Thanks again Quincey. Just for curiousity, if I do the line:
const uint8_t* buffer = [fileData bytes]; it will not read the entire file? or these address are pointed direct on disk so they are load on demand? also, just to mean if I understand here: CC_MD5_Update(&md5, buffer + byteOffset, byteLength); in the sum "buffer + byteOffset", in case, adding a number to an array pointer will change it offset? --- Wilker Lúcio http://about.me/wilkerlucio/bio Kajabi Consultant +55 81 82556600 On Wed, Jun 29, 2011 at 2:53 PM, Quincey Morris <quinceymor...@earthlink.net > wrote: > On Jun 29, 2011, at 07:08, Wilker wrote: > > > char buffer[CHUNK_SIZE]; > > > [fileData getBytes:buffer range:NSMakeRange(0, CHUNK_SIZE)]; > > CC_MD5_Update(&md5, buffer, CHUNK_SIZE); > > [fileData getBytes:buffer range:NSMakeRange(MAX(0, [fileData length] > - CHUNK_SIZE), CHUNK_SIZE)]; > > CC_MD5_Update(&md5, buffer, CHUNK_SIZE); > > Personally, having tried to get the file reads as cheaply as possible**, > I'd just do this: > > const uint8_t* buffer = [fileData bytes]; > // calculate byteLength and byteOffset (see below) > > CC_MD5_Update(&md5, buffer, byteLength); > CC_MD5_Update(&md5, buffer + byteOffset, byteLength); > > [fileData self]; > > to avoid copying the raw data unnecessarily. > > Your code still has the bug in the MAX parameters (if [fileData length] > > CHUNK_SIZE, then [fileData length] - CHUNK_SIZE is a *very* large positive > number), and you've introduced a new one -- 'getBytes:range:' will crash if > the range goes past the end of the NSData data, so I'd calculate the length > and offset like this: > > NSUInteger byteLength = [fileData length]; > NSUInteger byteOffset = 0; > > if (byteLength > CHUNK_SIZE) { // *** > byteLength = CHUNK_SIZE; > byteOffset = byteLength - CHUNK_SIZE; > } > > > > ** There are some drawbacks to using very large stack-based structures. At > least: > > a. If this code is ever part of a background thread, the default maximum > stack size isn't large enough to accommodate 65K data structures. > > b. If this is running under garbage collection, the collector must scan the > entire 65K every time it runs, uselessly and wastefully. > > What's the usual way to solve these problems? To move the data structure > off the stack into the heap. But you already have the data in the heap, in > your NSData object. > > *** Someone once pointed out on this list that you should never use MIN and > MAX (and other such convenience macros) in production code, because you are > never quite certain how they're implemented (in terms of handling mismatched > sign expressions, for example) and how they might be implemented on some > future platform you might want to port this code to. Therefore, he said, you > should write the code you want, not hope for the code you need. I think > maybe his point was a little obsessive, but I have to admit I now cringe and > hesitate before writing MIN or MAX. :) _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
