Problem with string replacement

[Fred Kiefer]

Yesterday I noticed that I have a few additional failing tests for base. 
Most of these come from the test for NSJSONSerialization. After a closer 
look I was able to restrict the issue down to the following code, which 
hasn't anything to do with JSON any more.
In the JSON test code we had a string that contained the character " and 
the unicode value 1 and the serialisation tries to deal with these value 
by replacing them.


  NSMutableString *str1, *str2;
  unichar chars[3];

  chars[0] = '\"';
  chars[1] = 1;
  str1 = [NSMutableString stringWithCharacters: chars length: 2];
  [str1 replaceOccurrencesOfString: @"\""
                       withString: @"\\\""
                          options: 0
                            range: NSMakeRange(0, [str1 length])];

  chars[0] = '\\';
  chars[1] = '\"';
  chars[2] = 1;
  str2 = [NSMutableString stringWithCharacters: chars length: 3];
  NSLog(@"string 1 %@ string 2 %@", str1, str2);
  PASS([str1 isEqual: str2],
    "string occurrences replacement works");

To me this looks like a bug in the code of 
replaceOccurrencesOfString:... I tried to understand that code but 
completely failed inside the GSeq.h functions. At least I noticed a few 
things, first we end of using the function strRangeCsNs, this is because 
the searched string is an NSConstantString and all the optimisations 
don't kick in for that case. The second is that this function returns 
twice the range {0, 1} although the second time the search range is {2, 1}.

Could somebody please explain this to me? If this is an actual problem I 
would like to add the above code into the NSMutableString tests.

Fred