AIS-5214: Clamp ending overlap in add/remove fold

src/helpers.ts

@@ -228,21 +228,23 @@ export const concatChanges = (baseChanges: Change[], changesToAdd: Change[]) =>
 
       // Search for overlap between the two changes for simplification
       if (isAdd(prevChange) && isRemove(change)) {
+        const minLength = Math.min(getChangeLength(prevChange), getChangeLength(change))
         const beginningOverlap = getBeginningOverlap(getChangeText(prevChange), getChangeText(change))
-        const endingOverlap = getEndingOverlap(getChangeText(prevChange), getChangeText(change))
+        // Clamp so a repeated boundary char (' ' vs '  ') isn't counted as both beginning and ending overlap
+        const endingOverlap = Math.min(
+          getEndingOverlap(getChangeText(prevChange), getChangeText(change)),
+          minLength - beginningOverlap
+        )
 
         if (beginningOverlap > 0 || endingOverlap > 0) {
-          const minLength = Math.min(getChangeLength(prevChange), getChangeLength(change))
           const getNonOverlap = (text: string) =>
             text.slice(beginningOverlap, endingOverlap > 0 ? -endingOverlap : Infinity)
           const newChanges = [
             getChangeText(change).slice(0, beginningOverlap),
             replaceChangeText(prevChange, getNonOverlap(getChangeText(prevChange))),
             replaceChangeText(change, getNonOverlap(getChangeText(change))),
           ]
-          if (endingOverlap > 0 && minLength !== beginningOverlap) {
-            newChanges.push(getChangeText(change).slice(-Math.min(endingOverlap, minLength - beginningOverlap)))
-          }
+          if (endingOverlap > 0) newChanges.push(getChangeText(change).slice(-endingOverlap))
 
           changes.splice(index - 1, 2, ...newChanges.filter(getChangeLength))
           index--

tests/index.test.ts

@@ -1,4 +1,5 @@
-import { createStringTracker, StringOp, StringTracker } from '../src'
+import { Change, createStringTracker, StringOp, StringTracker } from '../src'
+import { assertValidTracker } from './helpers'
 
 const str1 = 'this is my word'
 
@@ -180,6 +181,24 @@ it('should remove subsequent adds/strings with a remove in between in changes ar
   expect(tracker.getChanges()).toStrictEqual(['asd', [1, 'a '], 'pog'])
 })
 
+it.each<[string, () => StringTracker, Change[]]>([
+  [
+    'add shorter than remove',
+    () => createStringTracker('  0').remove(2, 3).add(2, 'x  ').remove(0, 4),
+    [' ', [StringOp.Remove, ' 0']],
+  ],
+  ['add longer than remove', () => createStringTracker('aa').add(0, 'aaa').remove(3, 5), ['aa', [StringOp.Add, 'a']]],
+  [
+    'overlaps cross mid-text',
+    () => createStringTracker('aab').add(0, 'ab').remove(2, 5),
+    ['a', [StringOp.Remove, 'a'], 'b'],
+  ],
+])('should not drop repeated chars when folding an overlapping add and remove (%s)', (_, build, changes) => {
+  const tracker = build()
+  expect(tracker.getChanges()).toStrictEqual(changes)
+  assertValidTracker(tracker)
+})
+
 // getIndexOnOriginal
 it('getIndexOnOriginal should throw a RangeError when called with a negative number', () => {
   expect(() => createStringTracker('asd').getIndexOnOriginal(-1))