Re: Create NSStrings from a mapped NSData object - safe?

FROM : Daniel Vollmer
DATE : Tue May 13 21:55:43 2008

On May 13, 2008, at 17:00, Jens Alfke wrote:

>
> On 12 May '08, at 11:38 PM, Daniel Vollmer wrote:
>

>> I'm parsing a rather large text-file (usually >20MB) and in doing 
>> so I'm iterating over its lines with [String 
>> getParagraphStart::::]. I've found a rather noticeable speed-up in 
>> the parsing operation if I create the string in question from an 
>> NSData object (created via initWithContentsOfMappedFile) using 
>> [String initWithData:encoding:].

>
> It sounds like you're creating a single NSString containing the 
> entire contents of the file, then?


Yes. Is that something I shouldn't do? I mean, I feel a tiny bit silly 
creating such huge strings but I didn't find a nice alternative (e.g. 
like the Ruby for each line iterators on file objects).

>> 2) Are substrings created from the original string (e.g. 
>> substringWithRange etc.) still backed properly after the original 
>> string and the NSData object are released?

>
> Yes. Even if the NSString is still using the NSData's contents for 
> its buffer, it retained them, so releasing the NSData won't make it 
> go away until the string is done with it.


But now that means that the strings are "endangered" from in-place 
file modification for the lifetime of my objects created during 
parsing, not just the initial parsing itself, correct?
Also, it feels a bit silly to have a retain on the 20MB NSData object 
while I still hold references to about 5KB of string bytes from 
various places in the file. Usually all this "behind-the-scenes" 
storage retaining doesn't matter much, but I'd quite like to make sure 
I drop most of the 20MB once I'm done parsing. This question of course 
also applies if I'm not mapping the file and creating a String from it 
directly


FWIW, my current iteration looks like this (String being the big 20MB 
one);

NSUInteger length = [String length];
NSUInteger paraStart = 0, paraEnd = 0, contentsEnd = 0;

while (paraEnd < length)
{
   [String getParagraphStart:&paraStart end:&paraEnd 
contentsEnd:&contentsEnd forRange:NSMakeRange(paraEnd, 0)];
   line = [String substringWithRange:NSMakeRange(paraStart, contentsEnd 
- paraStart)];
   // do lots of menial parsing of line
}

If I leave the mmaped reading in, it sounds like a sensible idea to 
check whether the file is on the same drive as the app. So thanks for 
that suggestion.


Thanks for any further insight,
   Daniel.