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>

LICENSE
    Copyright (C) 2002-2008 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.

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

VERSION
    Version: 0.08

    Date: 2008 June 04