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

[Daniel Vollmer]

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.
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