OpenEuphoria

Re: Interesting Experiment With String/Sequence Slicing

• Posted by Derek Parnell <ddparnell at bigpond.com> Aug 20, 2001
• 431 views

> From: slie at theage.fairfax.com.au
> To: EUforum <EUforum at topica.com>
> Reply-To: EUforum at topica.com
> Subject: Re: Interesting Experiment With 
> String/Sequence Slicing
> Date: 21/08/2001 12:58:45 PM
> 
> 
> i am just wild guessing here, but could it 
> be true that a function can be
> specifically written to deal with string 
> type sequences and so speeding up
> the slicing function?
> 

Hi Sam,
we have discovered that the fastest method of inserting, deleting, and
replacing text in sequences is to minimize the number and size of
concatenations.

Consider the general case where you have a text of <aaa><bbb><ccc> and a
replacement text of <ddd> which is going to replace <bbb>.

If the length of <ddd> is longer than <bbb> then create a new sequence (
using the repeat() function) with the length of <aaa> + <ddd> + <ccc> then
copy these sub-strings into the new sequence and return the new sequence. 

Where <ddd> is shorter than <bbb>, you can avoid the overhead of creating a
new sequence and reuse the space in the exiting string. The trick here is to
move either <aaa> closer to <ccc> or <ccc> closer to <aaa>, depending on
which is smaller, and leaving enough room for <ddd> to slide in between
them. Then you just return the truncated sequence. If you didn't move <aaa>
then that is S[1 .. length(<aaa>)+length(<ddd>)+length(<ccc>)], but if you
did move <aaa> it is S[length(S) -
(length(<aaa>)+length(<ddd>)+length(<ccc>)) .. length(S)].

There are further optimisations if <aaa>, <ccc>, or <ddd> are empty
sequences.

I know this looks complex but it is almost always faster than creating
temporary sequences and concatenating them, depending on the length of the
sequences involved.
----
Derek


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