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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

			    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, $utilization_ratio, $collision_pct,
		   $mean, $stddev, @length_counts) = bucket_info($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: Average bucket length
	       7: Standard Deviation of bucket lengths.
	       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.

       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, List::Util and "Algorithmic Complexity Attacks" in
       perlsec.

       Hash::Util::FieldHash.

perl v5.18.2			  2013-11-14		     Hash::Util(3perl)
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