ObjectiveC: Guard against high-surrogate(null)low-surrogate

Author: dmsnell

objectivec/DiffMatchPatch.m

@@ -1492,8 +1492,24 @@ - (NSString *)diff_decodeURIWithText:(NSString *)percentEncoded
     @catch (NSException *e) {
         return nil;
     }
-    
-    return [NSString stringWithCharacters:decoded length:output];
+
+    // 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 consumping 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"];
 }
 
 /**

objectivec/Tests/DiffMatchPatchTest.m

@@ -814,6 +814,10 @@ - (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 - _ . ! ~ * ' ( ) ; / ? : @ & = + $ , # "]];