From: Eric Mahurin <eric_mahurin@...>
Date: 2005-04-29T02:43:56+09:00
Subject: Re: cursor/stream/c++ iterator class?


--- Simon Strandgaard <neoneye@gmail.com> wrote:
> On 4/28/05, Eric Mahurin <eric_mahurin@yahoo.com> wrote:
> > --- Simon Strandgaard <neoneye@gmail.com> wrote:
> > > I have made a lib containing some external iterators,
> see:
> > > http://aeditor.rubyforge.org/iterator/
> > >
> > > download it here:
> > >
> http://rubyforge.org/frs/download.php/704/iterator-0.8.zip
> > 
> > Thanks Simon.  I just looked through what you have.  You
> gave
> > me a few ideas.  But, here are a few things I don't like:
> > 
> > * for efficiency I would like the ability to read/write
> > multiple elements at a time.  I want the API to have a
> superset
> > of the features in IO if possible.
> 
> True, I didn't made mine for speed.
> 
>  
> > * instead of calling has_next? I'm just having next return
> nil
> > or an empty string/array (for getting N elements).  If the
> > collection can have nil elements, you'll have to ask for N
> > elements (could be 1) so that you get an empty array at the
> > end.
> 
> What if you are iterating through an array that
> coincidentially
> contains a nil.. like this [42, nil, 42]  ?

For collections that can contain nil, you'll have to pass the
optional length argument (=1 if you want 1 element).  So in the
above, consecutive calls to get(1) will return [42], [nil],
[42], and [] (or nil - haven't decided).  Calls to get() will
return 42, nil (you'll think your're at the end), 42, nil.  So
get(length) is like read(length) (returns a String) and get()
is like getc() (returns a character/Fixnum).

> > * no standard insert/delete operations.  I don't need them
> now,
> > but they might be useful to others.
> 
> Yeah, I don't have neither insert nor delete. 
> I didn't needed it when I made the lib. I though about it,
> but couldn't decide how to approach this problem.

So that the number of methods doesn't blow up, I decided to
have my "get" and "put" methods take a flags argument to
provide many functions:

forward/reverse
move/hold
read(return original value)/ignore(return length)
scan or replace vs. delete or insert

Last I counted, I have 12 get functions and 10 put functions
(all of which can operator on individual elements or a
string/array of them).  The base class will base all of these
on 4 primitives (getdelete before/after and insert
before/after), but most classes will want to override most of
it.

> > * like the C++ iterator, yours standardizes on
> > allowing/creating/comparing multiple iterators on the same
> data
> > structure.  I don't want to do this because many sequential
> > data structures can't handle this.  The primary example
> would
> > be an IO (especially a non-file) which only allows you to
> > read/write at one location at a time.  Another would be a
> text
> > editor buffer where you can only insert/delete at the
> cursor
> > position.  Instead of allowing multiple iterators for a
> data
> > structure, you'll be able to get and set the position where
> > "position" is an object that you might be able to compare
> > and/or do arithmetic on (depends on the data structure).
> 
> This sounds interesting.  I would be interesting at looking
> at how you would approach this.

The best analogy to this is simply the IO tell/pos and
seek/pos= methods.   For this type of position (an integer),
you can compare and add/subtract, but other position objects
may not be so flexible.  I don't think I'll specify what these
position objects look like.  Some classes where the underlying
data-structure is unidirectional (i.e. IO pipes) will use this
to signal when to start/stop buffering so that you can go back
to the original position.

> > I think I'll call the class I'm doing "Cursor" because it
> most
> > resembles what you can do at the cursor in a text editor
> and
> > I've also seen the term "cursor" used for external
> iterator.
> > 
> > I might take some of the ideas in your Iterator classes
> also
> > (some from C++ STL iterator it looks like), if you don't
> mind.
> 
> Feel free to use all of it as you like. 
> Is your project on rubyforge ?

not yet.



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