Re: problem with applying md5 to data
Re: problem with applying md5 to data
- Subject: Re: problem with applying md5 to data
- From: Wilker <email@hidden>
- Date: Wed, 29 Jun 2011 16:26:13 -0300
Nice :)
This is my latest code, I mean it should be right now, hehe (and yeth, Im
using GC):
+(NSString *)generateHashFromPath:(NSString *)path {
const NSUInteger CHUNK_SIZE = 65536;
NSError *error = nil;
NSData *fileData = [NSData dataWithContentsOfFile:path
options:NSDataReadingMapped | NSDataReadingUncached error:&error];
if (error) {
return nil;
}
const uint8_t* buffer = [fileData bytes];
NSUInteger byteLength = [fileData length];
NSUInteger byteOffset = 0;
if (byteLength > CHUNK_SIZE) {
byteOffset = byteLength - CHUNK_SIZE;
byteLength = CHUNK_SIZE;
}
CC_MD5_CTX md5;
CC_MD5_Init(&md5);
CC_MD5_Update(&md5, buffer, (unsigned int) byteLength);
CC_MD5_Update(&md5, buffer + byteOffset, (unsigned int) byteLength);
unsigned char digest[CC_MD5_DIGEST_LENGTH];
CC_MD5_Final(digest, &md5);
NSMutableString *ret = [NSMutableString
stringWithCapacity:CC_MD5_DIGEST_LENGTH * 2];
for (int i = 0; i < CC_MD5_DIGEST_LENGTH; i++) {
[ret appendFormat:@"x", digest[i]];
}
return [ret lowercaseString];
}
I tried it against a video of 8.5GB, stored on external drive (at wifi with
Airport Extreme), and it's blazing fast :)
Thanks a lot
---
Wilker Lúcio
http://about.me/wilkerlucio/bio
Kajabi Consultant
+55 81 82556600
On Wed, Jun 29, 2011 at 4:18 PM, Quincey Morris <email@hidden
> wrote:
> On Jun 29, 2011, at 11:32, Wilker wrote:
>
> > 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?
>
> [fileData bytes] is a pointer to some memory address or other, in your
> application's virtual memory address space. The actual pages of data don't
> exist yet, they are indeed "loaded" on demand. The demand will happen when
> CC_MD5_Update tries to retrieve bytes to update its calculations. As its
> internal pointer increments into each new page, its data accesses will cause
> VM faults, which will cause the pages to be read from disk, which in this
> case is your video file.
>
> That's why this is efficient. A "normal" read will transfer pages of data
> into the system's disk cache, then transfer some of it again into your
> application's address space, and if those pages in your address space happen
> to be swapped out sometime, your data is written *back* to disk (in the
> system VM backing store) for later rereading. Using mapped reads with no
> caching avoids all of that.
>
> > 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?
>
> 'buffer' is not an array, it's just a pointer. It's a pointer to an array
> of bytes, if you choose to think of it that way, but so are all pointers, if
> you choose to think of them that way.
>
> Think of it this way. You were using
> 'getBytes:range:NSMakeRange(offset,length)' to copy the bytes to a local
> buffer. That buffer must have copied the bytes *from* somewhere. The place
> where the bytes are being copied from is, literally, '[fileData bytes] +
> offset', and NSData's API makes it perfectly legal to go to the source
> yourself.
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