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1 | use strict; |
2 | package Tie::Memoize; |
3 | use Tie::Hash; |
4 | our @ISA = 'Tie::ExtraHash'; |
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5 | our $VERSION = '1.0'; |
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6 | |
7 | our $exists_token = \undef; |
8 | |
9 | sub croak {require Carp; goto &Carp::croak} |
10 | |
11 | # Format: [0: STORAGE, 1: EXISTS-CACHE, 2: FETCH_function; |
12 | # 3: EXISTS_function, 4: DATA, 5: EXISTS_different ] |
13 | |
14 | sub FETCH { |
15 | my ($h,$key) = ($_[0][0], $_[1]); |
16 | my $res = $h->{$key}; |
17 | return $res if defined $res; # Shortcut if accessible |
18 | return $res if exists $h->{$key}; # Accessible, but undef |
19 | my $cache = $_[0][1]{$key}; |
20 | return if defined $cache and not $cache; # Known to not exist |
21 | my @res = $_[0][2]->($key, $_[0][4]); # Autoload |
22 | $_[0][1]{$key} = 0, return unless @res; # Cache non-existence |
23 | delete $_[0][1]{$key}; # Clear existence cache, not needed any more |
24 | $_[0][0]{$key} = $res[0]; # Store data and return |
25 | } |
26 | |
27 | sub EXISTS { |
28 | my ($a,$key) = (shift, shift); |
29 | return 1 if exists $a->[0]{$key}; # Have data |
30 | my $cache = $a->[1]{$key}; |
31 | return $cache if defined $cache; # Existence cache |
32 | my @res = $a->[3]($key,$a->[4]); |
33 | $_[0][1]{$key} = 0, return unless @res; # Cache non-existence |
34 | # Now we know it exists |
35 | return ($_[0][1]{$key} = 1) if $a->[5]; # Only existence reported |
36 | # Now know the value |
37 | $_[0][0]{$key} = $res[0]; # Store data |
38 | return 1 |
39 | } |
40 | |
41 | sub TIEHASH { |
42 | croak 'syntax: tie %hash, \'Tie::AutoLoad\', \&fetch_subr' if @_ < 2; |
43 | croak 'syntax: tie %hash, \'Tie::AutoLoad\', \&fetch_subr, $data, \&exists_subr, \%data_cache, \%existence_cache' if @_ > 6; |
44 | push @_, undef if @_ < 3; # Data |
45 | push @_, $_[1] if @_ < 4; # exists |
46 | push @_, {} while @_ < 6; # initial value and caches |
47 | bless [ @_[4,5,1,3,2], $_[1] ne $_[3]], $_[0] |
48 | } |
49 | |
50 | 1; |
51 | |
52 | =head1 NAME |
53 | |
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54 | Tie::Memoize - add data to hash when needed |
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55 | |
56 | =head1 SYNOPSIS |
57 | |
58 | require Tie::Memoize; |
59 | tie %hash, 'Tie::Memoize', |
60 | \&fetch, # The rest is optional |
61 | $DATA, \&exists, |
62 | {%ini_value}, {%ini_existence}; |
63 | |
64 | =head1 DESCRIPTION |
65 | |
66 | This package allows a tied hash to autoload its values on the first access, |
67 | and to use the cached value on the following accesses. |
68 | |
69 | Only read-accesses (via fetching the value or C<exists>) result in calls to |
70 | the functions; the modify-accesses are performed as on a normal hash. |
71 | |
72 | The required arguments during C<tie> are the hash, the package, and |
73 | the reference to the C<FETCH>ing function. The optional arguments are |
74 | an arbitrary scalar $data, the reference to the C<EXISTS> function, |
75 | and initial values of the hash and of the existence cache. |
76 | |
77 | Both the C<FETCH>ing function and the C<EXISTS> functions have the |
78 | same signature: the arguments are C<$key, $data>; $data is the same |
79 | value as given as argument during tie()ing. Both functions should |
80 | return an empty list if the value does not exist. If C<EXISTS> |
81 | function is different from the C<FETCH>ing function, it should return |
82 | a TRUE value on success. The C<FETCH>ing function should return the |
83 | intended value if the key is valid. |
84 | |
85 | =head1 Inheriting from B<Tie::Memoize> |
86 | |
87 | The structure of the tied() data is an array reference with elements |
88 | |
89 | 0: cache of known values |
90 | 1: cache of known existence of keys |
91 | 2: FETCH function |
92 | 3: EXISTS function |
93 | 4: $data |
94 | |
95 | The rest is for internal usage of this package. In particular, if |
96 | TIEHASH is overwritten, it should call SUPER::TIEHASH. |
97 | |
98 | =head1 EXAMPLE |
99 | |
100 | sub slurp { |
101 | my ($key, $dir) = shift; |
102 | open my $h, '<', "$dir/$key" or return; |
103 | local $/; <$h> # slurp it all |
104 | } |
105 | sub exists { my ($key, $dir) = shift; return -f "$dir/$key" } |
106 | |
107 | tie %hash, 'Tie::Memoize', \&slurp, $directory, \&exists, |
108 | { fake_file1 => $content1, fake_file2 => $content2 }, |
109 | { pretend_does_not_exists => 0, known_to_exist => 1 }; |
110 | |
111 | This example treats the slightly modified contents of $directory as a |
112 | hash. The modifications are that the keys F<fake_file1> and |
113 | F<fake_file2> fetch values $content1 and $content2, and |
114 | F<pretend_does_not_exists> will never be accessed. Additionally, the |
115 | existence of F<known_to_exist> is never checked (so if it does not |
116 | exists when its content is needed, the user of %hash may be confused). |
117 | |
118 | =head1 BUGS |
119 | |
120 | FIRSTKEY and NEXTKEY methods go through the keys which were already read, |
121 | not all the possible keys of the hash. |
122 | |
123 | =head1 AUTHOR |
124 | |
125 | Ilya Zakharevich L<mailto:perl-module-hash-memoize@ilyaz.org>. |
126 | |
127 | =cut |
128 | |