Update patch for fixing (null) bug

Older versions of the Objective C library would occasionally create
patches from invalid separations of the surrogate pairs.

It would start by finding the longest common prefix which split a
surrogate pair in half when inserting in between existing surrogates
sharing the same high surrogate:

`\ud83c\udd70\ud83c\udd71` -> `\ud83c\udd70\ud83c\udd72\ud83c\udd71`

`\ud83c\udd70\ud83c` + `\udd72\ud83c` + `\udd71`

Next it would try to create diff groups from these and fail because of
the unpaired surrogates. The middle group is entirely wrong and gets
replaced with `(null)` while the other surrogate halves stick around _as
counts_ since that's how `toDelta()` converts them.

`=3\t+(null)\t=1`

When the libraries receive this patch they end up reconstructing an
invalid Unicode sequence because of the numbers which now instruct it to
split those same surrogate pairs.

---

In this patch we're identifying this specific sequence in `fromDelta()`
and removing the additional breakage by eliminating the `(null)` group
and re-joining the split surrogate halves. You can see that this will
effectively undo the change operation because that `(null)` is where the
new character was inserted. _There is no way to avoid this_ as the
problem occurred in the past and we have lost information to `(null)`.

In this patch we are merely removing the vestige of a failure that would
lead to additional failures if we passed on the data.

java/src/name/fraser/neil/plaintext/diff_match_patch.java

@@ -1576,16 +1576,7 @@ private String decodeURI(String text) throws IllegalArgumentException {
       throw new IllegalArgumentException();
     }
 
-    // some objective-c versions of the library produced patches with
-    // (null) in the place where surrogates were split across diff
-    // boundaries. if we leave those in we'll be stuck with a
-    // high-surrogate (null) low-surrogate pattern that will break
-    // deeper in the library or consuming application. we'll "fix"
-    // these by dropping the (null) and re-joining the surrogate halves
-    return decoded.toString().replaceAll(
-      "([\\uD800-\\uDBFF])\\(null\\)([\\uDC00-\\uDFFF])",
-      "$1$2"
-    );
+    return decoded.toString();
   }
 
   /**
@@ -1601,7 +1592,8 @@ public LinkedList<Diff> diff_fromDelta(String text1, String delta)
     LinkedList<Diff> diffs = new LinkedList<Diff>();
     int pointer = 0;  // Cursor in text1
     String[] tokens = delta.split("\t");
-    for (String token : tokens) {
+    for (int x = 0; x < tokens.length; x++) {
+      String token = tokens[x];
       if (token.length() == 0) {
         // Blank tokens are ok (from a trailing \t).
         continue;
@@ -1637,6 +1629,31 @@ public LinkedList<Diff> diff_fromDelta(String text1, String delta)
               "Negative number in diff_fromDelta: " + param);
         }
         String text;
+
+        // some objective-c versions of the library produced patches with
+        // (null) in the place where surrogates were split across diff
+        // boundaries. if we leave those in we'll be stuck with a
+        // high-surrogate (null) low-surrogate pattern that will break
+        // deeper in the library or consuming application. we'll "fix"
+        // these by dropping the (null) and re-joining the surrogate halves
+        if (
+          x + 2 < tokens.length &&
+          Character.isHighSurrogate(text1.charAt(pointer + n - 1)) &&
+          tokens[x + 1].substring(1).equals("(null)") &&
+          Character.isLowSurrogate(text1.charAt(pointer + n))
+        ) {
+          n -= 1;
+          tokens[x + 1] = "+";
+          int m;
+          try {
+            m = Integer.parseInt(tokens[x + 2].substring(1));
+          } catch (NumberFormatException e) {
+            throw new IllegalArgumentException(
+              "Invalid number in diff_fromDelta: " + tokens[x + 2].substring(1), e);
+          }
+          tokens[x + 2] = tokens[x + 2].charAt(0) + String.valueOf(m + 1);
+        }
+
         try {
           text = text1.substring(pointer, pointer += n);
         } catch (StringIndexOutOfBoundsException e) {

java/tests/name/fraser/neil/plaintext/diff_match_patch_test.java

@@ -376,6 +376,12 @@ public static void testDiffText() {
     LinkedList<Diff> diffs = diffList(new Diff(EQUAL, "jump"), new Diff(DELETE, "s"), new Diff(INSERT, "ed"), new Diff(EQUAL, " over "), new Diff(DELETE, "the"), new Diff(INSERT, "a"), new Diff(EQUAL, " lazy"));
     assertEquals("diff_text1:", "jumps over the lazy", dmp.diff_text1(diffs));
     assertEquals("diff_text2:", "jumped over a lazy", dmp.diff_text2(diffs));
+
+    assertEquals(
+      "diff_text2: Objective-C (null) bug",
+      "🙂🙁",
+      dmp.diff_text2(dmp.diff_fromDelta("🙂🙁", "=3\t+(null)\t=1"))
+    );
   }
 
   public static void testDiffDelta() {
@@ -460,12 +466,6 @@ public static void testDiffDelta() {
       dmp.diff_toDelta(dmp.diff_fromDelta("\ud83c\udd70", "=1\t-1\t+%ED%B5%B1"))
     );
 
-    assertEquals(
-      "diff_fromDelta: Invalid diff from objective-c with (null) string",
-      diffList(new Diff(INSERT, "\ud83c\udd70")),
-      dmp.diff_fromDelta("", "+%ED%A0%BC%28null%29%ED%B5%B0")
-    );
-
     // Verify pool of unchanged characters.
     diffs = diffList(new Diff(INSERT, "A-Z a-z 0-9 - _ . ! ~ * ' ( ) ; / ? : @ & = + $ , # "));
     String text2 = dmp.diff_text2(diffs);

javascript/diff_match_patch_uncompressed.js

@@ -1512,13 +1512,7 @@ diff_match_patch.prototype.decodeURI = function(text) {
       throw new URIError('URI malformed');
     }
 
-    // some objective-c versions of the library produced patches with
-    // (null) in the place where surrogates were split across diff
-    // boundaries. if we leave those in we'll be stuck with a
-    // high-surrogate (null) low-surrogate pattern that will break
-    // deeper in the library or consuming application. we'll "fix"
-    // these by dropping the (null) and re-joining the surrogate halves
-    return decoded.replace(/([\uD800-\uDBFF])\(null\)([\uDC00-\uDFFF])/g, "$1$2");
+    return decoded;
   }
 };
 
@@ -1556,6 +1550,28 @@ diff_match_patch.prototype.diff_fromDelta = function(text1, delta) {
         if (isNaN(n) || n < 0) {
           throw new Error('Invalid number in diff_fromDelta: ' + param);
         }
+
+        // some objective-c versions of the library produced patches with
+        // (null) in the place where surrogates were split across diff
+        // boundaries. if we leave those in we'll be stuck with a
+        // high-surrogate (null) low-surrogate pattern that will break
+        // deeper in the library or consuming application. we'll "fix"
+        // these by dropping the (null) and re-joining the surrogate halves
+        if (
+          x + 2 < tokens.length &&
+          this.isHighSurrogate(text1[pointer + n - 1]) &&
+          '(null)' === tokens[x + 1].substring(1) &&
+          this.isLowSurrogate(text1[pointer + n])
+        ) {
+          n -= 1;
+          tokens[x + 1] = "+";
+          var m = parseInt(tokens[x + 2].substring(1),10);
+          if (isNaN(m) || m < 0) {
+            throw new Error('Invalid number in diff_fromDelta: ' + tokens[x + 2].substring(1));
+          }
+          tokens[x + 2] = tokens[x + 2][0] + (m + 1).toString();
+        }
+
         var text = text1.substring(pointer, pointer += n);
         if (tokens[x].charAt(0) == '=') {
           diffs[diffsLength++] = new diff_match_patch.Diff(DIFF_EQUAL, text);

javascript/tests/diff_match_patch_test.js

@@ -443,6 +443,12 @@ function testDiffText() {
   assertEquals('jumps over the lazy', dmp.diff_text1(diffs));
 
   assertEquals('jumped over a lazy', dmp.diff_text2(diffs));
+
+  // diff_text2: Objective-C (null) bug
+  assertEquals(
+    '\ud83d\ude42\ud83d\ude41',
+    dmp.diff_text2(dmp.diff_fromDelta('\ud83d\ude42\ud83d\ude41', '=3\t+(null)\t=1'))
+  );
 }
 
 function testDiffDelta() {
@@ -605,15 +611,6 @@ function testDiffDelta() {
     assertEquals('Swap surrogate pair', 'crashed');
   }
 
-  try {
-    assertEquivalent(
-      dmp.diff_fromDelta('', '+%ED%A0%BC%28null%29%ED%B5%B0'),
-      [[DIFF_INSERT, '\ud83c\udd70']]
-    );
-  } catch ( e ) {
-    assertEquals('Invalid diff from objective-c with (null) string' );
-  }
-
   // Empty diff groups
   assertEquivalent(
     dmp.diff_toDelta([[DIFF_EQUAL, 'abcdef'], [DIFF_DELETE, ''], [DIFF_INSERT, 'ghijk']]),

objectivec/DiffMatchPatch.m

@@ -1493,23 +1493,7 @@ - (NSString *)diff_decodeURIWithText:(NSString *)percentEncoded
         return nil;
     }
 
-    // some objective-c versions of the library produced patches with
-    // (null) in the place where surrogates were split across diff
-    // boundaries. if we leave those in we'll be stuck with a
-    // high-surrogate (null) low-surrogate pattern that will break
-    // deeper in the library or consuming application. we'll "fix"
-    // these by dropping the (null) and re-joining the surrogate halves
-    NSString *result = [NSString stringWithCharacters:decoded length:output];
-    NSRegularExpression *replacer = [NSRegularExpression
-                                     regularExpressionWithPattern:@"([\\x{D800}-\\x{DBFF}])\\(null\\)([\\x{DC00}-\\x{DFFF}])"
-                                     options:0
-                                     error:nil];
-
-    return [replacer
-            stringByReplacingMatchesInString:result
-            options:0
-            range:NSMakeRange(0, [result length])
-            withTemplate:@"$1$2"];
+    return [NSString stringWithCharacters:decoded length:output];
 }
 
 /**
@@ -1526,10 +1510,12 @@ - (NSMutableArray *)diff_fromDeltaWithText:(NSString *)text1
 {
   NSMutableArray *diffs = [NSMutableArray array];
   NSUInteger thisPointer = 0;  // Cursor in text1
-  NSArray *tokens = [delta componentsSeparatedByString:@"\t"];
+  NSMutableArray *tokens = [NSMutableArray arrayWithArray:([delta componentsSeparatedByString:@"\t"])];
   NSInteger n;
   NSDictionary *errorDetail = nil;
-  for (NSString *token in tokens) {
+  NSInteger tokenCount = [tokens count];
+  for (NSInteger x = 0; x < tokenCount; x++) {
+    NSString *token = tokens[x];
     if (token.length == 0) {
       // Blank tokens are ok (from a trailing \t).
       continue;
@@ -1572,6 +1558,44 @@ - (NSMutableArray *)diff_fromDeltaWithText:(NSString *)text1
           }
           return nil;
         }
+
+        // some objective-c versions of the library produced patches with
+        // (null) in the place where surrogates were split across diff
+        // boundaries. if we leave those in we'll be stuck with a
+        // high-surrogate (null) low-surrogate pattern that will break
+        // deeper in the library or consuming application. we'll "fix"
+        // these by dropping the (null) and re-joining the surrogate halves
+        if (x + 2 < tokenCount &&
+            CFStringIsSurrogateHighCharacter([text1 characterAtIndex:(thisPointer + n - 1)]) &&
+            [@"(null)" isEqualToString:([tokens[x + 1] substringFromIndex:1])] &&
+            CFStringIsSurrogateLowCharacter([text1 characterAtIndex:(thisPointer + n)])
+            ) {
+          n -= 1;
+          tokens[x + 1] = @"+";
+
+          NSInteger m = [[tokens[x + 2] substringFromIndex:1] integerValue];
+
+          if (m == 0) {
+            if (error != NULL) {
+              errorDetail = [NSDictionary dictionaryWithObjectsAndKeys:
+                  [NSString stringWithFormat:NSLocalizedString(@"Invalid number in diff_fromDelta: %@", @"Error"), param],
+                  NSLocalizedDescriptionKey, nil];
+              *error = [NSError errorWithDomain:@"DiffMatchPatchErrorDomain" code:100 userInfo:errorDetail];
+            }
+            return nil;
+          } else if (m < 0) {
+            if (error != NULL) {
+              errorDetail = [NSDictionary dictionaryWithObjectsAndKeys:
+                  [NSString stringWithFormat:NSLocalizedString(@"Negative number in diff_fromDelta: %@", @"Error"), param],
+                  NSLocalizedDescriptionKey, nil];
+              *error = [NSError errorWithDomain:@"DiffMatchPatchErrorDomain" code:101 userInfo:errorDetail];
+            }
+            return nil;
+          }
+
+          tokens[x + 2] = [NSString stringWithFormat:@"=%ld", m + 1];
+        }
+
         NSString *text;
         @try {
           text = [text1 substringWithRange:NSMakeRange(thisPointer, (NSUInteger)n)];

objectivec/Tests/DiffMatchPatchTest.m

@@ -686,6 +686,8 @@ - (void)test_diff_textTest {
 
   XCTAssertEqualObjects(@"jumped over a lazy", [dmp diff_text2:diffs], @"Compute the source and destination texts #2");
 
+  XCTAssertEqualObjects(@"🙂🙁", [dmp diff_text2:([dmp diff_fromDeltaWithText:@"🙂🙁" andDelta:@"=3\t+(null)\t=1" error:nil])]);
+
   [dmp release];
 }
 
@@ -814,10 +816,6 @@ - (void)test_diff_deltaTest {
                                             [Diff diffWithOperation:DIFF_INSERT andText:[NSString stringWithFormat:@"%C", 0xdd71]],
                                             nil])]);
 
-    // Invalid diff from objective-c with (null) string
-    XCTAssertEqualObjects([dmp diff_fromDeltaWithText:@"" andDelta:@"+%ED%A0%BC%28null%29%ED%B5%B0" error:nil],
-                          ([NSMutableArray arrayWithObjects:[Diff diffWithOperation:DIFF_INSERT andText:@"🅰"],nil]));
-
   // Verify pool of unchanged characters.
   diffs = [NSMutableArray arrayWithObject:
        [Diff diffWithOperation:DIFF_INSERT andText:@"A-Z a-z 0-9 - _ . ! ~ * ' ( ) ; / ? : @ & = + $ , # "]];