From: Rick DeNatale <rick.denatale@...>
Date: 2006-09-14T05:03:01+09:00
Subject: Re: How about Enumerable#find_pattern?

On 9/12/06, A. S. Bradbury <asbradbury@tekcentral.org> wrote:
> On Tuesday 12 September 2006 19:50, Rick DeNatale wrote:
> > Nice, but it can fall down if the pattern contains a regexp which
> > matches more than one element:
> >
> > a = "aaaabbabbab".split(//)
> > a.find_pattern /aab/,  'b'  #=> nil
> >
> > There's probably a way to fix that, but I'm not sure I can see how.
>
> I'd actually argue that's expected behaviour and you're just using it
> wrong(!). /aab/ simply doesn't match any element, so it *should* fail.

Well, I'd expect that if you allow regexps that you should allow
regexps! But see below.

> The
> main problem with my current implementation is it doesn't really skip ahead
> when it should do, it just continues to the next iteration. This is where
> it's easier when you're using an index and a while loop (and the object
> responds_to []), but we can't assume that for an Enumerable-compatible
> implementation.
>
> The Enumerable mixin provides #to_a, but relying on this seems like a poor
> solution...

Not sure why.

> > I did a pretty slavish translation to ruby of the KMP algorithm as
> > given in the Wikipedia article.  It involved turning the enumerable
> > into either an array or a string so that it could be indexed instead
> > of using each.  That didn't allow regexps at all though.
>
> > I've been working on hacking that to work with regexps, but that would
> > really only work if the enumerable was a string anyway, so it would
> > probably be better to do a specialized implementation in String.
>
> The string approach really doesn't seem like the right way to go about this,
> if I understand you correctly. Not for the problem I'm trying to solve at
> least. The idea of #find_pattern is that it will work for any arbitrary
> object.

But then regexps won't work except for a string.


Also, strings are a funny kind of enumerable here, since by default,
they just yield themselves in each, unless they include newlines.  I'd
think that the general use of find_pattern in a string would be to
search for the pattern in the string, not as a pattern of the lines in
the string.


> Perhaps you should be able to specify a comparison function in a block upon
> calling find_pattern.

Of course this would require careful interface design to expand the
generality while preserving the efficiency.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/