NAME
    Array::Unique - Tie-able array that allows only unique values

SYNOPSIS
     use Array::Unique;
     tie @a, 'Array::Unique';

     Now use @a as a regular array.

DESCRIPTION
    This package lets you create an array which will allow only one
    occurrence of any value.

    In other words no matter how many times you put in 42 it will keep only
    the first occurrence and the rest will be dropped.

    You use the module via tie and once you tied your array to this module
    it will behave correctly.

    Uniqueness is checked with the 'eq' operator so among other things it is
    case sensitive.

    As a side effect the module does not allow undef as a value in the
    array.

EXAMPLES
     use Array::Unique;
     tie @a, 'Array::Unique';

     @a = qw(a b c a d e f);
     push @a, qw(x b z);
     print "@a\n";          # a b c d e f x z

DISCUSSION
     When you are collecting a list of items and you want 
     to make sure there is only one occurrence of each item,
     you have several option:

    1) using an array and extracting the unique elements later
         You might use a regular array to hold this unique set of values
         and either remove duplicates on each update by that keeping the array
         always unique or remove duplicates just before you want to use the 
         uniqueness feature of the array. In either case you might run a 
         function you call @a = unique_value(@a);

         The problem with this approach is that you have to implement 
         the unique_value function (see later) AND you have to make sure you 
         don't forget to call it. I would say don't rely on remembering this.
 
         There is good discussion about it in the 1st edition of the 
         Perl Cookbook of O'Reilly. I have copied the solutions here, 
         you can see further discussion in the book.

         ----------------------------------------
         Extracting Unique Elements from a List (Section 4.6 in the Perl Cookbook 1st ed.)

         # Straightforward

         %seen = ();
         @uniq = ();
         foreach $item (@list) [
             unless ($seen{$item}) {
               # if we get here we have not seen it before
               $seen{$item} = 1;
               push (@uniq, $item);
            }
         } 

         # Faster
         %seen = ();
         foreach $item (@list) {
           push(@uniq, $item) unless $seen{$item}++;
         }

         # Faster but different
         %seen;
         foreach $item (@list) {
           $seen{$item}++;
         }
         @uniq = keys %seen;

         # Faster and even more different
         %seen;
         @uniq = grep {! $seen{$_}++} @list;

         ----------------------------------------

    2) using a hash
         Some people use the keys of a hash to keep the items and
         put an arbitrary value as the values of the hash:

         To build such a list:
         %unique = map { $_ => 1 } qw( one two one two three four! );

         To print it:
         print join ", ", sort keys %unique;

         To add values to it:
         $unique{$_}=1 foreach qw( one after the nine oh nine );

         To remove values:
         delete @unique{ qw(oh nine) };

         To check if a value is there:
         $unique{ $value };        # which is why I like to use "1" as my value

         (thanks to Gaal Yahas for the above examples)

         There are three drawbacks I see:
         1) You type more.
         2) Your reader might not understand at first why did you use hash 
            and what will be the values.
         3) You lose the order.

         Usually non of them is critical but when I saw this the 10th time
         in a code I had to understand with 0 documentation I got frustrated.

    3) using Array::Unique
         So I decided to write this module because I got frustrated
         by my lack of understanding what's going on in that code
         I mentioned.
         In addition I thought it might be interesting to write this and
         then benchmark it.
         Additionally it is nice to have your name displayed in 
         bright lights all over CPAN ... or at least in a module.

         Array::Unique lets you tie an array to hmmm, itself (?)
         and makes sure the values of the array are always unique.

         Since writing this I am not sure if I really recommend its usage.
         I would say stick with the hash version and document that the
         variable is aggregating a unique list of values.

    4) Using real SET
        There are modules on CPAN that let you create and maintain SETs. I
        have not checked any of those but I guess they just as much of an
        overkill for this functionality as Unique::Array.

BUGS
     use Array::Unique;
     tie @a, 'Array::Unique';

     @c = @a = qw(a b c a d e f b);
 
     @c will contain the same as @a AND two undefs at the end because
     @c you get the same length as the right most list.

TODO
     Test:
       Change size of the array
       Elements with false values ('', '0', 0) 
       splice:
       splice @a;
       splice @a,  3;
       splice @a, -3;
       splice @a,  3,  5;
       splice @a,  3, -5;
       splice @a, -3,  5;
       splice @a, -3, -5;
       splice @a,  ?,  ?, @b;

     Benchmark speed

     Add faster functions that don't check uniqueness so if I 
       know part of the data that comes from a unique source then
       I can speed up the process,
       In short shoot myself in the leg.

     Enable optional compare with other functions

     Write even better implementations.

AUTHOR
     Gabor Szabo <gabor@pti.co.il>

     Copyright (C) 2002-2004 Gabor Szabo <gabor@pti.co.il>
     All rights reserved.
     http://www.pti.co.il/

     You may distribute under the terms of either the GNU 
     General Public License or the Artistic License, as 
     specified in the Perl README file.

     No WARRANTY whatsoever.

SUPPORT
     There is no official support for this package but
     you can send bug reports directly to the author.

     To get other support answers you should use either
     the Hungarian Perl mailing list at http://www.perl.org.hu/
     if you want to ask in Hungarian or the Israeli Perl 
     mailing list at http://www.perl.org.il/ in English.

CREDITS
     Thanks for suggestions and bug reports to 
     Szabo Balazs (dLux)
     Shlomo Yona
     Gaal Yahas
     Jeff 'japhy' Pinyan
     Werner Weichselberger

VERSION
     Version: 0.06
     Date:    2004.10.02

