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Re: [zzdev] Re: Hack for Linked Lists
- To: zzdev@xxxxxxxxxx
- Subject: Re: [zzdev] Re: Hack for Linked Lists
- From: Ted Nelson <ted@xxxxxxxxxx>
- Date: Mon, 21 Dec 1998 03:15:01 -0800
- Cc: ted@xxxxxxxxxx
- In-reply-to: <19981220062254.9293.qmail@xxxxxxxxxx>
- References: <Your message of "Sat, 19 Dec 1998 19:57:13 PST." <22.214.171.124.19981219195713.00a512e0@xxxxxxxxxxxxxxxxxxx>
- Reply-to: zzdev@xxxxxxxxxx
Yeah, I figured it would be something like that.
Still, had to mention it.
At 01:22 AM 12/20/98 -0500, you wrote:
>> a game programmer told me a great efficiency hack for
>> linked lists. He said it's well known among game programmers
>> but not otherwise.
>I'd say it's well-known among assembly-language programmers.
>High-level languages usually go to some trouble to make a distinction
>between numbers and pointers, because it reduces the chance of errors
>and increases portability to different systems.
>The benefit usually claimed for this trick is that a to make alist
>that is linked in both directionsm, you normally need to store two
>pointers per item, one pointing to the next item in each direction.
>But with the trick, you only store one pointer.
>Until now, I have never heard anyone claim that this promoted a time
>efficiency, and I am very doubtful that it would actually deliver on
>that promise. Following a pointer in the normal way is not
>an expensive operation.
>Also, it isn't applicable to ZigZag, because it is only useful for
>complete traversals of a list. If you are at the nth item in the
>list, and you just came from the (n-1)th item, you use the trick to
>find the (n+1)th item. But if you happen to be at the nth item, and
>you don't know what the (n-1)th item is, there is absolutely no way to
>find the (n+1)th item. But this situation occurs all the time in
>> However, maybe we could figure some way
>> to crank this up for greater efficiency ...
>In my experience, this kind of extremely low-level micro-optimization
>is usually a mistake even in the advanced stages of a project that is
>already almost complete. The existing system already performs
>adequately. I think we should be considering issues of scalability
>and function before we worry about how to save a few bytes in the
Theodor Holm Nelson, Visiting Professor of Environmental Information,
Keio University, Shonan Fujisawa Campus, Fujisawa, Japan _____
Permanent Address: Project Xanadu, 3020 Bridgeway #295, Sausalito CA 94965
_____ World Wide Fax +1-415-332-0136 World Wide Voicemail:
+1-415-331-4422 _____ PERMANENT E-MAIL: ted@xxxxxxxxxx
Quotation of the day, 19 December 98:
"Denial is the longest river in the world." TN94