NAME
Digest::FNV::XS - Fowler/Noll/Vo (FNV) hashes
SYNOPSIS
use Digest::FNV::XS; # nothing exported by default
DESCRIPTION
This module is more or less a faster version of Digest::FNV, that
additionally supports binary data, incremental hashing, more FNV
variants and more. The API isn't compatible (and neither are the
generated hash values. The hash values computed by this module match the
official FNV hash values as documented on
<
http://www.isthe.com/chongo/tech/comp/fnv/>).
$hash = Digest::FNV::XS::fnv1a_32 $data[, $init]
$hash = Digest::FNV::XS::fnv1a_64 $data[, $init]
Compute the 32 or 64 bit FNV-1a hash of the given string.
$init is the optional initialisation value, allowing incremental
hashing. If missing or "undef" then the appropriate FNV constant is
used.
The 64 bit variant is only available when perl was compiled with 64
bit support.
The FNV-1a algorithm is the preferred variant, as it has slightly
higher quality and speed then FNV-1.
$hash = Digest::FNV::XS::fnv1_32 $data[, $init]
$hash = Digest::FNV::XS::fnv1_64 $data[, $init]
Compute the 32 or 64 bit FNV-1 hash of the given string.
$init is the optional initialisation value, allowing incremental
hashing. If missing or "undef" then the appropriate FNV constant is
used.
The 64 bit variant is only available when perl was compiled with 64
bit support.
The FNV-1a variant is preferable if you can choose.
$hash = Digest::FNV::XS::fnv0_32 $data[, $init]
$hash = Digest::FNV::XS::fnv0_64 $data[, $init]
The obsolete FNV-0 algorithm. Same as calling the FNV1 variant with
"$init = 0".
$init is the optional initialisation value, allowing incremental
hashing. If missing or "undef" then the appropriate FNV constant is
used.
The 64 bit variant is only available when perl was compiled with 64
bit support.
$hash = Digest::FNV::XS::xorfold_32 $hash, $bits
$hash = Digest::FNV::XS::xorfold_64 $hash, $bits
XOR-folds the 32 (64) bit FNV hash to $bits bits, which can be any
value between 1 and 32 (64) inclusive.
XOR-folding is a good method to reduce the FNV hash to a power of
two range.
$hash = Digest::FNV::XS::reduce_32 $hash, $range
$hash = Digest::FNV::XS::reduce_64 $hash, $range
These two functions can be used to reduce a 32 (64) but FNV hash to
an integer in the range 0 .. $range, using the retry method, which
distributes any bias more evenly.
INCREMENTAL HASHING
You can hash data incrementally by feeding the previous hahs value as
$init argument for the next call, for example:
$hash = fnv1a_32 $data1;
$hash = fnv1a_32 $data2, $hash; # and so on
Or in a loop (relying on the fact that $hash is "undef" initially):
my $hash;
$hash = fnv1a_32 $_, $hash
for ...;
REDUCIDNG THE HASH VALUE
A common problem is to reduce the 32 (64) bit FNV hash value to a
smaller range, 0 .. $range.
The easiest method to do that, is to mask (For power of two) or modulo
(for other values) the hash value, i.e.:
$inrage = $hash & ($range - 1) # for $range values that are power of two
$inrage = $hash % $range # for any range
This is called the lazy mod mapping method, which creates small biases
that rarely cause any problems in practise.
Nevertheless, you can improve the distribution of the bias by using *XOR
folding*, for power of two ranges (and 32 bit hashews, there is also
"forfold_64")
$inrage = Digest::FNV::XS::xorfold_32 $hash, $log2_of_range
And, using the retry method, for generic ranges (and 32 bit hashes,
there is also "reduce_64"):
$inrange = Digest::FNX::XS::reduce_32 $hash, $range
AUTHOR
Marc Lehmann <
[email protected]>
http://software.schmorp.de/pkg/Digest-FNV-XS.html
