Hello,
If memory serves well, CRC-32 is quite fast. How large are the BLOBs you're
trying to compare? If they're not too large, the following may work for you:
#include <zlib.h>
@implementation NSData (CRC32)
- (uint32_t)CRC32
{
uLong crc = crc32(0L, NULL, 0);
crc = crc32(crc, [self bytes], [self length]);
return crc;
}
@end
--Tito
On Nov 29, 2013, at 12:36 PM, Kyle Sluder <k...@ksluder.com> wrote:
>> On Nov 29, 2013, at 11:58 AM, Graham Cox <graham....@bigpond.com> wrote:
>
>>
>> Another general question.
>>
>> Does anyone have a quick-and-dirty (but functional) way to hash NSData? I’m
>> currently using SHA-1 but it’s quite slow. All I need is a way to determine
>> whether one block of data is identical to another or not - but every bit
>> counts.
>
> Just for the sake of the list, though I'm certain you're aware of this
> yourself: you can't use a hash function to check for equality unless it is a
> perfect hash function. You can use it to reduce the number of comparisons per
> candidate, but you'll always wind up doing at least O(M+N) comparisons (where
> M is the size of your input and N is the length of your hash output).
>
> Often, it's better to improve the data structure before trying a different
> hash function. It's much easier to reason about how to improve a solution in
> terms of algorithm analysis of your data structure rather than it is to
> perform an empirical analysis of a specific hash function's
> collision-to-speed tradeoffs.
>
> Long way of saying: what's the actual problem you're trying to solve? Maybe a
> hash table is a poor fit.
>
> --Kyle Sluder
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