Re: Fast hash of NSData?
Re: Fast hash of NSData?
- Subject: Re: Fast hash of NSData?
- From: Kyle Sluder <email@hidden>
- Date: Fri, 29 Nov 2013 15:41:36 -0800
> On Nov 29, 2013, at 3:35 PM, Graham Cox <email@hidden> wrote:
>
>
> On 29 Nov 2013, at 11:19 pm, Graham Cox <email@hidden> wrote:
>
>>> You also have another, damn-quick "hash key" that takes zero disk access to compute: -[NSData length].
>>
>> Yep, that’s a great idea.
>
>
> OK, I’ve implemented this much as your suggestion, and I’m also saving the hash object along with the data so I can avoid recomputing it when the file is imported.
That’s not a good idea. If the file on disk changes out from under you, the cache no longer reflects the data. Reminiscent of the old adage: “There are only two hard problems in computer science: naming things, cache invalidation, and off-by-one errors.”
If you really want to avoid computing hashes for all your embedded image data at load, you might consider deferring hash computation until absolutely necessary—that is, when inserting a file into a loaded document, only compute hashed for data objects which have the same length as the one you're in inserting.
--Kyle Sluder
_______________________________________________
Cocoa-dev mailing list (email@hidden)
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:
This email sent to email@hidden