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Hash::Util(3pm)        Perl Programmers Reference Guide        Hash::Util(3pm)




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
                          );

         %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);

           hash_value() returns the current perl's internal hash value for a
           given string.

           Returns a 32 bit integer representing the hash value of the string
           passed in. This 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.

   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.


SEE ALSO

       Scalar::Util(3), List::Util(3) and "Algorithmic Complexity Attacks" in
       perlsec(1).

       Hash::Util::FieldHash(3).



perl v5.24.0                      2016-03-01                   Hash::Util(3pm)

perl 5.24 - Generated Sun Nov 13 14:33:41 CST 2016
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