Add property test for textChangeToEdits and applyTextEdits. (#9131)

Add a test for some functionality introduced in #9055.

app/gui2/shared/util/data/__tests__/text.test.ts

@@ -1,5 +1,34 @@
-import { expect, test } from 'vitest'
-import { applyTextEditsToSpans, textChangeToEdits, trimEnd } from '../text'
+import { fc, test } from '@fast-check/vitest'
+import { expect } from 'vitest'
+import { applyTextEdits, applyTextEditsToSpans, textChangeToEdits, trimEnd } from '../text'
+
+test.prop({
+  before: fc.array(fc.boolean(), { minLength: 32, maxLength: 64 }),
+  after: fc.array(fc.boolean(), { minLength: 32, maxLength: 64 }),
+})('textChangeToEdits / applyTextEdits round-trip', ({ before, after }) => {
+  // Generate strings composed of a mix of only two characters so that `textChangeToEdits` will find a variety of
+  // similarities between the inputs.
+  const stringFromBools = (bools: Array<boolean>) =>
+    bools.map((bool) => (bool ? 't' : 'f')).join('')
+  const beforeString = stringFromBools(before)
+  const afterString = stringFromBools(after)
+  const edits = textChangeToEdits(beforeString, afterString)
+  expect(applyTextEdits(beforeString, edits)).toBe(afterString)
+})
+
+/** Test that `textChangeToEdits` and `applyTextEdits` work when inputs contain any special characters representable by
+ *  a `string`, including newlines and even incomplete surrogate pairs (invalid Unicode).
+ */
+test.prop({
+  before: fc.array(fc.string16bits(), { maxLength: 8 }),
+})('textChangeToEdits / applyTextEdits round-trip: Special characters', ({ before }) => {
+  const beforeString = before.join('\n')
+  // Produce the after-string by rearranging the lines of the before-string, so that the edit-relationship between them
+  // is non-trivial.
+  const afterString = before.sort().join('\n')
+  const edits = textChangeToEdits(beforeString, afterString)
+  expect(applyTextEdits(beforeString, edits)).toBe(afterString)
+})
 
 /** Tests that:
  *  - When the code in `a[0]` is edited to become the code in `b[0]`,