perl/lib/DBM_Filter.pm

package DBM_Filter ;

use strict;
use warnings;
our $VERSION = '0.01';

package Tie::Hash ;

use strict;
use warnings;

use Carp;


our %LayerStack = ();
our %origDESTROY = ();

our %Filters = map { $_, undef } qw(
            Fetch_Key
            Fetch_Value
            Store_Key
            Store_Value
        );

our %Options = map { $_, 1 } qw(
            fetch
            store
        );

#sub Filter_Enable
#{
#}
#
#sub Filter_Disable
#{
#}

sub Filtered
{
    my $this = shift;
    return defined $LayerStack{$this} ;
}

sub Filter_Pop
{
    my $this = shift;
    my $stack = $LayerStack{$this} || return undef ;
    my $filter = pop @{ $stack };

    # remove the filter hooks if this is the last filter to pop
    if ( @{ $stack } == 0 ) {
        $this->filter_store_key  ( undef );
        $this->filter_store_value( undef );
        $this->filter_fetch_key  ( undef );
        $this->filter_fetch_value( undef );
        delete $LayerStack{$this};
    }

    return $filter;
}

sub Filter_Key_Push
{
    &_do_Filter_Push;
}

sub Filter_Value_Push
{
    &_do_Filter_Push;
}


sub Filter_Push
{
    &_do_Filter_Push;
}

sub _do_Filter_Push
{
    my $this = shift;
    my %callbacks = ();
    my $caller = (caller(1))[3];
    $caller =~ s/^.*:://;
 
    croak "$caller: no parameters present" unless @_ ;

    if ( ! $Options{lc $_[0]} ) {
        my $class = shift;
        my @params = @_;

        # if $class already contains "::", don't prefix "DBM_Filter::"
        $class = "DBM_Filter::$class" unless $class =~ /::/;
    
        # does the "DBM_Filter::$class" exist?
        if ( ! defined %{ "${class}::"} ) {
            # Nope, so try to load it.
            eval " require $class ; " ;
            croak "$caller: Cannot Load DBM Filter '$class': $@" if $@;
        }
    
        no strict 'refs';
        my $fetch  = *{ "${class}::Fetch"  }{CODE};
        my $store  = *{ "${class}::Store"  }{CODE};
        my $filter = *{ "${class}::Filter" }{CODE};
        use strict 'refs';

        my $count = defined($filter) + defined($store) + defined($fetch) ;

        if ( $count == 0 )
          { croak "$caller: No methods (Filter, Fetch or Store) found in class '$class'" }
        elsif ( $count == 1 && ! defined $filter) {
           my $need = defined($fetch) ? 'Store' : 'Fetch';
           croak "$caller: Missing method '$need' in class '$class'" ;
        }
        elsif ( $count >= 2 && defined $filter)
          { croak "$caller: Can't mix Filter with Store and Fetch in class '$class'" }

        if (defined $filter) {
            my $callbacks = &{ $filter }(@params);
            croak "$caller: '${class}::Filter' did not return a hash reference" 
                unless ref $callbacks && ref $callbacks eq 'HASH';
            %callbacks = %{ $callbacks } ;
        }
        else {
            $callbacks{Fetch} = $fetch;
            $callbacks{Store} = $store;
        }
    }
    else {
        croak "$caller: not even params" unless @_ % 2 == 0;
        %callbacks = @_;
    }
    
    my %filters = %Filters ;
    my @got = ();
    while (my ($k, $v) = each %callbacks )
    {
        my $key = $k;
        $k = lc $k;
        if ($k eq 'fetch') {
            push @got, 'Fetch';
            if ($caller eq 'Filter_Push')
              { $filters{Fetch_Key} = $filters{Fetch_Value} = $v }
            elsif ($caller eq 'Filter_Key_Push')
              { $filters{Fetch_Key} = $v }
            elsif ($caller eq 'Filter_Value_Push')
              { $filters{Fetch_Value} = $v }
        }
        elsif ($k eq 'store') {
            push @got, 'Store';
            if ($caller eq 'Filter_Push')
              { $filters{Store_Key} = $filters{Store_Value} = $v }
            elsif ($caller eq 'Filter_Key_Push')
              { $filters{Store_Key} = $v }
            elsif ($caller eq 'Filter_Value_Push')
              { $filters{Store_Value} = $v }
        }
        else
          { croak "$caller: Unknown key '$key'" }

        croak "$caller: value associated with key '$key' is not a code reference"
            unless ref $v && ref $v eq 'CODE';
    }

    if ( @got != 2 ) {
        push @got, 'neither' if @got == 0 ;
        croak "$caller: expected both Store & Fetch - got @got";
    }

    # remember the class
    push @{ $LayerStack{$this} }, \%filters ;

    my $str_this = "$this" ; # Avoid a closure with $this in the subs below

    $this->filter_store_key  ( sub { store_hook($str_this, 'Store_Key')   });
    $this->filter_store_value( sub { store_hook($str_this, 'Store_Value') });
    $this->filter_fetch_key  ( sub { fetch_hook($str_this, 'Fetch_Key')   });
    $this->filter_fetch_value( sub { fetch_hook($str_this, 'Fetch_Value') });

    # Hijack the callers DESTROY method
    $this =~ /^(.*)=/;
    my $type = $1 ;
    no strict 'refs';
    if ( *{ "${type}::DESTROY" }{CODE} ne \&MyDESTROY )
    {
        $origDESTROY{$type} = *{ "${type}::DESTROY" }{CODE};
        no warnings 'redefine';
        *{ "${type}::DESTROY" } = \&MyDESTROY ;
    }
}

sub store_hook
{
    my $this = shift ;
    my $type = shift ;
    foreach my $layer (@{ $LayerStack{$this} })
    {
        &{ $layer->{$type} }() if defined $layer->{$type} ;
    }
}

sub fetch_hook
{
    my $this = shift ;
    my $type = shift ;
    foreach my $layer (reverse @{ $LayerStack{$this} })
    {
        &{ $layer->{$type} }() if defined $layer->{$type} ;
    }
}

sub MyDESTROY
{
    my $this = shift ;
    delete $LayerStack{$this} ;

    # call real DESTROY
    $this =~ /^(.*)=/;
    &{ $origDESTROY{$1} }($this);
}

1;

__END__

=head1 NAME

DBM_Filter -- Filter DBM keys/values 

=head1 SYNOPSIS

    use DBM_Filter ;
    use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File

    $db = tie %hash, ...

    $db->Filter_Push(Fetch => sub {...},
                     Store => sub {...});

    $db->Filter_Push('my_filter1');
    $db->Filter_Push('my_filter2', params...);

    $db->Filter_Key_Push(...) ;
    $db->Filter_Value_Push(...) ;

    $db->Filter_Pop();
    $db->Filtered();

    package DBM_Filter::my_filter1;
    
    sub Store { ... }
    sub Fetch { ... }

    1;

    package DBM_Filter::my_filter2;

    sub Filter
    {
        my @opts = @_;
        ...
        return (
            sub Store { ... },
            sub Fetch { ... } );
    }

    1;

=head1 DESCRIPTION

This module provides an interface that allows filters to be applied
to tied Hashes associated with DBM files. It builds on the DBM Filter
hooks that are present in all the *DB*_File modules included with the
standard Perl source distribution from version 5.6.1 onwards. In addition
to the *DB*_File modules distributed with Perl, the BerkeleyDB module,
available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter>
for more details on the DBM Filter hooks.

=head1 What is a DBM Filter?

A DBM Filter allows the keys and/or values in a tied hash to be modified
by some user-defined code just before it is written to the DBM file and
just after it is read back from the DBM file. For example, this snippet
of code

    $some_hash{"abc"} = 42;

could potentially trigger two filters, one for the writing of the key
"abc" and another for writing the value 42.  Similarly, this snippet

    my ($key, $value) = each %some_hash

will trigger two filters, one for the reading of the key and one for
the reading of the value.

Like the existing DBM Filter functionality, this module arranges for the
C<$_> variable to be populated with the key or value that a filter will
check. This usually means that most DBM filters tend to be very short.

=head2 So what's new?

The main enhancements over the standard DBM Filter hooks are:

=over 4

=item *

A cleaner interface.

=item *

The ability to easily apply multiple filters to a single DBM file.

=item *

The ability to create "canned" filters. These allow commonly used filters
to be packaged into a stand-alone module.

=back

=head1 METHODS

This module will arrange for the following methods to be available via
the object returned from the C<tie> call.

=head2 $db->Filter_Push()

=head2 $db->Filter_Key_Push()

=head2 $db->Filter_Value_Push()

Add a filter to filter stack for the database, C<$db>. The three formats
vary only in whether they apply to the DBM key, the DBM value or both.

=over 5

=item Filter_Push

The filter is applied to I<both> keys and values.

=item Filter_Key_Push

The filter is applied to the key I<only>.

=item Filter_Value_Push

The filter is applied to the value I<only>.

=back


=head2 $db->Filter_Pop()

Removes the last filter that was applied to the DBM file associated with
C<$db>, if present.

=head2 $db->Filtered()

Returns TRUE if there are any filters applied to the DBM associated
with C<$db>.  Otherwise returns FALSE.


=head1 Writing a Filter

Filters can be created in two main ways

=head2 Immediate Filters

An immediate filter allows you to specify the filter code to be used
at the point where the filter is applied to a dbm. In this mode the
Filter_*_Push methods expects to receive exactly two parameters.

    my $db = tie %hash, 'SDBM_File', ...
    $db->Filter_Push( Store => sub { },
                      Fetch => sub { });

The code reference associated with C<Store> will be called before any
key/value is written to the database and the code reference associated
with C<Fetch> will be called after any key/value is read from the
database.

For example, here is a sample filter that adds a trailing NULL character
to all strings before they are written to the DBM file, and removes the
trailing NULL when they are read from the DBM file

    my $db = tie %hash, 'SDBM_File', ...
    $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
                      Fetch => sub { s/\x00$// ;    });


Points to note:

=over 5

=item 1.

Both the Store and Fetch filters manipulate C<$_>.

=back

=head2 Canned Filters

Immediate filters are useful for one-off situations. For more generic
problems it can be useful to package the filter up in its own module.

The usage is for a canned filter is:

    $db->Filter_Push("name", params)

where

=over 5

=item "name"

is the name of the module to load. If the string specified does not
contain the package separator characters "::", it is assumed to refer to
the full module name "DBM_Filter::name". This means that the full names
for canned filters, "null" and "utf8", included with this module are:

    DBM_Filter::null
    DBM_Filter::utf8

=item params

any optional parameters that need to be sent to the filter. See the
encode filter for an example of a module that uses parameters.

=back

The module that implements the canned filter can take one of two
forms. Here is a template for the first

    package DBM_Filter::null ;

    use strict;
    use warnings;

    sub Store 
    {
        # store code here    
    }

    sub Fetch
    {
        # fetch code here
    }

    1;


Notes:

=over 5

=item 1.

The package name uses the C<DBM_Filter::> prefix.

=item 2.

The module I<must> have both a Store and a Fetch method. If only one is
present, or neither are present, a fatal error will be thrown.

=back

The second form allows the filter to hold state information using a
closure, thus:

    package DBM_Filter::encoding ;

    use strict;
    use warnings;

    sub Filter
    {
        my @params = @_ ;

        ...
        return {
            Store   => sub { $_ = $encoding->encode($_) },
            Fetch   => sub { $_ = $encoding->decode($_) }
            } ;
    }

    1;


In this instance the "Store" and "Fetch" methods are encapsulated inside a
"Filter" method.


=head1 Filters Included

A number of canned filers are provided with this module. They cover a
number of the main areas that filters are needed when interfacing with
DBM files. They also act as templates for your own filters.

The filter included are:

=over 5

=item * utf8

This module will ensure that all data written to the DBM will be encoded
in UTF-8.

This module needs the Encode module.

=item * encode

Allows you to choose the character encoding will be store in the DBM file.

=item * compress

This filter will compress all data before it is written to the database
and uncompressed it on reading.

This module needs Compress::Zlib. 

=item * int32

This module is used when interoperating with a C/C++ application that
uses a C int as either the key and/or value in the DBM file.

=item * null

This module ensures that all data written to the DBM file is null
terminated. This is useful when you have a perl script that needs
to interoperate with a DBM file that a C program also uses. A fairly
common issue is for the C application to include the terminating null
in a string when it writes to the DBM file. This filter will ensure that
all data written to the DBM file can be read by the C application.

=back

=head1 NOTES

=head2 Maintain Round Trip Integrity

When writing a DBM filter it is I<very> important to ensure that it is
possible to retrieve all data that you have written when the DBM filter
is in place. In practice, this means that whatever transformation is
applied to the data in the Store method, the I<exact> inverse operation
should be applied in the Fetch method.

If you don't provide an exact inverse transformation, you will find that
code like this will not behave as you expect.

     while (my ($k, $v) = each %hash)
     {
         ...
     }

Depending on the transformation, you will find that one or more of the
following will happen

=over 5

=item 1

The loop will never terminate.

=item 2

Too few records will be retrieved.

=item 3

Too many will be retrieved.

=item 4

The loop will do the right thing for a while, but it will unexpectedly fail. 

=back

=head2 Don't mix filtered & non-filtered data in the same database file. 

This is just a restatement of the previous section. Unless you are
completely certain you know what you are doing, avoid mixing filtered &
non-filtered data.

=head1 EXAMPLE

Say you need to interoperate with a legacy C application that stores
keys as C ints and the values and null terminated UTF-8 strings. Here
is how you would set that up

    my $db = tie %hash, 'SDBM_File', ...

    $db->Filter_Key_Push('int32') ;

    $db->Filter_Value_Push('utf8');
    $db->Filter_Value_Push('null');

=head1 SEE ALSO

<DB_File>,  L<GDBM_File>, L<NDBM_File>, L<ODBM_File>, L<SDBM_File>, L<perldbmfilter>

=head1 AUTHOR

Paul Marquess <pmqs@cpan.org>

Revision:	1.1
Committed:	Thu Jun 30 14:27:08 2005 UTC (21 years, 4 months ago) by root
Branch:	MAIN
CVS Tags:	PERL-5-8-7, HEAD
Branch point for:	PERL
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	package DBM_Filter ;
2
3			use strict;
4			use warnings;
5			our $VERSION = '0.01';
6
7			package Tie::Hash ;
8
9			use strict;
10			use warnings;
11
12			use Carp;
13
14
15			our %LayerStack = ();
16			our %origDESTROY = ();
17
18			our %Filters = map { $_, undef } qw(
19			Fetch_Key
20			Fetch_Value
21			Store_Key
22			Store_Value
23			);
24
25			our %Options = map { $_, 1 } qw(
26			fetch
27			store
28			);
29
30			#sub Filter_Enable
31			#{
32			#}
33			#
34			#sub Filter_Disable
35			#{
36			#}
37
38			sub Filtered
39			{
40			my $this = shift;
41			return defined $LayerStack{$this} ;
42			}
43
44			sub Filter_Pop
45			{
46			my $this = shift;
47			my $stack = $LayerStack{$this} \|\| return undef ;
48			my $filter = pop @{ $stack };
49
50			# remove the filter hooks if this is the last filter to pop
51			if ( @{ $stack } == 0 ) {
52			$this->filter_store_key ( undef );
53			$this->filter_store_value( undef );
54			$this->filter_fetch_key ( undef );
55			$this->filter_fetch_value( undef );
56			delete $LayerStack{$this};
57			}
58
59			return $filter;
60			}
61
62			sub Filter_Key_Push
63			{
64			&_do_Filter_Push;
65			}
66
67			sub Filter_Value_Push
68			{
69			&_do_Filter_Push;
70			}
71
72
73			sub Filter_Push
74			{
75			&_do_Filter_Push;
76			}
77
78			sub _do_Filter_Push
79			{
80			my $this = shift;
81			my %callbacks = ();
82			my $caller = (caller(1))[3];
83			$caller =~ s/^.*:://;
84
85			croak "$caller: no parameters present" unless @_ ;
86
87			if ( ! $Options{lc $_[0]} ) {
88			my $class = shift;
89			my @params = @_;
90
91			# if $class already contains "::", don't prefix "DBM_Filter::"
92			$class = "DBM_Filter::$class" unless $class =~ /::/;
93
94			# does the "DBM_Filter::$class" exist?
95			if ( ! defined %{ "${class}::"} ) {
96			# Nope, so try to load it.
97			eval " require $class ; " ;
98			croak "$caller: Cannot Load DBM Filter '$class': $@" if $@;
99			}
100
101			no strict 'refs';
102			my $fetch = *{ "${class}::Fetch" }{CODE};
103			my $store = *{ "${class}::Store" }{CODE};
104			my $filter = *{ "${class}::Filter" }{CODE};
105			use strict 'refs';
106
107			my $count = defined($filter) + defined($store) + defined($fetch) ;
108
109			if ( $count == 0 )
110			{ croak "$caller: No methods (Filter, Fetch or Store) found in class '$class'" }
111			elsif ( $count == 1 && ! defined $filter) {
112			my $need = defined($fetch) ? 'Store' : 'Fetch';
113			croak "$caller: Missing method '$need' in class '$class'" ;
114			}
115			elsif ( $count >= 2 && defined $filter)
116			{ croak "$caller: Can't mix Filter with Store and Fetch in class '$class'" }
117
118			if (defined $filter) {
119			my $callbacks = &{ $filter }(@params);
120			croak "$caller: '${class}::Filter' did not return a hash reference"
121			unless ref $callbacks && ref $callbacks eq 'HASH';
122			%callbacks = %{ $callbacks } ;
123			}
124			else {
125			$callbacks{Fetch} = $fetch;
126			$callbacks{Store} = $store;
127			}
128			}
129			else {
130			croak "$caller: not even params" unless @_ % 2 == 0;
131			%callbacks = @_;
132			}
133
134			my %filters = %Filters ;
135			my @got = ();
136			while (my ($k, $v) = each %callbacks )
137			{
138			my $key = $k;
139			$k = lc $k;
140			if ($k eq 'fetch') {
141			push @got, 'Fetch';
142			if ($caller eq 'Filter_Push')
143			{ $filters{Fetch_Key} = $filters{Fetch_Value} = $v }
144			elsif ($caller eq 'Filter_Key_Push')
145			{ $filters{Fetch_Key} = $v }
146			elsif ($caller eq 'Filter_Value_Push')
147			{ $filters{Fetch_Value} = $v }
148			}
149			elsif ($k eq 'store') {
150			push @got, 'Store';
151			if ($caller eq 'Filter_Push')
152			{ $filters{Store_Key} = $filters{Store_Value} = $v }
153			elsif ($caller eq 'Filter_Key_Push')
154			{ $filters{Store_Key} = $v }
155			elsif ($caller eq 'Filter_Value_Push')
156			{ $filters{Store_Value} = $v }
157			}
158			else
159			{ croak "$caller: Unknown key '$key'" }
160
161			croak "$caller: value associated with key '$key' is not a code reference"
162			unless ref $v && ref $v eq 'CODE';
163			}
164
165			if ( @got != 2 ) {
166			push @got, 'neither' if @got == 0 ;
167			croak "$caller: expected both Store & Fetch - got @got";
168			}
169
170			# remember the class
171			push @{ $LayerStack{$this} }, \%filters ;
172
173			my $str_this = "$this" ; # Avoid a closure with $this in the subs below
174
175			$this->filter_store_key ( sub { store_hook($str_this, 'Store_Key') });
176			$this->filter_store_value( sub { store_hook($str_this, 'Store_Value') });
177			$this->filter_fetch_key ( sub { fetch_hook($str_this, 'Fetch_Key') });
178			$this->filter_fetch_value( sub { fetch_hook($str_this, 'Fetch_Value') });
179
180			# Hijack the callers DESTROY method
181			$this =~ /^(.*)=/;
182			my $type = $1 ;
183			no strict 'refs';
184			if ( *{ "${type}::DESTROY" }{CODE} ne \&MyDESTROY )
185			{
186			$origDESTROY{$type} = *{ "${type}::DESTROY" }{CODE};
187			no warnings 'redefine';
188			*{ "${type}::DESTROY" } = \&MyDESTROY ;
189			}
190			}
191
192			sub store_hook
193			{
194			my $this = shift ;
195			my $type = shift ;
196			foreach my $layer (@{ $LayerStack{$this} })
197			{
198			&{ $layer->{$type} }() if defined $layer->{$type} ;
199			}
200			}
201
202			sub fetch_hook
203			{
204			my $this = shift ;
205			my $type = shift ;
206			foreach my $layer (reverse @{ $LayerStack{$this} })
207			{
208			&{ $layer->{$type} }() if defined $layer->{$type} ;
209			}
210			}
211
212			sub MyDESTROY
213			{
214			my $this = shift ;
215			delete $LayerStack{$this} ;
216
217			# call real DESTROY
218			$this =~ /^(.*)=/;
219			&{ $origDESTROY{$1} }($this);
220			}
221
222			1;
223
224			__END__
225
226			=head1 NAME
227
228			DBM_Filter -- Filter DBM keys/values
229
230			=head1 SYNOPSIS
231
232			use DBM_Filter ;
233			use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File
234
235			$db = tie %hash, ...
236
237			$db->Filter_Push(Fetch => sub {...},
238			Store => sub {...});
239
240			$db->Filter_Push('my_filter1');
241			$db->Filter_Push('my_filter2', params...);
242
243			$db->Filter_Key_Push(...) ;
244			$db->Filter_Value_Push(...) ;
245
246			$db->Filter_Pop();
247			$db->Filtered();
248
249			package DBM_Filter::my_filter1;
250
251			sub Store { ... }
252			sub Fetch { ... }
253
254			1;
255
256			package DBM_Filter::my_filter2;
257
258			sub Filter
259			{
260			my @opts = @_;
261			...
262			return (
263			sub Store { ... },
264			sub Fetch { ... } );
265			}
266
267			1;
268
269			=head1 DESCRIPTION
270
271			This module provides an interface that allows filters to be applied
272			to tied Hashes associated with DBM files. It builds on the DBM Filter
273			hooks that are present in all the DB_File modules included with the
274			standard Perl source distribution from version 5.6.1 onwards. In addition
275			to the DB_File modules distributed with Perl, the BerkeleyDB module,
276			available on CPAN, supports the DBM Filter hooks. See L<perldbmfilter>
277			for more details on the DBM Filter hooks.
278
279			=head1 What is a DBM Filter?
280
281			A DBM Filter allows the keys and/or values in a tied hash to be modified
282			by some user-defined code just before it is written to the DBM file and
283			just after it is read back from the DBM file. For example, this snippet
284			of code
285
286			$some_hash{"abc"} = 42;
287
288			could potentially trigger two filters, one for the writing of the key
289			"abc" and another for writing the value 42. Similarly, this snippet
290
291			my ($key, $value) = each %some_hash
292
293			will trigger two filters, one for the reading of the key and one for
294			the reading of the value.
295
296			Like the existing DBM Filter functionality, this module arranges for the
297			C<$_> variable to be populated with the key or value that a filter will
298			check. This usually means that most DBM filters tend to be very short.
299
300			=head2 So what's new?
301
302			The main enhancements over the standard DBM Filter hooks are:
303
304			=over 4
305
306			=item *
307
308			A cleaner interface.
309
310			=item *
311
312			The ability to easily apply multiple filters to a single DBM file.
313
314			=item *
315
316			The ability to create "canned" filters. These allow commonly used filters
317			to be packaged into a stand-alone module.
318
319			=back
320
321			=head1 METHODS
322
323			This module will arrange for the following methods to be available via
324			the object returned from the C<tie> call.
325
326			=head2 $db->Filter_Push()
327
328			=head2 $db->Filter_Key_Push()
329
330			=head2 $db->Filter_Value_Push()
331
332			Add a filter to filter stack for the database, C<$db>. The three formats
333			vary only in whether they apply to the DBM key, the DBM value or both.
334
335			=over 5
336
337			=item Filter_Push
338
339			The filter is applied to I<both> keys and values.
340
341			=item Filter_Key_Push
342
343			The filter is applied to the key I<only>.
344
345			=item Filter_Value_Push
346
347			The filter is applied to the value I<only>.
348
349			=back
350
351
352			=head2 $db->Filter_Pop()
353
354			Removes the last filter that was applied to the DBM file associated with
355			C<$db>, if present.
356
357			=head2 $db->Filtered()
358
359			Returns TRUE if there are any filters applied to the DBM associated
360			with C<$db>. Otherwise returns FALSE.
361
362
363
364			=head1 Writing a Filter
365
366			Filters can be created in two main ways
367
368			=head2 Immediate Filters
369
370			An immediate filter allows you to specify the filter code to be used
371			at the point where the filter is applied to a dbm. In this mode the
372			Filter_*_Push methods expects to receive exactly two parameters.
373
374			my $db = tie %hash, 'SDBM_File', ...
375			$db->Filter_Push( Store => sub { },
376			Fetch => sub { });
377
378			The code reference associated with C<Store> will be called before any
379			key/value is written to the database and the code reference associated
380			with C<Fetch> will be called after any key/value is read from the
381			database.
382
383			For example, here is a sample filter that adds a trailing NULL character
384			to all strings before they are written to the DBM file, and removes the
385			trailing NULL when they are read from the DBM file
386
387			my $db = tie %hash, 'SDBM_File', ...
388			$db->Filter_Push( Store => sub { $_ .= "\x00" ; },
389			Fetch => sub { s/\x00$// ; });
390
391
392			Points to note:
393
394			=over 5
395
396			=item 1.
397
398			Both the Store and Fetch filters manipulate C<$_>.
399
400			=back
401
402			=head2 Canned Filters
403
404			Immediate filters are useful for one-off situations. For more generic
405			problems it can be useful to package the filter up in its own module.
406
407			The usage is for a canned filter is:
408
409			$db->Filter_Push("name", params)
410
411			where
412
413			=over 5
414
415			=item "name"
416
417			is the name of the module to load. If the string specified does not
418			contain the package separator characters "::", it is assumed to refer to
419			the full module name "DBM_Filter::name". This means that the full names
420			for canned filters, "null" and "utf8", included with this module are:
421
422			DBM_Filter::null
423			DBM_Filter::utf8
424
425			=item params
426
427			any optional parameters that need to be sent to the filter. See the
428			encode filter for an example of a module that uses parameters.
429
430			=back
431
432			The module that implements the canned filter can take one of two
433			forms. Here is a template for the first
434
435			package DBM_Filter::null ;
436
437			use strict;
438			use warnings;
439
440			sub Store
441			{
442			# store code here
443			}
444
445			sub Fetch
446			{
447			# fetch code here
448			}
449
450			1;
451
452
453			Notes:
454
455			=over 5
456
457			=item 1.
458
459			The package name uses the C<DBM_Filter::> prefix.
460
461			=item 2.
462
463			The module I<must> have both a Store and a Fetch method. If only one is
464			present, or neither are present, a fatal error will be thrown.
465
466			=back
467
468			The second form allows the filter to hold state information using a
469			closure, thus:
470
471			package DBM_Filter::encoding ;
472
473			use strict;
474			use warnings;
475
476			sub Filter
477			{
478			my @params = @_ ;
479
480			...
481			return {
482			Store => sub { $_ = $encoding->encode($_) },
483			Fetch => sub { $_ = $encoding->decode($_) }
484			} ;
485			}
486
487			1;
488
489
490			In this instance the "Store" and "Fetch" methods are encapsulated inside a
491			"Filter" method.
492
493
494			=head1 Filters Included
495
496			A number of canned filers are provided with this module. They cover a
497			number of the main areas that filters are needed when interfacing with
498			DBM files. They also act as templates for your own filters.
499
500			The filter included are:
501
502			=over 5
503
504			=item * utf8
505
506			This module will ensure that all data written to the DBM will be encoded
507			in UTF-8.
508
509			This module needs the Encode module.
510
511			=item * encode
512
513			Allows you to choose the character encoding will be store in the DBM file.
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 <pmqs@cpan.org>
605