NAME
Text::SpeedyFx - tokenize/hash large amount of strings efficiently
VERSION
version 0.011
SYNOPSIS
use Data::Dumper;
use Text::SpeedyFx;
my $sfx = Text::SpeedyFx->new;
my $words_bag = $sfx->hash('To be or not to be?');
print Dumper $words_bag;
#$VAR1 = {
# '1422534433' => '1',
# '4120516737' => '2',
# '1439817409' => '2',
# '3087870273' => '1'
# };
my $feature_vector = $sfx->hash_fv("thats the question", 8);
print unpack('b*', $feature_vector);
# 01001000
DESCRIPTION
XS implementation of a very fast combined parser/hasher which works
well on a variety of bag-of-word problems.
Original implementation
<http://www.hpl.hp.com/techreports/2008/HPL-2008-91R1.pdf> is in Java
and was adapted for a better Unicode compliance.
METHODS
new([$seed, $bits])
Initialize parser/hasher, can be customized with the options:
$seed
Hash seed (default: 1).
$bits
How many bits do represent one character. The default value, 8,
sacrifices Unicode handling but is fast and low on memory footprint.
The value of 18 encompasses Basic Multilingual, Supplementary
Multilingual and Supplementary Ideographic planes. See also "UNICODE
SUPPORT"
hash($octets)
Parses $octets and returns a hash reference (not exactly; see "CAVEAT")
where keys are the hashed tokens and values are their respective count.
$octets are assumed to represent UTF-8 string unless Text::SpeedyFx is
instantiated with "$bits" == 8 (which forces Latin-1 mode, see "UNICODE
SUPPORT"). Note that this is the slowest form due to the
(computational) complexity of the associative array
<https://en.wikipedia.org/wiki/Associative_array> data structure
itself: hash_fv()/hash_min() variants are up to 260% faster!
hash_fv($octets, $n)
Parses $octets and returns a feature vector (string of bits) with
length $n. $n is supposed to be a multiplier of 8, as the length of the
resulting feature vector is ceil($n / 8). See the included utilities
cosine_cmp and uniq_wc.
hash_min($octets)
Parses $octets and returns the hash with the lowest value. Useful in
MinHash <http://en.wikipedia.org/wiki/MinHash> implementation. See also
the included minhash_cmp utility.
UNICODE SUPPORT
Due to the nature of Perl, Unicode support is handled differently from
the original implementation. By default, Text::SpeedyFx recognizes
UTF-8 encoded code points in the range 00000-2FFFF:
* Plane 0, the Basic Multilingual Plane (BMP, 0000–FFFF)
* Plane 1, the Supplementary Multilingual Plane (SMP, 10000–1FFFF)
* Plane 2, the Supplementary Ideographic Plane (SIP, 20000–2FFFF)
* There are planes up to 16; however, as in Perl v5.16.2, there are
no code points matching isALNUM_utf8() there (so it's irrelevant for
proper algorithm operation).
Although, there is a major drawback: in this mode, each instance
allocates up to 1 MB of memory.
If the application doesn't need to support code points beyond the Plane
0 (like the original SpeedyFx implementation) it is possible to
constraint the address space to 16 bits, which lowers memory allocation
to up to 256 KB. In fact, Text::SpeedyFx constructor accepts bit range
between 8 and 18 to address code points.
LATIN-1 SUPPORT
8 bit address space has one special meaning: it completely disables
multibyte support. In 8 bit mode, each instance will only allocate 256
bytes and hashing will run up to 340% faster! Tokenization will
fallback to ISO 8859-1 West European languages (Latin-1) character
definitions.
BENCHMARK
The test platform configuration:
* Intel® Xeon® E5620 CPU @ 2.40GHz (similar to the one cited in the
reference paper);
* Debian 6.0.6 (Squeeze, 64-bit);
* Perl v5.16.1 (installed via perlbrew);
* enwik8 from the Large Text Compression Benchmark
<https://cs.fit.edu/~mmahoney/compression/text.html>.
Rate murmur_utf8 hash_utf8 hash_min_utf8 hash hash_fv hash_min
murmur_utf8 6 MB/s -- -79% -86% -89% -97% -97%
hash_utf8 30 MB/s 376% -- -35% -47% -84% -85%
hash_min_utf8 47 MB/s 637% 55% -- -18% -76% -77%
hash 58 MB/s 803% 90% 23% -- -70% -72%
hash_fv 194 MB/s 2946% 541% 313% 237% -- -6%
hash_min 206 MB/s 3143% 582% 340% 259% 6% --
All the tests except the ones with _utf8 suffix were made in Latin-1
mode. For comparison, murmur_utf8 was implemented using
Digest::MurmurHash hasher and native regular expression tokenizer:
++$fv->{murmur_hash(lc $1)}
while $data =~ /(\w+)/gx;
See also the eg/benchmark.pl script.
CAVEAT
For performance reasons, hash() method returns a tied hash which is an
interface to nedtries
<http://www.nedprod.com/programs/portable/nedtries/>. The interesting
property of a trie data structure <https://en.wikipedia.org/wiki/Trie>
is that the keys are "nearly sorted" (and the first key is guaranteed
to be the lowest), so:
# This:
$fv = $sfx->hash($data);
($min) = each %$fv;
# Is the same as this:
($min) = $sfx->hash_min($data);
# (albeit the later being 2x faster)
The downside is the magic involved, the delete breaking the key order,
and the memory usage. The hardcoded limit is 524288 unique keys per
result, which consumes ~25MB of RAM on a 64-bit architecture. Exceeding
this will croak with the message "too many unique tokens in a single
data chunk". The only way to raise this limit is by recompilation of
the XS module:
perl Makefile.PL DEFINE=-DMAX_TRIE_SIZE=2097152
make
make test
make install
REFERENCES
* Extremely Fast Text Feature Extraction for Classification and
Indexing <http://www.hpl.hp.com/techreports/2008/HPL-2008-91R1.pdf>
by George Forman <http://www.hpl.hp.com/personal/George_Forman/> and
Evan Kirshenbaum <http://www.kirshenbaum.net/evan/index.htm>
* MinHash — выявляем похожие множества
<http://habrahabr.ru/post/115147/>
* Фильтр Блума <http://habrahabr.ru/post/112069/>
* cosine_cmp, minhash_cmp and uniq_wc utilities from this
distribution
AUTHOR
Stanislaw Pusep <stas@sysd.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2014 by Stanislaw Pusep.
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.