Hash::Util(3perl) Perl Programmers Reference Guide Hash::Util(3perl)
NAME
Hash::Util - A selection of general-utility hash subroutines
SYNOPSIS
# Restricted hashes
use Hash::Util qw(
fieldhash fieldhashes
all_keys
lock_keys unlock_keys
lock_value unlock_value
lock_hash unlock_hash
lock_keys_plus
hash_locked hash_unlocked
hashref_locked hashref_unlocked
hidden_keys legal_keys
lock_ref_keys unlock_ref_keys
lock_ref_value unlock_ref_value
lock_hashref unlock_hashref
lock_ref_keys_plus
hidden_ref_keys legal_ref_keys
hash_seed hash_value hv_store
bucket_stats bucket_info bucket_array
lock_hash_recurse unlock_hash_recurse
lock_hashref_recurse unlock_hashref_recurse
hash_traversal_mask
);
my %hash = (foo => 42, bar => 23);
# Ways to restrict a hash
lock_keys(%hash);
lock_keys(%hash, @keyset);
lock_keys_plus(%hash, @additional_keys);
# Ways to inspect the properties of a restricted hash
my @legal = legal_keys(%hash);
my @hidden = hidden_keys(%hash);
my $ref = all_keys(%hash,@keys,@hidden);
my $is_locked = hash_locked(%hash);
# Remove restrictions on the hash
unlock_keys(%hash);
# Lock individual values in a hash
lock_value (%hash, 'foo');
unlock_value(%hash, 'foo');
# Ways to change the restrictions on both keys and values
lock_hash (%hash);
unlock_hash(%hash);
my $hashes_are_randomised = hash_seed() !~ /^\0+$/;
my $int_hash_value = hash_value( 'string' );
my $mask= hash_traversal_mask(%hash);
hash_traversal_mask(%hash,1234);
DESCRIPTION
"Hash::Util" and "Hash::Util::FieldHash" contain special functions for
manipulating hashes that don't really warrant a keyword.
"Hash::Util" contains a set of functions that support restricted hashes.
These are described in this document. "Hash::Util::FieldHash" contains
an (unrelated) set of functions that support the use of hashes in
inside-out classes, described in Hash::Util::FieldHash.
By default "Hash::Util" does not export anything.
Restricted hashes
5.8.0 introduces the ability to restrict a hash to a certain set of
keys. No keys outside of this set can be added. It also introduces the
ability to lock an individual key so it cannot be deleted and the
ability to ensure that an individual value cannot be changed.
This is intended to largely replace the deprecated pseudo-hashes.
lock_keys
unlock_keys
lock_keys(%hash);
lock_keys(%hash, @keys);
Restricts the given %hash's set of keys to @keys. If @keys is not
given it restricts it to its current keyset. No more keys can be
added. delete() and exists() will still work, but will not alter the
set of allowed keys. Note: the current implementation prevents the
hash from being bless()ed while it is in a locked state. Any attempt
to do so will raise an exception. Of course you can still bless()
the hash before you call lock_keys() so this shouldn't be a problem.
unlock_keys(%hash);
Removes the restriction on the %hash's keyset.
Note that if any of the values of the hash have been locked they
will not be unlocked after this sub executes.
Both routines return a reference to the hash operated on.
lock_keys_plus
lock_keys_plus(%hash,@additional_keys)
Similar to lock_keys(), with the difference being that the optional
key list specifies keys that may or may not be already in the hash.
Essentially this is an easier way to say
lock_keys(%hash,@additional_keys,keys %hash);
Returns a reference to %hash
lock_value
unlock_value
lock_value (%hash, $key);
unlock_value(%hash, $key);
Locks and unlocks the value for an individual key of a hash. The
value of a locked key cannot be changed.
Unless %hash has already been locked the key/value could be deleted
regardless of this setting.
Returns a reference to the %hash.
lock_hash
unlock_hash
lock_hash(%hash);
lock_hash() locks an entire hash, making all keys and values read-
only. No value can be changed, no keys can be added or deleted.
unlock_hash(%hash);
unlock_hash() does the opposite of lock_hash(). All keys and values
are made writable. All values can be changed and keys can be added
and deleted.
Returns a reference to the %hash.
lock_hash_recurse
unlock_hash_recurse
lock_hash_recurse(%hash);
lock_hash() locks an entire hash and any hashes it references
recursively, making all keys and values read-only. No value can be
changed, no keys can be added or deleted.
This method only recurses into hashes that are referenced by another
hash. Thus a Hash of Hashes (HoH) will all be restricted, but a
Hash of Arrays of Hashes (HoAoH) will only have the top hash
restricted.
unlock_hash_recurse(%hash);
unlock_hash_recurse() does the opposite of lock_hash_recurse(). All
keys and values are made writable. All values can be changed and
keys can be added and deleted. Identical recursion restrictions
apply as to lock_hash_recurse().
Returns a reference to the %hash.
hashref_locked
hash_locked
hashref_locked(\%hash) and print "Hash is locked!\n";
hash_locked(%hash) and print "Hash is locked!\n";
Returns true if the hash and its keys are locked.
hashref_unlocked
hash_unlocked
hashref_unlocked(\%hash) and print "Hash is unlocked!\n";
hash_unlocked(%hash) and print "Hash is unlocked!\n";
Returns true if the hash and its keys are unlocked.
legal_keys
my @keys = legal_keys(%hash);
Returns the list of the keys that are legal in a restricted hash.
In the case of an unrestricted hash this is identical to calling
keys(%hash).
hidden_keys
my @keys = hidden_keys(%hash);
Returns the list of the keys that are legal in a restricted hash but
do not have a value associated to them. Thus if 'foo' is a "hidden"
key of the %hash it will return false for both "defined" and
"exists" tests.
In the case of an unrestricted hash this will return an empty list.
NOTE this is an experimental feature that is heavily dependent on
the current implementation of restricted hashes. Should the
implementation change, this routine may become meaningless, in which
case it will return an empty list.
all_keys
all_keys(%hash,@keys,@hidden);
Populates the arrays @keys with the all the keys that would pass an
"exists" tests, and populates @hidden with the remaining legal keys
that have not been utilized.
Returns a reference to the hash.
In the case of an unrestricted hash this will be equivalent to
$ref = do {
@keys = keys %hash;
@hidden = ();
\%hash
};
NOTE this is an experimental feature that is heavily dependent on
the current implementation of restricted hashes. Should the
implementation change this routine may become meaningless in which
case it will behave identically to how it would behave on an
unrestricted hash.
hash_seed
my $hash_seed = hash_seed();
hash_seed() returns the seed bytes used to randomise hash ordering.
Note that the hash seed is sensitive information: by knowing it one
can craft a denial-of-service attack against Perl code, even
remotely, see "Algorithmic Complexity Attacks" in perlsec for more
information. Do not disclose the hash seed to people who don't need
to know it. See also "PERL_HASH_SEED_DEBUG" in perlrun.
Prior to Perl 5.17.6 this function returned a UV, it now returns a
string, which may be of nearly any size as determined by the hash
function your Perl has been built with. Possible sizes may be but
are not limited to 4 bytes (for most hash algorithms) and 16 bytes
(for siphash).
hash_value
my $hash_value = hash_value($string);
my $hash_value = hash_value($string, $seed);
hash_value($string) returns the current perl's internal hash value
for a given string. "hash_value($string, $seed)" returns the hash
value as if computed with a different seed. If the custom seed is
too short, the function errors out. The minimum length of the seed
is implementation-dependent.
Returns a 32-bit integer representing the hash value of the string
passed in. The 1-parameter value is only reliable for the lifetime
of the process. It may be different depending on invocation,
environment variables, perl version, architectures, and build
options.
Note that the hash value of a given string is sensitive information:
by knowing it one can deduce the hash seed which in turn can allow
one to craft a denial-of-service attack against Perl code, even
remotely, see "Algorithmic Complexity Attacks" in perlsec for more
information. Do not disclose the hash value of a string to people
who don't need to know it. See also "PERL_HASH_SEED_DEBUG" in
perlrun.
bucket_info
Return a set of basic information about a hash.
my ($keys, $buckets, $used, @length_counts)= bucket_info($hash);
Fields are as follows:
0: Number of keys in the hash
1: Number of buckets in the hash
2: Number of used buckets in the hash
rest : list of counts, Kth element is the number of buckets
with K keys in it.
See also bucket_stats() and bucket_array().
bucket_stats
Returns a list of statistics about a hash.
my ($keys, $buckets, $used, $quality, $utilization_ratio,
$collision_pct, $mean, $stddev, @length_counts)
= bucket_stats($hashref);
Fields are as follows:
0: Number of keys in the hash
1: Number of buckets in the hash
2: Number of used buckets in the hash
3: Hash Quality Score
4: Percent of buckets used
5: Percent of keys which are in collision
6: Mean bucket length of occupied buckets
7: Standard Deviation of bucket lengths of occupied buckets
rest : list of counts, Kth element is the number of buckets
with K keys in it.
See also bucket_info() and bucket_array().
Note that Hash Quality Score would be 1 for an ideal hash, numbers
close to and below 1 indicate good hashing, and number significantly
above indicate a poor score. In practice it should be around 0.95 to
1.05. It is defined as:
$score= sum( $count[$length] * ($length * ($length + 1) / 2) )
/
( ( $keys / 2 * $buckets ) *
( $keys + ( 2 * $buckets ) - 1 ) )
The formula is from the Red Dragon book (reformulated to use the
data available) and is documented at
<http://www.strchr.com/hash_functions>
bucket_array
my $array= bucket_array(\%hash);
Returns a packed representation of the bucket array associated with
a hash. Each element of the array is either an integer K, in which
case it represents K empty buckets, or a reference to another array
which contains the keys that are in that bucket.
Note that the information returned by bucket_array is sensitive
information: by knowing it one can directly attack perl's hash
function which in turn may allow one to craft a denial-of-service
attack against Perl code, even remotely, see "Algorithmic Complexity
Attacks" in perlsec for more information. Do not disclose the
output of this function to people who don't need to know it. See
also "PERL_HASH_SEED_DEBUG" in perlrun. This function is provided
strictly for debugging and diagnostics purposes only, it is hard to
imagine a reason why it would be used in production code.
bucket_stats_formatted
print bucket_stats_formatted($hashref);
Return a formatted report of the information returned by
bucket_stats(). An example report looks like this:
Keys: 50 Buckets: 33/64 Quality-Score: 1.01 (Good)
Utilized Buckets: 51.56% Optimal: 78.12% Keys In Collision: 34.00%
Chain Length - mean: 1.52 stddev: 0.66
Buckets 64 [0000000000000000000000000000000111111111111111111122222222222333]
Len 0 Pct: 48.44 [###############################]
Len 1 Pct: 29.69 [###################]
Len 2 Pct: 17.19 [###########]
Len 3 Pct: 4.69 [###]
Keys 50 [11111111111111111111111111111111122222222222222333]
Pos 1 Pct: 66.00 [#################################]
Pos 2 Pct: 28.00 [##############]
Pos 3 Pct: 6.00 [###]
The first set of stats gives some summary statistical information,
including the quality score translated into "Good", "Poor" and
"Bad", (score<=1.05, score<=1.2, score>1.2). See the documentation
in bucket_stats() for more details.
The two sets of barcharts give stats and a visual indication of
performance of the hash.
The first gives data on bucket chain lengths and provides insight on
how much work a fetch *miss* will take. In this case we have to
inspect every item in a bucket before we can be sure the item is not
in the list. The performance for an insert is equivalent to this
case, as is a delete where the item is not in the hash.
The second gives data on how many keys are at each depth in the
chain, and gives an idea of how much work a fetch *hit* will take.
The performance for an update or delete of an item in the hash is
equivalent to this case.
Note that these statistics are summary only. Actual performance will
depend on real hit/miss ratios accessing the hash. If you are
concerned by hit ratios you are recommended to "oversize" your hash
by using something like:
keys(%hash)= keys(%hash) << $k;
With $k chosen carefully, and likely to be a small number like 1 or
2. In theory the larger the bucket array the less chance of
collision.
hv_store
my $sv = 0;
hv_store(%hash,$key,$sv) or die "Failed to alias!";
$hash{$key} = 1;
print $sv; # prints 1
Stores an alias to a variable in a hash instead of copying the
value.
hash_traversal_mask
As of Perl 5.18 every hash has its own hash traversal order, and
this order changes every time a new element is inserted into the
hash. This functionality is provided by maintaining an unsigned
integer mask (U32) which is xor'ed with the actual bucket id during
a traversal of the hash buckets using keys(), values() or each().
You can use this subroutine to get and set the traversal mask for a
specific hash. Setting the mask ensures that a given hash will
produce the same key order. Note that this does not guarantee that
two hashes will produce the same key order for the same hash seed
and traversal mask, items that collide into one bucket may have
different orders regardless of this setting.
bucket_ratio
This function behaves the same way that scalar(%hash) behaved prior
to Perl 5.25. Specifically if the hash is tied, then it calls the
SCALAR tied hash method, if untied then if the hash is empty it
return 0, otherwise it returns a string containing the number of
used buckets in the hash, followed by a slash, followed by the total
number of buckets in the hash.
my %hash=("foo"=>1);
print Hash::Util::bucket_ratio(%hash); # prints "1/8"
used_buckets
This function returns the count of used buckets in the hash. It is
expensive to calculate and the value is NOT cached, so avoid use of
this function in production code.
num_buckets
This function returns the total number of buckets the hash holds, or
would hold if the array were created. (When a hash is freshly
created the array may not be allocated even though this value will
be non-zero.)
Operating on references to hashes
Most subroutines documented in this module have equivalent versions that
operate on references to hashes instead of native hashes. The following
is a list of these subs. They are identical except in name and in that
instead of taking a %hash they take a $hashref, and additionally are not
prototyped.
lock_ref_keys
unlock_ref_keys
lock_ref_keys_plus
lock_ref_value
unlock_ref_value
lock_hashref
unlock_hashref
lock_hashref_recurse
unlock_hashref_recurse
hash_ref_unlocked
legal_ref_keys
hidden_ref_keys
CAVEATS
Note that the trapping of the restricted operations is not atomic: for
example
eval { %hash = (illegal_key => 1) }
leaves the %hash empty rather than with its original contents.
BUGS
The interface exposed by this module is very close to the current
implementation of restricted hashes. Over time it is expected that this
behavior will be extended and the interface abstracted further.
AUTHOR
Michael G Schwern <schwern@pobox.com> on top of code by Nick Ing-Simmons
and Jeffrey Friedl.
hv_store() is from Array::RefElem, Copyright 2000 Gisle Aas.
Additional code by Yves Orton.
Description of "hash_value($string, $seed)" by Christopher Yeleighton
<ne01026@shark.2a.pl>
SEE ALSO
Scalar::Util, List::Util and "Algorithmic Complexity Attacks" in
perlsec.
Hash::Util::FieldHash.
perl v5.40.1 2026-08-30 Hash::Util(3perl)
Generated by dwww version 1.16 on Sat Oct 3 07:43:13 CEST 2026.