=head1 NAME

Tie::ListKeyedHash - A system allowing the use of anonymous arrays as keys to a hash.

=head1 SYNOPSIS

   use Tie::ListKeyedHash;

   [$X =] tie %hash,  'Tie::ListKeyedHash';


   my $live_key     = ['key','items','live'];
   $hash{$live_key} = 'Hello!';

   $hash{['key','trees','grow']} = 'Goodbye!';

   print $hash{['key','items','live']},"\n";
   delete $hash{$live_key};

   my @list =  keys %{$hash{['key']}};
   print "@list\n";

   untie %hash ;

Alternatively keys are accessible as:

   $hash{'key','items','live'} = 'Hello!';

(a bare list/array for the key rather than using an anon list/array reference).

But that slows down the accesses by around 10% and cannot
be used for keys that conflict with the value of the $; special
variable.

Also usable via the object interface methods 'put',
'get','exists','delete','clear'. The object interface
is about 2x as fast as the tied interface.

=head1 DESCRIPTION

Tie::ListKeyedHash ties a hash so that you can use a reference to an
array as the key of the hash. It otherwise behaves exactly like
a normal hash (including all caveats about trying to use a key
as both a hash reference and a scalar value).

This frees you from needing to 'hardwire' hash references in code or
having to write tree traversal code to reach arbitrary points in a
hash tree.

Example:

                   ########################

 #!/usr/bin/perl

 use strict;
 use warnings;

 use Data::Dumper;

 use Tie::ListKeyedHash;

 my %example;
 tie (%example, 'Tie::ListKeyedHash');

 %example = (
     'a' => {
       'b0' => {
         'c' => 'value of c',
         'd' => 'value of d',
         'e' => {
           'f' => 'value of f',
         },
       },
       'b1' => {
         'g' => 'value of g',
       },
     },
     'h' => 'r',
 );

 my $b_key = ['a','b0'];

 my $d_key = [@$b_key,'d'];
 my $d     = $example{$d_key};
 print "d = $d\n";

 my $e_key = [@$b_key, 'e'];
 my $e     = $example{$e_key};
 print 'e = ' . Dumper ($e);

 my $f_key = [@$b_key, 'e','f'];
 my $f     = $example{$f_key};
 print "f = $f\n";

 my $h_key = ['h'];
 my $h     = $example{$h_key};
 print "h = $h\n";

                   ########################

The virtues of this particular way of accessing hash-of-hashes (HoH) vs bare
hardwired derefererences or 'tree crawling' are as follows:

1) As the number of levels in a HoH increases,
the tied object asymptotically approaches the speed of hardwired hash
dereferencing without the loss of flexibility penalty of having to hardwire the
keys into code in advance.

This gives an important property that it I<gets faster> the
deeper a HoH becomes as compared with the speed of software driven tree traveral.

So you can build and access arbitrarily structured HoH and still access deeply buried
elements in the tree I<quickly>.

2) The format was designed to use memory efficiently. It takes only a
few hundred extra bytes over the size of an untied HoH in memory or
when serialized (via Data::Dumper or Storable for example) regardless
of how deep the hash is.

3) A reference to an existing HoH can be passed into Tie::ListKeyedHash->new and
all of the OO key lists access methods will B<"just work">.

 Example:

    use Tie::ListKeyedHash;

    my %hash = ( 'a' => { 'b' => 'c' } );
    my $obj = Tie::ListKeyedHash->new(\%hash);

    my $b_value = $obj->get(['a','b']);

=cut

=head1 CHANGES

 1.03 08 Oct 2020 - Relicensed under MIT License. Maintainer info updated.
                    Build configs updated. GitHub repo meta added.
                    Added 'use warnings'

 1.02 17 Sep 2005 - Extended tests to 100% coverage. Fixed bugs in 'exists'.
                    Fixed returned value for undefined values and handling
                    of setting undefined values. Miscellaneous code cleanups
                    and streamlining. Added examples to clarify what the
                    module actually does and why it is useful.

                    Extended the 'new' method to permit the efficient importing
                    of a pre-existing hash or hash reference.

 1.01 16 Sep 2005 - Restoring POD was that went missing from 1.00 upload.
                    Added META.yml to MANIFEST.
 
 1.00 15 Sep 2005 - Reorganized and clarified documentation, added build tests.
                    Fixed bug in 'exists' support.

 0.41 09 Jun 1999 - Minor documentation changes.

 0.40 04 May 1999 - Renamed to 'Tie::ListKeyedHash' after discussion on
                    comp.lang.perl.module and added (on the suggestion of
                    Ilya Zakharevich, <ilya@math.ohio-state.edu>) support
                    for using the tie hash as $hash{'key1','key2','key3'}
                    as well as its native mode of $hash{['key1','key2','key3']}

 0.20 30 Apr 1999 - Initial public release as 'Tie::ArrayHash'

=cut

#######################################################################

=head1 METHODS

=over 4

=item new([\%hash]);

Returns an object reference for the hash accessor.

 my $obj = Tie::ListKeyedHash->new;

You can, optionally, pass a reference to a pre-existing hash in and the returned object
will provide access to its contents via the Tie::ListKeyedHash object instance.

Example:

 my %hash = ( 'a' => { 'b' => 'c' }};
 my $obj  = Tie::ListKeyedHash->new(\%hash);

=back

=cut

#######################################################################

=over 4

=item clear;

Clears the entire hash.

Example:

  $obj->clear;

=back

=cut

#######################################################################

=over 4

=item exists(\@key_list);

Returns true of the specified hash element exists, false if it does not.
Just as with normal hashes, intermediate elements will be created if
they do not already exist.

Example:

  my $key = ['a','b','c'];
  if ($obj->exists($key)) {
    # Stuff
  }

=back

=cut

#######################################################################

=over 4

=item get(\@key_list);

Returns the contents of the object field denoted by the @key_list.
This is a way of making arbitrary keys that act like hashes, with
the 'hardwiring' requirements of hashes. The routine returns the
the contents addressed by 'key1','key2','key3',...

The special case of $obj->get([]); returns a reference to the anon
hash containing the hash data. This is actually a blessed reference, but
you can use normal hash operators on it (such as 'keys', 'values' and 'each').

Note that returned anon hashes I<will not> be themselves be automatically
blessed into Tie::ListKeyedHash. You get back I<exactly> whatever you stored
originally. This is true of retrieving values via the tied interface as well.

Example:

  my $key = ['a','b','c'];
  my $value = $obj->get($key);

=back

=cut

#######################################################################

=over 4

=item put(\@key_list, $value);

Sets $value as the contents of the object field denoted by the @key_list.

This is a way of making arbitrary keys that act like hashes, without
the 'hardwiring' requirements of hashes.

Example:

  my $key = ['a','b','c'];
  $obj->put($key => 'help me');

=back

=cut

#######################################################################

=over 4

=item delete(\@key_list);

Deletes the object field denoted by the @key_list.

This is a way of making arbitrary keys that act like hashes, without
the 'hardwiring' requirements of hashes.

  my $key = ['a','b','c'];
  $obj->delete($key);

=back

=cut

#######################################################################

=head1 BUGS

None known.

=head1 TODO

Nothing planned.

=head1 AUTHORS

Jerilyn Franz <cpan@jerilyn.info>

=head1 VERSION

Version 1.03 - 2020.10.08

=head1 SEE ALSO

perl perltie

=head1 COPYRIGHT

Copyright 1999-2020 Jerilyn Franz, <cpan@jerilyn.info>. All Rights Reserved.

=head1 LICENSE

MIT License

Copyright (c) 2020 Jerilyn Franz

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

=head1 DISCLAIMER

THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS
OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE.

Use of this software in any way or in any form, source or binary,
is not allowed in any country which prohibits disclaimers of any
implied warranties of merchantability or fitness for a particular
purpose or any disclaimers of a similar nature.

IN NO EVENT SHALL I BE LIABLE TO ANY PARTY FOR DIRECT, INDIRECT,
SPECIAL, INCIDENTAL,  OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
USE OF THIS SOFTWARE AND ITS DOCUMENTATION (INCLUDING, BUT NOT
LIMITED TO, LOST PROFITS) EVEN IF I HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGE

=cut

1;
