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iPhone: fast hash function for storing web images (url) as files (hashed filenames)

Answer 1

+1 A:

I think the NSObject already has a hash function. And NSUrl or NSString can override it, can you try with those things. I think in most of cases, it is fast enough, like we put NSString into NSDictionary:) NSObject hash

vodkhang 2010-05-29 12:18:43

As a return value I get an Integer value, which I can convert to a string and use it as a filename. However, is that "strong" enough to differentiate between the many different urls out there? How likely is it that two different urls result in the same hash?

znq 2010-05-29 14:32:26

I can not find any documents on google for that :(. But I think if you have a small number of urls (10 - 100, I just guessed), it can be ok. But I found out that usually people use MD5 to generate the hash, so performance may not be a big problem with MD5

vodkhang 2010-05-29 14:59:58

Thanks. I actually checked the time to execute both, and MD5 is pretty much the same as [myObject hash]

znq 2010-05-29 16:32:52

Answer 2

+3 A:

MD5 may be broken for security purposes, but it works well for the situation you describe. Here's a thread on how to implement it on iPhone. Check out Vroomtrap's post. For posterity, here's my own version of that code:

- (NSString *)MD5Hash {
    const char *cStr = [self UTF8String];
    unsigned char result[CC_MD5_DIGEST_LENGTH];

    CC_MD5( cStr, strlen(cStr), result );

    return [NSString stringWithFormat: @"%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X%02X",
        result[0], result[1], result[2], result[3], result[4], result[5], result[6], result[7],
        result[8], result[9], result[10], result[11], result[12], result[13], result[14], result[15] ];
}

You'll need to import the CommonCrypto/CommonDigest.h header.

warrenm 2010-05-29 15:37:47

I found this one here very helpful: http://www.saobart.com/md5-has-in-objective-c/

znq 2010-05-29 16:33:08

I'd recommend using `dataUsingEncoding:` instead of `UTF8String`. `strlen` is not cheap, since it has to walk the entire string to find the end of it to know how long it is. The NSData object knows how long the data is.

Peter Hosey 2010-05-30 06:52:26

You're welsome to do so. My empirical testing showed that using dataUsingEncoding performed the same as the above method on strings of moderate length (200K) and substantially worse on large strings (2M).

warrenm 2010-05-31 03:36:58

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iPhone: fast hash function for storing web images (url) as files (hashed filenames)

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