[14489] | 1 | package DBM_Filter ;
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| 2 |
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| 3 | use strict;
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| 4 | use warnings;
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| 5 | our $VERSION = '0.01';
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| 6 |
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| 7 | package Tie::Hash ;
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| 8 |
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| 9 | use strict;
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| 10 | use warnings;
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| 11 |
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| 12 | use Carp;
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| 13 |
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| 14 |
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| 15 | our %LayerStack = ();
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| 16 | our %origDESTROY = ();
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| 17 |
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| 18 | our %Filters = map { $_, undef } qw(
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| 19 | Fetch_Key
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| 20 | Fetch_Value
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| 21 | Store_Key
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| 22 | Store_Value
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| 23 | );
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| 24 |
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| 25 | our %Options = map { $_, 1 } qw(
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| 26 | fetch
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| 27 | store
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| 28 | );
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| 29 |
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| 30 | #sub Filter_Enable
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| 31 | #{
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| 32 | #}
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| 33 | #
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| 34 | #sub Filter_Disable
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| 35 | #{
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| 36 | #}
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| 37 |
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| 38 | sub Filtered
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| 39 | {
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| 40 | my $this = shift;
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| 41 | return defined $LayerStack{$this} ;
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| 42 | }
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| 43 |
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| 44 | sub Filter_Pop
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| 45 | {
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| 46 | my $this = shift;
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| 47 | my $stack = $LayerStack{$this} || return undef ;
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| 48 | my $filter = pop @{ $stack };
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| 49 |
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| 50 | # remove the filter hooks if this is the last filter to pop
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| 51 | if ( @{ $stack } == 0 ) {
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| 52 | $this->filter_store_key ( undef );
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| 53 | $this->filter_store_value( undef );
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| 54 | $this->filter_fetch_key ( undef );
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| 55 | $this->filter_fetch_value( undef );
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| 56 | delete $LayerStack{$this};
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| 57 | }
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| 58 |
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| 59 | return $filter;
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| 60 | }
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| 61 |
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| 62 | sub Filter_Key_Push
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| 63 | {
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| 64 | &_do_Filter_Push;
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| 65 | }
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| 66 |
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| 67 | sub Filter_Value_Push
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| 68 | {
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| 69 | &_do_Filter_Push;
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| 70 | }
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| 71 |
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| 72 |
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| 73 | sub Filter_Push
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| 74 | {
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| 75 | &_do_Filter_Push;
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| 76 | }
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| 77 |
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| 78 | sub _do_Filter_Push
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| 79 | {
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| 80 | my $this = shift;
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| 81 | my %callbacks = ();
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| 82 | my $caller = (caller(1))[3];
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| 83 | $caller =~ s/^.*:://;
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| 84 |
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| 85 | croak "$caller: no parameters present" unless @_ ;
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| 86 |
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| 87 | if ( ! $Options{lc $_[0]} ) {
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| 88 | my $class = shift;
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| 89 | my @params = @_;
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| 90 |
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| 91 | # if $class already contains "::", don't prefix "DBM_Filter::"
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| 92 | $class = "DBM_Filter::$class" unless $class =~ /::/;
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| 93 |
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| 94 | # does the "DBM_Filter::$class" exist?
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| 95 | if ( ! defined %{ "${class}::"} ) {
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| 96 | # Nope, so try to load it.
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| 97 | eval " require $class ; " ;
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| 98 | croak "$caller: Cannot Load DBM Filter '$class': $@" if $@;
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| 99 | }
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| 100 |
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| 101 | no strict 'refs';
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| 102 | my $fetch = *{ "${class}::Fetch" }{CODE};
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| 103 | my $store = *{ "${class}::Store" }{CODE};
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| 104 | my $filter = *{ "${class}::Filter" }{CODE};
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| 105 | use strict 'refs';
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| 106 |
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| 107 | my $count = defined($filter) + defined($store) + defined($fetch) ;
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| 108 |
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| 109 | if ( $count == 0 )
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| 110 | { croak "$caller: No methods (Filter, Fetch or Store) found in class '$class'" }
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| 111 | elsif ( $count == 1 && ! defined $filter) {
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| 112 | my $need = defined($fetch) ? 'Store' : 'Fetch';
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| 113 | croak "$caller: Missing method '$need' in class '$class'" ;
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| 114 | }
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| 115 | elsif ( $count >= 2 && defined $filter)
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| 116 | { croak "$caller: Can't mix Filter with Store and Fetch in class '$class'" }
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| 117 |
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| 118 | if (defined $filter) {
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| 119 | my $callbacks = &{ $filter }(@params);
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| 120 | croak "$caller: '${class}::Filter' did not return a hash reference"
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| 121 | unless ref $callbacks && ref $callbacks eq 'HASH';
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| 122 | %callbacks = %{ $callbacks } ;
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| 123 | }
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| 124 | else {
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| 125 | $callbacks{Fetch} = $fetch;
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| 126 | $callbacks{Store} = $store;
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| 127 | }
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| 128 | }
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| 129 | else {
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| 130 | croak "$caller: not even params" unless @_ % 2 == 0;
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| 131 | %callbacks = @_;
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| 132 | }
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| 133 |
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| 134 | my %filters = %Filters ;
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| 135 | my @got = ();
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| 136 | while (my ($k, $v) = each %callbacks )
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| 137 | {
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| 138 | my $key = $k;
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| 139 | $k = lc $k;
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| 140 | if ($k eq 'fetch') {
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| 141 | push @got, 'Fetch';
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| 142 | if ($caller eq 'Filter_Push')
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| 143 | { $filters{Fetch_Key} = $filters{Fetch_Value} = $v }
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| 144 | elsif ($caller eq 'Filter_Key_Push')
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| 145 | { $filters{Fetch_Key} = $v }
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| 146 | elsif ($caller eq 'Filter_Value_Push')
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| 147 | { $filters{Fetch_Value} = $v }
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| 148 | }
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| 149 | elsif ($k eq 'store') {
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| 150 | push @got, 'Store';
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| 151 | if ($caller eq 'Filter_Push')
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| 152 | { $filters{Store_Key} = $filters{Store_Value} = $v }
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| 153 | elsif ($caller eq 'Filter_Key_Push')
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| 154 | { $filters{Store_Key} = $v }
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| 155 | elsif ($caller eq 'Filter_Value_Push')
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| 156 | { $filters{Store_Value} = $v }
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| 157 | }
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| 158 | else
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| 159 | { croak "$caller: Unknown key '$key'" }
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| 160 |
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| 161 | croak "$caller: value associated with key '$key' is not a code reference"
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| 162 | unless ref $v && ref $v eq 'CODE';
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| 163 | }
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| 164 |
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| 165 | if ( @got != 2 ) {
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| 166 | push @got, 'neither' if @got == 0 ;
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| 167 | croak "$caller: expected both Store & Fetch - got @got";
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| 168 | }
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| 169 |
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| 170 | # remember the class
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| 171 | push @{ $LayerStack{$this} }, \%filters ;
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| 172 |
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| 173 | my $str_this = "$this" ; # Avoid a closure with $this in the subs below
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| 174 |
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| 175 | $this->filter_store_key ( sub { store_hook($str_this, 'Store_Key') });
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| 176 | $this->filter_store_value( sub { store_hook($str_this, 'Store_Value') });
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| 177 | $this->filter_fetch_key ( sub { fetch_hook($str_this, 'Fetch_Key') });
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| 178 | $this->filter_fetch_value( sub { fetch_hook($str_this, 'Fetch_Value') });
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| 179 |
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| 180 | # Hijack the callers DESTROY method
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| 181 | $this =~ /^(.*)=/;
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| 182 | my $type = $1 ;
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| 183 | no strict 'refs';
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| 184 | if ( *{ "${type}::DESTROY" }{CODE} ne \&MyDESTROY )
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| 185 | {
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| 186 | $origDESTROY{$type} = *{ "${type}::DESTROY" }{CODE};
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| 187 | no warnings 'redefine';
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| 188 | *{ "${type}::DESTROY" } = \&MyDESTROY ;
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| 189 | }
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| 190 | }
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| 191 |
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| 192 | sub store_hook
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| 193 | {
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| 194 | my $this = shift ;
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| 195 | my $type = shift ;
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| 196 | foreach my $layer (@{ $LayerStack{$this} })
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| 197 | {
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| 198 | &{ $layer->{$type} }() if defined $layer->{$type} ;
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| 199 | }
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| 200 | }
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| 201 |
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| 202 | sub fetch_hook
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| 203 | {
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| 204 | my $this = shift ;
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| 205 | my $type = shift ;
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| 206 | foreach my $layer (reverse @{ $LayerStack{$this} })
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| 207 | {
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| 208 | &{ $layer->{$type} }() if defined $layer->{$type} ;
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| 209 | }
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| 210 | }
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| 211 |
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| 212 | sub MyDESTROY
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| 213 | {
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| 214 | my $this = shift ;
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| 215 | delete $LayerStack{$this} ;
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| 216 |
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| 217 | # call real DESTROY
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| 218 | $this =~ /^(.*)=/;
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| 219 | &{ $origDESTROY{$1} }($this);
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| 220 | }
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| 221 |
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| 222 | 1;
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| 223 |
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| 224 | __END__
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| 225 |
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| 226 | =head1 NAME
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| 227 |
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| 228 | DBM_Filter -- Filter DBM keys/values
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| 229 |
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| 230 | =head1 SYNOPSIS
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| 231 |
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| 232 | use DBM_Filter ;
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| 233 | use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File
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| 234 |
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| 235 | $db = tie %hash, ...
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| 236 |
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| 237 | $db->Filter_Push(Fetch => sub {...},
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| 238 | Store => sub {...});
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| 239 |
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| 240 | $db->Filter_Push('my_filter1');
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| 241 | $db->Filter_Push('my_filter2', params...);
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| 242 |
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| 243 | $db->Filter_Key_Push(...) ;
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| 244 | $db->Filter_Value_Push(...) ;
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| 245 |
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| 246 | $db->Filter_Pop();
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| 247 | $db->Filtered();
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| 248 |
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| 249 | package DBM_Filter::my_filter1;
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| 250 |
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| 251 | sub Store { ... }
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| 252 | sub Fetch { ... }
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| 253 |
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| 254 | 1;
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| 255 |
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| 256 | package DBM_Filter::my_filter2;
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| 257 |
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| 258 | sub Filter
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| 259 | {
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| 260 | my @opts = @_;
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| 261 | ...
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| 262 | return (
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| 263 | sub Store { ... },
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| 264 | sub Fetch { ... } );
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| 265 | }
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| 266 |
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| 267 | 1;
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| 268 |
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| 269 | =head1 DESCRIPTION
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| 270 |
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| 271 | This module provides an interface that allows filters to be applied
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| 272 | to tied Hashes associated with DBM files. It builds on the DBM Filter
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| 273 | hooks that are present in all the *DB*_File modules included with the
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| 274 | standard Perl source distribution from version 5.6.1 onwards. In addition
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| 275 | to the *DB*_File modules distributed with Perl, the BerkeleyDB module,
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| 276 | available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter>
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| 277 | for more details on the DBM Filter hooks.
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| 278 |
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| 279 | =head1 What is a DBM Filter?
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| 280 |
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| 281 | A DBM Filter allows the keys and/or values in a tied hash to be modified
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| 282 | by some user-defined code just before it is written to the DBM file and
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| 283 | just after it is read back from the DBM file. For example, this snippet
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| 284 | of code
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| 285 |
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| 286 | $some_hash{"abc"} = 42;
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| 287 |
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| 288 | could potentially trigger two filters, one for the writing of the key
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| 289 | "abc" and another for writing the value 42. Similarly, this snippet
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| 290 |
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| 291 | my ($key, $value) = each %some_hash
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| 292 |
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| 293 | will trigger two filters, one for the reading of the key and one for
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| 294 | the reading of the value.
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| 295 |
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| 296 | Like the existing DBM Filter functionality, this module arranges for the
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| 297 | C<$_> variable to be populated with the key or value that a filter will
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| 298 | check. This usually means that most DBM filters tend to be very short.
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| 299 |
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| 300 | =head2 So what's new?
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| 301 |
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| 302 | The main enhancements over the standard DBM Filter hooks are:
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| 303 |
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| 304 | =over 4
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| 305 |
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| 306 | =item *
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| 307 |
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| 308 | A cleaner interface.
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| 309 |
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| 310 | =item *
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| 311 |
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| 312 | The ability to easily apply multiple filters to a single DBM file.
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| 313 |
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| 314 | =item *
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| 315 |
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| 316 | The ability to create "canned" filters. These allow commonly used filters
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| 317 | to be packaged into a stand-alone module.
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| 318 |
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| 319 | =back
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| 320 |
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| 321 | =head1 METHODS
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| 322 |
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| 323 | This module will arrange for the following methods to be available via
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| 324 | the object returned from the C<tie> call.
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| 325 |
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| 326 | =head2 $db->Filter_Push()
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| 327 |
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| 328 | =head2 $db->Filter_Key_Push()
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| 329 |
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| 330 | =head2 $db->Filter_Value_Push()
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| 331 |
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| 332 | Add a filter to filter stack for the database, C<$db>. The three formats
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| 333 | vary only in whether they apply to the DBM key, the DBM value or both.
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| 334 |
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| 335 | =over 5
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| 336 |
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| 337 | =item Filter_Push
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| 338 |
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| 339 | The filter is applied to I<both> keys and values.
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| 340 |
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| 341 | =item Filter_Key_Push
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| 342 |
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| 343 | The filter is applied to the key I<only>.
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| 344 |
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| 345 | =item Filter_Value_Push
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| 346 |
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| 347 | The filter is applied to the value I<only>.
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| 348 |
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| 349 | =back
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| 350 |
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| 351 |
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| 352 | =head2 $db->Filter_Pop()
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| 353 |
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| 354 | Removes the last filter that was applied to the DBM file associated with
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| 355 | C<$db>, if present.
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| 356 |
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| 357 | =head2 $db->Filtered()
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| 358 |
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| 359 | Returns TRUE if there are any filters applied to the DBM associated
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| 360 | with C<$db>. Otherwise returns FALSE.
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| 361 |
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| 362 |
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| 363 |
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| 364 | =head1 Writing a Filter
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| 365 |
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| 366 | Filters can be created in two main ways
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| 367 |
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| 368 | =head2 Immediate Filters
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| 369 |
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| 370 | An immediate filter allows you to specify the filter code to be used
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| 371 | at the point where the filter is applied to a dbm. In this mode the
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| 372 | Filter_*_Push methods expects to receive exactly two parameters.
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| 373 |
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| 374 | my $db = tie %hash, 'SDBM_File', ...
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| 375 | $db->Filter_Push( Store => sub { },
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| 376 | Fetch => sub { });
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| 377 |
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| 378 | The code reference associated with C<Store> will be called before any
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| 379 | key/value is written to the database and the code reference associated
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| 380 | with C<Fetch> will be called after any key/value is read from the
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| 381 | database.
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| 382 |
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| 383 | For example, here is a sample filter that adds a trailing NULL character
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| 384 | to all strings before they are written to the DBM file, and removes the
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| 385 | trailing NULL when they are read from the DBM file
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| 386 |
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| 387 | my $db = tie %hash, 'SDBM_File', ...
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| 388 | $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
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| 389 | Fetch => sub { s/\x00$// ; });
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| 390 |
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| 391 |
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| 392 | Points to note:
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| 393 |
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| 394 | =over 5
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| 395 |
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| 396 | =item 1.
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| 397 |
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| 398 | Both the Store and Fetch filters manipulate C<$_>.
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| 399 |
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| 400 | =back
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| 401 |
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| 402 | =head2 Canned Filters
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| 403 |
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| 404 | Immediate filters are useful for one-off situations. For more generic
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| 405 | problems it can be useful to package the filter up in its own module.
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| 406 |
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| 407 | The usage is for a canned filter is:
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| 408 |
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| 409 | $db->Filter_Push("name", params)
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| 410 |
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| 411 | where
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| 412 |
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| 413 | =over 5
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| 414 |
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| 415 | =item "name"
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| 416 |
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| 417 | is the name of the module to load. If the string specified does not
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| 418 | contain the package separator characters "::", it is assumed to refer to
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| 419 | the full module name "DBM_Filter::name". This means that the full names
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| 420 | for canned filters, "null" and "utf8", included with this module are:
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| 421 |
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| 422 | DBM_Filter::null
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| 423 | DBM_Filter::utf8
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| 424 |
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| 425 | =item params
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| 426 |
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| 427 | any optional parameters that need to be sent to the filter. See the
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| 428 | encode filter for an example of a module that uses parameters.
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| 429 |
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| 430 | =back
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| 431 |
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| 432 | The module that implements the canned filter can take one of two
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| 433 | forms. Here is a template for the first
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| 434 |
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| 435 | package DBM_Filter::null ;
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| 436 |
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| 437 | use strict;
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| 438 | use warnings;
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| 439 |
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| 440 | sub Store
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| 441 | {
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| 442 | # store code here
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| 443 | }
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| 444 |
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| 445 | sub Fetch
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| 446 | {
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| 447 | # fetch code here
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| 448 | }
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| 449 |
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| 450 | 1;
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| 451 |
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| 452 |
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| 453 | Notes:
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| 454 |
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| 455 | =over 5
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| 456 |
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| 457 | =item 1.
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| 458 |
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| 459 | The package name uses the C<DBM_Filter::> prefix.
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| 460 |
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| 461 | =item 2.
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| 462 |
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| 463 | The module I<must> have both a Store and a Fetch method. If only one is
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| 464 | present, or neither are present, a fatal error will be thrown.
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| 465 |
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| 466 | =back
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| 467 |
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| 468 | The second form allows the filter to hold state information using a
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| 469 | closure, thus:
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| 470 |
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| 471 | package DBM_Filter::encoding ;
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| 472 |
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| 473 | use strict;
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| 474 | use warnings;
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| 475 |
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| 476 | sub Filter
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| 477 | {
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| 478 | my @params = @_ ;
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| 479 |
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| 480 | ...
|
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| 481 | return {
|
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| 482 | Store => sub { $_ = $encoding->encode($_) },
|
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| 483 | Fetch => sub { $_ = $encoding->decode($_) }
|
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| 484 | } ;
|
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| 485 | }
|
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| 486 |
|
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| 487 | 1;
|
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| 488 |
|
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| 489 |
|
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| 490 | In this instance the "Store" and "Fetch" methods are encapsulated inside a
|
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| 491 | "Filter" method.
|
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| 492 |
|
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| 493 |
|
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| 494 | =head1 Filters Included
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| 495 |
|
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| 496 | A number of canned filers are provided with this module. They cover a
|
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| 497 | number of the main areas that filters are needed when interfacing with
|
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| 498 | DBM files. They also act as templates for your own filters.
|
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| 499 |
|
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| 500 | The filter included are:
|
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| 501 |
|
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| 502 | =over 5
|
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| 503 |
|
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| 504 | =item * utf8
|
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| 505 |
|
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| 506 | This module will ensure that all data written to the DBM will be encoded
|
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| 507 | in UTF-8.
|
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| 508 |
|
---|
| 509 | This module needs the Encode module.
|
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| 510 |
|
---|
| 511 | =item * encode
|
---|
| 512 |
|
---|
| 513 | Allows you to choose the character encoding will be store in the DBM file.
|
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| 514 |
|
---|
| 515 | =item * compress
|
---|
| 516 |
|
---|
| 517 | This filter will compress all data before it is written to the database
|
---|
| 518 | and uncompressed it on reading.
|
---|
| 519 |
|
---|
| 520 | This module needs Compress::Zlib.
|
---|
| 521 |
|
---|
| 522 | =item * int32
|
---|
| 523 |
|
---|
| 524 | This module is used when interoperating with a C/C++ application that
|
---|
| 525 | uses a C int as either the key and/or value in the DBM file.
|
---|
| 526 |
|
---|
| 527 | =item * null
|
---|
| 528 |
|
---|
| 529 | This module ensures that all data written to the DBM file is null
|
---|
| 530 | terminated. This is useful when you have a perl script that needs
|
---|
| 531 | to interoperate with a DBM file that a C program also uses. A fairly
|
---|
| 532 | common issue is for the C application to include the terminating null
|
---|
| 533 | in a string when it writes to the DBM file. This filter will ensure that
|
---|
| 534 | all data written to the DBM file can be read by the C application.
|
---|
| 535 |
|
---|
| 536 | =back
|
---|
| 537 |
|
---|
| 538 | =head1 NOTES
|
---|
| 539 |
|
---|
| 540 | =head2 Maintain Round Trip Integrity
|
---|
| 541 |
|
---|
| 542 | When writing a DBM filter it is I<very> important to ensure that it is
|
---|
| 543 | possible to retrieve all data that you have written when the DBM filter
|
---|
| 544 | is in place. In practice, this means that whatever transformation is
|
---|
| 545 | applied to the data in the Store method, the I<exact> inverse operation
|
---|
| 546 | should be applied in the Fetch method.
|
---|
| 547 |
|
---|
| 548 | If you don't provide an exact inverse transformation, you will find that
|
---|
| 549 | code like this will not behave as you expect.
|
---|
| 550 |
|
---|
| 551 | while (my ($k, $v) = each %hash)
|
---|
| 552 | {
|
---|
| 553 | ...
|
---|
| 554 | }
|
---|
| 555 |
|
---|
| 556 | Depending on the transformation, you will find that one or more of the
|
---|
| 557 | following will happen
|
---|
| 558 |
|
---|
| 559 | =over 5
|
---|
| 560 |
|
---|
| 561 | =item 1
|
---|
| 562 |
|
---|
| 563 | The loop will never terminate.
|
---|
| 564 |
|
---|
| 565 | =item 2
|
---|
| 566 |
|
---|
| 567 | Too few records will be retrieved.
|
---|
| 568 |
|
---|
| 569 | =item 3
|
---|
| 570 |
|
---|
| 571 | Too many will be retrieved.
|
---|
| 572 |
|
---|
| 573 | =item 4
|
---|
| 574 |
|
---|
| 575 | The loop will do the right thing for a while, but it will unexpectedly fail.
|
---|
| 576 |
|
---|
| 577 | =back
|
---|
| 578 |
|
---|
| 579 | =head2 Don't mix filtered & non-filtered data in the same database file.
|
---|
| 580 |
|
---|
| 581 | This is just a restatement of the previous section. Unless you are
|
---|
| 582 | completely certain you know what you are doing, avoid mixing filtered &
|
---|
| 583 | non-filtered data.
|
---|
| 584 |
|
---|
| 585 | =head1 EXAMPLE
|
---|
| 586 |
|
---|
| 587 | Say you need to interoperate with a legacy C application that stores
|
---|
| 588 | keys as C ints and the values and null terminated UTF-8 strings. Here
|
---|
| 589 | is how you would set that up
|
---|
| 590 |
|
---|
| 591 | my $db = tie %hash, 'SDBM_File', ...
|
---|
| 592 |
|
---|
| 593 | $db->Filter_Key_Push('int32') ;
|
---|
| 594 |
|
---|
| 595 | $db->Filter_Value_Push('utf8');
|
---|
| 596 | $db->Filter_Value_Push('null');
|
---|
| 597 |
|
---|
| 598 | =head1 SEE ALSO
|
---|
| 599 |
|
---|
| 600 | <DB_File>, L<GDBM_File>, L<NDBM_File>, L<ODBM_File>, L<SDBM_File>, L<perldbmfilter>
|
---|
| 601 |
|
---|
| 602 | =head1 AUTHOR
|
---|
| 603 |
|
---|
| 604 | Paul Marquess <[email protected]>
|
---|
| 605 |
|
---|