NAME
Algorithm::PageRank::XS - A Fast PageRank implementation
DESCRIPTION
This module implements a simple PageRank algorithm in C. The goal is to
quickly get a vector that is closed to the eigenvector of the stochastic
matrix of a graph.
Algorithm::PageRank does some pagerank calculations, but it's slow and
memory intensive. This module was developed to compute pagerank on
graphs with millions of arcs. This module will not, however, scale up to
quadrillions of arcs (see TODO).
SYNOPSYS
use Algorithm::PageRank::XS;
my $pr = Algorithm::PageRank::XS->new();
$pr->graph([
'John' => 'Joey',
'John' => 'James',
'Joey' => 'John',
'James' => 'Joey',
]
);
$pr->results();
# {
# 'James' => '0.569840431213379',
# 'Joey' => '1',
# 'John' => '0.754877686500549'
# }
#
#
# The following simple program takes up arcs and prints the ranks.
use Algorithm::PageRank::XS;
my $pr = Algorithm::PageRank::XS->new();
while (<>) {
chomp;
my ($from, to) = split(/\t/, $_);
$pr->add_arc($from, $to);
}
while (my ($name, $rank) = each(%{$pr->results()})) {
print("$name,$rank\n");
}
METHODS
new %PARAMS
Create a new PageRank object. Possible parameters:
alpha
This is (1 - how much people can move from one node to another
unconnected one randomly). Decreasing this number makes convergence
more likely, but brings us further from the true eigenvector.
max_tries
The maximum number of tries until we give up trying to achieve
convergence.
convergence
The maximum number the difference between two subsequent vectors
must be before we say we are "convergent enough". The convergence
rate is the rate at which "alpha^t" goes to 0. Thus, if you set
"alpha" to 0.85, and "convergence" to 0.000001, then you will need
85 tries.
add_arc
Add an arc to the pagerank object before running the computation. The
actual values don't matter. So you can run:
$pr->add_arc("Apple", "Orange");
and you mean that "Apple" links to "Orange".
graph
Add a graph, which is just an array of from, to combinations. This is
equivalent to calling "add_arc" a bunch of times, but may be more
convenient.
results
Compute the pagerank vector, and return it as a hash.
Whatever you called the nodes when specifying the arcs will be the keys
of this hash, where the values will be the vector.
The result vector is normalized such that the maximum value is 1. This
is to prevent extremely small values for large data sets. You can
normalize it any other way you like if you don't like this.
BUGS
None known.
TODO
* We may want to support "double" values rather than single floats
* We may or may not want to adjust the weighting of individual arcs, as
you cannot do now.
* At present the indexes are "unsigned int", rather than "size_t". Thus
this will not scale with 64-bit architectures.
* It'd be nice to be able to use mmap(2) to efficiently use the hard
drive to scale to places where memory can't take us.
PERFORMANCE
This module is pretty fast. I ran this on a 1 million node set with 4.5
million arcs in 57 seconds on my 32-bit 1.8GHz laptop. Let me know if
you have any performance tips. It's orders of magnitude faster than
Algorithm::PageRank, but performance tests will be here shortly.
SEE ALSO
Algorithm::PageRank
AUTHOR
Michael Axiak
COPYRIGHT
Copyright (C) 2008 by Michael Axiak
This package is free software; you can redistribute it and/or modify it
under the same terms as Perl itself