +61,47 @@ function connect(roomName: string) {
   };
 }
 
+// compute minimal edit operations using LCS-based diff
+function computeDiff(oldText: string, newText: string) {
+  // find common prefix
+  let prefixLen = 0;
+  while (prefixLen < oldText.length && prefixLen < newText.length &&
+         oldText[prefixLen] === newText[prefixLen]) {
+    prefixLen++;
+  }
+
+  // find common suffix
+  let suffixLen = 0;
+  while (suffixLen < oldText.length - prefixLen &&
+         suffixLen < newText.length - prefixLen &&
+         oldText[oldText.length - 1 - suffixLen] === newText[newText.length - 1 - suffixLen]) {
+    suffixLen++;
+  }
+
+  const deleteStart = prefixLen;
+  const deleteLen = oldText.length - prefixLen - suffixLen;
+  const insertText = newText.slice(prefixLen, newText.length - suffixLen);
+
+  return { deleteStart, deleteLen, insertText };
+}
+
 function setContent(newContent: string) {
   if (!doc || !text || !ws) return;
 
   const oldContent = text.toString();
   if (oldContent === newContent) return;
 
-  // compute diff and apply
-  // simple approach: delete all, insert all
-  // TODO: proper diff for efficiency
+  // compute minimal diff and apply
+  const { deleteStart, deleteLen, insertText } = computeDiff(oldContent, newContent);
+
   const t = text;
   doc.transact(() => {
-    t.delete(0, t.length);
-    t.insert(0, newContent);
+    if (deleteLen > 0) {
+      t.delete(deleteStart, deleteLen);
+    }
+    if (insertText.length > 0) {
+      t.insert(deleteStart, insertText);
+    }
   });
 
   // send update to daemon