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use strict;
package Tie::Memoize;
use Tie::Hash;
our @ISA = 'Tie::ExtraHash';
our $VERSION = '1.1';
our $exists_token = \undef;
sub croak {require Carp; goto &Carp::croak}
# Format: [0: STORAGE, 1: EXISTS-CACHE, 2: FETCH_function;
# 3: EXISTS_function, 4: DATA, 5: EXISTS_different ]
sub FETCH {
my ($h,$key) = ($_[0][0], $_[1]);
my $res = $h->{$key};
return $res if defined $res; # Shortcut if accessible
return $res if exists $h->{$key}; # Accessible, but undef
my $cache = $_[0][1]{$key};
return if defined $cache and not $cache; # Known to not exist
my @res = $_[0][2]->($key, $_[0][4]); # Autoload
$_[0][1]{$key} = 0, return unless @res; # Cache non-existence
delete $_[0][1]{$key}; # Clear existence cache, not needed any more
$_[0][0]{$key} = $res[0]; # Store data and return
}
sub EXISTS {
my ($a,$key) = (shift, shift);
return 1 if exists $a->[0]{$key}; # Have data
my $cache = $a->[1]{$key};
return $cache if defined $cache; # Existence cache
my @res = $a->[3]($key,$a->[4]);
$a->[1]{$key} = 0, return unless @res; # Cache non-existence
# Now we know it exists
return ($a->[1]{$key} = 1) if $a->[5]; # Only existence reported
# Now know the value
$a->[0]{$key} = $res[0]; # Store data
return 1
}
sub TIEHASH {
croak 'syntax: tie %hash, \'Tie::AutoLoad\', \&fetch_subr' if @_ < 2;
croak 'syntax: tie %hash, \'Tie::AutoLoad\', \&fetch_subr, $data, \&exists_subr, \%data_cache, \%existence_cache' if @_ > 6;
push @_, undef if @_ < 3; # Data
push @_, $_[1] if @_ < 4; # exists
push @_, {} while @_ < 6; # initial value and caches
bless [ @_[4,5,1,3,2], $_[1] ne $_[3]], $_[0]
}
1;
=head1 NAME
Tie::Memoize - add data to hash when needed
=head1 SYNOPSIS
require Tie::Memoize;
tie %hash, 'Tie::Memoize',
\&fetch, # The rest is optional
$DATA, \&exists,
{%ini_value}, {%ini_existence};
=head1 DESCRIPTION
This package allows a tied hash to autoload its values on the first access,
and to use the cached value on the following accesses.
Only read-accesses (via fetching the value or C<exists>) result in calls to
the functions; the modify-accesses are performed as on a normal hash.
The required arguments during C<tie> are the hash, the package, and
the reference to the C<FETCH>ing function. The optional arguments are
an arbitrary scalar $data, the reference to the C<EXISTS> function,
and initial values of the hash and of the existence cache.
Both the C<FETCH>ing function and the C<EXISTS> functions have the
same signature: the arguments are C<$key, $data>; $data is the same
value as given as argument during tie()ing. Both functions should
return an empty list if the value does not exist. If C<EXISTS>
function is different from the C<FETCH>ing function, it should return
a TRUE value on success. The C<FETCH>ing function should return the
intended value if the key is valid.
=head1 Inheriting from B<Tie::Memoize>
The structure of the tied() data is an array reference with elements
0: cache of known values
1: cache of known existence of keys
2: FETCH function
3: EXISTS function
4: $data
The rest is for internal usage of this package. In particular, if
TIEHASH is overwritten, it should call SUPER::TIEHASH.
=head1 EXAMPLE
sub slurp {
my ($key, $dir) = shift;
open my $h, '<', "$dir/$key" or return;
local $/; <$h> # slurp it all
}
sub exists { my ($key, $dir) = shift; return -f "$dir/$key" }
tie %hash, 'Tie::Memoize', \&slurp, $directory, \&exists,
{ fake_file1 => $content1, fake_file2 => $content2 },
{ pretend_does_not_exists => 0, known_to_exist => 1 };
This example treats the slightly modified contents of $directory as a
hash. The modifications are that the keys F<fake_file1> and
F<fake_file2> fetch values $content1 and $content2, and
F<pretend_does_not_exists> will never be accessed. Additionally, the
existence of F<known_to_exist> is never checked (so if it does not
exists when its content is needed, the user of %hash may be confused).
=head1 BUGS
FIRSTKEY and NEXTKEY methods go through the keys which were already read,
not all the possible keys of the hash.
=head1 AUTHOR
Ilya Zakharevich L<mailto:perl-module-hash-memoize@ilyaz.org>.
=cut
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