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.\" ========================================================================
.\"
.IX Title "Hash::Util 3pm"
.TH Hash::Util 3pm "2013-03-04" "perl v5.16.3" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
Hash::Util \- A selection of general\-utility hash subroutines
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&  # Restricted hashes
\&
\&  use Hash::Util qw(
\&                     hash_seed all_keys
\&                     lock_keys unlock_keys
\&                     lock_value unlock_value
\&                     lock_hash unlock_hash
\&                     lock_keys_plus hash_locked
\&                     hidden_keys legal_keys
\&                   );
\&
\&  %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, \*(Aqfoo\*(Aq);
\&  unlock_value(%hash, \*(Aqfoo\*(Aq);
\&
\&  # Ways to change the restrictions on both keys and values
\&  lock_hash  (%hash);
\&  unlock_hash(%hash);
\&
\&  my $hashes_are_randomised = hash_seed() != 0;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`Hash::Util\*(C'\fR and \f(CW\*(C`Hash::Util::FieldHash\*(C'\fR contain special functions
for manipulating hashes that don't really warrant a keyword.
.PP
\&\f(CW\*(C`Hash::Util\*(C'\fR contains a set of functions that support
restricted hashes. These are described in
this document.  \f(CW\*(C`Hash::Util::FieldHash\*(C'\fR contains an (unrelated)
set of functions that support the use of hashes in
\&\fIinside-out classes\fR, described in Hash::Util::FieldHash.
.PP
By default \f(CW\*(C`Hash::Util\*(C'\fR does not export anything.
.SS "Restricted hashes"
.IX Subsection "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.
.PP
This is intended to largely replace the deprecated pseudo-hashes.
.IP "\fBlock_keys\fR" 4
.IX Item "lock_keys"
.PD 0
.IP "\fBunlock_keys\fR" 4
.IX Item "unlock_keys"
.PD
.Vb 2
\&  lock_keys(%hash);
\&  lock_keys(%hash, @keys);
.Ve
.Sp
Restricts the given \f(CW%hash\fR's set of keys to \f(CW@keys\fR.  If \f(CW@keys\fR is not
given it restricts it to its current keyset.  No more keys can be
added. \fIdelete()\fR and \fIexists()\fR will still work, but will not alter
the set of allowed keys. \fBNote\fR: the current implementation prevents
the hash from being \fIbless()\fRed while it is in a locked state. Any attempt
to do so will raise an exception. Of course you can still \fIbless()\fR
the hash before you call \fIlock_keys()\fR so this shouldn't be a problem.
.Sp
.Vb 1
\&  unlock_keys(%hash);
.Ve
.Sp
Removes the restriction on the \f(CW%hash\fR's keyset.
.Sp
\&\fBNote\fR that if any of the values of the hash have been locked they will not be unlocked
after this sub executes.
.Sp
Both routines return a reference to the hash operated on.
.IP "\fBlock_keys_plus\fR" 4
.IX Item "lock_keys_plus"
.Vb 1
\&  lock_keys_plus(%hash,@additional_keys)
.Ve
.Sp
Similar to \f(CW\*(C`lock_keys()\*(C'\fR, 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
.Sp
.Vb 1
\&  lock_keys(%hash,@additional_keys,keys %hash);
.Ve
.Sp
Returns a reference to \f(CW%hash\fR
.IP "\fBlock_value\fR" 4
.IX Item "lock_value"
.PD 0
.IP "\fBunlock_value\fR" 4
.IX Item "unlock_value"
.PD
.Vb 2
\&  lock_value  (%hash, $key);
\&  unlock_value(%hash, $key);
.Ve
.Sp
Locks and unlocks the value for an individual key of a hash.  The value of a
locked key cannot be changed.
.Sp
Unless \f(CW%hash\fR has already been locked the key/value could be deleted
regardless of this setting.
.Sp
Returns a reference to the \f(CW%hash\fR.
.IP "\fBlock_hash\fR" 4
.IX Item "lock_hash"
.PD 0
.IP "\fBunlock_hash\fR" 4
.IX Item "unlock_hash"
.PD
.Vb 1
\&    lock_hash(%hash);
.Ve
.Sp
\&\fIlock_hash()\fR locks an entire hash, making all keys and values read-only.
No value can be changed, no keys can be added or deleted.
.Sp
.Vb 1
\&    unlock_hash(%hash);
.Ve
.Sp
\&\fIunlock_hash()\fR does the opposite of \fIlock_hash()\fR.  All keys and values
are made writable.  All values can be changed and keys can be added
and deleted.
.Sp
Returns a reference to the \f(CW%hash\fR.
.IP "\fBlock_hash_recurse\fR" 4
.IX Item "lock_hash_recurse"
.PD 0
.IP "\fBunlock_hash_recurse\fR" 4
.IX Item "unlock_hash_recurse"
.PD
.Vb 1
\&    lock_hash_recurse(%hash);
.Ve
.Sp
\&\fIlock_hash()\fR 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.
.Sp
\&\fBOnly\fR 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.
.Sp
.Vb 1
\&    unlock_hash_recurse(%hash);
.Ve
.Sp
\&\fIunlock_hash_recurse()\fR does the opposite of \fIlock_hash_recurse()\fR.  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 \fIlock_hash_recurse()\fR.
.Sp
Returns a reference to the \f(CW%hash\fR.
.IP "\fBhash_unlocked\fR" 4
.IX Item "hash_unlocked"
.Vb 1
\&  hash_unlocked(%hash) and print "Hash is unlocked!\en";
.Ve
.Sp
Returns true if the hash and its keys are unlocked.
.IP "\fBlegal_keys\fR" 4
.IX Item "legal_keys"
.Vb 1
\&  my @keys = legal_keys(%hash);
.Ve
.Sp
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).
.IP "\fBhidden_keys\fR" 4
.IX Item "hidden_keys"
.Vb 1
\&  my @keys = hidden_keys(%hash);
.Ve
.Sp
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
\&\*(L"hidden\*(R" key of the \f(CW%hash\fR it will return false for both \f(CW\*(C`defined\*(C'\fR
and \f(CW\*(C`exists\*(C'\fR tests.
.Sp
In the case of an unrestricted hash this will return an empty list.
.Sp
\&\fB\s-1NOTE\s0\fR 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.
.IP "\fBall_keys\fR" 4
.IX Item "all_keys"
.Vb 1
\&  all_keys(%hash,@keys,@hidden);
.Ve
.Sp
Populates the arrays \f(CW@keys\fR with the all the keys that would pass
an \f(CW\*(C`exists\*(C'\fR tests, and populates \f(CW@hidden\fR with the remaining legal
keys that have not been utilized.
.Sp
Returns a reference to the hash.
.Sp
In the case of an unrestricted hash this will be equivalent to
.Sp
.Vb 5
\&  $ref = do {
\&      @keys = keys %hash;
\&      @hidden = ();
\&      \e%hash
\&  };
.Ve
.Sp
\&\fB\s-1NOTE\s0\fR 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.
.IP "\fBhash_seed\fR" 4
.IX Item "hash_seed"
.Vb 1
\&    my $hash_seed = hash_seed();
.Ve
.Sp
\&\fIhash_seed()\fR returns the seed number used to randomise hash ordering.
Zero means the \*(L"traditional\*(R" random hash ordering, non-zero means the
new even more random hash ordering introduced in Perl 5.8.1.
.Sp
\&\fBNote that the hash seed is sensitive information\fR: by knowing it one
can craft a denial-of-service attack against Perl code, even remotely,
see \*(L"Algorithmic Complexity Attacks\*(R" in perlsec for more information.
\&\fBDo not disclose the hash seed\fR to people who don't need to know it.
See also \*(L"\s-1PERL_HASH_SEED_DEBUG\s0\*(R" in perlrun.
.IP "\fBhv_store\fR" 4
.IX Item "hv_store"
.Vb 4
\&  my $sv = 0;
\&  hv_store(%hash,$key,$sv) or die "Failed to alias!";
\&  $hash{$key} = 1;
\&  print $sv; # prints 1
.Ve
.Sp
Stores an alias to a variable in a hash instead of copying the value.
.SS "Operating on references to hashes."
.IX Subsection "Operating on references to hashes."
Most subroutines documented in this module have equivalent versions
that operate on references to hashes instead of native hashes.
The following is a list of these subs. They are identical except
in name and in that instead of taking a \f(CW%hash\fR they take a \f(CW$hashref\fR,
and additionally are not prototyped.
.IP "lock_ref_keys" 4
.IX Item "lock_ref_keys"
.PD 0
.IP "unlock_ref_keys" 4
.IX Item "unlock_ref_keys"
.IP "lock_ref_keys_plus" 4
.IX Item "lock_ref_keys_plus"
.IP "lock_ref_value" 4
.IX Item "lock_ref_value"
.IP "unlock_ref_value" 4
.IX Item "unlock_ref_value"
.IP "lock_hashref" 4
.IX Item "lock_hashref"
.IP "unlock_hashref" 4
.IX Item "unlock_hashref"
.IP "lock_hashref_recurse" 4
.IX Item "lock_hashref_recurse"
.IP "unlock_hashref_recurse" 4
.IX Item "unlock_hashref_recurse"
.IP "hash_ref_unlocked" 4
.IX Item "hash_ref_unlocked"
.IP "legal_ref_keys" 4
.IX Item "legal_ref_keys"
.IP "hidden_ref_keys" 4
.IX Item "hidden_ref_keys"
.PD
.SH "CAVEATS"
.IX Header "CAVEATS"
Note that the trapping of the restricted operations is not atomic:
for example
.PP
.Vb 1
\&    eval { %hash = (illegal_key => 1) }
.Ve
.PP
leaves the \f(CW%hash\fR empty rather than with its original contents.
.SH "BUGS"
.IX Header "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.
.SH "AUTHOR"
.IX Header "AUTHOR"
Michael G Schwern <schwern@pobox.com> on top of code by Nick
Ing-Simmons and Jeffrey Friedl.
.PP
\&\fIhv_store()\fR is from Array::RefElem, Copyright 2000 Gisle Aas.
.PP
Additional code by Yves Orton.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Scalar::Util, List::Util and \*(L"Algorithmic Complexity Attacks\*(R" in perlsec.
.PP
Hash::Util::FieldHash.
