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) result in calls to the functions; the modify-accesses are performed as on a normal hash. The required arguments during C are the hash, the package, and the reference to the Cing function. The optional arguments are an arbitrary scalar $data, the reference to the C function, and initial values of the hash and of the existence cache. Both the Cing function and the C functions have the same signature: the arguments are C<$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 function is different from the Cing function, it should return a TRUE value on success. The Cing function should return the intended value if the key is valid. =head1 Inheriting from B 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 and F fetch values $content1 and $content2, and F will never be accessed. Additionally, the existence of F 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. =cut