Make equal-segment cursor recovery robust

Author: iSapozhnik

Sources/TextDiff/AppKit/DiffRevertActionResolver.swift

@@ -57,12 +57,20 @@ enum DiffRevertActionResolver {
             case .equal:
                 originalRange = NSRange(location: originalCursor, length: textLength)
                 updatedRange = NSRange(location: updatedCursor, length: textLength)
-                if textMatches(segment.text, source: originalNSString, at: originalCursor) {
-                    originalCursor += textLength
-                }
-                if textMatches(segment.text, source: updatedNSString, at: updatedCursor) {
-                    updatedCursor += textLength
-                }
+                let originalMatches = textMatches(segment.text, source: originalNSString, at: originalCursor)
+                let updatedMatches = textMatches(segment.text, source: updatedNSString, at: updatedCursor)
+                #if !TESTING
+                assert(
+                    originalMatches,
+                    "Equal segment text mismatch in original at \(originalCursor) for segment \(index): \(segment.text)"
+                )
+                assert(
+                    updatedMatches,
+                    "Equal segment text mismatch in updated at \(updatedCursor) for segment \(index): \(segment.text)"
+                )
+                #endif
+                originalCursor += textLength
+                updatedCursor += textLength
             case .delete:
                 originalRange = NSRange(location: originalCursor, length: textLength)
                 updatedRange = NSRange(location: updatedCursor, length: 0)

Tests/TextDiffTests/DiffRevertActionResolverTests.swift

@@ -25,6 +25,29 @@ func candidatesBuildPairedReplacementForAdjacentDeleteInsert() throws {
     #expect(action.resultingUpdated == "old")
 }
 
+@Test
+func indexedSegmentsAdvancePastEqualSegmentsEvenWhenTextValidationFails() {
+    let segments = [
+        DiffSegment(kind: .equal, tokenKind: .word, text: "abc"),
+        DiffSegment(kind: .delete, tokenKind: .word, text: "X"),
+        DiffSegment(kind: .insert, tokenKind: .word, text: "Y")
+    ]
+
+    let indexed = DiffRevertActionResolver.indexedSegments(
+        from: segments,
+        original: "zzzX",
+        updated: "zzzY"
+    )
+
+    #expect(indexed.count == 3)
+    #expect(indexed[0].originalRange == NSRange(location: 0, length: 3))
+    #expect(indexed[0].updatedRange == NSRange(location: 0, length: 3))
+    #expect(indexed[1].originalRange == NSRange(location: 3, length: 1))
+    #expect(indexed[1].updatedRange == NSRange(location: 3, length: 0))
+    #expect(indexed[2].originalRange == NSRange(location: 4, length: 0))
+    #expect(indexed[2].updatedRange == NSRange(location: 3, length: 1))
+}
+
 @Test
 func candidatesDoNotPairWhenAnySegmentExistsBetweenDeleteAndInsert() {
     let segments = [