[mendoza] Smarter rebasing of strings

packages/mendoza/src/incremental-patcher.ts

@@ -1,3 +1,5 @@
+/* eslint-disable max-statements */
+/* eslint-disable yoda */
 /* eslint-disable default-case */
 /* eslint-disable class-methods-use-this */
 /* eslint-disable no-else-return */
@@ -6,7 +8,7 @@
 import {ObjectModel} from './object-model'
 import {RawPatch} from './patch'
 import {Patcher} from './internal-patcher'
-import {utf8charSize, utf8stringSize} from './utf8'
+import {utf8charSize, utf8stringSize, commonPrefix, commonSuffix} from './utf8'
 
 // The incremental patcher allows you to apply multiple patches and tracks the history of every element.
 // It also allows you to extract a simple diff between the documents.
@@ -410,11 +412,35 @@ export function rebaseValue<T>(left: Value<T>, right: Value<T>): Value<T> {
     }
     case 'null':
     case 'boolean':
-    case 'number':
-    case 'string': {
+    case 'number': {
       if (unwrap(left) === unwrap(right)) return left
       break
     }
+    case 'string': {
+      let leftRaw = unwrap(left) as string
+      let rightRaw = unwrap(right) as string
+      if (leftRaw === rightRaw) return left
+
+      let result = rightModel.copyString(null)
+      let prefix = commonPrefix(leftRaw, rightRaw)
+      let suffix = commonSuffix(leftRaw, rightRaw)
+
+      let rightLen = utf8stringSize(rightRaw)
+      let leftLen = utf8stringSize(leftRaw)
+
+      if (0 < prefix) {
+        rightModel.stringAppendSlice(result, left, 0, prefix)
+      }
+      if (prefix < rightLen - suffix) {
+        rightModel.stringAppendSlice(result, right, prefix, rightLen - suffix)
+      }
+      if (leftLen - suffix < leftLen) {
+        rightModel.stringAppendSlice(result, left, leftLen - suffix, leftLen)
+      }
+      let value = rightModel.finalize(result)
+      if (unwrap(value) !== rightRaw) throw new Error('incorrect string rebase')
+      return value
+    }
   }
 
   return right

packages/mendoza/src/utf8.ts

@@ -35,3 +35,31 @@ export function utf8resolveIndex(str: string, idx: number, start = 0) {
 
   return ucsIdx
 }
+
+export function commonPrefix(str: string, str2: string) {
+  let len = Math.min(str.length, str2.length)
+  let b = 0
+  for (let i = 0; i < len; ) {
+    let aPoint = str.codePointAt(i)!
+    let bPoint = str2.codePointAt(i)!
+    if (aPoint !== bPoint) return b
+    let size = utf8charSize(aPoint)
+    b += size
+    i += size === 4 ? 2 : 1
+  }
+  return b
+}
+
+export function commonSuffix(str: string, str2: string) {
+  let len = Math.min(str.length, str2.length)
+  let b = 0
+  for (let i = 0; i < len; ) {
+    let aPoint = str.codePointAt(str.length - 1 - i)!
+    let bPoint = str2.codePointAt(str2.length - 1 - i)!
+    if (aPoint !== bPoint) return b
+    let size = utf8charSize(aPoint)
+    b += size
+    i += size === 4 ? 2 : 1
+  }
+  return b
+}