NAME
Algorithm::SkipList - Perl implementation of skip lists
REQUIREMENTS
The following non-core modules are used:
Tree::Node
INSTALLATION
Installation can be done using the traditional Makefile.PL or the newer
Build.PL method .
Using Makefile.PL:
perl Makefile.PL
make test
make install
(On Windows platforms you should use nmake instead.)
Using Build.PL (if you have Moddule::Build installed):
perl Build.PL
perl Build test
perl Build install
SYNOPSIS
my $list = new Algorithm::SkipList();
$list->insert( 'key1', 'value' );
$list->insert( 'key2', 'another value' );
$value = $list->find('key2');
$list->delete('key1');
DESCRIPTION
This is an implementation of skip lists in Perl.
Skip lists are an alternative to balanced trees. They are ordered linked
lists with random links at various *levels* that allow searches to skip
over sections of the list, like so:
4 +---------------------------> +----------------------> +
| | |
3 +------------> +------------> +-------> +-------> +--> +
| | | | | |
2 +-------> +--> +-------> +--> +--> +--> +-------> +--> +
| | | | | | | | |
1 +--> +--> +--> +--> +--> +--> +--> +--> +--> +--> +--> +
A B C D E F G H I J NIL
A search would start at the top level: if the link to the right exceeds
the target key, then it descends a level.
Skip lists generally perform as well as balanced trees for searching but
do not have the overhead with respect to reblanacing the structure. And
on average, they use less memory than trees.
They also use less memory than hashes, and so are appropriate for large
collections.
KNOWN ISSUES
Developer Release
This is a developer release, with many changes to improve
performance. Some of these changes may cause incompatabilities. See
the Changes file.
Algorithm::SkipList::PurePerl is not included with this release, and
code which relies on it may fail.
The documentation has largely been copied from the 1.02 release, and
may not accurately reflect changes in this release.
Undefined Values
Certain methods such as the find and delete entries elsewhere in
this document will return the the value associated with a key, or
`undef' if the key does not exist. However, if the value is `undef',
then these functions will appear to claim that the key cannot be
found.
In such circumstances, use the exists method to test for the
existence of a key.
Duplicate Keys
Duplicate keys are an experimental feature in this module, since
most methods have been designed for unique keys only.
Access to duplicate keys is akin to a stack. When a duplicate key is
added, it is always inserted *before* matching keys. In searches, to
find duplicate keys one must use find_with_finger or the
find_duplicates method.
The copy method will reverse the order of duplicates.
The behavior of the the merge and append entries elsewhere in this
document methods is not defined for duplicates.
Non-Determinism
Skip lists are non-deterministic. Because of this, bugs in programs
that use this module may be subtle and difficult to reproduce
without many repeated attempts. This is especially true if there are
bugs in a custom node.
Additional issues may be listed on the CPAN Request Tracker at
http://rt.cpan.org/NoAuth/Bugs.html?Dist=Algorithm-SkipList or
http://rt.cpan.org/NoAuth/Bugs.html?Dist=List-SkipList.
AUTHOR
Robert Rothenberg <rrwo at cpan.org>
Acknowledgements
Carl Shapiro for introduction to skip lists.
Suggestions and Bug Reporting
Feedback is always welcome. Please use the CPAN Request Tracker at
http://rt.cpan.org to submit bug reports.
LICENSE
Copyright (c) 2003-2005, 2008-2010 Robert Rothenberg. All rights
reserved. This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.
SEE ALSO
See the article by William Pugh, "A Skip List Cookbook" (1989), or
similar ones by the author at http://www.cs.umd.edu/~pugh/ which discuss
skip lists.
Another article worth reading is by Bruce Schneier, "Skip Lists: They're
easy to implement and they work", Doctor Dobbs Journal, January 1994.
Tie::Hash::Sorted maintains a hash where keys are sorted. In many cases
this is faster, uses less memory (because of the way Perl5 manages
memory), and may be more appropriate for some uses.
If you need a keyed list that preserves the order of insertion rather
than sorting keys, see List::Indexed or Tie::IxHash.